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Maturation impact on polyaromatic hydrocarbons and organosulfur compounds in the Carboniferous Keluke Formation from Qaidam basin, NW China Zongxing Li, Haiping Huang, Chuan He, and Xinxin Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00403 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Maturation impact on polyaromatic hydrocarbons and organosulfur compounds in the Carboniferous

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Keluke Formation from Qaidam basin, NW China

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Zongxing Li †, Haiping Huang ‡, §*, Chuan He ‡, §, Xinxin Fang †

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† The

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Academy of Geological Sciences, Beijing, China;

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‡

School of Energy Resource, China University of Geosciences, Beijing 100083, China

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§

Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4,

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Canada

Key Laboratory of Shale Oil and Gas Geological Survey, Institute of Geomechanics, Chinese

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* Corresponding author. Department of Geoscience, University of Calgary, 2500 University Drive NW,

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Calgary, AB T2N 1N4, Canada. Email: [email protected]. Tel. 1-403-2208396 (H. Huang).

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Abstract

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A maturity sequence from the Keluke Formation of the Upper Carboniferous marine-continental

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transitional depositional environment in the Qaidam basin, NW China has been geochemically

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characterized by bulk and molecular compositions, especially the behavior of polyaromatic hydrocarbons

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and organosulfur compounds. Some commonly used maturity parameters such as methylnaphthalene ratio

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(MNR), dimethylnaphthalene ratio (DNR), methylphenanthrene ratio (MPR), methylphenanthrene index

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3 (MPI-3), dimethylphenanthrene index 2 (DMPI-2), methyldibenzothiophene ratio (MDR) and

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dimethyldibenzothiophene (DMDBT) ratios (4,6-/16,-+1,8-+l,4-DMDBT and 2,4-/1,6-+1,8-+l,4-

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DMDBT), Rock-Eval Tmax and measured vitrinite reflectance (%Ro) increase gradually with burial depth,

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whereas others such as trimethylnaphthalene ratio (TMNr), tetramethylnaphthalene ratio (TeMNr), MPI-1

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and DMPI-1 show no correlation with these maturity indicators. The calculated equivalent Ro values from

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MPR and MDR based on empirical correlation reported in the literature overestimate the maturity level.

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The degree of alkylation plays the dominant role in molecular compositional variation and maturity 1

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parameter validity, which is in turn controlled by the nature of organic input, depositional environment

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and lithology rather than solely controlled by maturation. Dealkylation of alkylnaphthalenes at Ro ~1.0%

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removes most thermally unstable isomers from C3- and C4 homologues, which makes parameters based

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upon them lost the sensitivity. The proportion of phenanthrene varies greatly in marine-continental

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transitional depositional system and the involvement of phenanthrene in the formulation makes MPI-1

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and DMPI-1 fail to reflect maturity level. Overestimation of maturity level based on the degree of

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isomerization is caused by high catalytic effect in marine-continental transitional depositional system,

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which facilitates isomerization and dealkylation processes.

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1 Introduction

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The search for reliable thermal maturity indicators to determine the maturity of oil and source rock is an

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important objective in petroleum geochemistry especially for shale resource where most biomarkers are

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unreliable. 1-4 Polyaromatic hydrocarbons (PAHs) and organosulfur compounds (OSCs) formed by

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complex thermal bond-breaking and dehydrogenation reactions from biogenic precursors during

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diagenetic and catagenetic processes or pyrolytic process are widely distributed in highly matured oils,

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source rocks and coals and their relative abundances and structural isomer distributions can serve as

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reliable maturity assessment tool. 5-12 Generally, these maturity parameters rely either on the degree of

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alkylation of a given parent compound or a shift in the isomer distribution of homologues towards

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thermally more stable isomers. It has been assumed that significant quantities of alkylaromatic

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hydrocarbons in sediments may be derived by alkylation of parent aromatic hydrocarbons 6, 13-15 and the

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degree of alkylation may reflect level of maturity. However, dealkylation of highly alkylated homologue

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to less alkylated homologue or parent compound occurs at high maturity stage. 6-10, 16-17 The link between

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degrees of alkylation and dealkylation to assess maturity levels has rarely been investigated. 11

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The most widely used maturation parameters are derived from the isomer distribution of homologues of

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alkylnaphthalenes, alkylphenanthrenes and alkyldibenzothiophenes. During maturation, thermally less 2


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stable isomers with an α-position substituent will be transferred to thermally more stable isomers in the β-

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position. A number of maturity parameters have been demonstrated based on this principle. 5-10, 18-21 In

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contrast to biomarkers (rapid depletion of concentrations and early equilibrium isomerization reactions

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with increasing maturity), substantially high concentrations of the key alkyl-aromatics persist at elevated

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maturation levels and maturity parameters derived from PAHs and OSCs are typically effective across the

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entire oil generation window. 5-10, 22-24

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In addition to thermal maturation, it has been suggested that the abundance and distribution of PAHs and

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OSCs in sediment extracts or oils of different ages are influenced by the origin of the organic matter and

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depositional environments. The irregular trends in various aromatic hydrocarbon maturity parameters

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have been observed in many case studies. 8, 22, 25-27 Kruge 23 reported that tri- and tetramethylnaphthalenes

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are useful molecular maturity parameters, while the relative distributions of C0–C3 alkylphenanthrenes,

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dibenzothiophene, methyldibenzothiophenes and methyldibenzofurans are affected by organic matter type

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differences. Fabianska and Kruszewska 28 found that methydibenzothiophene ratio (MDR) and

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methylphenanthrene index-1 (MPI-1) show good correlation with vitrinite reflectance but

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trimethylnaphthatene ratio (TMR-2), tetramethylnaphthalene ratio (TeMNr) and dimethylphenenthrene

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ratio (DMPR) have lower correlation coefficients. Borrego et al. 29 noted the correlation between

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alkylphenanthrenes and vitrinite reflectance is fairly good but the correlation between

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alkyldibenzothiophenes and vitrinite reflectance is poor. Requejo et al. 30 found that the aromatic

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parameters based on trimethylnaphthalene and methyldibenzothiophene distributions vary with estimated

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level of maturity, while parameters based on dimethylnaphthalene and methylphenanthrene distributions

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do not accurately reflect maturity level. Radke et al. 8 proposed that maturity parameters based on the

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methylphenanthrenes are applicable only to kerogen Types III and II/III. Budzinski et al. 22 suggested that

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based on the presence of specific methyl-, dimethyl-, and trimethylphenanthrene isomers maturation

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effect can be separated from organic input influence. Kruge 23 further recommended adding the ratio of

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dibenzothiophenes to dibenzofurans to help discriminate organic type from maturation. Cassini et al. 25 3


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attributed anomalous MPI values to lithologic changes as a result of variations in the organic matter input

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during deposition and they suggested that methylphenanthrene index could be a useful parameter for the

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thermal maturity estimation at high level of thermal maturation. Similarly, Radke 9 suggested that MPI-1

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may be useful in the maturity evaluation of post-mature crude oils and condensates. In summary, aromatic

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hydrocarbon maturity parameters are controlled by organofacies and diagenetic and catagenetic processes.

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Parameters applicable in one case history may be invalid for other cases and no universal correlation

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between aromatic component ratios and vitrinite reflectance can be established.

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While aromatic maturity parameters have been expected to be valid at high maturity stage since the

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impact of the organic facies might be reduced with increasing maturity 9, 25, the actual causes for

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aforementioned failures of maturity assessment vary from case to case. It is now important to understand

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how and to what extent factors of organofacies may influence the distributions of PAHs and OSCs. The

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Carboniferous strata in the Qaidam basin NW China is coal bearing source rocks which are sufficiently

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different from marine and lacustrine source rocks in their organic matter characteristics. The possible

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impact of source input and lithology on thermal maturation has been taken into account during the present

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interpretation of molecular data. The objective of this study is to test the suitability of the distributions of

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PAHs and OSCs in determining thermal maturity levels in marine-continental transitional system. The

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observations presented in this paper enable us to have a closer look at the mechanisms that are involved in

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determining the relative abundances and isomer distributions of PAHs and OSCs in core extracts and

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further verify their applicability in maturity assessment.

