Sources of natural gases in the Xihu Sag, East China Sea Basin

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Sources of natural gases in the Xihu Sag, East China Sea Basin: insights from stable carbon isotopes and confined system pyrolysis Xiong Cheng, Dujie Hou, Zhe Zhao, Xiaodong Chen, and Hui Diao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00090 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Energy & Fuels

Sources of natural gases in the Xihu Sag, East China Sea Basin: insights from stable carbon isotopes and confined system pyrolysis Xiong Cheng a,b *, Dujie Hou a,b *, Zhe Zhao a,b, Xiaodong Chen c, Hui Diao c

a School

of Energy Resources, China University of Geosciences, Beijing 100083,

China b Key

Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation

Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083, China c Exploration

and Development Research Institute, Shanghai Branch of China

National Offshore Oil Corporation, Shanghai 200050, China

* Corresponding authors. E-mail address: [email protected] (X. Cheng), [email protected] (D. Hou) ORCID X. Cheng: 0000-0002-2991-1137 D. Hou: 0000-0003-2001-4082

1 ACS Paragon Plus Environment

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ABSTRACT

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The Xihu Sag is the most gas-rich area in the East China Sea Basin. However, the origin

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of the natural gases is still controversial. Twenty-seven natural gas samples collected

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from the Xihu Sag were analysed for chemical and stable carbon isotopic compositions.

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In addition, three source rock samples (a coal, a carbonaceous mudstone, and a

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mudstone) from the Middle–Upper Eocene Pinghu Formation were pyrolysed in a

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closed system using a gold tube. The stable carbon isotopes of pyrolysis gaseous

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hydrocarbons were analysed for gas-source correlation. Five distinct gas families (A to

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E) were classified based on stable carbon isotopic compositions of methane (δ13C1),

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ethane (δ13C2), and propane (δ13C3) using the natural gas plot and δ13C2-δ13C1 versus

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δ13C3-δ13C2. Family A contains 10 samples which are widespread in the Pinghu Slope;

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family B consists of 12 samples mainly from southern parts of the Central Inverse

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Anticline Belt; family C consists of three samples and occurs in the Pinghu Slope;

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families D and E both contain only one gas and occur in the Pinghu Slope. Families A,

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B, C, and D are thermogenic gas derived from type III kerogens. Family D may have

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been subjected to alteration by diffused 12C-rich methane. C2+ of family E were mainly

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generated by oil-prone kerogen, while the C1 is associated with type III kerogen.

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Thermal maturity of the gases was reassessed based on newly proposed empirical

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isotope-maturity models for the Xihu Sag. Results suggest that the gases were generated

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during the late oil window to late wet gas window. The δ13C values of C1–C3 from the


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pyrolysis experiments are useful in direct correlations with natural gas accumulations.

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Gas-source correlation suggests that families A and B were derived from coal and

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mudstone of the Middle–Upper Eocene Pinghu Formation, respectively. Family C could

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be an admixture of gases generated by mudstone and carbonaceous mudstone of the

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Pinghu Formation. The C2+ in family D were mainly derived from the more mature

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Lower–Middle Eocene Baoshi Formation, while the 12C-rich C1 might be diffused

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upward from greater depth. Family E is mixture of sapropelic gas probably from the

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Paleocene source rock and high maturity humic gas probably from the Baoshi

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Formation. Gas-source rock correlation indicates that the Pinghu Formation is the main

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gas source rock unit in the Xihu Sag and gas distribution is source controlled.

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Confirmation of the Lower–Middle Eocene Baoshi Formation and Paleocene as

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additional effective source rocks in the Xihu Sag suggests that new exploration plays

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targeting these alternative hydrocarbon source rocks are possible and worth exploring.

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Keywords: gas family; gas origin; pyrolysis; stable carbon isotope; maturity; gas-source

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correlation; Xihu Sag

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1. INTRODUCTION

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Pyrolysis methods, including either open or closed system pyrolysis as well as

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hydrous or anhydrous pyrolysis, have been widely employed to characterize geological


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macromolecules, and to simulate organic matter maturation and hydrocarbon

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generation1-9. Previous studies have demonstrated that the molecular and isotopic

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compositions of gases generated by laboratory simulations on source rock can be

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compared with those of natural gases. Hydrous pyrolysis8 and microscale sealed vessel

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(MSSV) pyrolysis9 of potential source rocks combined with stable carbon isotope

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analysis of generated gas were demonstrated to be useful for gas-source rock

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correlation.

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The Xihu Sag is the most gas-rich region in the East China Sea Basin. Following

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recent gas exploration activities, some natural gas geochemistry studies have been

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performed, but large controversies still exist in terms of the maturity and origin of the

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natural gases. Previous research suggested that the gases were mainly derived from

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humic organic matter during thermal cracking and a few small gas shows were

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generated from microbial activity and inorganic processes, based on molecular and

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stable carbon isotopic compositions of the gases10-14. In these studies, the gas maturity

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was assessed by using various empirical isotope–maturity models based on study of

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other basins, which might not be suitable for the Xihu Sag. As a result, varying maturity

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levels for the gases were deduced, such as 1.6%–2.0% (vitrinite reflectance scale, Ro)10,

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1.30%–1.95%11, two maturity stages, i.e. 0.6%–1.0% and 1.1%–2.0%12, and mature to

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post mature14. Comparison of the calculated Ro values of the gases with measured Ro

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values of available source rocks of different ages allowed these studies to conclude that

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the gases were derived from (1) the deep buried Middle–Upper Eocene Pinghu and


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Oligocene Huagang formations which have not been penetrated in center parts of the

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XihuSag10, (2) the Lower Eocene11, (3) the Pinghu and Huagang formations (mature

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gases) and Palaeocene–Lower Eocene (high mature gases) source rocks, respectively12-

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13,

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and (4) the Pinghu Formation and underlying source rocks14. In previous studies, neither a main source contributor nor a specific kind of source

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rock (coal, carbonaceous mudstone or mudstone) were determined. Moreover, direct

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evidence, e.g. a thermal simulation experiment and product analyses, were lacking for

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reliable gas-source correlation. Recently, gas exploration in the Xihu Sag encounters

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severe challenges, such as the sources of the natural gases, the contributions of coal

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versus mudstone, and the exploration potential of the Baoshi Formation and the

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Palaeocene as source rocks. Therefore, detailed geochemical study of natural gases and

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gas-source correlation are urgently needed for identifying their origin and providing

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implications for gas exploration. In this study, molecular and stable carbon isotope

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compositions were analysed for gas family assignment, source type determination, and

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thermal maturity assessment. In addition, three potential source rock samples, a coal, a

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carbonaceous mudstone, and a mudstone from the Pinghu Formation, were subjected to

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closed-system pyrolysis to simulate gas generation and to attain gas carbon isotope data

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for direct gas-source correlation.

