Thermal cracking of oil under water pressure up to 900 bar at high

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Thermal cracking of oil under water pressure up to 900 bar at high thermal maturities: 2. Insight from light hydrocarbon generation and carbon isotope fractionation Liujuan Xie, Yongge Sun, Clement N. Uguna, Youchuan Li, Colin E. Snape, and will meredith Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01697 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Energy & Fuels

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Thermal cracking of oil under water pressure up to 900 bar at high

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thermal maturities: 2. Insight from light hydrocarbon generation and

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carbon isotope fractionation

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Liujuan Xie1, 2, Yongge Sun2*, Clement N. Uguna3,, Youchuan Li4,

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Colin E. Snape3, Will Meredith3

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1 Qingdao

Institute of Marine Geology, China Geological Survey, Qingdao, 266071,

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China 2 Institute

of Environmental and Biogeochemistry (EBIG), School of Earth Sciences,

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Zhejiang University, Hangzhou 310027, China. 3Faculty

of Engineering, University of Nottingham, Energy Technologies Building,

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Triumph Road, Nottingham NG7 2TU, UK. 4 Beijing

Research Center of CNOOC China Ltd., Beijing 100027, China.

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*Corresponding author. E-mail address: [email protected] (Y. Sun), Zhejiang

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University, Tel: +86-571-87951336, Fax: +86-571-87951336.

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Abstract

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In this study, pyrolysis experiments were conducted with a saturate-rich Tertiary

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source rock-derived oil from the South China Sea basin, using a fixed-volume pressure

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vessel at temperatures from 350 to 425 °C for 24 h (0.92–1.85% Easy Ro) to investigate

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pressure effects up to 900 bar on the generation and stable carbon isotopic fractionation

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of light hydrocarbons in the C6–C7 range. The results demonstrate that the pressure

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retards oil cracking to light hydrocarbons, but the retardation depends on the thermal

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evolution. In the peak oil to early wet gas stage (350 °C and 373 °C, 0.92–1.15% Easy

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Ro), the light hydrocarbon generation is low but it is still suppressed by increasing

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pressure. In the late stages of the wet gas window (390 °C, 405 °C and 425 °C, 1.35–

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1.85% Easy Ro), the light hydrocarbon generation is suppressed significantly from 200

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to 470 bar, followed by promotion and promotion-suppression as pressure is increased

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up to 900 bar. Meanwhile, the distributions of branched alkanes, cycloalkanes and

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aromatic hydrocarbons are pressure-dependent. The medium to high pressures result in

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increasing Mango K1 values and toluene/n-C7 ratios, and decreasing n-

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C7/methylcyclohexane ratios, suggesting that pressure benefits the occurrence of

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cyclization and aromatization during oil cracking, probably involving bimolecular

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reaction pathways. Preferential aromatization and isomerization with increasing

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pressure lead to significant carbon isotopic fractionations of aromatic hydrocarbons and

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branched alkanes as up to 4‰ and 2‰, respectively. However, stable carbon isotopic

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compositions of cycloalkanes show almost no fractionation under pressurized cracking.

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Therefore, caution must be taken in respect to the application of light hydrocarbon-

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derived parameters in deep petroleum reservoirs usually at high temperatures and

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pressures. The carbon isotopes of branched alkanes and aromatic hydrocarbons could

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be potential measures to identify the pressure effects, while carbon isotopes of

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cycloalkanes could be an effective index for oil-oil / oil-source correlations.

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Keywords: Oil cracking; hydrous pyrolysis; light hydrocarbon; pressure retardation;

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carbon isotope fractionation.

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1. Introduction

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With the expansion of deep petroleum exploration around the world, attention has

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focused on identifying and evaluating the thermal stability of deep-buried crude oil

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under high temperature/pressure (HT/HP) conditions.1-7 Oil cracking is a complicated

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and progressive process, and controlled by many factors as oil components, temperature,

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pressure, mineral matrix, metal elements, etc.8-13 Pyrolysis experiments have been

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traditionally conducted to compare products changes and describe oil-cracking

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processes, thereafter extracting molecular and isotopic information to evaluate deep-

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buried oil reservoirs within geological context.6, 14-20

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It is well known that temperature plays a key role during oil cracking. However, it

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has long been recognized that pressure is another important factor influencing the

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thermal stability of crude oils since oil to gas process is an endothermic volume

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expansion reaction.21 Previous studies have widely investigated pressure effects for

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crude oils and model compounds.6, 15-17, 22-26 It has been generally accepted that the

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thermal decomposition of hydrocarbons, such as n-alkanes, can be described by a

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mechanism consisting of free radical reactions. Two types of radicals are involved in

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the pyrolysis mechanism of saturated hydrocarbons1, 27, 28: radicals that decompose by

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unimolecular reactions and radicals that react by bimolecular reactions. In the

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unimolecular reactions, the volume of transition state is larger than the volume of the

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initial species. In the bimolecular reactions, the volume of the transition state is smaller

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than the sum of the volume of the reactants. Under high temperature and low pressure,

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unimolecular radical decomposition reactions are favored over bimolecular reactions


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because of the low reactant concentrations and their higher activation energies.29

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However, under low temperature and high pressure, bimolecular reactions are favored

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over radical decomposition reactions.30 Michels et al.31 proposed that it was the

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proportion of unimolecular vs bimolecular reactions controlled the extent of

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conversions of n-alkanes under different temperature and pressure regimes. However,

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in terms of crude oils, results from pressurized-pyrolysis experiments are still

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contradictory to date in respect to the effects of pressure on oil cracking (“retardation

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vs acceleration”) and remain ambiguous.

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Gasoline-range hydrocarbons, usually called light hydrocarbons in the range of C5–

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C12 (LHs), are important components of crude oil. The molecular and stable carbon

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isotopic compositions of LHs have been widely used for oil/gas-source correlations,32-

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38

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crude oil,42-44 and in-reservoir secondary alteration identification, e.g., biodegradation,

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evaporation, water washing, thermochemical sulfate reduction.45-50 Hunt51 proposed

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that LHs are formed from the combined effects of various biological, geological and

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chemical processes on the dispersed organic matter in sediments. Three types of

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reactions are involved in LH generation, including biodegradation of sedimentary

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organic matter in the biochemical and early diagenetic stages, low-temperature

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chemical degradation of kerogen and sedimentary organic matter in the diagenetic stage,

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and high-temperature thermal degradation of kerogen, bitumen and crude oil in the

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catagenetic and metagenetic stages. High-temperature thermal degradation was

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considered to be the main process accounting for LH generation in crude oil, although

maturity assessment of crude oil,39-41 migration pathway and accumulation history of


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it has been challenged by transition metal catalysis theory proposed by Mango in

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~1990s.52-54 However, the composition of cutting gas and numerical models based on

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chemical thermodynamics55,

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transitional metal catalysis under geological conditions.

