Hydrocarbon Generation Kinetics And Geochemical Characteristics Of Coaly Source Rocks In The Kuqa Depression

A huge amount of natural gas has been found in the Kuqa Depression,which mainly originated from the continental sedimentary strata in the area—Triassic–Jurassic coaly strata.The sourceRocks are distributed in Middle–Upper Triassic Kelamayi Formation?T₂–₃k?,the Upper Triassic Huangshanjie?T₃h?and Taliqike formations?T₃t?,the Lower Jurassic Yangxia Formation?J₁y?,and the Middle Jurassic Kezilenuer?J₂k?and Qiakemake formations?J₂q?.The Taliqike?T₃t?,Yangxia?J₁y?and Kezilenuer?J₂k?formations are coal-bearing strata.How to properly evaluate the petroleum generation potentials for coaly source rocks remains unresolved.In this dissertation,the hydrocarbon generation potentials and kinetic parameters of the seven Triassic–Jurassic coal samples were determined by confined pyrolysis experiments?gold capsules?and Rock–Eval pyrolysis experiment.Three of the coal samples?JKC1,JKC2 and JKC3?are located within the Middle Jurassic Kezilenuer Formation?J2k?.The hydrogen index?HI?,Tmax and%Ro value of the three Jurassic coals are ranging from 57 to 183 mg HC/g TOC,424 to 437?C and 0.58 to 0.66%,respectively.The other four coals?TTC1,TTC4,TTC11 and TTC18?are located within the Upper Triassic Taliqike Formation?T3t?.The hydrogen index?HI?,Tmax and%Ro value of the four Triassic coals are ranging from 223 to 278 mg HC/g TOC,433 to 458?C and0.58 to 0.74%,respectively.The oil and gas hydrocarbon yields and hydrocarbon generation kinetics characteristics of seven coal samples can be summarized as follows:?1?The maximum oil yields?S_o?and gas yields??C₁–₅?of four Triassic coals?TTC1,TTC4,TTC11 and TTC18?are in the range from 46.39 to 87.50 mg/g TOC and 107.20 to 120.94 mg/g TOC,respectively.The maximum oil yields?S_o?and gas yields??C₁–₅?of three Jurassic coals?JKC1,JKC2 and JKC3?are in the range from 14.3 to 39.78 mg/g TOC and 70.1to 95.06 mg/g TOC,respectively.?2?Mass balance results from seven coals within the oil generative window demonstrate that only a portion?38–53%?of the releasable moieties measured by Rock-Eval pyrolysis contributed to the formation of oil and gaseous hydrocarbons,while the other portion?47–62%?of these moieties was rearranged and incorporated into polyaromatic residual solids.?3?At high maturities?EASY%Ro>1.87?,gas generative potentials??C₁–₅,GGP?for the residual solid are very similar among the seven coals,and are substantially higher than the QI=?S₁+S₂?/TOC values of the residual solid with the differences ranging from 20 to 40 mg HC/gTOC.This can be ascribed to the differences both in methane formation mechanisms and thermal stress level betweenRock-Eval pyrolysis?EASY%Ro 2.25%?and confined pyrolysis?EASY%Ro 4.44%?.?4?For the four Triassic coals that are effective oil source rocks with maximum oil yield>40 mg/g TOC,the weighted average activation energies for oil generation range from 51.64 to 52.96 kcal/mol with frequency factors ranging from 9.61×10¹² s^–11 to 1.70×10¹³ s^–1.The distributions of activation energies for oil generation from these four coals are narrow,indicating that the precursors have similar chemical bonds that are cleaved for oil generation.In addition,this feature is also related to the hydrocarbon generation characteristics of coals,only a limited portion of the bound alkanes in the initial coal can be released as oil components,the major portion of the bound alkanes remains attaching to kerogen and become the precursors for gas generation.?5?The weighted average activation energies for gas generation range from 64.72 to 65.33 kcal/mol with frequency factors ranging from 8.25×10¹³ s^–1 to 1.22×10¹⁴ s^–1 for the three Jurassic coals and range from62.78 to 65.02 kcal/mol with frequency factors ranging from 8.21×10¹³ s^–1 to 1.67×10¹⁴s^–11 for four Triassic coals.Seven coal samples all have the characteristics of late gas generation:Only a minor portion