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2 Geologic setting

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The Qaidam basin is the largest active inter-mountain basin in the northern part of the Qinghai–Tibet

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plateau with an average elevation of ~2800 m and an area of 121,000 km2. It is bounded by the Altyn

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Tagh fault in the west, the Qilian thrust belt in the north, and the Kunlun thrust belt in the south (Fig. 1).

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The Proterozoic-Paleozoic basement mainly consists of metamorphic rocks, flysch, and carbonate and the 4


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marine sedimentation ceased by the end of the Triassic as a result of the amalgamation of the Qiangtang

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terrane to the southern margin of Asia. 31 The basin underwent three developmental stages: (1) Lower

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Paleozoic to Carboniferous characterized by a simple synclinal depression; (2) Lower Permian to

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Cretaceous compressional uplifting and denudation marked by the occurrence of reverse faults and (3)

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Paleogene compressional strike-slip depressions and Neogene–Quaternary compressional folding and

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depressions, featured intra-basinal deformation and uplift. 32

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The Carboniferous strata are mainly deposited during marine transgression and regression cycle. The

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platform facies carbonate and marine-terrigenous transitional facies coal bearing depositions are well

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developed in the study area. 33 The Lower Carboniferous consists of Chuanshangou, Chengqianggou and

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Huaitoutala formations (from oldest to youngest). The Chuanshangou Formation is composed of oolitic

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limestone and bioclastic limestone interbedded with calcareous shale and siltstone. The Chengqianggou

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Formation consists of gray mudstone, silty limestone and bioclastic limestone, which can hardly be

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differentiated from the Chuanshangou Formation in most area. The Huaitoutala Formation is composed of

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chert limestone and bioclastic limestone interbedded with silty mudstone. The Upper Carboniferous

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contains the Keluke and Zabusagaxiu formations. The Keluke Formation consists of marl, dark-colored

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shale, carbonaceous shale and coal, which is the main organic-rich source rock section developed in the

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Carboniferous (Fig. 2). 34-35 The Zabusagaxiu Formation is composed of massive bioclastic limestone

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interbedded with siltstone, carbonaceous shale and coal, which is overlaid by the Jurassic unconformity.

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While no commercial oil and gas have been produced from the Carboniferous strata, oil seeps found

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within interlayers and fractures of the thick bioclastic limestone in the Huaitoutala Formation in periphery

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outcrops show close affinity biomarker compositions with source rocks in the Keluke Formation.34

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3 Samples and Experimental Methods

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Ten core samples have been collected from the Upper Carboniferous Keluke Formation in well ZK5-1

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(Fig. 1; Table 1). The crushed samples (about 70 mg) have been pyrolyzed by using Rock-Eval 6 5


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apparatus in programmed pyrolysis oven from ambient temperature to 650 °C at 25 °C/min. A flame

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ionization detector measured the hydrocarbons generated from these rocks and an infrared detector

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detector measured the CO2 generated from the rocks. Rock-Eval pyrolysis parameters, such as S1, S2, S3

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and Tmax can be obtained from direct measurement.

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Measurements of mean random vitrinite reflectance (Ro) were done at 546 nm in oil with an Axioplan–

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Opton microphotometer and Opton 20 Microscope System Processor exanimated on whole rock. Sample

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preparation and point counts were carried out in accordance with the procedure of International

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Committee of Organic Petrology.

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Extractable organic matter (EOM) of powdered rock (80-100 mesh) samples was determined by Soxhlet

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extraction with dichloromethane (93% v) and methanol (7% v) for 72 h. Elemental sulfur was removed

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during the extraction procedure with activated copper strips. The weight of extract was determined by

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home developed method to avoid solvent evaporation and light end loss. The extracts were separated into

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saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes by C18 non-endcapped (NEC)

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SPE cartridge followed by silver nitrate impregnated silica SPE column. 36

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Gas chromatography-mass spectrometry was accomplished using an Agilent 5975C MSD system

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interfaced to an Agilent 7890A gas chromatograph. A DB‒1MS fused silica capillary column (60 m ×

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0.32 mm i.d. × 0.25 μm film thickness) and a HP‒5MS column (60 m × 0.32 m × 0.25 μm) were used for

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saturated and aromatic hydrocarbon fractions, respectively without any solvent evaporation. The oven

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temperature was initially set at 40 °C for 5 min, programmed to 325 °C at 3 °C/min and held for 20 min.

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Helium was used as the carrier gas with constant flow rate of 1 ml/min. Both interface temperature and

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injector temperature were 300 °C. The ion source was operated in the electron ionization mode at 70 eV

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and selected ion monitoring was used. Peak area relative to an appropriate internal standard was used for

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concentration calculation and no response factor calibration has been applied.

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6


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4 Result

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4.1 Bulk organic matter characterization

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Rock-Eval pyrolysis results for the studied samples are listed in Table 1. Total organic carbon (TOC)

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values vary in the range of 1.21‒6.83 wt% with an average of 2.96%, indicating fairly high organic

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content. Pure mudstone and marl have higher TOC content than silty mudstones. However, about 95% of

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organic matter determined by TOC is residual carbon (RC), indicating hydrocarbon generation potential

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has largely been exhausted.

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The existing free hydrocarbon in the source rocks (S1 peak) released during thermal extraction at 300 °C

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is fairly low ranging from 0.02 to 0.61 mg/g rock with an average value of 0.15 mg/g rock. The remaining

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generation capacity of kerogen (S2) released during pyrolysis varies from 0.35 to 5.11 mg/g rock with an

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average value of 1.62 mg/g rock, indicating poor quality. Solvent extractable organic matter (EOM) also

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shows low absolute yields ranging from 0.18 to 1.38 mg/g with an average value of 0.42 mg/g (Table 1).

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Calculated hydrogen indices (HI = S2/TOC × 100) for the studied samples are in the range of 25 to 84 mg

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HC/g TOC. Very low HI value may indicate poor kerogen quality or exhausted generation potential. Total

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hydrocarbon generation potential (S1 + S2) varies in the range of 0.37 to 5.72 mg/g rock with an average

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value of 1.77 mg/g rock. The production index [PI = S1/(S1 + S2)] shows unusually low values ranging

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5.3% to 14.3%, suggesting migration loss of generated hydrocarbons.

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Rock Eval Tmax values, the temperature corresponding to the maximum generation of pyrolytic

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hydrocarbons (S2), are in the range of 455 and 488 °C and show systematic increase with burial depth

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(Fig. 3a). Vitrinite reflectance (Ro) is the most common and trustworthy approach for the determination of

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thermal maturity. Ro values for the studied samples are in the range of 0.79% to 1.45%, which is also

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linearly correlated with burial depth (Fig. 3b). The linear correlation coefficient between Tmax and %Ro is

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0.94. Both measurements suggest that burial depth exerts the dominant control on maturity levels and

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organic matters in the Keluke Formation are currently at mature to overmature thermal evolution stage.

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However, to interpret the maturity trends properly, the data must be placed in the context of the burial 7


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history of the sediments. If sedimentation has been fairly continuous from initiation to present day and

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there have been no major changes in heat flow, the vitrinite reflectance trend indicates current maturity.

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But when a unconformity is present, as shown in the present study, the vitrinite reflectance trend indicates

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the maximum maturity level of the strata have ever reached before the uplift and erosion. While the

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reconstruction of burial history is out of the scope of current study, the depth of oil and gas generation

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window needs to be adjusted for the sediment lost. The measured data indicate the top of the oil window

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at 0.6% Ro has been eroded and the current samples cover the peak oil generation to the end of oil

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generation interval.