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demonstrated that LHs is unlikely formed from

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If in-reservoir oil cracking occurs in deep-buried basins, the LHs must be the

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intermediate products carrying important structural information. Previous studies

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mainly focused on analyzing C12+ hydrocarbons or C1–C5 gas hydrocarbons when

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considering oil cracking during pressurized pyrolysis experiments,15, 17, 18, 26, 57 and little

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is known about the evolution regularities of LHs with respect to pressure. Although

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there are clear descriptions on how temperature influences the molecular and stable

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carbon isotopic compositions of LHs during oil cracking,43, 58 pressure can theoretically

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complicate oil cracking and result in different pathways so affecting molecular

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compositions and stable carbon isotopes of LHs. To the best of our knowledge, no

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attempt has been made to probe this issue and the mechanisms involved. In the early

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work of this study, using a C9- free of saturate-rich oil derived from Tertiary source rock

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in South China Sea, fixed-volume pressure vessel pyrolysis experiments at a range of

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temperature from 350 to 425 °C were conducted under up to 900 bar water-pressure

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condition, C1–C5 gas compositions and carbon isotopes were reported.57 The present

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study further addresses the yields and stable carbon isotopic compositions of different

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compound classes (n-, iso-, cyclo-alkanes, and aromatics) in the LH range for the same

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pressurized oil cracking experiments. The objectives are to (1) identify the pressure

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effect on oil cracking processes by monitoring the generation of LHs, (2) determine the


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carbon isotopic fractionation of individual LHs with increasing pressure at different

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thermal stage, and (3) probe the mechanisms involved in the pressure effect on LH

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generation. The study of how LH generation responds to high pressure and high

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temperature not only expands our understanding of the evolutionary behavior of LHs,

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but also provide possibility to explore LHs parameters for the use of oil cracking

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evaluation in high pressure and high temperature basins and potentially to monitor the

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refining processes of crude oils.

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2. Experimental section

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2.1 Oil sample

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The crude oil used in this study was collected from the Pearl River Mouth Basin

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(The Zhujiangkou Basin), South China Sea with API gravity of 22º (density of 0.921

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g/mL). It is saturate-rich oil with 52.3% saturates, 24.6% aromatics, 10.1% resins and

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7.1% asphaltenes. This crude oil was specially selected to investigate the generation

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and destruction of LHs because of LHs depletion, which might result from slight

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biodegradation and/or natural volatilization during sample collection, transportation

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and storage.

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2.2 High water-pressure pyrolysis experiments

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The pyrolysis equipment comprised a 25 mL Hastalloy cylindrical pressure vessel

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rated to 1400 bar at 420 °C connected to a pressure gauge and rupture disc rated to 950

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bar.18 The experiments were conducted using 1.20 g of crude oil at five temperature

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points for 24 h under low pressure hydrous (200 bar) and high liquid water pressure

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(470 bar, 750 bar and 900 bar), as described in details in Xie et al.57 Briefly,


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temperatures points were set at 350, 373, 390, 405, and 425 °C (accuracy ±1 °C). The

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equivalent vitrinite reflectance values (Easy%Ro) were calculated following the method

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developed by Sweeney and Burnham59. The calculated vitrinite reflectance of the set

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temperatures are 0.92, 1.15, 1.35, 1.56, 1.85% Ro, respectively. Therefore, the thermal

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maturity reached during the experiments covers the peak oil generation to an elevated

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stage in the gas window.

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2.3 Chemical and isotopic analyses

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After the experiments, the generated gas was collected as described in Xie et al.57 The residue oil was collected for GC and GC-ir-MS analysis respectively.

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The GC analyses of the whole oil were carried out using an Agilent 7890A GC

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fitted with a DB-1MS capillary column (60 m × 0.32 mm × 0.25 μm). The temperature

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was kept constantly at 30 °C (15 min hold), increased to 295 °C at 4 °C/min (30 min

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hold). Nitrogen was used as the carrier gas with a flow rate of 1 mL/min. The

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concentrations of LHs with different chemical structures were determined with the

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response coefficients relative to an internal standard (n-C24D50), which were described

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in Xiao60.

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The stable carbon isotopes analyses of individual LHs were carried out using a GV

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Instruments IsoPrime mass spectrometer interfaced to HP6890 gas chromatography.

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The GC was fitted with a CP-Sil 5 CB capillary column (50 m × 0.32 mm × 0.40 μm).

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The temperature was kept constantly at 35 °C (15 min hold), increased to 145 °C at

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2 °C/min, and then increased at 15 °C/min to 295 °C (30 min hold). Helium was used

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as the carrier gas with a flow rate of 1.2 mL/min. The injection of samples was


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conducted in the split ratio of 3:1. The isotope values were calibrated against the

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reference gas and are reported in the usual delta notation relative to the VPDB. At least

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two measurements were performed for each sample to confirm the standard deviation

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was less than 0.5‰ for most of the compounds.

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3. Results

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3.1 The distribution of light hydrocarbons and yields of C6–C7 range

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hydrocarbons

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The distributions of LHs in the pristine oil and residual oils after pyrolysis as a

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function of temperature at a pressure of 200 bar are presented in Figure 1. Individual

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compound identification was achieved by comparison with GC retention time published

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by Ten Haven32 and George et al.48 The abbreviations used in the text, figures and tables

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for individual LHs are listed in Table 1.

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LHs with six to seven carbon atoms have been most widely used in petroleum

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geochemistry,42, 44, 48-50 therefore the C6–C7 range LHs are mainly investigated here.