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2. GEOLOGICAL SETTING

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The East China Sea is located between the SE coast of mainland China and the

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Ryukyu Islands of Japan (Figure 1a). The East China Sea Basin is one of the major

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sedimentary basins in the East China Sea, and is bounded by the Zhejiang-Fujian

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Mesozoic Volcanic Belt to the west and the Diaoyu Island Uplift to the east. The Xihu

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Sag, covering an area of 46,000 km2, is located in northeast of the East China Sea

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Basin. It is bounded by Diaoyu Island Uplift to the east, the Hupijiao, Haijiao, and

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Yushan uplifts to the west, the Fujiang Sag to the north, and the Diaobei Sag to the

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south. Structurally, the northeast-trending Xihu Sag can be divided into five tectonic

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units from west to east, namely the Western Slope Belt, the Western Sub-Sag, the

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Central Inverse Anticline Belt, the Eastern Sub-Sag, and the Eastern Steep Slope Fault-

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Uplift Belt (Figure 1b). The Western Slope Belt is divided into, from north to south, the

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Hangzhou, Pinghu, and Tiantai slopes (Figures 1b and 1c).

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Since the Cenozoic, the Xihu Sag experienced a fault-subsidence stage from the

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Late Cretaceous to Eocene, a stage of regional uplift without significant compression of

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tectonic movement at the end of the Eocene to early Oligocene, a local subsidence stage

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from the Oligocene to the Miocene, and a stage of regional subsidence after the Late

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Miocene15. The stratigraphy in the Xihu Sag, with a maximum thickness of

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approximately 10,000 m, consists of the Paleocene, the Eocene Baoshi and Pinghu

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formations, the Oligocene Huagang Formation, the Miocene Longjing, Yuquan, and

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Liulang formations, the Pliocene Santan Formation, and the Quaternary Donghai Group

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(Figure 1d). Of these strata, the Paleocene and the Baoshi, Pinghu, and Huagang


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formations are potential source rock-bearing intervals. The Oligocene Huagang Formation was deposited in humid fan, fan delta, and

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lacustrine sedimentary environments16. The coal-bearing source rocks within this

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formation consist of grey mudstone, carbonaceous mudstone, and coal. The upper part

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of the Huagang Formation is immature to low maturity, and the lower part is low

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maturity to mature at present with respect to the oil window17. The Eocene coal-bearing

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source rocks of Pinghu Formation are extensively distributed and very thick and were

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deposited in a lacustrine-swamp facies environment within a semi-enclosed bay18. The

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sediments mainly consist of siltstone, grey mudstone, carbonaceous mudstone, and coal,

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intercalated with fluvial sandstone and fan sand bodies. Source rocks (mudstone,

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carbonaceous mudstone, and coal) are commonly mature to highly mature, and are

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postmature in the center parts of the sub-sags17. The coal seams typically have an

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accumulative thickness of 30 to 50 m18. The Baoshi Formation was deposited in a semi-

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closed shallow sea environment. Coal-bearing source rocks of the Baoshi Formation

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have been penetrated in 11 wells in the Western slope belt of the Xihu Sag. Although

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the deeply buried Paleocene has not been drilled in the Xihu Sag, seismic data and

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geological analyses have confirmed the existence of this stratum. Chen and Wang19

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proposed that the Paleocene is very likely to have been deposited in a marine

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environment in the Xihu Sag.

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The discovered gas and condensate accumulations mainly occur in the Pinghu slope of the Western Slope Belt, and to a lesser extent in the southern parts of the Central


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Inverse Anticline Belt. There are four hydrocarbon-bearing intervals, namely, the

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Baoshi, Pinghu, Huagang, and Longjing formations. However, economic hydrocarbon

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accumulations mostly occur in the Pinghu Formation in the Pinghu Slope and the

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Huagang Formation in southern parts of the Central Inverse Anticline Belt.

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3. SAMPLES AND METHODS

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3.1. Natural Gas and Source Rock Samples

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Twenty-seven natural gas samples were collected from 18 wells in the Pinghu Slope

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of the Western Slope Belt and southern parts of the Central Inverse Anticline Belt, Xihu

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Sag, East China Sea Basin (Figure 1). These selected wells all have relatively high

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production, and those with small gas shows were excluded. Therefore, the gases should

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be representative of the major characteristics of natural gases in the Xihu Sag. Gas

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samples were collected from a separator after flushing the lines and metal container (1L

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or 2L) for 5 min to remove air contamination. The pressure in the metal container was

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higher than 1 MPa to prevent possible leakage of atmosphere into the container. General

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location information for the sampling wells and reservoir depth are given in Table 1.

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Although inconsistent, most previous studies have suggested that the Pinghu

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Formation is most like to be the major source rock unit in the Xihu Sag. In addition,

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only limited source rock samples are available for the offshore wells. In order to better

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characterize the generated gas from the Pinghu Formation, three source rocks including


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a coal, a carbonaceous mudstone, and a mudstone from the PE, N25-1, and D-1 wells,

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respectively (Figure 1c), were selected for pyrolysis experiments and products analyses.

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The source rock samples are cuttings and were washed with water and handpicked, prior

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to crushing and geochemical analysis. The vitrinite reflectance and total organic carbon

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content of for the samples are provided in Table 2.

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3.2. Confined System Pyrolysis

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The source rock samples were finely ground (< 100 mesh) and extracted with

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chloroform using a Soxhlet apparatus. Kerogens were isolated from the powdered

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samples using HCl/HF digestion. A gold tube confined system non-isothermal pyrolysis

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technique4 was used. Kerogen powder, 20–50 mg depending on the final temperature,

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was put into gold tubes which were welded under an argon atmosphere after complete

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removal of air. Two heating programs (2 °C/h and 20 °C/h) were used for the pyrolysis

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experiments. A pressure of 50 MPa was maintained during pyrolysis. Note that the

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samples were not pyrolysed in a batch; the temperatures were slightly different (Table

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3).

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3.3. Molecular Composition Analysis

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The molecular compositions of 20 natural gases were analysed using an Agilent


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6890N gas chromatograph (GC) equipped with a flame ionization detector (FID) and a

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thermal conductivity detector (TCD). The other seven natural gases were analysed by

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GC only using an FID. Therefore, the contents of non-hydrocarbon gases in these

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samples are not available (see Table 1). Individual gas components (C1–C5 alkanes, N2,

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CO2) were separated using a capillary column (PLOT Al2O3 50 m × 0.53 mm). The GC

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oven temperature was set at 30 °C for 10 min, and then ramped to 180 °C at 10 °C/min

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with a final hold of 20 min.