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4.2 Geochemical characteristics of aromatic hydrocarbon fraction

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Total ion chromatograms (TIC) of the aromatic hydrocarbon fraction of core extracts from the

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Carboniferous sediments are illustrated in Fig. 4. The extracted aromatic hydrocarbons are dominated by

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2–4 ring components mainly composed by alkylnaphthalenes, alkylbiphenyls, alkylphenanthrenes,

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alkyldibenzothiophenes and alkylchrysenes. The monocyclic aromatics (alkylbenzenes), pentacyclic

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aromatics (benzofluoranthenes, benzopyrenes) are present in very low relative abundance. No aromatic

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steroids (mono- and tri-) have been detected. The variation of aromatic hydrocarbon distribution patterns

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can be attributed to both maturation impacts and some differences in organic facies and/or lithology. The

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current study focuses on the naphthalene, phenanthrene, dibenzothiophene and their alkyl-substituted

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compounds. Detailed alkylnaphthalenes distributions in studied samples are shown by partial

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reconstructed mass chromatograms of m/z (128, 142, 156, 170, 184 and 198) in Fig. 5. All samples show

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the predominance of naphthalene (N) and methylnaphthalenes (MN) and their relative abundances

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decrease with increasing substituted carbon numbers. Relatively high abundance of C3 alkylnaphthalenes

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(C3N, TMN) only occurs in samples at 253 m and 392 m. The C4 alkylnaphthalenes (C4N, TeMN) can

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hardly be observed in summed ion chromatograms but their enlarged fragmentograms (m/z 184) show

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very similar distribution patterns with 1,3,4,5- (peak 1) and 1,3,6,7-TeMN (peak 2) as dominant isomers. 8


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The C5 alkylnaphthalenes (C5N, PMN) only occur in 3 shallowest samples and their geochemical feature

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has been excluded in the present study due to their unusually low concentrations.

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Several maturity parameters related to alkylnaphthalene isomer distributions are available in literature and

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have been applied in the studied section (Fig. 6). The methylnaphthalene ratio (MNR = 2-MN/1-MN,

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Radke et al. 6) varies from 2.2 to 3.1 in well ZK5-1 and shows a general increase trend with burial depth

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(Fig. 6A). A linear correlation between MNR and Tmax derives the equation of Tmax = 29.9 × MNR +

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390.7 with correlation coefficient (R2) of 0.73, indicating maturation exerts dominant control on

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methylnaphthalene isomer distribution. Dimethylnaphthalene ratio [DNR = (2,6-DMN + 2,7-DMN)/1,5-

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DMN] defined by Radke et al. 8 increases from 10.2 at the shallowest sample to 30.7 at the deepest

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sample (Fig. 6B). A linear correlation with Tmax gives the equation of Tmax = 1.43 × DNR + 442.6 and R2

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of 0.69. Dimethylnaphthalene isomer distributions still show a good maturation response but the

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correlation coefficient with Tmax is lower than methylnaphthalene one. Trimethylnaphthalene isomers in

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the studied samples are dominated by 1,3,7- (peak 1), 1,3,6- (peak 2) and 2,3,6-TMN (peak 4) with very

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low relative abundance of 1,2,5-TMN (peak 8) (C3N, Fig. 5). The trimethylnaphthalene ratio [TMNr =

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1,3,7-TMN /(1,3,7- + 1,2,5-TMN), van Aarssen et al. 10] varies from 0.86 to 0.94, indicating quite high

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maturity level, but shows no correlation with burial depth (Fig. 6C). A linear correlation between TMNr

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and Tmax derives the R2 of < 0.01. Other trimethylnaphthalene ratios [TNR1 = 2,3,6-TMN/(1,3,6-TMN +

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1,4,6-TMN + 1,3,5-TMN) and TNR2 = (1,3,7-TMN + 2,3,6-TMN)/(1,3,5-TMN + 1,4,6-TMN + 1,3,6-

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TMN), Radke et al. 8] also show no correlation with burial depth, Tmax and vitrinite reflectance (data not

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shown). Among the tetramethylnaphthalenes, 1,3,6,7-TeMN (peak 2) is the most thermally stable isomer

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whereas 1,2,5,6-TeMN (peak 9) (C4N, Fig. 5) is the least thermally stable isomer. Under the

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chromatographic conditions used here, 1,2,5,6-TeMN co-elutes with 1,2,3,5-TeMN, which is in fact very

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similar in stability to each other. The tetramethylnaphthalene ratio [TeMNr = 1,3,6,7-TeMN/(1,3,6,7- +

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1,2,5,6 + 1,2,3,5)-TeMN, van Aarssen et al. 10] ranges from 0.76 to 0.95 in the studied sample suite and

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shows no correlation with burial depth (Fig. 6D), Tmax or vitrinite reflectance (data not shown). The 9


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failure of tri- and tetramethylnaphthalene isomer distributions as maturity indicator is possible due to

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dealkylation.

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Partial reconstructed mass chromatograms of m/z (178, 192, 206 and 220) show the distribution of

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phenanthrene (P), methylphenanthrenes (MP), C2 alkylphenanthrenes (dimethylpheuanthrenes, DMP and

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ethylpheuanthrenes, EP) and C3 alkylphenanthrenes (trimethylpheuanthrenes, TMP and methyl- +

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ethylpheuanthrenes), respectively (Fig. 7). Both methylphenanthrenes and C2 alkylphenanthrenes are well

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identified, whereas the identification of individual isomers in the C3 alkylphenanthrenes has not been

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performed in this study. Similar to alkylnaphthalenes distributions, relative abundance of individual

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isomer and degree of alkylation show significant variations in the studied samples, which may reflect the

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influence from maturation and/or organic input.

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The maturity differences were quantified by ratios of the relative concentrations of the more thermally

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stable isomers to the less stable ones. The methylphenanthrene Index [MPI-1 = 1.5 × (3- + 2-MP)/(P + 1-

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MP + 9-MP), Radke and Welte 7], is probably one of the most widely used aromatic maturity parameters

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to estimate the level of thermal maturity. The MPI-1 values in the studied samples vary from 0.66 to 1.27

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but show no correlation with burial depth (Fig. 8A). Similar change can be observed from MPI-2 [3 × (2-

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MP)/(P + 1-MP + 9-MP)] (data not shown). The erratic variation of MPI-1 and MPI-2 values with burial

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depth and other measured maturity parameters (Tmax and Ro) suggest their invalidity as maturity indicator

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due to the involvement of phenanthrene in the formula. The MPI-3 [= (3-MP + 2-MP)/(9-MP + 1-MP)],

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based on the distribution of 4 methylphenanthrene isomers, varies from 1.1 at the top to 4.08 at the

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bottom (Fig. 8B). A linear correlation between MPI-3 and Tmax derives the equation of Tmax = 11.233 ×

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MPI-3 + 446.0 and correlation coefficient of 0.91. Similarly, the MPR (= 2-MP/1-MP, Radke et al. 6) uses

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only 2 methylphenanthrene isomers varies from 1.58 to 5.85 in the studied section (data not shown). The

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correlation between MPR and Tmax gives the equation of Tmax = 7.67 × MPR + 447.3 and correlation

248

coefficient of 0.89. Distribution of C2-alkylphenanthrene isomers is also sensitive to thermal maturation.

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There are two dimethylphenanthrene indexes [DMPI-1 = 4 × (2,6- + 2,7- + 3,5- + 3,6-DMP + 1- + 2- + 910


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EP)/(P + 1,3- + 1,6- + 1,7- + 2,5- + 2,9- + 2,10+ 3,9- + 3,10-DMP) and DMPI-2 = (2,6- + 2,7- + 3,5-

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DMP)/(1,3- + 1,6- + 2,5- + 2,9- + 2,10- + 3,9- + 3,10-DMP), Radke et al. 5, 6]. The DMPI-1 values mimic

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the change of MPI-1 and show no correlation with depth (Fig. 8C), while DMPI-2 values increase

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gradually from 0.32 at the top to 1.22 at bottom (Fig. 8D). The correlation between DMPI-2 and Tmax

254

gives the equation of Tmax = 38.39 × DMPR-2 + 446.2 and coefficient of 0.89. Ratios of 2,7-DMP/1,2-

255

DMP proposed by Radke et al. (1982a, b) as dimethylphenanthrene ratio (DMPR) vary from 4.56 at the

256

top to 22.16 at the bottom and also show linear correction with Tmax (data not shown). Data in the present

257

study clearly indicate that distributions of both methylphenanthrene and dimethylphenanthrene isomers

258

are dominantly controlled by maturation and parameters based on thermal stability of substitution serves

259

valid maturity indicator while the involvement of phenanthrene in the formulation breaks the validity of

260

maturity parameter.