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The yields of the C6–C7 range LHs from the pyrolysis experiments are presented in

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Table 2 and Figure 2. With increasing thermal stress, the total C6–C7 yield at 200 bar

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increases from 4.5 mg/goil at equivalent vitrinite reflectance of 0.92% Ro to a maximum

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of 94.2 mg/goil at 1.56% Ro, then decrease to 84.6 mg/goil at 1.85% Ro (Table 2, Figure

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2). As shown in the conceptual model (Figure 3), LHs are intermediate products during

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oil-cracking and can be progressively thermally-degraded to methane and pyrobitumen

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in highly to over mature stages, accounting for the decrease of total C6–C7 yield at high

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pyrolysis temperature (corresponding to Ro >1.56%). The C6–C7 yields under a liquid-


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water pressure of 750 bar has the same trend as that of 200 bar, reaching a maximum at

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the temperature of 405 °C. However, the yields at liquid-water pressures of 470 bar and

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900 bar show different trends upon temperature sequence. No significant change occurs

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between 350 and 373 °C, followed by a remarkable increase at 390 °C with little further

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change at 425 °C.

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Whatever the pressure used, a significant increase in C6–C7 yields occurs at 390 °C

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(equivalent vitrinite reflectance of 1.35% Ro; Figure 2). However, at each temperature,

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the C6–C7 yields at low pressure (200 bar) are higher than that at high pressures (470–

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900 bar), although trends show small changes at the different temperatures used. At 350

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°C, the total C6–C7 yield is reduced by 67% from 4.5 to 1.5 mg/goil as the pressure

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increases from 200 to 900 bar. At the higher temperatures of 373, 390, and 405 °C, the

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total C6–C7 yields are highest at 200 bar, then decrease significantly as pressure

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increases to 470 bar, followed by temperature-dependent distributions as the pressure

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reaches up to 900 bar. That is, at 373 °C and 405 °C, the total C6–C7 yields at 750 bar

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are higher than that at 470 bar, and then decrease when pressure increases to 900 bar,

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while the yield at 750 bar and 390 °C is lower than that at 470 bar and 900 bar. At

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425 °C, as the pressure increases from 200 to 470 bar, the total C6–C7 yield is reduced

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by 71% from 84.6 to 24.9 mg/goil, and then remain nearly constant as the pressure

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increases to 900 bar.

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Compound groups in the C6–C7 range, namely n-alkanes, branched alkanes,

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cycloalkanes and aromatics, show distinct patterns upon temperature sequence at the

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different pressures used although yields of these fractions are still much higher at 200


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bar than that at the higher pressures (Figure 4a, b, c, d). At 200 bar, the yields of the

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compound groups show a normal trend in that yields of n-alkanes and branched alkanes

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increase first followed by a decrease at 425 °C due to cracking, cyclization and

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aromatization (Figure 4a, b). This is typically evidenced by enrichment of cycloalkanes

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and aromatics at 425 °C, with the proportion of total cycloalkanes and aromatics up to

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69% (Figure 4c, d). As similar to the total C6–C7 yields, the distribution patterns of the

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compound groups at 750 bar show the same trend as at 200 bar and reach a maximum

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at 405 °C. No significant change occurs on the yields of n-alkanes and cycloalkanes at

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pressures of 470 and 900 bar at 350 and 373 °C. This is followed by a remarkable

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increase at 390 °C with the yield stabilizing at 425 °C (Figure 4a, c). However, regarding

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the yields of branched alkanes and aromatics, these demonstrate the same trends as 200

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bar, but reach their maximum yields at 390 °C (Figure 4b, d).

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3.2 Compound-specific stable carbon isotopic compositions of light hydrocarbons

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In terms of the concentrations needed for reliable isotopic measurements for

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individual compounds in the C6–C8 range from the oil cracking experiments, only the

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δ13C values of n-C6 to n-C8, benzene (Ben), cyclohexane (CH), methylcyclopentane

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(MCP), 3-methylhexane (3-MH), methylcyclohexane (MCH), and toluene (Tol) are

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reported and listed in Table 3. As presented in Figure 5, the δ13C values of these

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compounds generally show 13C enrichment with increasing temperature. In early stages

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of oil-cracking at 350 °C and 373 °C, less isotopic fractionation occurs as evidenced by

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low C6–C8 yields (Figure 5). However, as the temperature increases from 373 to 425 °C,

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the stable carbon isotopic fractionation of n-alkanes, cycloalkanes and aromatics


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reaches up to 11‰, 5‰ and 6‰, respectively (Figure 5a, b, c). One exception is the

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δ13C value of cyclohexane that demonstrates a relatively stable ratio upon temperature

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sequence under different pressure regimes.

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4. Discussion

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Due to the experiments under hydrous conditions, supercritical water (> 374 °C)

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effect on the oil cracking process is a first challenge. Although the supercritical water

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system is characterized by favorable transport properties and high diffusivities61, the

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upgrading of oil in sub- and supercritical water are both dominated by free radical

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reaction mechanisms.62 In our previous work57, parallel hydrous and anhydrous gold

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tube pyrolysis with fixed pressure of 450 bar for 24 h at 370, 390, and 405 °C were

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conducted in order to clarify this issue. The results demonstrated that supercritical water

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did not have a significant effect on the hydrocarbon-cracking reaction pathway.

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Therefore, the oil-cracking experiments here still can be used to evaluate the pressure

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effect on oil cracking processes.

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4.1 High pressure retardation on light hydrocarbon generation

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The lack of C9- LHs in the pristine oil provides an excellent framework to assess the

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pressure effects on the LH generation during oil-cracking. As shown in Table 2 and

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Figure 2, the total C6–C7 yields at low pressure (200 bar) are much higher than those

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under high liquid water pressure (470, 750 and 900 bar) at all temperatures, indicating

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that increasing pressure definitely retards oil cracking. This is in agreement with

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previous studies.18,

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depends on the thermal stage of evolution.

24, 63

However, the retardation of high pressure on oil cracking


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At the peak oil to early wet gas stage (350 to 373 °C, 0.92–1.15% Ro), the total C6–

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C7 yields are very low making the trends with increasing pressure less significant than

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that at higher temperatures, although they generally demonstrate a decreasing trend with

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increasing pressure from 200 to 900 bar (Figure 2). The trend in the total C6–C7 yields

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for the initial stages of oil cracking is consistent to those for gas yields observed in

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previous studies from the pyrolysis of coals, oil and n-hexadecane between 175 and 900

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bar water pressure at 350 °C.18,

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hydrocarbons to generate free radicals which is suppressed. In terms of cracking-

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kinetics, the effect of activation volume during pressure variation could account for the

255

retardation of oil-cracking28.

