Formation Conditions And Enrichment Models Of Shale Oil In The Chang-7 Oil Layer Of Ordos Basin

The seventh member of the Upper Triassic Yanchang Formation（Chang 7 Member）is an important position for shale oil exploration and development in Ordos Basin,and the oil source material is composed of widely distributed black shale and dark-grey mudstone.Most of studies focused on the black shale,but ignored the study of dark-grey mudstone in this member.Currently,there are still many problems to be solved in the aspects of the difference of organic matter enrichment mechanism,hydrocarbon generation characteristics,shale oil accumulation conditions and genetic mechanism of these differences between the two types of source rocks.Moreover,the study on enrichment model and enrichment characteristics of shale oil in the member and its main control factors are also controversial.In view of these problems existing in the Chang 7 Member black shale and dark-grey mudstone,the paper adopts organic geochemistry,organic petrology,element geochemistry and other methods to analyze their differences in sedimentary environment,organic matter enrichment mechanism and hydrocarbon generation characteristics in the study area,discusses the origin of their differences in hydrocarbon generation characteristics,and reveals the maturity range conducive to the formation and distribution of shale oil.The paper proposes the quantitative evaluation method and parameter system of oil content of black shale and dark-grey mudstone,analyzes the main controlling factors of shale oil enrichment,and establishes the shale oil enrichment model of the Chang 7 Member shale in the study area.The main achievements and understandings are described as follows:The black shale and some dark-grey mudstone in the Chang 7 Member have favorable oil source conditions for shale oil formation,but their geochemical characteristics,organic matter enrichment conditions,hydrocarbon generation potential and the role in shale oil formation are different.The differences in these aspects are the affected by the palaeoclimatic conditions,geological events,organic matter source composition,palaeoproductivity,sedimentary environment and organic matter preservation conditions during the sedimentary period.The hydrocarbon generation potential of black shale is significantly higher than that of dark-grey mudstone.The black shale has a high abundance of organic matter,and the kerogen is dominated by typeⅠand typeⅡ₁,and the black shale was classified as good–very good source rocks.The organic matter abundance of dark-grey mudstone is relatively high,and the kerogen is dominated by typeⅡ₁ and typeⅡ₂,and the dark-grey mudstone was classified as poor–good source rocks.The thermal maturity of the Chang 7 Member source rock ranges from low-mature to mature.All black shales and some dark-grey mudstones have reached the source rock criteria of shale oil,and only some samples in the two types of source rocks reached the mobility criteria of shale oil.The palaeoclimatic conditions are warm and humid during the deposition of black shale and dark-grey mudstone in the Chang 7 Member.The organic matter of black shale mainly originated from aquatic organisms and deposited in reducing environment.The high palaeoproductivity caused by volcanic ash fall and hydrothermal intrusion and the anoxic sedimentary environment caused by sulfur-containing gases with volcanic eruption are the main controlling factors for the organic matter enrichment of black shale.The organic matter of dark-grey mudstone originated from the mixture of aquatic organisms and terrestrial higher plants,which are mainly deposited under weakly reducing to reducing conditions.The moderate palaeoproductivity caused by hydrothermal intrusion and the relatively high sedimentation rate caused by the massive input of terrigenous debris are the main reasons for the relatively poor organic matter enrichment conditions of dark-grey mudstone compared with black shale.The organic matter enrichment model of the black shale in the Chang 7 Member is preservation and productivity mixing model,while the organic matter enrichment model of the dark-grey mudstone is productivity and sedimentation rate model.The hydrocarbon generation process of the black shale is characterized by high hydrocarbon generation threshold,fast transformation rate and short hydrocarbon generation cycle,while the hydrocarbon generation process of dark-grey mudstone is opposite to that of black shale.The hydrocarbon generation peak of black shale is earlier than that of dark-grey mudstone,but the maturity range that can provide oil source for shale oil formation is smaller than that of dark-grey mudstone.The differences in organic matter source composition and organic macerals composition are the main reasons for these obvious differences between the two types of source rocks in the study area.The organic macerals of black shale are dominated by algae,and the relatively single organic macerals make the distribution of activation energy concentrated.The algae,vitrinite and inertinite are distributed in the organic macerals of dark-grey mudstone,and the relatively complex organic macerals make the distribution of activation energy more dispersed than that of black shale.The difference of these characteristics results in that the dark-grey mudstone has the characteristics of low hydrocarbon generation threshold value and longer hydrocarbon generation cycle compared with black shale,thus causing that the maturity range of dark-grey mudstone that can provide oil source for shale oil formation is larger than that of black shale.The oil content of black shale is higher than that of dark-grey mudstone in the study area.Shale oil is mainly distributed in black shale and dark-grey mudstone of the Chang 7² and Chang 7³ sub-members in the vertical direction,and in the Jiyuan and Longdong areas in the plane.The development of high-quality source rocks and maturity of organic matter control the formation and distribution of shale oil,and the hydrocarbon expulsion controls the distribution characteristics of shale oil in the source of different lithology associations of the Chang 7 Member.The shale oil enrichment characteristics are obviously controlled by the lithology associations.There are six different shale oil enrichment models in the Chang 7 Member of the study area,of which shale oil mainly concentrated in the shale of lithology associations with the thick shale,and the shale oil mainly concentrated in sandstone adjacent to shale of lithology associations with the thin shale.The most favorable lithology association is thick sandstone adjacent to thick mudstone（shale）,which is mainly distributed in the Dingbian–Wuqi and Huachi–Zhengning areas.The research results of this paper can provide geological reference for shale oil exploration in the study are,and have reference value for shale oil exploration in other continental petroliferous basins.