Lacustrine fine-grained sedimentary features and organicrich shale distribution pattern: A case study of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin, NW China

The Chang 7 Member of Triassic Yanchang Formation in Ordos Basin is taken as an example to study the distribution rule and major controlling factors of the fine-grained sedimentary system by core description, thin section observation, X-ray diffraction, geochemical testing and TOC well logging quantitative calculation. The main sedimentary pattern of organic-rich shale in Chang 7 Member is transgression–water stratification. The sedimentary facies, water depth, anoxic condition and lacustrine flow are the major controlling factors for the formation and distribution of organic-rich shale. During the deposition of the Chang 73 Member, lake water invaded rapidly, lake depth and scope sharply increased. Because of the temperature difference, the circulation between upper surface water and lower water in deep lake was restrained, then large-area anoxic environment was formed in deep lake, which was favorable for the development of organic-rich shale. In silent deep lake where is far away from the delta front, organic-rich shale well developed, with high organic carbon content and mainly type I kerogen. In deep lake where sandy debris flows developed, organic-rich shale was inter-bedded with sandstone, which contains high organic carbon content, mainly with type I-II1 kerogen. In semi-deep lake close to the delta front, wavy-massive silty mudstone developed, mainly with type II kerogen.