64

Oil cracking to gas occurs via beta scission of

256

At 390 °C (1.35% Ro), increasing pressure has a strong suppression first from 200

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to 750 bar, then followed by a significant promotion from 750 to 900 bar (Figure 2).

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However, the total C6–C7 yields at 405 °C (1.56% Ro) show a strong suppression-slight

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promotion-slight suppression sequence with increasing pressure (Figure 2). At this

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highly mature stage, cage and possibly diffusional effects play important roles to

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account for the lower yields from 470 to 900 bar, which generally suppress reaction

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rates with increasing pressure due to the activation volume dynamics.1, 28, 65, 66 On the

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other hand, although the collision rate among reactants is mainly temperature-

264

dependent, it is also pressure sensitive and has a pressure threshold for its maximum.66

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Therefore, the overall reaction rates depend on competition between collision rates and

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cage/diffusional effects.57

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As pressure increases, the collision rate will reach a maximum at a certain pressure


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threshold, resulting in increased overall reaction rates.65 This could be the case of higher

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yields at 900 bar than that at 750 bar at 390 °C. With increasing temperature up to

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405 °C, the pressure threshold for maximum collision rate should shift to lower

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pressures compared to 390 °C, resulting in higher yield at 750 bar. Overall reaction

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rates can be quantitatively evaluated by calculation of activation volume.1, 28, 66 When

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the activation volume is negative, higher pressure increases the rate constant and thus

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enhances the reaction.28 Hill et al.66 found that the gas generation rates increase from

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345 to 690 bar at the temperatures of 350, 380 and 400 °C, and the average activation

276

volume values for methane generation were estimated to be ΔV‡ = -14 cm3/mol.

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However, further work is required to acquire the specific activation volumes of the

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reactions of oil cracking under different pressure regimes.

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At 425 °C (1.85% Ro), the LHs experience significantly secondary cracking and

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more complicated processes are involved. Pressure gives a suppression first followed

281

by a stable cracking level from 470 to 900 bar (Figure 2). At this over mature stage, the

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pressure effect is less significant and subordinate to the temperature.

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4.2 Pressure effects on the distribution of light hydrocarbons and implications

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The relative abundances of the C7 branched alkanes, cycloalkanes, and aromatics

285

are presented in Figure 6. Because LHs experience significant secondary cracking at

286

425 °C (Figure 2), the composition distribution at 425 °C will not be discussed here. As

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shown in Figure 6, at each pressure, the relative abundances of branched alkanes

288

generally increase first and then decrease with increasing temperature, while those of

289

the cycloalkanes generally decrease and the aromatics also show an increasing trend.


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At 200 bar, the relative abundance of branched alkanes increases from 17.1% to 18.6%

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from 350 to 373 °C, then shows a decrease at higher temperatures. However, at 470,

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750 and 900 bar, the branched alkanes decrease until the temperature reaches up to 390

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°C. The decreases in the relative abundance of branched alkanes at higher temperatures

294

(Figure 6a) indicate its significant contribution to the formation of other light

295

components, including gases. This is consistent with the pyrolysis results of Fabuss et

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al.22 and Hill et al.17 Furthermore, the relative abundances of branched alkanes begin to

297

decrease at higher temperatures at higher pressures than 200 bar (Figure 6a), suggesting

298

two possible scenarios: (a) branched alkanes cleavage require much higher activation

299

energies at medium to high liquid water pressure; or, (b) the generation rates of

300

branched alkanes is higher than the rates of themselves’ cracking with increasing

301

pressure. In addition to the aromatization of cycloalkanes, aromatics may also be

302

generated from branched alkanes via C–C bond cleavage followed by direct

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aromatization. This is evidenced by an increase in their yields and relative abundances

304

of as branched alkanes decrease with increasing temperature (Figure 4b, d and Figure

305

6a, c), in agreement with the previous study by Qin et al.,67 probably due to higher

306

thermal stability and lower cracking rates of aromatic hydrocarbons.

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As shown in Figure 6, the percentages of branched alkanes, cycloalkanes and

308

aromatics generally are higher at 470 to 900 bar compared to that at 200 bar, especially

309

at the lower temperatures, probably due to the higher generation rates of their precursors’

310

cracking than the rates of themselves’ cracking. In this situation, it can be assumed that

311

increasing pressure could promote the isomerization, cyclization and aromatization.


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Energy & Fuels

312

However, at 405 °C, no significant difference occurs in terms of the percentages of

313

cycloalkanes and aromatics with increasing pressure, indicating that temperature starts

314

to dominate the cracking process while pressure becomes secondary at higher thermal

315

evolution. The big difference of branched alkanes yields between low and the higher

316

pressures could be induced by preferential isomerization during cracking. It is

317

interesting to note that the cycloalkane yields show less difference between 200 bar and

318

medium to high liquid water pressure, while the branched alkanes and aromatics

319

percentages display a big difference, probably suggesting that the pressure effects on

320

cracking favor isomerization followed by a direct aromatization.

