Lithofacies characteristics and sedimentary environment of Chang 7 black shale in the Yanchang Formation,Ordos Basin
WANG Lan1, LI Wenhou2, LIU Qun3, WANG Daxing4, ZHANG Mengbo4, BAI Bin1
1 PetroChina Research Institute of Petroleum Exploration & Development,Beijing 100083,China; 2 Department of Geology/State Key Laboratory of Continental Dynamics,Northwest University,Xi'an 710069,China; 3 School of Environmental Science,Nanjing Xiaozhuang University,Nanjing 211171,China; 4 Research Institute of Exploration and Development, PetroChina Changqing Oilfield Company,Xi'an 710018,China
Abstract：Chang 7 black shale of the Yanchang Formation,Ordos Basin,has a wide distribution area,large thickness and high organic matter content,which has a broad shale oil exploration prospect. Based on the analysis of a large number of core,thin section and geochemical data,the lithofacies types,geological characteristics and distribution laws of Chang 7 black shale are analyzed,and the palaeoenvironment and shale genetic mechanism during its formation period are studied by using trace element indicators. According to the classification standard of “organic matter abundance-rock mineral composition-sedimentary structure”,Chang 7 is divided into three types of lithofacies,in which the bottom of Chang 73 is developed with very rich organic matter laminated shale,and Chang 72 is transformed into organic matter rich laminated clay/silty shale,and Chang 71 is developed with poor organic matter massive silty mudstone. From Chong 73 to Chang 71,as the lake basin shrinks and the delta advances,the content of clay minerals,organic matter and pyrite and the development of laminae gradually decrease. The subtropical warm and humid climate and long-term stable deep-water lake environment create conditions for the large-scale development of algae. The trace element study shows that Chang 73 extremely rich organic shale was formed in a strong reducing environment with extremely high productivity in the lake basin,oxygen deficiency and even sulfurization in the water body,and was affected by volcanic activity and lake bottom hydrothermal solution. Therefore,it is rich in collophanite,Ni,Cu,Mo and other nutrients,and is a rare uranium rich shale in China. Based on the comprehensive analysis of palaeogeomorphology,climate and hydrological conditions,it is considered that the genetic mechanism of the Chang 7 black shale is the water stratification model of the depressed lake basin under the strong tectonic movement of volcanic and hydrothermal activities.
WANG Lan,LI Wenhou,LIU Qun et al. Lithofacies characteristics and sedimentary environment of Chang 7 black shale in the Yanchang Formation,Ordos Basin[J]. JOPC, 2023, 25(3): 598-613.
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