261

Partial reconstructed mass chromatograms of m/z (184, 198, 212 and 226) show the distribution of

262

dibenzothiophene (DBT), methyldibenzothiophenes (MDBT), C2-alkyldibenzothiophenes

263

(dimethyldibenzothiophenes, DMDBT and ethyldibenzothiophenes, EDBT) and C3-

264

alkyldibenzothiophenes (TMDBT, no identification of individual isomers has been performed in this

265

study), respectively (Fig. 9). Overall abundance of alkyldibenzothiophenes as compared to

266

alkylnaphthalenes is quite low (Fig. 5) and their isomer distributions and degree of alkylation show some

267

similarity as alkylnaphthalenes and alkylphenanthrenes (Fig. 5 & 7).

268

The methyldibenzothiophene ratios (MDR = 4-MDBT/1-MDBT, Radke et al. 8) increase from 10.8 to

269

45.0 in the studied samples (Fig. 10A). While data show some scattering on depth profile, a linear

270

correlation between MDR and Tmax with equation of Tmax = 0.73 × MDR + 454.3 and correlation

271

coefficient of 0.57 has been observed. For the C2-alkyldibenzothiophenes, two maturity parameters

272

proposed by Chakhmakhchev et al. 20 are 4,6-/l,4-DMDBT and 2,4-/l,4-DMDBT ratios obtained from m/z

273

212 mass fragmentograms. Due to co-elution of 1,6- and 1,8-DMDBT with 1,4-DMDBT (peak 7, Fig. 9),

274

ratios of 4,6-/l,4-DMDBT and 2,4-/l,4-DMDBT cannot be obtained in the present study. We assume that 11


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275

1,6- and 1,8-DMDBT have similar stability as 1,4-DMDBT and the co-eluted peak are directly used form

276

calculation. The 4,6-/1,6-+1,8-+1,4-DMDBT ratio increases from 2.1 to 9.5 and 2,4-/1,6-+1,8-+1,4-

277

DMDBT ratio from 1.7 to 6.1 in well ZK5-1 (Fig. 10B & C). The linear correlation between 4,6-/1,6-

278

+1,8-+1,4-DMDBT and Tmax gives the coefficient of 0.53 while that between the ratio of 2,4-/1,6-+1,8-

279

+1,4-DMDBT and Tmax derives the coefficient of 0.81. These two ratios are well correlated to the MDR

280

over a wide range of catagenesis from moderately mature to overmature stage, verifying their similar

281

behavior with increasing maturity.

282 283

5 Discussion

284

5.1 Variation of organic source input and its impact on degree of alkylation

285

Molecular compositions of oil and source rock extracts are widely used to correlate the depositional

286

environment and organic source input. n-Alkanes, the most abundant hydrocarbons in bacteria, marine

287

and terrestrial organisms and organic matter, are the most commonly used molecular indicator. The

288

dominant n-alkanes in marine organic matter are n-C15 to n-C19 while terrigenous organic matter shows

289

high abundance of n-C27 to n-C31.37-38 n-Alkanes extracted from the m/z 85 fragmentogram of the GC/MS

290

traces in the studied samples are mainly distributed in the range of n-C10 to n-C31 and are dominated by

291

short-chain n-alkanes (n-C13 to n-C20). The unimodal n-alkane distributions in the studied samples,

292

generally peaking between C15 and C17, are likely indicative of a uniform source, where algal and

293

bacterial lipids could be the major contributors of n-alkanes. However, interpretation of source input

294

using the distribution of n-alkanes should be used with caution, because n-alkanes are affected by several

295

limitations, including biodegradation, maturation, and migration fractionation. As all studied samples are

296

highly matured (Ro > 0.7%), a unimodal distribution of n-alkanes bias to low molecular weight

297

compounds can also result from thermal cracking of the higher molecular weight n-alkanes during

298

maturation and cannot be directly applied for source input diagnosis. The depth profile of Σn-C20-/Σn-C21+

299

ratio defined by summed short-chain to summed long chain n-alkanes 38 can depict both source and 12


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300

maturation influence. This ratio varies greatly in the studied samples ranging from 1.6 to 6.9. Overall high

301

Σn-C20-/Σn-C21+ ratios are consistent with high maturity regime and a general increase trend from top to

302

bottom can be interpreted as a maturation sequence. Therefore, the outliers from maturity trend may

303

indicate source input variation. The highest value of 6.9 at 350 m indicates the dominance of marine algal

304

while the lowest value of 1.6 at 494 m suggests the most significant influence of terrestrial higher plant

305

input (Fig. 11A). Organic source inputs for other samples fall in between these two end member

306

situations.

307

The pristane/phytane (Pr/Ph) ratio is one of the most widely used geochemical parameters and has been

308

commonly found a good indicator to estimate depositional environments of organic matter. 39 Pristane and

309

phytane were primarily derived from chlorophyll which phytyl side. Under anoxic conditions in

310

sediments, the phytyl side chain is cleaved to yield phytol, which is reduced to dihydrophytol and then

311

phytane. Under oxic conditions, phytol is oxidized to phytenic acid, decarboxylated to pristene and then

312

reduced to pristane. Generally, Pr/Ph ratios greater than 3.0 have been documented to be related to

313

terrestrial organic matter deposited under oxic and suboxic conditions. Low Pr/Ph ratios (less than 1)

314

indicate anoxic conditions while Pr/Ph ratios between 1 and 3 have been found in marine oxic and

315

suboxic conditions. 36, 39 The Pr/Ph ratios in all samples range from 0.6-1.2, suggesting normal marine

316

depositional environment under anoxic conditions (Fig. 11B).

317

Pr/n-C17 and Ph/n-C18 ratios can also be used to indicate the source of organic matter and are sometimes

318

used in correlation studies. Lijmbach 40 indicated that oils originating from organic matter deposited in

319

marine conditions are characterized by Pr/n-C17 ratios lower than 0.5, while oils from continental

320

environments have ratios higher than 1. Ratios between 0.5 and 1.0 indicate transitional depositional

321

environments. Didyk et al. 39 suggested that high Pr/n-C17 ratios (less than 1) for crude oils indicate that

322

non-marine plants have a contribution in the origin of the crude oil and organic matter. The high ratios of

323

Ph/n-C18 are used as indicator to oxic conditions. The ratios Pr/n-C17 and Ph/n-C18 are considered

324

sensitive to thermal maturity when organic matter types are constant, and decrease with increasing 13


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325

maturation. Generally low Pr/n-C17 and Ph/n-C18 ratios (mainly < 0.5) are consistent with anoxic

326

deposition environment but Pr/n-C17 and Ph/n-C18 ratios are not sensitive to organic input diagnosis due to

327

high maturity levels (Fig. 11B).

