The Original Organism Assemblages and Kerogen Carbon Isotopic Compositions of the Early Paleozoic Source Rocks in the Tarim Basin, China

Original organisms are the biological precursors of organic matter in source rocks. Original organisms in source rocks are informative for oil-source rock correlation and hydrocarbon potential evaluation, especially for source rocks which have high-over level of thermal maturity. Systematic identification of original organism assemblages of the Lower Paleozoic potential source rocks and detailed carbon isotopic composition of kerogen analyses were conducted for four outcrop sections in the Tarim basin. Results indicated that the original organism assemblages of the lower part of the Lower Cambrian were composed mainly of benthic algae, whereas those of the Upper Cambrian and the Ordovician were characterized by planktonic algae. Kerogen carbon isotopic data demonstrated that the δ13 Ckerogen values of source rocks dominated by benthic algae are lower than-34‰, whereas the δ13 Ckerogen values of source rocks dominated by planktonic algae are higher than-30‰ in general. We tentatively suggested that the carbon species those are utilized by algae and the carbon isotopic fractionation during photosynthesis are the major controls for the δ13 Ckerogen values in the Lower Paleozoic source rocks in the Tarim basin. Correlating the δ13 C values of oils exploited in the Tarim basin, the original organism assemblages, and δ13 Ckerogen values of source rocks, it implied that the Lower Paleozoic oils exploited in the Tarim basin should be sourced from the source rocks with original organism assemblages dominated by planktonic algae, and the hydrocarbon sourced from the Cambrian benthic algae should be of great exploration potential in future. Original organism assemblages in source rocks can provide important clues for oil-source rocks correlation, especially for the source rocks with high thermal maturity.

Distribution and geochemical significance of alkylbenzenes for crude oil with different depositional environments and thermal maturities

A total of 45 alkylbenzenes were detected and identified in crude oils with different depositional environments and thermal maturities from the Tarim Basin,Beibuwan Basin,and Songliao Basin using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry(GC×GCTOFMS).By analyzing the distribution characteristics of C0-C5alkylbenzenes,it is found that the content of some alkylbenzenes varies greatly in crude oils.Based on the distribution characteristics of 1,2,4,5-tetramethylbenzene(Te MB)and 1,2,3,4-Te MB,the ratio of 1,2,4,5-Te MB to 1,2,3,4-Te MB is proposed to indicate the organic matter origin and depositional environment of ancient sediments.Oil samples originated mainly from lower hydrobiont,algae,bacteria and source rocks deposited under reducing/anoxic conditions have low 1,2,4,5-/1,2,3,4-Te MB values(less than 0.6),while oil samples originated mainly from terrestrial higher plants and source rocks deposited under oxic/sub-oxic conditions have higher 1,2,4,5-/1,2,3,4-Te MB values(greater than 1.0).The significant difference of 1,2,4,5-/1,2,3,4-Te MB values is controlled by 1,2,4,5-Te MB content.1,2,4,5-Te MB content in oils derived from source rocks deposited in oxidized sedimentary environment(greater than 1.0 mg/g whole oil)is higher than that in oils from source rocks deposited in reduced sedimentary environment(less than 1.0 mg/g whole oil).1,2,4,5-/1,2,3,4-Te MB ratio might not or slightly be affected by evaporative fractionation,biodegradation and thermal maturity.1,2,4,5-/1,2,3,4-Te MB ratio and 1,2,4,5-Te MB content can be used as supplementary parameter for the identification of sedimentary environment and organic matter input.It should be noted that compared to the identification of organic matter sources,the 1,2,4,5-/1,2,3,4-Te MB parameter is more effective in identifying sedimentary environments.

Origin,Distribution and Geochemical Significance of Isopropyltoluene Isomers in Crude Oil

With comprehensive two-dimensional gas chromatography linked to time-of-flight mass spectrometry(GC × GC-TOFMS),ten light hydrocarbon(LH) compounds were qualitatively and quantitatively studied in light hydrocarbons(LHs) components of crude oils.For significant differences in the concentrations of 3-isoproyltoluene(3-iPT),4-isoproyltoluene(4-iPT) and 2-isoproyltoluene(2-iPT) in crude oils,and the 2-iPT probably derived mainly from similar skeleton monocyclic terpenoids via dehydrogenation and aromatization,the ratios of(3+4)-/2-iPT(iPTr),3-/2-iPT(iPTr1) and 4-/2-iPT(iPTr2) are proposed to distinguish the organic matter origin of crude oils.Relatively higher iPTr(>8.0),iPTr1(>7.0) and iPTr2(>4.0) values indicate that crude oils are sourced from the co-contribution of lower aquatic organisms,bacteria,algae,and terrestrial higher plants,whereas lower iPTr(<5.0),iPTr1(<3.0),iPTr2(<2.0) values suggest that crude oils originated from terrestrial higher plants.The iPTr,iPTr1,and iPTr2 values show notable distinction which is mainly controlled by 2-iPT concentrations,while the concentrations of 3-iPT and 4-iPT have similar distribution range in all studied oils.The 2-iPT depleted in marine oils from the Tarim Basin and lacustrine oils from the Beibuwan Basin is less than 0.30 mg/g LHs,whereas 2-iPT enriched in swamp oils from the Tarim Basin is greater than 0.50 mg/g LHs.The iPTr,iPTr1,and iPTr2 ratios and 2-iPT concentrations can be used to distinguish the organic matter origin of crude oils,especially for light oils and condensates with low concentrations of biomarkers.