321

Changes on the distributions of branched alkanes, cycloalkanes and aromatic

322

hydrocarbons at different pressures demonstrate a pressure-dependent generation of

323

LHs during oil cracking (Figure 6). This could be attributed to the changes in the

324

reactant concentration (or density) and/or in the rate constant of elementary reactions

325

with pressure.68 The rate constants of unimolecular reactions (positive activation

326

volume: a bond is broken) will decrease with increasing pressure, while those of

327

bimolecular reactions (negative activation volume: a bond is formed) will increase with

328

increasing pressure.20, 24, 28, 69

329

At 350 °C, as the pressure increases from 200 to 900 bar, there are insufficient

330

molecules/radicals to reach the high activation energy barriers for complete

331

decomposition of free radicals.65, 66 Bimolecular reactions could be favored over the

332

unimolecular radical decomposition when pressure increases. This means that the

333

radicals would tend to be stabilized (e.g., by hydrogen abstraction and radical addition)


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

334

rather than undergo further decomposition, resulting in the formation of cycloalkanes

335

and aromatics hydrocarbons. Because of the increased rates of bimolecular reactions

336

and decreased number of decomposition steps of radicals with increasing pressure, the

337

percentages of cycloalkanes and aromatic hydrocarbons would tend to increase (Figure

338

6b, c). Also, the decreased extent of unimolecular reactions (that is, depressed radical

339

decomposition rate) would result in lower yields of C1–C5 gases at higher pressure, as

340

shown in our previous results.57

341

Compared to 350 °C, the collision rates among radicals at 373 °C and 390 °C

342

become more intense, and unimolecular reactions become more favored. With

343

increasing pressure, the competition between the unimolecular and bimolecular

344

reactions would result in higher yields of total LHs at a given pressure threshold as

345

evidenced by Figure 2. Increasing pressure enhances the collision rates of free radicals

346

and accelerates their reaction rates, resulting in the generation of relatively abundant

347

branched alkanes. Moreover, the radicals formed by H-transfer reactions are

348

predominant in the bimolecular reactions, leading to the rapid increase in the relative

349

contents of branched alkanes with increasing pressure (Figure 6a). Up to 405 °C, as the

350

pressure increases from 200 to 900 bar, the concentrations of branched alkanes increase

351

significantly, cycloalkanes decrease, while aromatics slightly increase (Figure 6a, b, c).

352

Due to higher activation energies being reached at this stage, unimolecular reactions

353

may dominate, with bimolecular reactions playing less a role than that at low

354

temperatures. Therefore, the higher molecular weight compounds show quickly

355

decomposition rates to generate LHs, accompanied by cyclization and aromatization


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356

Energy & Fuels

reactions over the pressure regimes.

357

Branched alkanes begin to crack at this stage as shown in Figure 6a with decreasing

358

concentrations at 405 °C. The increase in the branched alkane concentrations with

359

pressure may result from the relative smaller proportion of bimolecular reactions at this

360

temperature. In contrast, the cracking reaction pathways of cycloalkanes are different

361

from the branched alkanes. The pronounced decrease in the percentages of cycloalkanes

362

can be explained by the relative larger in the rates of unimolecular reactions over the

363

pressure regimes (Figure 6b). Aromatic hydrocarbons are very stable, and there are a

364

variety of production pathways, e.g., dehydrogenation of cycloalkanes, therefore the

365

change in aromatic concentrations with the pressure is not very prominent and even

366

displaying a slight increase (Figure 6c).

367

Molecular parameters derived LHs are powerful tool in the field of petroleum

368

geochemistry.38, 39, 48, 54, 70, 71 Because of the pressure-dependent distributions of LHs

369

during oil cracking, caution must be taken in respect to the applications of LHs-

370

associated molecular parameters during pressurized oil cracking occurred in deep-

371

buried petroleum reservoirs.

372

Thompson

parameters,

including

heptane

value,

isoheptane

value,

n-

373

C7/methylcyclohexane and toluene/n-C7, can be used to identify the types and thermal

374

maturity of petroleum and evaluate the secondary alteration in reservoirs, such as

375

evaporative fractionation, water washing and biodegradation.39,

376

versus isoheptane values demonstrate that the residual oils are mature or supermature

377

oils (Figure 7a). The oils from 200 bar at 350, 373 and 390 °C are located in the


70

Plots of heptane

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

378

supermature part of Figure 7a. Most of the those from 470 to 900 bar at 350, 373 and

379

390 °C are located in the mature part, suggesting that high pressure suppresses the

380

thermal evolution process of oil cracking, which is consistent with the total yields of

381

LHs in C6–C7 range (Figure 2). All the residue oils from 405 °C are located in the

382

supermature part of Figure 7a, whereas the 200 bar sample is separated from the 470 to

383

900 bar samples, indicating that their thermal maturities are different. Toluene/n-C7 and

384

n-C7/methylcyclohexane ratios, usually used as aromaticity and paraffinicity,

385

respectively,39 can be used to evaluate the cyclization and aromatization reactions in oil

386

cracking. When pressure increases from 200 to 900 bar at each temperature, toluene/n-

387

C7 shows an increase and n-C7/methylcyclohexane a decrease (Table 4, Figure 7b),

388

indicating that pressure benefits the occurrence of cyclization and aromatization

389

reactions. This is consistent with the LH distributions in Figure 6b and c. Similarly, at

390

the higher temperature of 405 °C, n-C7/methylcyclohexane and toluene/n-C7 ratios

391

show smaller variations with the pressure, which can be attributed to the weaker

392

pressure effect at higher temperatures.

393

The Mango parameter52 is defined as (2-MH + 2,3-DMP)/ (3-MH + 2,4-DMP) =

394

K1, which can be explained by a steady state catalytic reaction mechanism. Usually, the

395

Mango parameter of a given crude oil fluctuates around 1.0, ranging from 0.9 to 1.1.

396

Figure 7c presents the relative amounts of 2-MH + 2,3-DMP versus 3-MH + 2,4-DMP

397

in the C7 fraction of residue oils. Except for the oil sample from 350 °C and 470 bar,

398

the residue oil from 200 bar at each temperature points are below the K1=1 line (K1

399

values: 0.84–0.90), but samples from 470 to 900 bar are above the K1=1 line (K1 value:


Page 18 of 44

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Energy & Fuels

400

1.02–1.18) (Table 4). It is also noted that the residual oil from 470 to 900 bar at 405 °C

401

are farther away from the K1=1 line than the other samples, with the K1 value up to

402

1.18. The results suggest that the medium to high pressures increase K1 values, being

403

more significant at higher temperature. The abnormal rise of Mango index may result

404

from the combination of temperature and pressure effects. On one hand, high

405

temperature promotes the fast rate of oil cracking reactions and the cracking of LHs

406

because the kinetic energy of molecule/species can be high enough to overcome the

407

reaction barrier; on the other hand, the increasing pressure benefits the generation of

408

branched alkanes, as evidenced by the increasing relatively content of branched alkanes

409

(Figure 6a).