328

Steranes are derived from steroids (sterols) in eukaryotic organisms such as diatoms, flagellates,

329

zooplanktons, and higher organisms during diagenesis and with low concentrations or absent in

330

prokaryotes. 39, 40 Generally, sterane distributions reflect the variations in algal input to source rocks and

331

can be used to differentiate crude oils based on genetic relationships. C29 steranes are often used as

332

indicators of land-plant-derived organic matter in source rocks and oils while C27 steranes are believed to

333

be derived from marine phytoplankton. An abundance of C28 compounds may indicate the influence of

334

lacustrine algae. 39, 41 While the abundance of C29 steranes have also been identified in many algal and

335

cyanobacteria sources not related to an influence of terrestrial organic matter, 40 the relative distribution of

336

C27, C28 and C29 steranes of the studied samples likely reflects the change source input. The predominance

337

of C29 (~ 60%) at the top sample and a sample at 494 m likely indicates the dominance of terrestrial high

338

plant contribution. The greater proportion of C28 steranes over C29 steranes at 350-355 m, coupled with

339

the highest Σn-C20-/Σn-C21+ ratio, verifies the algal input at this interval (Fig. 11C). Molecular indicators

340

from the saturated hydrocarbon fraction suggest a cyclic change of organic input from terrestrial plant

341

dominance at 494 m to algal dominance at 350-355 m, then the terrestrial plant become important again at

342

the top of the studied sample suite.

343

Alkylation of pre-existing aromatic structures with increasing thermal stress is a maturation related

344

process and forms a key, theoretically inferred component, controlling the behaviour of parameters such

345

as the methyl phenanthrene index,6 but has rarely been used as a proxy by itself, in defining the level of

346

maturity of source rocks or oils. Alkylation degree trends of alkylnaphthalenes, alkylphenanthrenes and

347

alkyldibenzothiophenes can be easily established in studied samples by the ratios of less alkylated

348

aromatic hydrocarbon homologues to their more highly alkylated counterparts. The ratio of MN/C3N

349

increases with burial depth, suggesting dealkylation plays an important role (Fig. 12A). The outlier 14


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Energy & Fuels

350

samples at 350-355 m reflect source input variation. The lowest degree of alkylation at this interval

351

reflects the marine algal input, which is consistent with n-alkane and biomarker diagnosis. However,

352

unusually low degree of alkylation for the top sample seems conflict with biomarker analysis. More

353

investigation for alkylnaphthalene distribution is still called for.

354

The P/C2P and DBT/C2DBT ratios show very similar variations in their burial depth profiles. A general

355

increase trend from top to bottom suggests that dealkylation exerts certain control on the distributions of

356

alkylphenanthrenes and alkyldibenzothiophenes. The outlier of maturation trend suggests source input

357

variation (Fig. 12A&B). Samples at depths of 350 and 355 m with abnormally low degree of alkylation

358

reflect the dominance of algae contribution, while other samples with normal alkylation sequence may

359

have similar organic source input which are mainly derived from continental plants.

360 361

5.2 Relationship between thermal maturity and alkylnaphthalene parameters

362

van Aarssen et al. 10 pointed out that maturity parameters derived from different classes of methylated

363

naphthalenes are closely related, and any deviations are caused by mixing and biodegradation processes.

364

However, the present study shows that MNR and DNR vary with maturity trend along predictable

365

pathways whereas the TMNr and TeMNr do not. Requejo et al. 30 had the opposite observations and they

366

claimed that trimethylnaphthalene distributions are maturity related but not for dimethylnaphthalene and

367

methylphenanthrene distributions. Such kind of data seems hard to be explained solely by organic type

368

influence on the distributions of methylated naphthalenes. Alkylnaphthalenes are likely derived from the

369

diagenetic aromatization of various terpenoids. 11, 21 Irregular changes in alkylnaphthalene parameters

370

during maturation can partially be attributed to variable organic matter type or depositional environments.

371

10, 21, 44

372

monoaromatic secohopanes, and are enriched in terrestrially originated source rocks and oils. 21, 45 The

373

proportion of 1,6-DMN in C2 alkylnaphthalenes varies from 14.4 to 18.6% with a slightly decline trend in

374

the depth profile (Fig. 13A). Slightly higher 1,6-DMN at 253 m may reflect more terrestrial influence,

1,6-DMN, 1,2,5-TMN and 1,2,5,6-TeMN are potentially derived from β-Amyrin and

15


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375

while lower 1,6-DMN at 350 and 355 m samples may suggest more algal input. Overall decrease trend

376

suggests the maturation effect plays the dominant role in C2 alkylnaphthalene isomer distribution, which

377

is verified by the DNR profile. In C3 alkylnaphthalenes, the relative proportion of 1,2,5-TMN, supposed

378

to reflect terrestrial organic matter influence, is only in the range of 1.3‒2.8% (Fig. 13B). Near

379

disappearance of 1,2,5-TMN results in high TMNr values but the correlation between TMNr and other

380

maturity parameters is poor. Overall very similar C3 alkylnaphthalene isomer distribution except the top

381

sample suggests some kind of equilibrium has been reached at the studied maturity level. Similarly, the

382

proportion of 1,2,5,6- + 1,2,3,5-TeMN in total C4 alkylnaphthalenes is generally < 5.0 % except the top

383

sample which is 9.4% (Fig. 13C). Unusually low proportion of 1,2,5,6- + 1,2,3,5-TeMN makes high

384

TeMNr values but insensitive to maturity variation.

385

Maturity parameters of alkylnaphthalenes rely on degree of isomerization assuming that the isomer

386

distributions reflect relative stability of isomers with diminished unstable isomers and elevated stable

387

isomers during increasing maturation process. 10, 11, 21 However, the lest stable isomers such as 1,2,5-TMN

388

and 1,2,5,6- + 1,2,3,5-TeMN are largely removed from C3- and C4 alkylnaphthalenes to form less

389

alkylated homologs. Dealkylation rather than isomerisation may play a critical role in controlling C3- and

390

C4 alkylnaphthalene isomer distributions. The loss of methyl groups from the methyl pool is the actual

391

cause of decrease in the degree of alkylation of alkylnaphthalenes. Dealkylation of alkylnaphthalenes

392

seems to occur at Ro < 1.0%, much earlier than Ro of 1.35% for alkylphenanthrenes. 5-9 Alternatively,

393

once the lest stable isomers in C3- and C4 alkylnaphthalenes approach disappearance at Ro ~1.0%, other

394

metastable and stable isomers reach an equilibrium without further isomerization. This may explain the

395

reason why maturity parameters based on the distribution of C3- and C4 alkylnaphthalenes showed

396

irregular variations in studied samples and no systematic correlation to the vitrinite reflectance can be

397

observed. For methylnaphthalenes and C2 alkylnaphthalenes, such equilibrium may occur at much higher

398

maturity level but their irregularity may occur at relative low maturity stage as observed by Requejo et al.

399

30

16


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Energy & Fuels

400 401

5.3 Relationship between thermal maturity and alkylphenanthrene parameters

402

MPI-1 and DMPI-1 as demonstrated for rock extracts in the present study show no correlation with

403

maturity. Changes in the type of organic matter have always been attributed to irregular behaviour of

404

methylphenanthrene indexes, 8, 22 but the reason behind seems not clearly explained. Alkylphenanthrenes

405

may be mainly derived from the aromatization of steroids during diagenetic and catagenetic processes. 46-

406

47

407

while 9-MP is dominated in type I and II kerogens. 8, 22, 48, 49 Organic input also exerts influence on

408

thermal behaviour of dimethylphenanthrene isomer distributions. The predominance of 1,7-DMP over the

409

other isomers can characterize the terrestrial source rocks, which is often related to natural precursors like

410

pimaric acid present in resin material. 49

411

Rock-Eval pyrolysis has no ability to differentiate kerogen type in the current sample suite due to high

412

maturity. Marine-continental transitional deposition during the Carboniferous should be characterized by

413

the predominance of admixed Type II/III to Type III kerogens with typical terrestrial plants as the

414

important source of the organic matter, however, typical terrestrial plant input derived markers such as

415

cadalene, retene and simonellite are largely absent or present in very low amount either due to the lack of

416

conifer resins during deposition or decomposed to other components due to high maturity. The

417

proportions of 1-MP in summed methylphenanthrenes and 1,7-DMP in summed C2 alkylphenanthrenes

418

are quantified here to illustrate subtle difference of organic matter input. Both ratios decrease gradually

419

with increasing burial depth. Lower 1-MP and 1,7-DMP in samples at 350 and 355 m than adjacent

420

samples may reflect relatively lower proportion of terrestrial input than other intervals (Fig. 14A & B).