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3.4. Stable Carbon Isotope Analysis

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Stable carbon isotope analyses of gases were performed using a Thermo Delta V

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Advantage isotope ratio mass spectrometer (IRMS) interfaced with a Thermo TRACE

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GC Ultra. A PLOT Q column (30 m × 0.32 mm × 0.25 μm) was used to separate the

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alkane gases and carbon dioxide. Helium was used as the carrier gas. The GC oven

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temperature was programmed to ramp from 50 °C (hold for 3 min) to 150 °C at

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4 °C/min, with a final hold of 8 min. A standard mixture of gaseous hydrocarbons (C1–

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C4), with known isotope compositions20, was used daily to calibrate the instrument. The

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results are reported in δ notation (δ13C, ‰) relative to the VPDB standard. The

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analytical precision is ±0.3‰.

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4. RESULTS

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4.1. Molecular Composition of Natural Gases

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Molecular compositions and geochemical indices for the analysed natural gases are

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given in Table 1. The molecular composition of the natural gas samples was primarily

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hydrocarbon gases (C1–5 varying from 92.8% to 99.2%, volume fraction) with minor

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proportions of non-hydrocarbon gases, i.e., nitrogen (N2) and carbon dioxide (CO2). The

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concentration of CO2 varies from 0 to 6.3%, with an average of 2.9%, and N2 content is

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in the range of 0.12–3.7%, with an average of 1.0%. The gas dry coefficient (C1/C1–5)

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varies between 0.75 and 0.94, indicating that the natural gases are wet gas. Hydrocarbon

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index (C1/C2–3) ranges from 3.9 to 17.4. Interestingly, the seven gases analysed using

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GC-FID have slightly lower dry coefficient and hydrocarbon index than their

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counterparts that have similar carbon isotope ratios.

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4.2. Stable Carbon Isotopic Composition of Natural Gases and Pyrolysis Gases

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Theδ13C1, δ13C2, and δ13C3 values of the natural gases increase with increasing

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carbon number, i.e., a normal stable carbon isotope trend, and range from −40.6‰ to

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−33.8‰, −30.9‰ to −22.7‰, and −27.8‰ to −20.8‰, respectively (Table 1). The δ13C

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values of carbon dioxide (δ13CCO2) range from −23.1‰ to −4.0‰.


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The stable carbon isotopes of the pyrolysis gases (C1–C3) are given in Table 3. The

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δ13C values of the generated alkane gases decrease in the early stage of pyrolysis and

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increase in the later stage, which is consistent with previous pyrolysis experiments21–23.

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This phenomenon could be explained by two or more precursors of gas generation with

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different isotopic compositions21.

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5. DISCUSSION

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5.1. Gas Family Assignments and Geographic Distributions Natural gas is dominated by low-molecular weight gaseous hydrocarbons whose

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genetic and post-genetic information is mainly obtained from stable carbon isotope

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compositions24–27. Chung et al.26 proposed a model, called a natural gas plot, to solve

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problems of gas-gas and gas-oil correlations. The model is based on plotting the carbon

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isotope ratio of individual gaseous molecules as a function of the inverse carbon number

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of the molecule. A linear fit to the data supports a cogenetic origin for the gas species,

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whereas a non-linear fit suggests that the gas accumulation is a mixture of gases, a gas

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derived from structurally heterogeneous carbon source, or a gas affected by secondary

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alteration23, 26.

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Based on this approach, the natural gases from the Xihu Sag were classified into

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five families (Figures 2a and 2b, Table 1). The isotope differences between C1 and C2

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and between C2 and C3 for the gas families are distinct (Figure 2c), suggesting that the


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assignments are reliable.

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Sublinear relationships were observed between the carbon isotopic ratios and the

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inverse carbon numbers of molecules for family A gases (Figure 2a). This gas family

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was further divided into two sub-families, A1 and A2 based on the systematic difference

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in δ13C1, δ13C2, and δ13C3 values. Each sub-family consists of five gases. Sub-family A1

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has slightly heavier carbon isotopes, and is widespread in the Pinghu Slope, while sub-

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family A2 is restricted to the middle of the Pinghu Slope (Figure 1). The systematically

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heavier carbon isotopes of sub-family A1 than that of sub-family A2 suggest slightly

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higher maturity of the former group. Family B gases show a suite of parallel and linear

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curves in the natural gas plot (Figure 2b). This family mainly consists of samples from

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the southern parts of the Central Inverse Anticline Belt (the H-A1, H-2, C-A1, C-A3, C-

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4, and D-1 wells), but it also includes three samples (PE-B, PE-C, and PE-D) from the

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PE well of the Pinghu Slope (Figure 1). Family C has almost identical sublinear isotope

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curves and is characterized by the lightest δ13C1 values (-40.6 to -40.4‰) of the sample

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set. Family C has almost identical δ13C2 and δ13C3 values with that of the light ends of

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family B, e.g. the PE-B, PE-C, PE-D, and D-1 gases. Family C was discovered in three

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wells (K-B2, N19-1, and PE) that are located in the northern, middle, and southern parts

248

of the Pinghu Slope, respectively.

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Both families D and E are represented by only one sample, and have doglegged

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isotope curves (Figure 2a). Family D occurs in the northernmost of the Pinghu Slope. It

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has the heaviest δ13C2 and δ13C3 values (-22.7‰ and -20.8‰), but abnormally negative


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δ13C1 value (-37.1‰). In contrast, family E has a more positive δ13C1 (-35.0‰) but

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more negative δ13C2 and δ13C3 values (-30.9‰ and -27.8‰) when compared with those

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gases having similar δ13C1 values. Family E occurs in the middle part of the Pinghu

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Slope (B-1 well).

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5.2. Source Type of Hydrocarbon Gases

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The Bernard diagram (i.e., plot of δ13C1 versus C1/(C2+C3)) is commonly used to

260

interpret the origin, maturity, and secondary alteration of hydrocarbon gases28. As

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shown in Figure 3, all the gas samples plot in the thermogenic-gas field, and all of them

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generally seen to follow the maturation trend for type III kerogen. However, the family

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D gas slightly deviates from this trend, as it has higher C1/(C2+C3) values.

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Dai et al.29 proposed a cut-off value of −29‰ for δ13C2 to differentiate the source

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type of thermogenic gases, as gases derived from humic kerogen generally have δ13C2

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values heavier than −29‰, while sapropelic kerogen-generated gases have lighter δ13C2

267

values. The gases in families A, B, C, and D have δ13C2 values heavier than −29‰

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(Table 1, Figure 4), indicating that they are derived from humic kerogens. In contrast,

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family E has a more negative δ13C2 value of −30.9‰, suggesting contribution from

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sapropelic kerogen.