410

4.3 Pressure effects on carbon isotopic compositions of light hydrocarbons and

411

implications

412

Theoretically, gasoline hydrocarbons from oil cracking are enriched in

413

compared to that in unaltered oil due to the isotopic kinetic fractionation induced by

414

preferential cleavage of

415

aromatization. Our high pressurized oil cracking experiments demonstrate that carbon

416

isotope fractionations of individual LHs can be complicated by pressure effects in deep

417

high temperature and high pressure petroleum reservoirs.

12C-12C

13C

bond followed by isomerization, cyclization and

418

The stable carbon isotopic compositions versus the relative percentages of n-C7, 3-

419

methylhexane, toluene and methylcyclohexane are shown in Figure 8. As earlier

420

indicated, the generation rate of LHs is pressure-dependent. Although pressure

421

retardation results in lower yields of LHs, and preferential isomerization and


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 20 of 44

422

aromatization further lead to a relative decrease of straight chain alkanes and a relative

423

increase of aromatics, significant carbon isotopic fractionation of straight chain alkanes

424

and aromatics occurs over the pressure regimes (Figure 8a, c). Stable carbon isotopic

425

compositions of n-C7 from different temperatures and pressures show that 1–2‰

426

enrichment of

427

fractionation could be mainly induced by accelerated aromatization. Stable carbon

428

isotopic compositions of 3-methylhexane is also enriched in

429

with the maximum isotopic fractionation reaching up to 2‰ (Figure 8b). Corresponding

430

to the straight and branched chain alkanes, stable carbon isotopic compositions of

431

individual aromatics, as revealed by toluene here, show a large fractionation at pressure

432

with the maximum isotopic fractionation reaching up to 4‰ (Figure 8c). The relative

433

percentages and stable carbon isotopic compositions of toluene demonstrate a

434

comparatively positive relationship. However, almost no fractionation occurs for

435

cycloalkanes under our experimental conditions as evidenced by measurements on

436

methylcyclohexane, although the higher temperature of 405 C induces ~1‰ isotopic

437

fractionation without pressure effects (Figure 8d). Overall, the results suggest that

438

pressurized oil cracking probably experiences C-C bond cleavage directly followed by

439

isomerization and preferential aromatization, rather than via cyclization as in normal

440

in-reservoir oil cracking.

13C

occurs at higher pressure (470, 750 and 900 bar, Figure 8a). This

13C

at higher pressures,

441

Stable carbon isotope values of individual LHs have been successfully applied in

442

oil-oil / oil-source correlations and identification of in-reservoir oil cracking.33, 48-50, 72

443

However, our high pressure water oil cracking experiments show the influence of


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Energy & Fuels

444

pressure effects on the carbon isotopic compositions of individual LHs. Aromatics are

445

enriched in

446

pressure regimes due to preferential isomerization and aromatization with increasing

447

pressure. While stable carbon isotopic compositions of cycloalkanes in LHs show no

448

isotopic fractionation under different pressure regimes, suggesting that isotopic

449

signatures of cycloalkanes (e.g., methylcyclohexane and cyclohexane) could be used as

450

an effective index to make oil-oil / oil-source correlations in deep buried petroleum

451

reservoirs with high pressure and high temperature background. While stable carbon

452

isotopic compositions of branched alkanes and aromatic hydrocarbons could be used to

453

identify the pressure effects.

454

5. Conclusions

13C

at medium to high water pressure regimes compared with at low

455

Oil cracking was retarded by pressure, as evidenced by the total C6–C7 yields with

456

remarkably lower under high liquid pressure (470, 750 and 900 bar) than at low pressure

457

(200 bar). However, the retardation of high pressure on oil cracking depends on the

458

thermal stage of evolution. In the peak oil to early wet gas stage (350 °C and 373 °C,

459

0.92-1.15% Easy Ro), total C6–C7 yields are very low and decrease with increasing

460

pressure. At 390 °C (1.35% Easy Ro), total C6–C7 yields are retarded by pressure from

461

200 to 750 bar, then followed by a significant promotion from 750 to 900 bar. At 405 °C

462

(1.56% Easy Ro), they display strong suppression-slight promotion-slight suppression

463

process along the pressure sequence (470, 750 and 900 bar). At 425 °C (1.85% Easy

464

Ro), the pressure shows a suppression first followed by a stable cracking level from 470

465

to 900 bar.


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

466

Changes in the distributions of branched alkanes, cycloalkanes and aromatics at

467

different pressures demonstrate the promotion effect of pressure on isomerization,

468

cyclization and aromatization reactions during oil cracking. The competition between

469

unimolecular reactions and bimolecular reactions could be used to explain dynamics on

470

the generation rates and subsequent LH distributions over the pressure range

471

investigates. Further, the LHs-derived molecular parameters as heptane versus

472

isoheptane value, toluene/n-C7 versus n-C7/methylcyclohexane, and the Mango

473

parameter are significantly affected by pressure. Therefore, caution must be taken in

474

respect to their application in deep-buried petroleum reservoirs at high temperatures

475

and pressures.

476

Stable carbon isotopic compositions for toluene and 3-methylhexane show large

477

fractionations over the pressure range and the maximum isotopic fractionations reaches

478

up to 4‰ and 2‰, respectively. However, almost no carbon isotopic fractionation

479

occurs for cycloalkanes (less than 1‰). The results suggest that isotopic signatures of

480

cycloalkanes could be used as an effective index to make oil-oil / oil-source correlations,

481

and the isotopic signatures of branched alkanes and aromatics in the LH range could be

482

used to identify the pressure effects in high pressure and high temperature in deep

483

petroleum reservoirs.

484

Acknowledgements

485

This work was collaboratively supported by the National Natural Science

486

Foundation of China (grant numbers 41602143, 41572101 and 41330313), the China

487

Postdoctoral Science Foundation (grant number 2016M590671), China geological


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Energy & Fuels

488

survey project (grant number DD20189503), and the cooperative research grant from

489

the University of Nottingham and Zhejiang University.

490

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Table Captions

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Table 1. List of C6–C7 light hydrocarbons and their abbreviations for the cracked oils.

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Table 2. Light hydrocarbon yields from the oil cracking experiments at different

696

temperatures and pressures.

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Table 3. Stable carbon isotopic compositions of individual C6–C7 compounds in the

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cracked oils.

