Geochemical Characteristics Of Hydrocarbon Source Rocks And Chang 8 Oil Source Tracing In The Northern Pingliang Area,Ordos Basin

The Ordos Basin has proved rich oil and gas resources under continuous exploration for many years,but the Northern Pingliang area is located in the southwest margin of the basin,far away from the center of the basin,the exploration degree is low,the geological understanding is not sufficient,the genetic type of crude oil in Chang 8 section of the Northern Pingliang area is not uniform,and the oil source is not clear.Based on the organic geochemical analysis technology and supplemented by geophysical logging technology,this paper clarifies the type and distribution of Chang 8 crude oil in the Northern Pingliang area,the effective source rock of Chang 8 crude oil and its geochemical characteristics and distribution,which provides a reliable basis for further clarifying the main controlling factors and accumulation mode of Chang 8 reservoir in this area,and provides a theoretical basis for the next oil and gas exploration and development deployment.Combined with core observation,through rock pyrolysis,total organic carbon TOC determination,soluble organic matter extraction and group separation quantitative analysis,vitrinite reflectance determination,kerogen carbon isotope determination,saturated hydrocarbon chromatography-mass spectrometry analysis,and other geochemical techniques,the Chang 8 crude oil type in the study area was determined.Comprehensive evaluation of Chang 7₃ black shale,Chang 7₃dark mudstone and Chang8 dark mudstone;the effective source rocks of Chang 8 crude oil were traced by oil source correlation method.The results indicate that the Chang 8 crude oil can be classified into three types:A,B,and C,with A being the most dominant.Type A crude oil is widely distributed in the north of Pingliang.The content of C₂₇ rearranged sterane is low,the content of C₂₉ norhopane is higher than that of C₂₉ Ts and C₃₀ RAH,and the relative content of Ts is lower than that of Tm.A small amount of type B crude oil is distributed in Pingliang area.The content of C₂₇rearranged sterane is high,the content of C₂₉ Ts is higher than that of C₂₉ norhopane and C₃₀ RAH,and the relative content of Ts is higher than that of Tm.A small amount of type C crude oil is distributed in the northern part of Pingliang area.The content of C₂₇ rearranged sterane is low,C₂₇-C₂₉ regular sterane is’L’type,the content of C₂₉ norhopane is higher than that of C₂₉Ts and C₃₀RAH,and the relative content of Ts is higher than that of Tm.The effective source rocks for all three types of crude oil are the black mudstones of the Chang 7₃sub-section.The primary source rocks for A-type crude oil are the black mudstones of the Chang 7₃sub-section within the basin and in the Mengba area,while those for B-type crude oil are the black mudstones of the Chang 7₃ sub-section in the Pingliang area.The C-type crude oil is formed by a mixture of A-and B-type crude oils.Among the source rocks,only the black mudstones of the Chang 7₃ sub-section are effective source rocks for the Chang 8 crude oil.The comprehensive geological evaluation rates them as very good source rocks with organic matter types of I-type to Ⅱ1-type and maturity ranging from low-maturity to mature stage.TheΔlogR method is used to calculate the organic carbon content of the source rocks in the Chang 7₃ sub-member of Meng 53 well,and the measured value of TOC is fitted to establish the TOC logging prediction model represented by Meng 53 well.The predicted TOC values from this model are highly correlated with the measured values,with a correlation coefficient of 0.883,indicating high reliability.The predicted map of the distribution of effective source rocks in the northern Pingliang area shows that the lower limit of TOC for effective source rocks is 3%,and they are mainly distributed in the eastern part of the region,with thickness gradually decreasing from east to west.