Organic Matter Enrichment Mechanism And Hydrocarbon Generation And Expulsion Characteristics Of The Black Shales In Chang 7 Member,Ordos Basin

Based on the principles and methods of organic geochemistry and elemental geochemistry,the relationships between organic matter content of black shale in Chang7 member of Ordos Basin and paleoproductivity,redox condition,volcanism influence,hydrothermal activity and sedimentary water conditions(salinity and sedimentation rate)are investigated.Further,the characteristics of hydrocarbon generation and expulsion of the Chang 7 black shales are explored on the basis of the changing and distribution of saturated and aromatic molecular markers in the source rock extract and expelled oil;accordingly,the relationship between hydrocarbon expulsion efficiency and the ratios of biomarkers.The results show that the organic matter content of Chang 7 black shale in Tongchuan Tangnihe area is higher than that in C75 well in the basin.TOC ranges from16.76% to 39.39%,with an average of 25.05%;S1 + S2 in a range from 61.81mg/g to178.79mg/g,with an average of 103.30mg/g.The input of algae and phytoplankton is significant,for instance,∑ C21-/ ∑ C22 + value being from 0.96 to 2.74,with an average of 1.68,and TAR value being from 0.17 to 0.37,with an average of 0.29,In addition,the occurrence of high abundance of bicyclic sesquiterpanes indicates the contribution from green algae,resulting in the predominance of C29 steranes in this profile.The productivity is higher,with Mo content from 76.4ug/g to 133.0ug/g,with an average of98.4ug/g.The water column is anoxic,with Pr / Ph value ranging from 0.79 to 0.99(in average 0.90),;and AIR value ranging from 0.73 to 3.74(in average 2.22).The influence of hydrothermal activity is stronger,with Ba / Sr value of 3.5 to 9.5 and average value of 5.3.Therefore,the deposition period of Chang 7 member of Tangnihe area in Tongchuan is a semi deep water to deep water lake,accompanied by strong hydrothermal activities,which brings higher amount of nutrients to enhance the primary productivity of the lake.Meanwhile,a large amount of H2 S gas makes the sedimentary environment more anoxic,providing favorable conditions for the formation and preservation of matter.The hydrocarbon expulsion efficiency of black shale has a good negative correlation with 4-MDBT / Phen and a good positive correlation with aromatic hydrocarbon ratios 3 / 1-MCH AND 4 / 1-MDBT AND benzo[k]fluoranthene/perylene,benzo[e]pyrene /Perylene and has a good positive correlation with the maturity Ro.It is expected to be a valuable molecular parameter for quantitative evaluation of hydrocarbon expulsion efficiency of the Chang 7 black shales.