of gaseous hydrocarbons,about 32%and 44%,respectively for the Jurassic coals and Tisassic coals is generated at up to EASY%Ro2.19%while the major portion is generated at higher maturities?EASY%Ro>2.19%?.?6?Kinetic parameters of the generalized coals JKC and TTC for oil and gas generation were determined from the average yields of the three Jurassic coals and four Triassic coals,respectively.Under natural conditions with a heating rate of 5°C/My,the generalized Jurassic coal JKC and Triassic coal TTC can be effective gas source rocks with gas yield>20 mg/g TOC at EASY%Ro>1.76 and 1.59%,respectively.The abundant gaseous hydrocarbons found in the Kuqa Depression can be mainly ascribed to high maturities of coal source rocks?%Ro>2.0?,in combination with excellent seal of thick salt and gypsum for the gas reservoirs.The geochemical characteristics of source rocks in Kuqa Depression have been studied by many people,but most of them focus on one or two strata.In this dissertation,the geochemical characteristics of Triassic–Jurassic coaly source rocks in Kuqa Depression are systematically analyzed to obtain gross and molecular geochemical parameters and isotopic compositions of mudstone source rocks in different strata collected from outcrops along the Kuqa river and coal samples from coal mines.The following results were observed.?1?Outcrop samples have Type III kerogen within the Jurassic Kezilenuer Formation?J₂k?,mainly Type III with minor Type II₂ kerogen within the Jurassic Yangxia Formation?J₁y?,mainly Type II2 with minor II1 kerogen within the Triassic Taliqike Formation?T₃t?and mainly Type III kerogen within the Triassic Huangshanjie Formation?T₃h?.?2?Measured%Ro and Tmax values are in the range of 0.58⁰.66 and 424⁴37°C,respectively,demonstrating a low maturity for mine coals within the Middle Jurassic Kezilenuer Formation?J₂k?while they are in the range of 0.58⁰.96 and 433⁴96°C,respectively,demonstrating maturities from early to bulk oil generation stage for mine coals within the Upper Triassic Taliqike Formation?T₃t?.Outcrop samples have Tmax values in the range of 428⁴51°C with the Middle Jurassic Kezilenuer Formation?J₂k?,436⁴87°C within the Lower Jurassic Yangxia Formation?J₁y?,447⁵85°C within the Upper Triassic Taliqike Formation?T₃t?and 442⁴58°C within the Huangshanjie Formation?T₃h?.This result demonstrates that maturities for the outcrop samples increase from the early oil generation stage within the Middle Jurassic Kezilenuer Formation?J₂k?to bulk oil generation stage within the Upper Triassic Taliqike Formation?T₃t?and Huangshanjie Formation?T₃h?.?3?Pr/Ph ratios increase from outcrop samples within the Upper Triassic Taliqike Formation?T₃t?and Huangshanjie Formation?T₃h?to those within the Middle Jurassic Kezilenuer Formation?J₂k?,suggesting depositional environmental changes from relatively reduced lacustrine to oxidized swamp facies.?4?Both the outcrop and mine coal samples are characterized with higher relative concentrations of C₁₉ and C₂₀ tricyclic terpaens and C₂₄ tetracyclic terpane,high concentration of C₂₉relative to C₂₇ and C₂₈ steranes,low relative concentration of gammacerane and high hopane/sterane ratio,demonstrating terrigenious precursors.However,outcrop samples within the Upper Triassic Taliqike Formation?T₃t?have higher relatively concentrations of C₃₀ diahopane and gammacerane,higher ratio of tricyclic terpanes to C₃₀ hopane but lower ratios of hopane to sterane and C₂₉ to C₂₇ and C₂₈ steranes,demonstrating a relatively higher amount of aquatic organism input,substantially different from those within the other formations.?5??¹³C values of individual n-alkanes increase from outcrop samples within the Upper Triassic Taliqike Formation?T₃t?and Huangshanjie Formation?T₃h?to those within the Middle Jurassic Kezilenuer Formation?J₂k?demonstrating an increasing trend of precursor input of terrisgenious high plants from the Late Triassic to the Middle Jurassic ages.