421

However, subtle deviation of 1-MP/ƩMPs and 1,7-DMP/ƩC2Ps ratios has been compensated by further

422

depletion of 1-MP and 1,7-DMP proportions in deep buried samples, suggesting maturation exerts the

423

dominant control on methylphenanthrene and C2 alkylphenanthrene isomer distributions. Almost linear

Methylphenanthrene distributions are source related: 1-MP is mainly derived from type III kerogen

17


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424

correlation between 1-MP/ƩMPs and 1,7-DMP/ƩC2Ps (Fig. 14C) in the studied samples suggests general

425

similar organic input in majority of the studied samples except two samples at 350 and 355 m.

426

A strong argument for using the phenanthrene isomer distribution as a maturity parameter can be

427

demonstrated by correlating MPR, MPI-3 and DMPI-2 with other maturity indicators. The invalidity of

428

MPI-1 and DMPI-1 as maturity indicator seems inconsistent with previous studies indicating irregular

429

evolution trends for MPI-1 only occur in type II organic matter. 8, 25 If samples at 350 and 355 m are

430

excluded from the sample suite, the correlation between MPI-1, DMPI-1 and other maturity parameters is

431

still weak. Abnormally low MPI-1 and DMPI-1 values in samples at 350 and 355 m are caused by the

432

involvement of phenanthrene in the ratios. Phenanthrene incorporated in MPI-1 formulation to

433

compensate the facies-dependent variations is based on the assumption that dealkylation of

434

alkylphenanthrenes occurs at Ro of 1.35%. 5-8 However, data presented in current study illustrate that

435

degree of alkylation is not solely controlled by maturity. Organic input is one possible process to affect

436

degree of alkylation. Püttmann et al. 50 noticed that alginate source is enriched in the relative amount of

437

phenanthrene and will result in unusually low MPI-1 and DMPI-1 values. Lithology variation may also

438

attribute to the anomalous MPI-1 and DMPI-1 values. Cassani et al. 25 noted that high MPI-1 values

439

correspond to high carbonate contents in the source rocks. Requejo et al. 26 reported that, under similar

440

thermal regime, the maximum degree of alkylation decreases from carbonate to siliciclastic to paralic

441

source environments. One sample at 392 m contains high proportion of carbonate whose MPI-1 value is

442

the second highest and DMPI-1 value is the highest one in studied sample suite. The lithological influence

443

is consistent with the impact of degree of alkylation. High degree of alkylation reduces the relative

444

amount of phenanthrene and results in high MPI-1 and DMPI-1 values no matter which type of organic

445

matter is involved. In typical marine depositional environment where alginate may coexist with carbonate

446

in type II kerogen, uncertain degree of alkylation might be the major causes for the failure of MPI-1 and

447

DMPI-1 as maturity indicator. In marine-continental transitional depositional system, lithology variation

448

form coal, carboniferous shale, mudstone to marl and limestone resulted in very different catalytic effect, 18


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Energy & Fuels

449

coupled with variable organic source input, leads to different kerogen structures and different levels of

450

alkylation, which is complicated by thermal evolution. While ratios based purely on isomer stability are

451

well correlated to maturity levels, the involvement of phenanthrene in the MPI-1 and DMPI-1 formulation

452

causes their failure as maturity indicator.

453 454

5.4 Validity of empirical correlation of PAH and OSC maturity parameters with vitrinite

455

reflectance

456

One advantage to study aromatic hydrocarbons is to estimate maturity level for samples without vitrinite

457

reflectance measurement based on empirical calibration. For instance, MPR values > 2.6 are indicative of

458

maturities between 1.4 and 2% Ro. 9 High maturity crude oils have MPI 3 > 1.0, medium maturity oils

459

have MPI 3 = 0.8–1.0, and immature crude oils have MPI 3 < 0.8. 17 The MDR values between 0.5 and

460

2.0 suggest late diagenesis to early catagenesis, values of 2.0–15 indicate oil window stage, and values of

461

> 15 suggest late catagenesis to early metagenesis. 8, 9

462

The most widely applied empirical estimation is derived from MPI-1. For organic matter of low thermal

463

maturity (Ro < 1.35%), the equation to estimate equivalent vitrinite reflectance is Rc = 0.6 × MPI-1 + 0.4,

464

while for the high maturity (Ro > 1.35%) the equation is Rc = ‒0.6 × MPI1 + 2.3. 7 Samples in the present

465

study are mainly in the low maturity range of this regression analysis. The calculated equivalent vitrinite

466

reflectance (Rc) values are in the range of 0.8 to 1.16%, slightly lower than measured ones. When this

467

value is plotted against burial depth, no correlation can be obtained (Fig. 15A). Radke 9 noticed an

468

exponential increase of MPR with Ro between 0.4 and 1.8% and established an empirical relationship of

469

Rc = 0.95 + 1.1 × Log(MPR). When this equation is applied in the present studied samples, the MPR

470

derived Rc is almost linearly correlated with the measured Ro. However, there is a systematic difference

471

between them. The calculated Rc is about 0.3% higher than measured Ro (Fig. 15A).

472

There is no doubt that the calibration of the MPI-1 against vitrinite reflectance will encounter some

473

problems with oil-prone marine source rocks. 8-9 The present study, integrated with data published in 19


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474

literature, illustrated that even within type III kerogens and coals, the empirical correlation should be

475

localized in the specific basin and strata. No universal applicable equation can be established because the

476

extent of phenanthrene alkylation and degree of isomerization are affected by multiple progresses and

477

factors. For instance, Boreham et al. 19 suggested a calibration of MPI1 in Australian coals and sediments

478

to vitrinite reflectance as follows: Rc = 0.7 × MPI-1 + 0.22 for Ro < 1.7%; Rc = -0.55 × MPI-1 + 3.0 for Ro

479

> 1.7%. Norgate et al. 37 found the regression equation of MPI1 vs %Ro is different from those reported

480

by Boreham et al. 19 and Radke and Welte. 7 Their linear regression for the Buller coals was expressed as:

481

Rc =1.1 × MPI-1 + 0.07. More empirical correlation between MPI-1 and vitrinite reflectance may exist in

482

literature even all their studied samples are coal with presumably the same nature of organic input. In the

483

current studied samples, MPR seems better reflect maturity trend than MPI-1 but we caution against to

484

build another equation.

485

The MDR relies on the same chemical basis as the MPR and can be used for Type II kerogen. 8 The

486

regression curve for Type I and II kerogens is: Rc (%) = 0.40 + 0.30 (MDR) - 0.094 (MDR)2 + 0.011

487

(MDR)3. 9 If this equation is applied to the present studied samples, the calculated Ro values range from

488

8.0 to over 600%, which is obviously meaningless. This probably indirectly confirms that the studied

489

organics matters in the Keluke Formation are dominated by type III kerogen. Equation for Type III

490

kerogens is: Rc (%) = 0.51 + 0.073 × MDR. 9 Apply this relationship to the studied samples get the

491

calculated Ro values in the range of 1.35 to 3.55%. While linear correlation between calculated and

492

measured Ro remains valid, the maturity levels are dramatically overestimated by calculated Ro (Fig.

493

15B). Our limited data derive the equation of Rc (%) = 0.4037 × ln(MDR) - 0.0493 with R² = 0.83.

494

Alkyldibenzothiophene distributions can be influenced in certain instances by depositional environment

495

and lithology to an extent that may affect maturity assessment. 8, 51, 52 Some discrepancies in calibration of

496

MDR with Ro have been reported by Dzou et al. 53 Chakhmakhchev et al. 20 observed that MDR

497

variations at early maturity stage (0.4−0.7% Ro) exhibit significantly different maturation trends for Type

498

II and III kerogens. Relative concentration of dibenzothiophene seems to be organofacies controlled and 20


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Energy & Fuels

499

does not show systematic change with increasing maturity. Huang and Pearson 27 found oils formed from

500

source rocks deposited in hypersaline environments, with high sulfur contents, have very low initial MDR

501

values, while oils originating from freshwater source rock environments with high wax content tend to

502

have very high initial MDR ratios. This study provided further evidence that thermal stress is not the sole

503

factor controlling the distribution patterns of dibenzothiophenes in oils and source extracts. While the

504

variation of isomer ratios is driven by maturation, caution should be taken when converting these ratios

505

into an equivalent vitrinite reflectance.