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Table 4. Molecular parameters for light hydrocarbons in the cracked oils. n-C7/MCH,

700

n-C7/ methylcyclohexane; Heptane value= 100 × n-heptane/ (∑cyclohexane through

701

methylcyclohexane); Isoheptane value = (2 + 3)-MHs/ (1,trans-3 + 1,cis-3 + 1,trans-2)-

702

DMCPs; Mango Index = (2-MH+2,3-DMP)/(3-MH+2,4-DMP).

703 704


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Table 1. List of C6–C7 light hydrocarbons and their abbreviations for the cracked oils. Peak Abbreviation Compound name Peak Abbreviation 1 2,3-DMB 2,3-Dimethylpropane 22 ECP 2 2-MP 2-Methylpentane 23 2,4-DMH 3 3-MP 3-Methylpentane 24 1,2,4-TMCP 4 n-C6 n-Hexane 25 1,2,3-TMCP 5 2,2-DMP 2,2-Dimethylpentane 26 Tol 6 MCP Methylcyclopentane 27 2-MHe 7 2,4-DMP 2,4-Dimethylpentane 28 3-MHe 8 2,2,3-TMB 2,2,3-Trimethylpentane 29 C1,3-DMCH 9 Ben Benzene 30 1,1-DMCH 10 3,3-DMP 3,3-Dimethylpentane 31 n-C8 11 CH Cyclohexane 32 2,6-DMHe 12 2-MH 2-Methylhexane 33 ECH 13 2,3-DMP 2,3-Dimethylpentane 34 3,5-DMHe 14 1,1-DMCP 1,1-Dimethylcyclopentane 35 1,1,3-TMCH 15 3-MH 3-Methylhexane 36 EB 16 C1,3-DMCP 1,cis-3-Dimethylcyclopentane 37 m-xylene + p-xylene 17 T1,3-DMCP 1,trans-3-Dimethylcyclopentane 38 o-xylene 18 T1,2-DMCP 1,trans-2-Dimethylcyclopentane 39 n-C9 19 n-C7 n-Heptane 40 n-C10 20 MCH Methylcyclohexane 41 n-C11 21 1,1,3-TMCP 1,1,3-Trimethylcyclopentane


Compound name Ethylcyclopentane 2,4-Dimethylhexane 1,trans,2,cis,4-Trimethylcyclopentane 1,trans,2,cis,3-Trimethylcyclopentane Toluene 2-Methylheptane 3-Methylheptane 1,cis-3-Dimethylcyclohexane 1,1-Dimethylcyclohexane n-Octane 2,6-Dimethyheptane Ethylcyclohexane 3,5-Dimethyheptane 1,1,3-Trimethylcyclohexane Ethylbenzene meta-Xylene + para-Xylene ortho-Xylene n-Nonane n-Decane n-Undecane

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

Table 2. Light hydrocarbon yields from the oil cracking experiments at different temperatures and pressures. Yields of light hydrocarbons produced from oil cracking (mg/goil) Temperatur Pressure C6–C7 C6–C7 C6–C7 C7 C7 C7 C6–C7 C6–C7 total C7 aromatic e (bar) normal branched aromatic normal branched cyclocycloalkanes hydrocarbons hydrocarbons alkanes alkanes hydrocarbons alkanes alkanes alkanes 200 1.36 0.92 2.04 0.27 4.5 0.86 0.52 1.51 0.16 470 0.57 1.04 2.38 0.31 4.3 0.36 0.60 1.73 0.19 350 C 750 0.64 0.67 1.67 0.21 3.2 0.40 0.39 1.28 0.15 0.92% Ro 900 0.30 0.36 0.72 0.10 1.5 0.19 0.21 0.53 0.07 200 4.2 2.7 4.1 0.6 11.6 2.45 1.28 2.85 0.32 470 0.8 0.9 1.2 0.2 3.1 0.50 0.48 0.83 0.16 373 C 750 1.1 1.3 2.8 0.5 5.8 0.71 0.77 1.61 0.39 1.15% Ro 900 0.6 0.7 1.0 0.2 2.6 0.42 0.43 0.71 0.15 200 17.4 9.4 15.4 3.5 45.7 9.8 4.1 10.3 2.1 470 5.4 6.3 8.2 1.9 21.7 3.2 3.4 5.8 1.5 390 C 750 4.1 4.2 6.0 1.4 15.6 2.7 2.6 4.4 1.2 1.35% Ro 900 6.3 9.0 9.6 2.3 27.1 3.6 4.6 6.7 2.0 200 33.9 13.6 34.8 11.9 94.2 18.2 5.5 22.5 8.3 470 5.0 4.3 7.6 2.3 19.1 2.7 2.1 4.6 1.9 405 C 750 8.8 9.2 12.0 3.6 33.5 4.8 4.5 7.6 3.2 1.56% Ro 900 6.3 6.1 8.2 2.5 23.1 3.4 2.9 4.9 2.2 200 19.9 6.3 39.2 19.3 84.6 8.4 2.2 22.0 14.6 470 5.2 2.9 10.4 6.5 24.9 2.6 1.2 5.7 5.8 425 C 750 5.2 3.9 9.7 5.9 24.9 2.7 1.7 5.1 5.4 1.85% Ro 900 4.5 2.5 9.1 6.1 22.1 2.7 1.3 5.1 5.6


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Table 3. Stable carbon isotopic compositions of individual C6–C7 compounds in the cracked oils. δ13C(‰) Pressure Temperature (bar) 3-MH MCH CH MCP BEN 200 –28.4 –27.7 –25.4 –27.5 –25.9 470 –27.7 –28.0 –26.1 –28.2 –28.8 350 C 750 –26.9 –27.1 –25.2 –27.7 –28.1 0.92% Ro 900 –26.5 –27.5 –25.6 –26.6 –28.5 200 –29.1 –28.2 –24.7 –29.1 –25.8 470 –27.8 –27.8 –24.3 –29.0 –28.1 373 C 1.15% Ro 750 –27.2 –27.8 –25.3 –29.1 –28.8 900 –27.8 –28.5 –25.2 –28.0 –28.2 200 –26.2 –28.6 –26.1 –29.1 –27.6 470 –25.4 –27.6 –26.8 –27.9 –27.4 390 C 1.35% Ro 750 –25.3 –28.0 –27.9 –26.8 –26.9 900 –25.5 –27.5 –26.2 –27.9 –27.5 200 –22.9 –27.2 –26.6 –28.0 –25.4 470 –22.9 –26.6 –25.2 –27.7 –26.7 405 C 1.56% Ro 750 –23.0 –26.9 –25.8 –26.9 –26.5 900 –22.9 –26.6 –25.3 –27.3 –27.1 200 –17.6 –23.6 –25.3 –25.1 –23.1 470 –16.5 –24.3 –23.8 –25.1 –25.0 425 C 1.85% Ro 750 –16.6 –24.3 –24.4 –25.4 –25.9 900 –15.7 –24.4 –24.3 –24.7 –25.7