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To further elucidate the kerogen type of the source rocks of the gas samples, δ13C1 versus δ13C2 was plotted with regression lines for alkane gases derived from type II and


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type III kerogens30-31 (Figure 4a). In this plot, the gases in families A, B, C and D

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distribute close to the regression lines for type III kerogens, and deviate from that of

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type II kerogen from the Delaware/Val Verde basins, supporting type III kerogens for

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the gases. Note that the family D gas slightly deviates from the major trend. In contrast,

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Family E is closer to the trend of gases derived from type II kerogen.

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The slight deviation of the family D gas from the major trend for type III kerogen

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might be caused by admixture of 12C-enriched C1 (see section 5.5). The contradictory

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results (type III versus type II kerogens) for the family E gas based on the Bernard

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diagram, δ13C2, and δ13C1 versus δ13C2 might reflect different sources for the C1 and C2.

282

This is consistent with the non-linear isotope curve in the natural gas plot (Figure 2a)

283

and the corresponding interpretation in section 5.5.

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Comprehensive interpretation of the source types of the gas families based on the

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above discussion enables the conclusion that family A, B, and C gaseswere generated

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from type III kerogens, family D is mainly a type III kerogen-derived gas, but was

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subjected to alteration by 12C-enriched C1, and family E is a mixture of type II and type

288

III kerogens generated gases.

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5.3. Origin of CO2

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CO2 is ubiquitous in natural gases and could be formed by varying pathways, such as thermal evolution of sedimentary organic matter, decarboxylation of lipids, microbial


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respiration, chemical equilibria between minerals, bacterial sulfate reduction,

295

thermochemical sulfate reduction, hydrocarbon oxidation by mineralized waters,

296

thermal decomposition of carbonate, carbonate hydrolysis, and endogenic (mantle,

297

volcanic) activities32-36.

298

Although the sources of CO2 are complicated, its origin can be determined by a

299

comprehensive study of molecular and isotopic compositions of gas samples37-39. The

300

content of CO2 in the analysed natural gases varies from 0 to 6.3%, and the δ13CCO2

301

values range from −19.8‰ to −4.0‰ (Table 1). Cross plots of δ13C1 versus δ13CCO2

302

(Figure 5a) and CO2 content versus δ13CCO2 (Figure 5b) were applied to interpret the

303

origin of carbon dioxide in the natural gases. The stable carbon isotopes of C1 and CO2

304

indicate that CO2 in family A (N13-2 and PF), family B (PE-B, PE-C, PE-D, and D-

305

1B), and family E (B-1) was generated during thermal transformation of organic matter.

306

In contrast, the data points for K-A1, K-B2, N25-1, and N25-2 (family A), H2-B, H-A1,

307

C-A1, and C-A3 (family B), K-B2 and PE-A (family C), and N13-1 (family D) gases

308

plot near the boundary or in the range of endogenic gases (Figure 5a). This suggests that

309

they might contain a partial endogenic component. This is consistent with the

310

distribution of the data points in the plot of δ13CCO2 versus CO2 content, where the

311

former gases plot in the area of organic origin, and the latter gases are near the boundary

312

of inorganic origin (Figure 5b).

313


Page 16 of 37

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314 315

Energy & Fuels

5.4. Thermal Maturity of Natural Gases Thermal maturity of natural gas is of great significance for determining gas origins

316

and gas-source correlation. Both laboratory pyrolysis experiments and analyses of

317

geological samples have confirmed that maturation of organic matter is accompanied by

318

isotopic fractionations in favor of the heavier isotope24. Based on this observation, a

319

suite of empirical carbon isotope-maturity relationships has been proposed for

320

evaluating the maturity of gases24, 40-43. However, due to different geological settings

321

and/or accumulation histories, the δ13C-maturity models in the Xihu Sag gave

322

misleading results (see Section 1). Recently, empirical carbon isotope-maturity models

323

for humic gases from the Xihu Sag were obtained from curve-fitting of δ13C values of

324

C1 and C2 versus gas-condensate maturity (unpublished). The equations are: δ13C1

325

(‰)=58.67×lgRo-44.37, δ13C2 (‰)=37.31×lgRo-32.80, and δ13C2=0.64δ13C1-4.2.

326

Based on the new δ13C1-Ro and δ13C2-Ro models, a maturity curve was drawn on

327

the δ13C1 and δ13C2 diagram (Figure 4a). Using this approach, (sub-) families A1, A2,

328

and B gases were estimated to have maturities of about 1.5%, 1.4%, and 1.3%–1.6%,

329

respectively. Families C, D, and E gases deviate more from the maturity curve. Based

330

on δ13C1, gas families C and D have Ro values of 1.16%–1.17% and 1.33%,

331

respectively; in contrast, based on δ13C2, the corresponding Ro values are ~1.3% and

332

1.9%, respectively. The more positive carbon isotope ratios for C2 and C3 of family C

333

gases than that of gases from other families might indicate a higher thermal maturity

334

(Figure 4b), which is consistent with the higher dryness coefficient (0.94, Table 2).


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

335

Family E is a mixture of sapropelic and humic kerogen-derived gas, so the models are

336

not applicable. However, a dryness coefficient of 0.89 for the family E gas might

337

suggest its products were generated mainly during the wet gas window. In summary, the

338

natural gases from the Xihu Sag were generated during the late oil window to the wet

339

gas window.

340 341

5.5. Gas-Source Correlation

342 343

Tao et al.17 reported that the source rocks of Pinghu Formation are highly mature to

344

overmature at present, while the overlying Oligocene Huagang Formation source rocks

345

are mainly in the oil window. Therefore, a major contribution from the Huagang

346

Formation can be precluded, as the natural gases were formed during the late oil

347

window to the wet gas window. The Pinghu Formation and underlying Baoshi

348

Formation and Paleocene source rocks are mature enough to have generated the gases.

349

However, we cannot determine which unit and lithology (coal, carbonaceous mudstone

350

or mudstone) is the main source contributor only based on thermal maturities of gases

351

and source rocks. In order to determine the sources of the gases, stable carbon isotopic

352

compositions of the natural gases were compared with that of the pyrolysis gases

353

yielded by the coal, carbonaceous mudstone and mudstone from the Pinghu Formation.