506 507

6 Conclusion

508

Source rocks in the Upper Carboniferous Keluke Formation from Qaidam basin, NW China are in the

509

mature to overmature stage as evidenced by Rock-Eval Tmax values of 455 and 488 °C and measured

510

vitrinite reflectance (Ro) values of 0.79% to 1.45%. n-Alkane and sterane distributions illustrated that

511

organic matters are dominated by type III kerogen except one algae dominated interval in the middle

512

section, which are deposited under highly reducing environment.

513

Distributions of polyaromatic hydrocarbons (PAHs) and organosulfur compounds (OSCs) in the source

514

rocks are mainly controlled by degree of thermal maturation, while the variations of organic facies and

515

lithology may exert certain impacts on the evolution trajectory.

516

The methylnaphthalene ratio (MNR) and dimethylnaphthalene ratio (DNR) increase gradually with burial

517

depth, Tmax and Ro while trimethylnaphthalene ratio (TMNr) and tetramethylnaphthalene ratio (TeMNr)

518

show irregular changes with these maturity indicators. Largely depleted relative abundance of C3- and C4

519

alkylnaphthalenes and the loss of thermally unstable isomers cause the failure of maturity sensitivity.

520

MNR and DNR may cover wider maturity range than TMNr and TeMNr.

521

Irregular maturation behaviour also occurs in methylphenanthrene index 1 (MPI-1) and

522

dimethylphenanthrene index 1 (DMPI-1) but does occur in isomer ratios of the same homologue

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

523

components such as MPI-3, methylphenanthrene ratio (MPR) and DMPI-2. The involvement of

524

phenanthrene in formulation makes MPI-1 and DMPI-1 invalid maturity indicator.

525

The methyldibenzothiophene ratio (MDR), 4,6-/l,4-DMDBT and 2,4-/l,4-DMDBT ratios show linear

526

correlation to burial depth, Tmax and Ro and can serve as reliable maturity indicator.

527

Calculated equivalent vitrinite reflectance based on empirical correlation of valid maturity parameters

528

(MPR and MDR) overestimates the maturity levels in the continental-marine transitional sediments.

529

Degree of alkylation exerts critical control on validity of aromatic hydrocarbon maturity parameters,

530

which shows great variability in marine-continental transitional depositional system due to complicated

531

source type, lithology and maturation interactions.

532 533

Acknowledgements

534

This work was supported by National Natural Science Foundation of China (Grant Number 41573035,

535

41873049) and the Mitacs project at University of Calgary. Prof Steve Larter from University of Calgary

536

is gratefully acknowledged for the discussions. Three reviewers are acknowledged for their constructive

537

comments that substantially improved the quality of this manuscript.

538 539

Reference

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Figure Caption Fig. 1. An outline of the structure of the eastern Qaidam basin and well location. Fig. 2. Generalized stratigraphic column of the Keluke Formation in the Qaidam basin. Fig. 3. Depth profiles of Rock-Eval Tmax and vitrinite reflectance (Ro) in the Keluke Formation. Fig. 4. Total ion chromatograms (TIC) of the aromatic hydrocarbon fraction of core extracts from the Keluke Formation. ABs: alkylbenzenes; N: naphthalene; MN: methylnaphthalenes; BP: biphenyl; C2N: C2 alkylnaphthalenes; MBP: methylbiphenyls; C3N: C3 alkylnaphthalenes; C4N: C4 alkylnaphthalenes; P: phenanthrene; MP: methylphenanthrenes; C2P: C2 alkylphenanthrenes; MPy: methylpyrenes; C3P: C3 alkylphenanthrenes; Ch: chrysene; MCh: methylchrysenes; BFl: benzofluoranthenes; BPy: benzopyrenes. Fig. 5. Partial reconstructed mass chromatograms of m/z (128, 142, 156, 170, 184 and 198) indicating alkylnaphthalene distributions in the studied samples. N: naphthalene; MN: methylnaphthalenes; C2N: C2 alkylnaphthalenes (1: 2-EN; 2: 1-EN; 3: 2,6-+2,7DMN; 4: 1,3-+1,7-DMN; 5: 1,6-DMN; 6: 1,4-+2,3-DMN; 7: 1,5-DMN; 8: 1,2-DMN); C3N: C3 alkylnaphthalenes (1: 1,3,7-TMN; 2: 1,3,6-TMN; 3: 1,3,5-+1,4,6-TMN; 4: 2,3,6-TMN; 5: 1,2,7-+1,6,7TMN; 6: 1,2,6-TMN; 7: 1,2,4-TMN; 8: 1,2,5-TMN); C4N (enlarged): C4 alkylnaphthalenes (1: 1,3,4,5TeMN; 2: 1,3,6,7-TeMN; 3: 1,2,4,6-+1,2,4,7-+1,4,6,7-TeMN; 4: 1,2,5,7-TeMN; 5: 2,3,6,7-TeMN; 6: 1,2,6,7-TeMN; 7: 1,2,3,7-TeMN ; 8: 1,2,3,6-TeMN; 9: 1,2,5,6-+1,2,3,5-TeMN); DBT: dibenzothiophene; C5N: C5 alkylnaphthalenes; MDBT: methyldibenzothiophenes. Fig. 6. Depth profiles of maturity parameters derived from alkylnaphthalene isomer distributions. (A) MNR; (B) DNR; (C) TMNr; (D) TeMNr. Fig. 7. Partial reconstructed mass chromatograms of m/z (178, 192, 206 and 220) indicating alkylphenanthrene distributions in the studied samples. P: phenanthrene; MP: methylphenanthrenes; C2P: C2 alkylphenanthrenes (1: 3-EP; 2: 3,6-+9,2-DMP+1EP; 3: 3,5-+2,6-DMP; 4: 2,7-DMP; 5: 1,3-+3,9-+2,10-+3,10-DMP; 6: 2,5-+2,9-+1,6-DMP; 7: 1,7-DMP; 8: 2,3-DMP; 9: 1,9-+4,9-+4,10-DMP; 10: 1,8-DMP; 11: 1,2-DMP); C3P: C3 alkylphenanthrenes.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 8. Depth profiles of maturity parameters derived from alkylphenanthrene isomer distributions. (A) MPI-1; (B) MPI-3; (C) DMPI-1; (D) DMPI-2. Fig. 9. Partial reconstructed mass chromatograms of m/z (184, 198, 212 and 226) indicating alkyldibenzothiophene distributions in the studied samples. DBT: dibenzothiophene; MDBT: methyldibenzothiophenes; C2DBT: C2 alkyldibenzothiophenes (1: 4EDBT; 2: 4,6-DMDBT; 3: 2,4-DMDBT; 4: 2,6-DMDBT+2-EDBT; 5: 3,6-DMDBT; 6: 2,8-+2,7-+3,7DMDBT; 7: 1,6-+1,8-+1,4-DMDBT; 8: 1,3-+3,4-DMDBT; 9: 1,7-DMDBT; 10: 2,3-DMDBT); C3DBT: C3 alkyldibenzothiophenes. Fig. 10. Depth profiles of maturity parameters derived from alkyldibenzothiophene isomer distributions. (A) MDR; (B) 4,6-/1,6-+1,8-+l,4-DMDBT; (C) 2,4-/1,6-+1,8-+l,4-DMDBT. Fig. 11. Variations of organic input and depositional environment with burial depth indicated by saturated hydrocarbons in the studied samples. (A) Ʃn-C20-/Ʃn-C21+; (B) Pr/Ph, Pr/n-C17 and Ph/n-C18; (C) relative percentage of C27, C28 and C29 regular steranes. Fig. 12. Variations of degree of alkylation with burial depth in the studied samples. (A) MN/C3N; (B) P/C2P; (C) DBT/C2DBT. Fig. 13. Maturation and organic matter input influence indicated by alkylnaphthalene isomer distributions. (A) 1,6-DMN in C2 alkylnaphthalenes; (B) 1,2,5-TMN in C3 alkylnaphthalenes; (C) 1,2,5,6- + 1,2,3,5TeMN in C4 alkylnaphthalenes. Fig. 14. Terrestrial originated organic matter influence indicated by alkylphenanthrene isomer distributions. (A) Depth profile of 1-MP/ƩMP; (B) Depth profile of 1,7-DMP/ƩC2P; (C) 1-MP/ƩMP vs. 1,7-DMP/ƩC2P. Fig. 15. Comparison of calculated equivalent vitrinite reflectance (Rc) values with measured ones. (A) Rc derived from MPI-1 and MPR; (B) Rc derived from MDR.


