High Resolution Research And Paleoenvironment Reconstruction In The Lower Jurassic Oil Shale Bearing Strata Of The Dachanggou Basin Xinjiang,China

Dachanggou Basin contains abundant organic rich deposits,such as oil shale,cannel boghead and humic coal,which provided an ideal carrier for high resolution analysis of the enrichment mechanism of organic matter and paleoenvironment reconstruction in oil shale bearing strata.Based on the methods of sedimentology,sequence stratigraphy,organic petrology,element geochemistry,organic geochemistry and isotopic geochemistry,the sedimentary environment,paleoclimatic characteristics,source,enrichment mechanism of organic matter and response of sedimentary organic matter to environmental change of oil shale bearing strata in Dachanggou Basin are well analyzed in this study.According to the core,profile and logging data,the main sedimentary facies types are identified as lake and delta facies which can be further divided into four subfacies including semi-deep to deep lake,shallow lake,delta front and delta plain,with eight microfacies.Oil shale and cannel boghead developed in semi-deep to deep lake,humic coal developed in delta plain.The results of sedimentary evolution analysis show that the source supply direction of sequence II is stable,and the main provenance area is the northeast of the basin.The thick oil shale is mainly developed in the semi-deep lake to deep lake during the HST stage of sequence II,with which candel algal coal is symbiotic.The test results show that the TOC content(average 13.0 wt.%)and hydrocarbon potential(average 77 mg/g)of oil shale exposed by core and profile samples is high in general.The cannel boghead and humic coal have the higher TOC content(average 51.6 wt.%),but the hydrocarbon potential(S₁+S₂)of cannel boghead(average 242 mg/g)is higher than that of humic coal(average 178mg/g).Cannel boghead and oil shale have similar Hydrogen index(HI:average 551 and 531 mg HC/g TOC,respectively),and the HI of humic coal is significantly lower than the former(average 268mg HC/g TOC).The type of organic matter in oil shale is type I and type II₁,that of cannel boghead is type II₁ and humic coal is type II_2?T_max and Ro showed that the maturity of organic matter in Badaowan Formation was in the immature to low maturity stage.Industrial analysis shows that the oil yield of cannel boghead is the highest(up to 24.4%,average 18.3%),followed by humic coal(up to 13.1%,average 12.2%)and oil shale(up to 12.7%,average 7.4%).The ash content of oil shale(average 75.8%)is higher than that of two kinds of coal(average 36.9%).Biomarkers,macerals and organic carbon isotopes were used to analyze the source of organic matter in oil shale,cannel boghead and humic coal.The results show that the main source of organic matter in oil shale is algae,supplemented by emergent plants and terrestrial higher plants.Higher plants are the main source of cannel boghead and humic coal,but the former has relatively high algae content.Therefore,the terrigenous organic matter in cannel boghead underwent transportation and separation,and then the hydrogen rich components were deposited in deeper water body,while the organic matter in humic coal was near source or in-situ deposition of higher plants.Based on the analysis of trace element enrichment factor EF,substitution index of pyritization(Dop_T)and biomarker parameters such as ratio of phytane to pristane(Pr/Ph)and relative content of rearranged steranes,the data show that the paleoxygenation facies of Badaowan Formation oil shale in the study area is from poor oxygen environment,cannel boghead is from poor oxygen to reduction environment.According to the analysis of salinity characteristics by Sr/Ba and Ca/Mg ratios,the water body shows a fresh condition during the oil shale deposition period,and the lower gammacerane index(GI)can also confirm this.During the cannel boghead deposition period,the water body was brackish to saline.Using element ratio C_-value and Sr/Cu,organic carbon isotope,sporopollen and clay mineral composition,the paleoclimatic characteristics during oil shale and coal profile are analyzed by various means.The results show that the oil shale and cannel boghead formed under warm and humid climatic background,but in contrast,cannel boghead is the product of humid and hot climate.The higher evaporation makes the water body of sedimentary environment shallower,the salinity increases,and the input of higher plants increases,which is conducive to the formation of cannel boghead.During the HST period of sequence I and II,the climate was the warmest and wettest,which led to the increase of endogenous productivity and algae input,promoting the formation of thick oil shale.And the paleoclimate is the primary factor controlling the sediment types and oil shale distribution characteristics of different sedimentary periods in sequence stratigraphic framework.The vertical variation trend of carbon isotope of nC_27,29,31 can better reflect the variation of paleoatmospheric CO₂ concentration during the sedimentary period.According to the response mechanism of carbon isotopic composition of C₃ plants to environmental CO₂ concentration,the calculated value of corresponding CO₂ concentration variation range of sedimentary period of oil shale profile is 593-2546 ppm,with an average of 1172ppm(+279,-135ppm),which is relatively high and has a large fluctuation range.In the early stage of oil shale deposition,there was relatively high atmospheric CO₂ concentration.The warm and humid climate background led to large-scale lake transgression and improved biological productivity.The high atmospheric CO₂ concentration at the initial stage corresponded to higher inertinite content,which may be due to the high atmospheric CO₂concentration increasing fire frequency.There is a certain coupling between the cannel boghead and the high value of CO₂,that is,the increase of CO₂ concentration is not only conducive to the development of higher plants,but also improves the biological productivity of the lake,and promotes the formation of lacustrine cannel boghead.