354 355

The carbon isotope ratios of the pyrolysis gases generated during the second stage were plotted versus the inverse carbon numbers of molecules (Figure 6). Generally


Page 18 of 37

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Energy & Fuels

356

sublinear or linear relationships were observed between the carbon isotope ratios and

357

the inverse carbon numbers of molecules. However, in detail the trends are quite

358

different for the gaseous hydrocarbons generated from the three source rocks. The

359

pyrolysis gases of the coal show a sublinear relationship for C1, C2, and C3. In contrast,

360

the carbonaceous mudstone and mudstone show good linearity for C1, C2, and C3. The

361

slope for the carbonaceous mudstone is steeper than that of the mudstone, which in turn

362

is steeper than the coal. The distinct carbon isotope signatures for the pyrolysis products

363

of varying source rocks suggest they could be used for gas-source rock correlation.As

364

shown in Figures 6a and 6b, the isotope curves for gases of sub-family A1 correlate well

365

with that of the pyrolysis gases originated from the coal at 433 °C and 480 °C with a

366

rate of 2 °C/h and 20 °C/h, respectively. Gases of sub-family A2 plot between 433 °C

367

and 457 °C and between 480 °C and 505 °C with a rate of 2 °C/h and 20 °C/h,

368

respectively. These suggest that gases of both sub-families A1 and A2 were derived from

369

coal of the Pinghu Formation, and that sub-family A1 has a higher maturity than that of

370

sub-family A2. In contrast, the isotope curves for family B correlate well with that of the

371

pyrolysis gases derived from the mudstone (Figures 6c and 6d), suggesting genetic

372

correlation between family B gases and the mudstone.

373

The family C gases have almost identical C2 and C3 isotope ratios with that of the

374

family B gases, but isotopically lighter C1 (Figure 2b). If a family B gas was exposed to

375

secondary alteration processes, such as mixing with bacterial gas or diffused 12C-rich C1

376

and migration fractionation, this would result in isotopically light C1 with negligible


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

377

alteration of C2 and C3 isotopes. However, the lower dryness coefficients of family C

378

gases (≤0.8, Table 1) compared with that of family B gases does not support these

379

hypotheses, as admixture with bacterial gas or 12C-rich C1 (both are dominated by C1)

380

and migration fractionation should elevate the content of C1. Furthermore, the reservoirs

381

for family B gases (PE-B, PE-C, and PE-D) and PE-A gas (family C) are closely

382

spaced, as indicated by their depth intervals (Table 1). The small differences in

383

migration distance among the gases in these reservoirs are unlikely to have caused

384

significant migration fractionation of the PE-A gas. Intriguingly, C1 and C2 isotopes of

385

family C gases correlate well with the pyrolysis gas of the carbonaceous mudstone

386

(Figures 6e and 6f, 409 °C and 440 °C, respectively), but the former has isotopically

387

lighter C3 than the latter. This observation combined with the partial correlation of

388

family C gases with the mudstone, may suggest that methane and ethane in family C

389

gases are related to the carbonaceous mudstone, while C2 and C3 were generated from

390

mudstone of the Pinghu Formation.

391

In contrast, the isotope compositions of families D and E do not match any pyrolysis

392

gases of the coal, carbonaceous mudstone, and mudstone (Figure 6), suggesting non-

393

genetic correlation between the gases and the source rocks. The positive δ13C2 and δ13C3

394

values and the negative δ13C1 value for the family D gas resulted in a non-linearity in

395

the natural gas plot (Fig 2a). This isotope curve resembles those of the mixtures of

396

thermogenic and bacteriogenic gases23, 26. The gas reservoir has a depth interval of

397

4150–4169 m and the present temperature is 130 °C. Therefore, it would be impossible


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Energy & Fuels

398

for there to have been generation of biogenic gas, due to 80 °C is the limit for

399

biodegradation and pasteurization44-46. Furthermore, no significant biogenic gas has

400

been discovered in the Xihu Sag.

401

Zhang and Krooss47 proposed that there is a significant depletion of the 13C in

402

diffused C1. Therefore, an alternative explanation for the gas compositions in the Xihu

403

Sag is that family D gas is mixed with 12C-rich C1 that escaped from deeper reservoirs.

404

The diffused gas would be rich in C1 and poor in C2+ components. As a result, the C2+

405

hydrocarbon gases suggest a pristine nature of the gases, while the C1is isotopically

406

lighter. The higher maturity, associated with coaly source rocks and the non-genetic

407

correlation with the Pinghu Formation, suggest that the C2 and C3 in the family D gas

408

could have been derived from the more mature coal-bearing Baoshi Formation.

409

The non-linear isotope curve for the family E gas could be interpreted as residual

410

gas altered by diffusion47-48 or mixing of gases23, 26. The remaining natural gas in the

411

reservoir would be depleted in C1 and enriched in C2+ gases and CO2 with increasing

412

degree of diffusion, due to the greater diffusion coefficient of C1 than that of C2+ gases

413

and CO2 49-50. In addition, the residual C1 will be progressively enriched in 13C, while

414

the stable carbon isotope compositions of C2 and C3 will be less altered23, 51. However,

415

the family E gas has a relatively high dry coefficient of 0.9 and a low CO2 content of

416

2.2%, suggesting that the relatively heavy δ13C1 value could not be the result of gas

417

diffusion.

418

Admixture of high maturity humic kerogen-derived gas may result in the downward


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

419

concave isotope curve of the family E gas (Figure 2a). If the high maturity gas is

420

dominated by isotopically heavy C1, admixture can result in a shift of δ13C1 towards

421

heavier values, yet negligible changes in the δ13C2 and δ13C3 of the original gas. In this

422

model, two end-member gases (EMG-A and EMG-B) are required: EMG-A should

423

have lighter δ13C2 (< -30.9‰) and δ13C3 values than that of the family E gas, while the

424

highly maturity EMG-B should have a more positive δ13C1 (> -35.0‰) value than the

425

family E gas. The isotopically light C2 and C3 suggest that the EMG-A is a sapropelic

426

gas, which is most likely to be derived from an oil-prone source rock. Based on the

427

geological setting (see section 2), the inferred oil-prone source rock could be of

428

Paleocene age. The highly mature EMG-B gas could be a highly mature humic kerogen

429

associated gas, similar to the unaltered family D gas from the Baoshi Formation,

430

because the original carbon isotope ratio of C1 would have been about -30.0‰, based on

431

extrapolating the carbon isotope ratios of C2 and C3 in Figure 2b.

432 433

5.6. Implications for Gas Exploration

434 435

Gas-source correlations indicate that the coal-bearing Pinghu Formation of Middle–

436

late Eocene age is the major source rock unit, and that both coals and mudstones are the

437

major source contributors. Coal-derived gases (Family A) occur in the Pinghu slope

438

where coal is more developed, while mudstone-generated gases (Family B) are mainly

439

distributed in the central parts of the sag, where mudstone is more developed,


Page 22 of 37

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Energy & Fuels

440

suggesting that the natural gas distribution is source controlled. Therefore, the regions

441

where good source rocks are well developed are more promising for gas exploration. In

442

addition, the underlying Baoshi Formation and Paleocene source rocks are also inferred

443

to have made contributions to gas accumulations. This is of great significance for gas

444

exploration in the Xihu Sag, where new exploration plays that target different

445

hydrocarbon sources may be established in the future. Although only limited

446

discoveries related to the Baoshi Formation and the Paleocene source rocks have been

447

achieved to date, we are confident in the exploration potential of these deep buried

448

intervals as source rocks.