TOL –29.6 –28.3 –28.6 –28.3 –31.3 –27.9 –28.7 –28.7 –32.2 –27.5 –27.4 –28.0 –29.1 –26.5 –27.0 –27.1 –25.6 –25.5 –26.5 –26.2

n-C6 –27.1 –27.7 –27.3 –27.8 –27.0 –27.2 –27.2 –27.3 –26.3 –26.2 –26.2 –26.7 –23.7 –23.4 –24.5 –24.3 –17.1 –18.8 –20.1 –19.0

n-C7 –27.6 –27.0 –26.9 –26.6 –27.0 –26.5 –26.5 –26.7 –26.8 –24.7 –25.3 –25.8 –23.7 –22.4 –23.3 –23.1 –16.5 –17.8 –18.7 –17.7

n-C8 –27.6 –27.2 –27.8 –27.7 –26.9 –27.2 –27.5 –27.3 –25.7 –26.2 –26.1 –26.4 –22.5 –23.7 –23.8 –23.2 –16.8 –18.6 –19.4 –18.7

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Table 4. Molecular parameters for light hydrocarbons in the cracked oils. n-C7/MCH, n-C7/ methylcyclohexane; Heptane value= 100 × n-heptane/ (∑cyclohexane through methylcyclohexane); Isoheptane value = (2 + 3)–MHs/ (1,trans-3 + 1,cis-3 + 1,trans-2)DMCPs; Mango Index = (2-MH+2,3-DMP)/(3-MH+2,4-DMP). Molecular parameters Temperatur Pressure Toluene/ Heptane Isoheptane Mango e (bar) n-C7/MCH n-C7 value value Index 200 0.22 1.01 32.28 1.51 0.89 470 0.47 0.78 26.47 1.51 0.94 350 C 0.92% Ro 750 0.45 0.56 21.57 1.46 1.02 900 0.48 0.57 21.49 1.56 1.02 200 0.13 1.88 41.65 1.75 0.89 470 0.38 1.30 30.38 1.64 1.04 373 C 1.15% Ro 750 0.40 0.96 26.47 1.60 1.04 900 0.45 1.44 30.79 1.71 1.05 200 0.25 2.31 45.47 1.32 0.84 470 0.50 1.42 30.03 1.57 1.09 390 C 1.35% Ro 750 0.48 1.60 32.04 1.58 1.07 900 0.58 1.43 26.91 1.70 1.08 200 0.43 2.24 45.19 0.73 0.90 470 0.74 1.54 33.02 1.25 1.15 405 C 1.56% Ro 750 0.69 1.85 32.57 1.49 1.18 900 0.63 2.15 35.05 1.48 1.16


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Figure Captions Figure 1. GC fingerprints of pristine oil and residual oils after pyrolysis as a function of temperature at 200 bar showing the distribution of light hydrocarbons. (a) pristine oil; (b)350 °C; (c)373 °C; (d)390 °C; (e)405 °C; (f)425 °C. The pristine oil is light hydrocarbons depletion, while the residual oils after pyrolysis are enriched in light hydrocarbons. Peak labels are defined in Table 1. Figure 2. Yields of total light hydrocarbons in C6–C7 range produced by oil cracking at different temperatures and pressures. Histogram showing the yields as a function of pressure at temperatures from 350 to 425 °C. Curves showing the yields as a function of temperature at the pressure of (a) 200 bar, (b) 470 bar, (c) 750 bar, and (d) 900 bar. Figure 3. Conceptual model of the pyrolysis sequence in oil cracking (after Hill et al.17). Light hydrocarbons are intermediate products during oil-cracking processes that can be progressively thermally-degraded to ethane-pentane, methane and pyrobitumen at higher temperatures and prolonged heating time. C15+NSO, heteroelement (N, S, O) compounds in C15+ fraction. Figure 4. Yields of compound groups in C6–C7 range produced by oil cracking at different temperatures and pressures. (a) normal alkanes; (b) branched alkanes; (c) cycloalkane; (d) aromatics. Histogram showing the yields as a function of pressure at temperatures from 350 to 425 °C. Curves showing the yields as a function of temperature at pressures from 200 to 900 bar. Figure 5. The effects of temperature and pressure on carbon isotope ratios of the compounds in the fractions: (a) chain alkanes (n-C6; n-C7; 3-MH: 3-Methylhexane), (b) cyclic alkanes (MCP: Methylcyclopentane; MCH: Methylcyclohexane; CH: Cyclohexane), and, (c) aromatics


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(Ben: Benzene; Tol: Toluene). Figure 6. Relative percentage of (a) branched alkanes, (b) cycloalkanes, (d) aromatics in the C7 fraction with increasing temperature at the different pressures. Figure 7. Plots of (a) the Heptane versus isoheptane values, (b) n-heptane/ methylcyclohexane versus toluene/n-heptane70, and (c) the relative amount of 3-MH + 2,4-DMP versus 2-MH + 2,3-DMP in the C7 fraction (c) in the pyrolysis experiments. Heptane value= 100 × n-heptane/ (∑cyclohexane through methylcyclohexane); Isoheptane value = (2 + 3)-MHs/ (1,trans-3 + 1,cis-3 + 1,trans-2)-DMCPs. Figure 8. Plots of relative percentages of individual compounds in the C7 fraction versus their stable carbon isotopic compositions. (a) n-C7; (b) 3-Methylhexane; (c) Toluene; (d) Methylcyclohexane.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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Thermal cracking of oil under water pressure up to 900 bar at high

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