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Fig. 1


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 2


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Page 33 of 46

Tmax (°C) 440

460

480

Ro (%) 500

0.5

1

1.5

0 100 200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

300 400 500 600 700

A

Fig. 3


B

20.0

25.0

30.0 35.0 40.0 45.0 Retention Time (min.)

55.0

60.0

MP

BFl BPy

392 m Ch MCh

C2P MPy C3P

DBT

ABs

C4N

P

C3N

C2N

MN N

BFl BPy

Ch 50.0

MCh

MP P

C2P MPy C3P

C3N

C4N

ABs

15.0

114 m

BP

MN N

BP C2N MBP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MBP

Energy & Fuels

210 m

494 m

253 m

505 m

350 m

610 m

355 m

624 m

65.0 70.0

15.0

20.0

25.0

Fig. 4


30.0


55.0

60.0

65.0

70.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

MN

2

N

C4N

MN

1

2-

3 45 6 8 7

12

34

114 m

N

2 1 34 5 678

C4N

C4N 3 45 6 8 7

3 4

C3N

5

C5N + MDBT

C3N 56 8 7

9

1

C2N 1-

C2N

1-

2

2-

1 2

2 1 4 3 56

6 78

78

C4N

DBT

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9

392 m C5N + MDBT

210 m

494 m

253 m

505 m

350 m

610 m

355 m

624 m

DBT

22.0

24.0

26.0

28.0 30.0 32.0 34.0 36.0 Retention Time (min.)

38.0

40.0

42.0

22.0

24.0

26.0

28.0

30.0

32.0

34.0

36.0

Retention Time (min.)

Fig. 5


38.0

40.0

42.0

Energy & Fuels

1.5

MNR 2.5

3.5

0

DNR 20

40

0.7

TMNR 0.8 0.9

1

0.6

TeMNR 0.8

1

0 100 200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 46

300 400 500 600 700

A

B

Fig. 6


C

D

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C2P

MP

P

3 4 1

42.0

2


97 89

5 1-

C3P 10 11

47.0

34 1

48.0

49.0

392 m

C2P

3-

6

1-

41.0

2-

114 m

3-

40.0

MP

P

5

2- 9-

2

6

7 8

C3P 910 11

210 m

494 m

253 m

505 m

350 m

610 m

355 m

624 m

50.0

40.0

41.0

42.0

Fig. 7


43.0


48.0

49.0

50.0

Energy & Fuels

0.5

MPI-1 1

1.5

1

MPI-3 3

5

0

DMPI-1 1

2

0

DMPI-2 0.5 1

1.5

0 100 200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 46

300 400 500 600 700

A

B

Fig. 8


C

D

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4-

MDBT

2

DBT

3-+2-

3 1-

39.0

40.0

4-

C2DBT5

1

C3DBT

4

114 m

MDBT

DBT

56 7 8 9 10

41.00 42.0 43.0 44.0 45.0 46.0 Retention Time (min.)

C3DBT

34 1-

47.0

392 m

C2DBT 2

3-+2-

1

56 7 8 9 10

210 m

494 m

253 m

505 m

350 m

610 m

355 m

624 m

48.0

49.0

39.0

40.0

41.00 42.0

43.0

44.0

45.0

46.0

Retention Time (min.)

Fig. 9


47.0

48.0

49.0

Energy & Fuels

MDR 0

25

50

0

4,6-/1,6+1,8+1,4-DMDBT 2 4 6 8 10

0

2,4-/1,6+1,8+1,4-DMDBT 2 4 6 8

0 100 200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 46

300 400 500 600 700

A

B

Fig. 10


C

Page 41 of 46

Ʃn-C20-/Ʃn-C21+ 2 4 6

0

8

0

0.5

Ratio 1

1.5

0

Percentage (%) 20 40 60

80

0 100

300 400 500 600 700

C27

Marine Algal

Depth (m)

200

Terrestrial Plant

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

C28 C29

Pr/Ph Pr/n-C17

Ph/n-C18

A

B

Fig. 11


C

Energy & Fuels

0

MN/C3N 5 10

P/C2P 15

0

2

4

6

0

DBT/C2DBT 1 2

3

0 100

200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 46

300

Source impact

Source impact

Source impact

400 500 600 700

Dealkylation A

Dealkylation

B

Fig. 12


Dealkylation

C

Page 43 of 46

1,6-DMN/ƩC2N (%) 12 16

20

0

1,2,5-TMN/ƩC3N (%) 4

8

1,2,5,6-TeMN/ƩC4N (%) 0 5 10

B

C

0 100 200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

300

400 500 600 700

A

Fig. 13


Energy & Fuels

0

1-MP/ƩMP 0.1

0.2

0.04

1,7-DMP/ƩC2Ps 0.08 0.12 0.12

0 100 1,7-DMP/ƩC2P

0.10

200 Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 46

300 400

0.08

0.06

500 350 & 355 m 600 700

A

B

Fig. 14


0.04 0.05

0.1 0.15 1-MP/ƩMP

0.2

C

Page 45 of 46

Ro (%) 0.5

1

1.5

2

0

1

Ro (%) 2

3

4

0 MPI-1

Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

MDR

100

MPR

200

Measured

Mearsured

300 400 500 600 700

A

Fig. 15


B

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 46

Table Table 1 Basic sample information and results from Rock-Eval pyrolysis, vitrinite reflectance (Ro) measurement and solvent extraction.

Depth (m)

Lithology

114 210 253 350 355 392 494 505 610 624

Mudstone S. mudstone Mudstone Mudstone Mudstone Marl S. mudstone S. mudstone Mudstone Mudstone

S1

S2

Tmax

PI

HI

OI

PC

RC

TOC

MINC

Ro

EOM

mg/g

mg/g

°C

%

mg/g.c

mg/g.c

%

%

%

%

%

mg/g

0.11 0.13 0.61 0.03 0.02 0.29 0.09 0.08 0.05 0.08

0.66 2.09 5.11 0.39 0.35 3.49 1.09 1.43 0.45 1.15

455 462 457 478 479 469 476 478 488 487

14.3 5.9 10.7 7.1 5.4 7.7 7.6 5.3 10.0 6.5

50 84 75 25 28 57 49 36 37 44

14 5 4 8 10 2 10 4 12 13

0.08 0.21 0.5 0.05 0.04 0.33 0.12 0.14 0.05 0.14

1.25 2.29 6.33 1.51 1.23 5.8 2.1 3.79 1.16 2.49

1.33 2.5 6.83 1.56 1.27 6.13 2.22 3.93 1.21 2.63

0.77 0.18 5.33 0.07 0.07 0.21 0.18 0.28 0.16 0.62

0.79 0.98

0.23 0.48 0.82 0.18 0.20 1.38 0.18 0.33 0.18 0.24

S. mudstone: silty mudstone.


1.23 1.29 1.37 1.45

Maturation impact on polyaromatic hydrocarbons ... - ACS Publications

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