449 450

6. CONCLUSIONS

451 452

The natural gases in the Xihu Sag have been classified into five distinct gas families

453

(A–E) based on their stable carbon isotope compositions. The carbon isotope curves

454

combined with geological information suggest that families A and B are primary, while

455

families C, D, and E have been subjected to secondary alteration, i.e., admixture of

456

additional gases. Molecular and isotopic compositions of the gases suggest that families

457

A, B, C, and D were derived from type III kerogens, while family E is a mixture of

458

humic gas with sapropelic gas. Thermal maturity assessment suggests that the gases

459

were generated during the late oil window to wet gas window. Gas-source rock

460

correlation by comparing stable carbon isotope ratios of natural gases with the pyrolysis


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

461

gases of three source rocks suggests that families A and B gases were generated from

462

coal and mudstone in the Middle–Upper Eocene Pinghu Formation, respectively.

463

Family C gases were suggested to have been derived from mudstone and carbonaceous

464

mudstone of the Pinghu Formation. Family D gas could be a mixture of humic gas

465

derived from the Lower–Middle Eocene Baoshi Formation, mixed with diffused gas

466

dominated by 12C-rich C1. Family E may be a mixture sourced from the Lower–Middle

467

Eocene Baoshi Formation and the Paleocene oil-prone source rocks. CO2 in some of the

468

analysed gas samples was generated during thermogenic processes of transformation of

469

organic matter, while the other natural gases contain components of CO2 from

470

endogenic processes. The confirmation of the Eocene Baoshi Formation and Paleoccene

471

as additional effective source rocks in the Xihu Sag suggests that new exploration plays

472

targeting these alternative hydrocarbon source rocks are possible and worth exploring.

473 474

Notes

475

The authors declare no competing financial interest.

476 477

ACKNOWLEDGEMENTS

478 479

This study was financially supported by the National Science and Technology Major

480

Project (Grant No. 2016ZX05027-001-003). We would like to thank Professor Simon

481

George and an anonymous reviewer for their valuable comments which helped to


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greatly improve the quality of the manuscript.

483 484

REFERENCES

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Comet, P. A.; Leeuw, J. W.; Spiro, B.; Illich, H.; Raiswell, R.; Eglinton, G.; Curtis, C.

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D.; McKenzie, D. P.; Murchison, D. G. PHILOS. T. R. SOC. A. 1985, 315, 123–134.

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(2) Horsfield, B.; Disko, U.; Leistner, F. Geologische Rundschau 1989, 78, 361–373.

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627–632 (in Chin. with Engl. abstract).

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Zhiqiang, F.; McMillen, K. J., Eds. Am. Assoc. Pet. Geol. Mem. 1992, 53, 17–25.

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(16) Chen, L.; Sun, B.; Wang, L.; Li, K. Complex Hydro. Reserv. 2016, 9, 5–9 (in Chin.

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with Engl. abstract).

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abstract).

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Wang, X.; Tang, Y. Chem. Geol. 2012, 310-311, 49-55.

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(25) Schoell, M. AAPG Bull. 1983, 67, 2225–2238.

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(27) Whiticar, M. J. Int. J. Coal Geol. 1996, 32, 191–215.

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(28) Whiticar, M. J. Chem. Geol. 1999, 161, 291–314.

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(31) Rooney, M. A.; Claypool, G. E.; Moses Chung, H. Chem. Geol. 1995, 126, 219–

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C. Chem. Geol. 2013, 339, 141–156.

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Energy & Fuels

566

Table captions

567

Table 1. Stable carbon isotopic and molecular compositions and gas indices of natural

568

gases from the Xihu Sag, East China Sea Basin.

569

Table 2. Geochemical characteristics of the source rocks of the Pinghu Formation used

570

for pyrolysis experiments.

571

Table 3. Stable carbon isotopic composition of pyrolysis gases from the source rocks of

572

the Pinghu Formation.

573 574

Figure captions

575

Figure 1. Location of the East China Sea Basin and the Xihu Sag within the East China

576

Sea Basin (a), structural units of the Xihu Sag (b), location of wells sampled here and

577

distribution of the gas families (c), and a generalized stratigraphy of the Xihu Sag (d).

578

Note that gases of different gas families occurred in the N19-1 and PE wells (Table 1),

579

so two labels were overlapped for them in Figure 1c.

580

Figure 2. Natural gas plots of gaseous hydrocarbons in the Xihu Sag gases (a and b)

581

and plot of δ13C3 - δ13C2 versus δ13C2 - δ13C1 values (c), showing family assignments of

582

the gases.

583

Figure 3. Plot of δ13C1 versus C1/(C2+C3) (Bernard diagram, after Whiticar28) showing

584

the origin of the studied natural gases.

585

Figure 4. Plots of δ13C2 versus δ13C1 values (a) and δ13C2 versus δ13C3 values (b) for the

586

Xihu Sag gases. Regression lines for alkane gases are derived from Type II kerogen


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

587

(Paleozoic marine shales from Delaware and Val Verde basins, West Texas) after

588

Jenden et al.30 and Type III kerogens (fluvio-deltaic shales from the Niger delta and the

589

Sacramento Basin) after Rooney et al.31.

590

Figure 5. Cross plots of δ13CCO2 versus δ13C1 (a) and δ13CCO2 versus CO2 content (b),

591

showing the origin of carbon dioxide in the analysed natural gases from the Xihu Sag.

592

Figure 6. Natural gas plots for varying gas families and pyrolysis gases from different

593

source rocks, showing genetic correlations between natural gas and source rocks.


Page 30 of 37

Page 31 of 37

594 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 40 595 41 42 43 44 45 46

Energy & Fuels

Table 1 Well

Sample code

Depth (m)

Formation

Gas

Stable carbon isotope (δ13C, ‰)

Molecular composition (%)

Gas indices

family

C1

C2

C3

CO2

C1

C2

C3

iC4

nC4

iC5

nC5

N2

CO2

C1/C1-5

C1/(C2+C3)

N13-1

N13-1

4150–4169

E2p5

D

-37.1

-22.7

-20.8

-7.0

89.1

4.5

0.63

0.16

0.10

0.05

0.03

0.36

5.0

0.94

17.4

N13-2

N13-2

4093.5–4148

E2b

A1

-34.6

-25.3

-24.5

-10.6

79.4

8.4

2.8

1.0

0.65

0.28

0.20

0.59

6.3

0.86

7.1

K-A1

K-A1

n.a.

n.a.

A1

-34.4

-25.6

-24.9

-5.9

83.1

9.5

2.9

1.2

0.73

0.46

0.25

n.d.

n.d.

