Differences in geochemistry and hydrocarbon generation of sourcerock samples dominated by telalginite and lamalginite: A case study on the Permian saline lacustrine source rocks in the Jimusaer Sag, NW China

The Middle Permian Lucaogou Formation in Northwestern China mainly contains typical saline lacustrine oil shale.To study the differences in geochemistry and hydrocarbon generation of source-rock samples in Lucaogou Formation in Jimusaer Sag,57 core samples from two boreholes were analyzed herein by performing total organic carbon(TOC)analysis,RockeEval pyrolysis,and gas chromatographicemass spectrometry experiments on saturated hydrocarbons.The kinetics of oil generation were studied using two samples comprising typical maceral components.The results showed that the hydrocarbons produced by telalginite are relatively rich in pristane(Pr),phytane(Ph),b-carotane,high-carbon normal alkanes,and C29 regular steranes.Hydrocarbons produced by lamalginite contain a significantly higher content of C20 tricyclic terpanes(TT),C21TT,C24 tetracyclic terpanes(TeT),C29 norhopane,and C28 regular sterane.Based on the pyrolysis and biomarker compound parameters,telalginite has a higher conversion rate for hydrocarbons than lamalginite in the low-mature to mature stage,which is consistent with their kinetic analysis.Lamalginite source rock displays a much narrower distribution of activation energies than telalginite source rocks.Such narrower activation energy distribution effectively narrows the main stage of hydrocarbon generation.In addition,the activation energy distribution of lamalginite concentrated in the high-value interval,indicating the characteristics of the relatively lagging hydrocarbon generation of lamalginite.

Accumulation model and geochemistry characteristics of oil occurring from Jurassic coal measures in the Huangling mining area of the Ordos Basin, China

The Ordos Basin is an important intracontinental sedimentary basin in China,containing a significant amount of coal,oil,and natural gas.This study analyzed the sedimentary environment,sedimentary facies,parent material type,maturity,and carbon isotopic composition of the coal-bearing organic matter using gas chromatography–mass spectrometry(GC–MS)and stable isotope ratio mass spectrometry.The source of oil occurring in the No.2 coal seam of the Jurassic Yan’an Formation(An-1 oil)and its accumulation model were also investigated.The results show that the relative abundances of C_(27),C_(28),and C_(29) steranes in the An-1 oil are 43.8%,33.0%,and 23.2%,respectively.The tricyclic terpanes,C_(29)20S/(20S+20R),and C_(29)ββ/(ββ+αα)contents of the An-1 oil are 31.4%−34.8%,0.85 and 0.81,respectively.Pr/n-C17,Ph/n-C18,and Pr/Ph values are 0.34,0.42,and 0.87,respectively.Biomarker parameters indicate that the An-1 oil mainly comes from the plankton source rock deposited in the freshwater lake facies and a reducing environment,which has evolved to maturity.The correlation of oil-oil indicates that the An-1 oil is homologous to the Chang-7 oil(Chang-7 member of the Triassic Yanchang Formation).The correlation of oil-source rock presents that the An-1 oil is generated from the Yanchang Formation(Chang-6 and Chang-7 source rocks)and occurred in the coal seam during the stage of stratum uplift since the Early of Late Cretaceous.The distribution characteristics of δ13C group components in the An-1 oil and Chang-7 oil also reveal the fractionation phenomenon during the migration of crude oil.