The Compositions Of Molecular Markers In The Mesoproterozoic Successions And Their Geochemical Significance In The Jibei-Liaoxi Area

The Meso-Neoproterozoic strata widely occur in the Jibei-Liaoxi area,which are ideal setting for the study of early life evolution,biological compositions and paleoenvironment.The discovery of a numerous oil and asphalt seepages demonstrates a good exploration potential and prospects of Proterozoic ultra-ancient petroleum resources.However,due to the constraints of geological samples,geological data,as well as experimental techniques,less comprehensive work has been done on the compositions of molecular markers for different sets of source rocks.The petroleum accumulation history of ancient oil reservoirs in the study area has also not been sysyematically studied.According to the systematic analyses of molecular markers and carbon isotopic of the Mesoproterozoic source rocks and related oil accumulations,this thesis discusses the characteristics of paleoenvironment and primitive biological community of the Mesoproterozoic Era in the Jibei-Liaoxi area.It also further determines the oil source of paleo-oil reservoirs by oil to source rocks correlation.Based on the geological and geochemical data,the hydrocarbon generation history of the Mesoproterozoic Gaoyuzhuang and Hoshuizhuang source rocks,the episodes and timing of oil accumulation were determined.The black argillaceous dolomite of the Gaoyuzhuang Formation and the black shale of the Hongshuizhuang Formation in the Jibei-Liaoxi area are effective source rocks,which contain moderate to extremely high organic matter abundance and are still in mature to high mature evolution stage.The Xiamaling Formation shale has low maturity and high organic matter abundance in the Xuanlong Depression,but it lost hydrocarbon-generating ability prematurely in the Jibei-Liaoxi area due to the influence of extensive magma intrusion.The molecular markers and carbon isotopic compositions indicates that the basin had a semi-enclosed,shallow,and high salinity water condition during the depositional period of the Gaoyuzhuang Formation.The primitive organisms were mainly derived from cyanobacteria,heterotrophic halophile and benthic macroalgae,but with a minor contribution of planktonic algae.However,during the depositional periods of the Hongshuizhuang and Xiamaling formations,the basin had an open and relatively deeper water conditions with lower salinity,and the biological community is mainly composed of cyanobacteria,bacteria,and other planktonic algae.In addition,the high abundance of C19-C20 TT,C24Te T,C18-C3313?(n-alkyl)-tricyclic terpanes and rearranged hopane series in the source rocks of the Hongshuizhuang and the Xiamaling formations may represent the contribution of one or more specific bacteria and/or algae,which were not prosperous during the depositional period of the Gaoyuzhuang Formation,probably resulting from high salinity stratified water condition.The investigation of reservoir rock samples,slices observation and molecular markers analyses indicate that the oils in the Wumishan Formation of Well XL1 and the upper sandstone of Luotuoling Formation of Well H1 are mainly derived from the Gaoyuzhuang Formation source rocks.While the oils in the Wumishan Formation of SD section,the Tieling Formation of Well JQ1 and the lower sandstone of Luotuoling Formation of Well H1 are generated from the Hongshuizhuang Formation source rocks.In addition,a bituminous sandstone at the base of the Xiamaling Formation of LTG section has mixed oil sources.In general,the Liaoxi Depression has experienced at least two oil generation phases and three oil accumulation episodes.The first oil generation phase mainly occurred from 1500 to 1300 Ma and the hydrocarbon was originated from the Gaoyuzhuang Formation source rock.Whereas the second phase is the oil generation process of the Hongshuizhuang Formation source rock during 250?230 Ma.The first oil accumulation episode occured during 1500?1300 Ma,and the hydrocarbons from the Gaoyuzhuang Formation source rock charged into the reservoirs of the Wumishan,Tieling and Xiamaling formations.The second oil accumulation episode(465?455 Ma)was resulted from the adjustment of early paleo-oil pools,and oil reservoirs were redistributed in Proterozoic to Ordovician.Moreover,the timing of the third oil accumulation episode is 240?230 Ma,and the hydrocarbons from the Hongshuizhuang Formation source rock charged into the reservoirs of Proterozoic to Triassic.The petroleum reservoirs formed in the third episode are relatively weakly affected by tectonic movements and have better accumulation and preservation conditions.Thus,they should be the primary petroleum exploration targets of Proterozoic ultra-ancient petroleum resources in the future.