0.85

6.7

K-B2

K-B2

n.a.

n.a.

C

-40.4

-28.1

-26.1

-7.8

77.7

10.7

5.5

1.7

1.5

0.71

0.46

n.d.

n.d.

0.79

4.8

N19-1

N19-1A

4359.0–4382.0

E2p4

A1

-35.4

-25.3

-24.3

n.d.

84.4

7.1

2.6

0.54

0.55

0.19

0.14

0.12

4.1

0.88

8.7

N19-1

N19-1B

4580.0–4620.0

E2p5

C

-40.6

-28.5

-26.3

n.d.

74.2

12.2

6.7

1.4

1.2

0.27

0.17

0.44

3.3

0.77

3.9

B-1

B-1

3953.2–3956.6

E2p5

E

-35.0

-30.9

-27.8

-19.8

86.6

5.8

3.1

0.64

0.62

0.12

0.08

0.89

2.2

0.89

9.7

B-A5

B-A5

n.a.

n.a.

A1

-34.4

-25.8

-25.0

-7.5

73.7

11.2

7.2

2.4

2.0

0.97

0.56

n.d.

n.d.

0.75

4.0

N25-1

N25-1A

3223.4–3242

E2p2

A2

-37.2

-27.4

-26.2

-7.0

81.8

7.0

4.5

1.4

1.0

0.36

0.19

1.3

2.3

0.85

7.1

N25-1

N25-1B

3310–3317

E2p2

A2

-35.5

-27.3

-26.2

-7.7

87.5

5.2

2.5

0.75

0.53

0.21

0.10

0.85

2.0

0.90

11.4

N25-2

N25-2A

n.a.

n.a.

A2

-36.0

-27.4

-26.3

-7.4

87.5

5.1

2.4

0.63

0.47

0.17

0.10

0.54

2.9

0.91

11.7

N25-2

N25-2B

n.a.

n.a.

A2

-36.2

-27.5

-26.2

n.d.

87.1

5.6

4.0

1.3

0.77

0.32

0.16

0.45

0.00

0.88

9.1

T-A3

T-A3

n.a.

n.a.

A2

-35.6

-27.1

-25.6

-7.1

77.5

9.1

6.5

2.2

1.8

0.83

0.48

n.d.

n.d.

0.79

5.0

PE

PE-A

2703.2–2726.5

E2h

C

-40.6

-28.5

-26.3

-8.2

75.4

8.5

4.0

2.6

1.8

1.2

0.68

3.7

1.6

0.80

6.0

PE

PE-B

3031–3038

E2p1

B

-37.4

-27.8

-25.7

-11.2

80.5

7.8

3.1

1.4

1.1

0.50

0.30

2.8

2.6

0.85

7.4

PE

PE-C

3402.2–3418.8

E2p2

B

-38.1

-28.6

-26.3

-13.4

79.8

8.1

3.6

1.4

1.3

0.58

0.43

1.0

3.8

0.84

6.8

PE

PE-D

3541.2–3553

E2p3

B

-37.8

-27.8

-25.9

-14.2

80.7

8.2

3.3

1.1

1.19

0.41

0.27

1.5

3.4

0.85

7.0

PF

PF

3279–3294

E2p2

A1

-36.4

-25.0

-23.9

-12.7

80.7

7.2

4.0

0.82

0.69

0.18

0.06

0.82

5.4

0.86

7.2

H-2

H-2A

3350-3361

E3hs

B

-35.0

-25.9

-23.8

n.d.

87.7

6.1

2.4

0.99

0.58

0.25

0.19

0.85

0.58

0.89

10.3

H-2

H-2B

3963.7-3979.7

E2hx

B

-34.2

-26.0

-24.2

-9.1

84.2

6.7

2.3

0.66

0.49

0.20

0.15

0.12

4.9

0.89

9.4

H-A1

H-A1

n.a.

Eh

B

-33.8

-25.3

-23.3

-7.4

86.5

5.6

3.3

1.2

0.98

0.50

0.31

n.d.

n.d.

0.88

9.7

C-4

C-4A

2721-2731

E3hs

B

-36.0

-27.5

-25.3

n.d.

78.4

9.7

4.8

1.3

1.1

0.42

0.28

0.95

2.2

0.82

5.4

C-4

C-4B

2734-2748

E3hs

B

-36.2

-27.2

-25.1

n.d.

81.8

8.1

4.3

1.3

1.1

0.41

0.27

0.55

1.7

0.84

6.6

C-A1

C-A1

n.a.

Eh

B

-35.7

-27.6

-25.5

-4.0

78.7

10.1

5.6

1.7

1.5

0.63

0.39

n.d.

n.d.

0.80

5.0

C-A3

C-A3

n.a.

Eh

B

-35.8

-27.1

-25.1

-4.6

77.6

10.6

5.7

2.4

1.4

0.69

0.38

n.d.

n.d.

0.79

4.8

D-1

D-1A

2512.5-2519.5

E3hs

B

-38.3

-28.5

-27.0

n.d.

78.8

8.1

6.2

2.0

1.6

0.54

0.35

0.87

0.85

0.81

5.5

D-1

D-1B

3031.5-3036

E2hx

B

-37.0

-28.8

-26.7

-16.0

83.0

6.0

3.2

1.2

0.86

0.36

0.22

1.7

3.23

0.87

9.0

n.a.: not available; n.d.: not determined; E2p5: the fifth member of the Eocene Pinghu Formation; E2b: the Eocene Baoshi Formation 31 ACS Paragon Plus Environment

Energy & Fuels

596 1 2 597 3 4 5 6 7 8 598 9 10 599 11 600 12 601 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 40 602 41 42 43 44 45 46

Page 32 of 37

Table 2 Well

Sample

depth (m)

TOC (%)

Ro (%)

PE

Coal

3570

63.2

0.72

N25-1

Carbonaceous mudstone

3230–3260

21.2

0.63

D-1

Mudstone

3717–3785

1.9

0.86

Table 3 Coal

Mudstone

Temperature

rate (°C/h)

(°C)

2

336

-36.8

-32.1

-29.0

336

-42.6

-31.7

-28.2

-38.7

n.d.

n.d.

2

360

-37.8

-32.3

-29.4

360

-44.7

-31.5

-27.3

-39.4

-33.0

-30.7

2

384

-38.5

-30.4

-29.0

384

-43.3

-29.3

-26.0

-41.2

-31.6

-29.4

2

409

-37.0

-28.8

-27.4

409

-41.2

-28.6

-25.5

-41.3

-30.4

-29.1

2

433

-35.5

-27.8

-26.4

433

-38.1

-26.6

-24.2

-40.5

-29.4

-27.6

2

457

-33.4

-23.5

-22.2

449

-35.7

-24.3

-20.3

-38.8

-28.7

-26.5

2

481

-31.7

-16.9

n.d.

