Molecular evolution of nitrogen-containing compounds in highly mature organic matter: Implications for the hydrocarbon generation potential

Evaluating the hydrocarbon generation potential of highly mature organic matter is a key and critically challenging area of research in petroleum geochemistry. To explore this issue, we used negative ion electrospray ionization-Fourier transform-ion cyclotron resonance-mass spectrometry to investigate the molecular evolution of N-containing compounds in Carboniferous-lower Permian source rocks with a range of maturities in the northwestern Junggar Basin, China. The N1compounds formed from on-fluorescent chlorophyll catabolites(NCCs), which record the characteristics of the residual soluble organic matter. These components remain in the source rocks after hydrocarbon generation and expulsion, and enable evaluation of the hydrocarbon generation potential. The newly defined indexes of molecular evolution, which are the polymerization index P1([DBE 18+DBE 15]/[DBE 12+DBE 9]_N1) and alkylation index R1(RC_(6–35)/RC_(0–5)), combined with the vitrinite reflectance(VR_(o)) and paleo-salinity index(β-carotane/n Cmax), can identify the factors that control the evolution of highly mature organic matter. The main factor for source rocks deposited in a weakly saline environment is the maturity, but for a highly saline environment both the maturity and salinity are key factors. The high salinity inhibits the molecular polymerization of organic matter and extends the oil generation peak. Given the differences in the bio-precursors in saline source rocks, we propose a new model for hydrocarbon generation that can be used to determine the oil generation potential of highly mature organic matter.

Characterizing the maturity of highly evolved organic matter based on aromatic hydrocarbons and optimization with pyrobitumen reflectance and Raman spectral parameters

Maturity evaluation of highly evolved organic matter is an important problem in oil and gas geochemistry,because conventional organic geochemical proxies are not applicable due to hydrocarbon generation and expulsion.In this study,we investigated the pyrobitumen in the reservoirs of the Sinian Dengying Formation in the Sichuan Basin,China.We focused on aromatic hydrocarbon biomarkers,and the calibration and application of aromatic maturity proxies using pyrobitumen reflectance and Raman spectral parameters.Our results show that only the thiophene compounds are effective maturity indicators,such as MDR(4-/1-MDBT),MDR′(4-MDBT/(1-MDBT+4-MDBT)),MDR1(1-MDBT/DBT),4,6-/(1,4+1,6)-DMDBT,(2,6+3,6)-/(1,4+1,6)-DMDBT,and[2,1]BNT/([2,1]BNT+[1,2]BNT),as these are largely independent of the optical anisotropy of pyrobitumen.The condensation and incorporation of thiophene compounds can lead to an increase in the reflectance of pyrobitumen,which means the thiophene compound parameters are positively or negatively correlated with pyrobitumen reflectance.New potential maturity parameters were proposed,including DMDBTs/MDBTs,TeMDBTs/(DBT+MDBTs+DMDBTs+TMDBTs+TeMDBTs),1-MDBT/MDBTs,and[2,1]BNT/BNTs.This method and these parameters are universally applicable to maturity characterization,especially in rocks that lack vitrinite,contain strongly anisotropic organic matter,and have high contents of thiophene compounds.

Investigation of Hydrous Pyrolysis on Type-Ⅱ Kerogen-bearing Source Rocks from Iran and its Application in Geochemical and Kinetic Analyses of Hydrocarbon Products

Hydrous pyrolysis(HP)practiced on type-Ⅱ kerogen-bearing oil shale samples from the Sargelu Formation in the Ghali-Kuh Area,western Iran,using a specially designed apparatus was performed at different temperatures(250-350℃),with hydrocarbon generation evaluated at each temperature.For comparison,the samples subjected to Rock-Eval pyrolysis before proceeding to HP resulted in T_(max)=418℃,HI=102,and TOC=4.33%,indicating immaturity and hence remarkable hydrocarbon(especially oil)generation potential,making them appropriate for HP.Moreover,the samples were deposited in a low-energy reductive marine environment,with maximum oil and gas generation(739 mg and 348 mg out of50 g of rock sample,respectively)observed at 330℃ and 350℃,respectively.The oil generated at 330℃ was subjected to gas chromatography(GC)and isotopic analyses to assess hydrocarbon quality and composition.The hydrocarbon generation data was devised to estimate kinetic indices of the Arrhenius equation and to investigate the gas-oil ratio(GOR)and overall conversion yield.Based on the producible hydrocarbon quantity and quality,the findings contribute to the economic assessment of oil shales across the study area.The developed kinetic model indicates the history of hydrocarbon generation and organic matter(OM)maturity.

Pore evolution characteristic of shale in the Longmaxi Formation,Sichuan Basin

Through the field emission scanning electronic microscope(FESEM)and the nitrogen adsorption test,pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied.Result showed that the pore type includes organic pore,intercrystalline pore,dissolution intracrystalline pore and interparticle pore,and the organic pore was one of major pore types;among the organic pore,the micropore had large pore volume and specific surface area,and was the main storage space of shale gas.Through study on effect of total organic carbon(TOC),organic matter maturity(Ro),diagenesis and tectonism on shale porosity,influence of TOC on porosity could be divided into four stages:the rapid increasing stage(TOC from 0 to 2%),the slow decreasing stage(TOC from 2 to 3%),the rapid increasing stage(TOC from 3 to 4%or 6%)and the rapid decreasing stage(TOC>4%or 6%);influence of the maturity on porosity of shale could be divided into three stages:the rapid decreasing stage(Ro from 1.5 to 2.2%),the rapid increasing stage(Ro from 2.2 to 2.7%)and the rapid decreasing stage(Ro>2.7%);during the high thermal evolution stage,the organic diagenesis was stronger than the inorganic diagenesis;the tectonism had a great impact on porosity,and the more intense the tectonism was,the smaller the porosity would be.The evolution of shale porosity of the Longmaxi Formation underwent five stages:the immature rapid compaction stage(Ro<0.7%),the mature hydrocarbon generation and dissolution stage(Ro from 0.7 to 1.3%),the high mature pore closed stage(Ro from 1.3 to 2.2%),the overmature secondary pyrolysis stage(Ro from 2.2 to 2.7%)and the overmature slow compaction stage(Ro>2.7%);among which the mature hydrocarbon generation and dissolution stage and the overmature secondary pyrolysis stage were the most favorable shale pore development stages。