464

-33.4

-20.1

n.d.

-36.8

-26.7

-25.6

481

-31.8

-16.4

n.d.

-35.1

-24.6

-23.4

δ13C1 (‰)

Temperature

Carbonaceous mudstone

Heating

δ13C2 (‰)

δ13C3 (‰)

2

(°C)

δ13C1 (‰)

δ13C2 (‰)

δ13C3 (‰)

δ13C1 (‰)

δ13C2 (‰)

δ13C3 (‰)

20

360

-34.7

-31.7

-28.8

360

-42.3

-32.2

-28.6

-37.4

-33.4

-31.4

20

370

-35.7

-31.6

-29.2

380

-44.2

-31.3

-27.6

-38.5

-33.5

-31.1

20

380

-37.1

-31.7

-29.3

400

-44.0

-31.1

-27.3

-40.2

-32.7

-31.0

20

410

-38.0

-31.1

-29.0

420

-43.2

-29.7

-26.4

-40.7

-32.0

-29.3

20

430

-37.6

-30.5

-28.3

440

-41.0

-28.9

-25.9

-41.1

-30.9

-28.8

20

460

-36.4

-29.3

-27.9

460

-38.7

-27.1

-24.6

-40.8

-29.9

-27.4

20

480

-35.2

-27.8

-27.7

480

-36.3

-25.5

-22.0

-39.4

-29.2

-26.3

20

505

-32.9

-23.5

n.d.

500

-34.0

-22.2

-15.9

-37.1

-27.6

-25.3

20

530

-31.9

-18.7

n.d.

520

-32.5

-16.5

n.d.

-35.7

-26.3

-23.7

540

-31.3

n.d.

n.d.

-33.6

-24.5

-20.8

20 n.d.: not determined


Page 33 of 37 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

Figure 1.


Energy & Fuels

0.0

0.2

0.4

1/Cn

0.6

A1 A2 E

-35.0 -40.0 -45.0

13

13

N13-2 K-A1 N19-1A B-A5 PF N25-2 N25-2 T-A3 N13-1 B-1

A1

-30 -30.0

-40.0

D E

-45.0

C2

C1

δ C3-δ C2 (‰)

13

(Sub-) Family

A1

2

A2 B C

1

D E

0 10 15 13 13 δ C2-δ C1 (‰)

PE-B,C & D H-2 H-A1 C-A1 C-A3 C-4 D-1 N19-1B K-B2 PE-A

C3

3

5

0.6

0.8

1.0

b

c

0

1/Cn

-15.0

-35.0

A2

C3

4

0.4

-25.0

δ C (‰)

-30.0

0.2

-20.0

D

-25.0

δ C (‰)

a

(Sub-) Family

-20.0

1.0 0.0

0.8

-15.0

13

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 34 of 37

20

Figure 2.


Family

B B C C C2

C1

Page 35 of 37

104 Migration

Ro

og

en

Migration

er

102

tio n

K

C1 /(C2 +C3 )

Ox id a

A1 A2 B C D E

Ty pe II

(Sub-) Family

103

THERMOGENIC

K

o er

ge

n

II eI p Ty

10

1 -60

-50

-40

13

δ C1 (‰)

-30

-20

Figure 3.

-20

-20

a

2.0%

Niger Delta (Type III)

-30

1.4% 1.2%

-40 -45

(Sub-) Family

-29

A1 A2 B C D E

Type II

-35

Delaware/Val Verde Basins (Type II)

-50

Type III

-30

13

13

1.1%

-35

Sacramento-25 Basin (Type III)

δ C2 (‰)

1.6%

-25

b

Ro

1.8%

δ C2 (‰)

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

-40

-35

-30

-40 -25 -35

13

-30

-25 13

δ C1 (‰)

δ C3 (‰)

Figure 4.


-20

-15

Energy & Fuels

-70

13

δ C1 (‰)

-50

Family N13-2 K-A1 PF A N25-1 N25-2 PE-B,C&D H-2B H-A1 B C-A1 C-A3 D-1B K-B2 C PE-A N13-1 D E B-1

M O icro of xida bial M tio eth n an e

a

Microbial Gases

-60

Thermogenic Gases

-40 20 0℃

-30

En 300 do ℃ ge 4 0 n ic

50 0 ℃ 0℃

-20 -30

-20

-10

Ga ses

0

10

13

δ CCO2 (‰)

-30

b

13

δ CCO2 (‰)

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 37

Organic CO2

-20

Coexisting area of organic and inorganic CO2 Mixture of organic and inorganic CO2

-10 Inorganic CO2

0 0

20

40

60

CO2 (%) Figure 5.


100 80

Page 37 of 37

1/Cn 0.2

0.4

1/Cn

0.6

1 0

0.8

0.2

0.4

1/Cn 0.6

0.8

1 0

-15.0

-15.0

-15.0

-20.0

Coal -20.0 2℃/h

Mudstone 2℃-20.0 /h

-25.0

-25.0

-40.0 -45.0 0

0.2

0.6

1 0 -45.0

0.8

-25.0

-25.0

-40.0 -45.0

410 °C 430 °C 460 °C 480 °C 505 °C Family A Family D Family E

PROPANE ETHANE

δ13C (‰)

-20.0

Coal 20℃/h -20.0

-35.0

0.4

-30.0 -35.0 -40.0 -45.0

METHANE

-30.0 -35.0 -40.0

1/Cn 0.6

0.8

-45.01 0

0.2

0.4

0.6

0.8

1

Carbonaceous mudstone 2℃/h 384 °C 409 °C 433 °C 449 °C 464 °C Family C Family D Family E

0.2

1/Cn 0.4

0.6

0.8

1

-15.0

-15.0

-15.0

-30.0

δ13C (‰)

-35.0 -40.0

1/Cn 0.4

-30.0

-25.0

Mudstone -20.0 20℃/h 420 °C 440 °C 460 °C 480 °C 500 °C 520 °C 540 °C Family B Family D Family E

Carbonaceous mudstone 20℃/h

-25.0 -30.0

13

-35.0

384 °C 409 °C 433 °C 457 °C Family A Family D Family E

384 °C 409 °C 433 °C 449 °C 464 °C 481 °C Family B Family D Family 0.2 E

δ C (‰)

-30.0

δ13C (‰)

δ13C (‰)

0

δ13C (‰)

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

Energy & Fuels

-35.0 -40.0 -45.0

PROPANE ETHANE

Figure 6.


METHANE

420 °C 440 °C 460 °C 480 °C 500 °C Family C Family D Family E

PROPANE ETHANE

METHANE

Sources of natural gases in the Xihu Sag, East China Sea Basin

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