Abstract:

Organicrich shale and mudstone developed well in Chang7 Member of the Triassic Yanchang Formation, which has become an important prospecting target for shale gas and shale oil. Based on fine description of core, microstructure observation of thin section and grain analysis, three kinds of lithofacies of shale were determined, including black shale, laminar mudstone, and silty mudstone. Brittle minerals content of silty mudstone is higher than that of black shale and laminar mudstone. Pyrite content of the latter is higher than that of the former, and clay minerals content of the three lithofacies are nearly the same. Black shale is the most favorable lithofacies which has high organic abundance and greatest kerogen type (mainly type I). Laminar mudstone is the second favorable lithofacies. During the deposition of Chang7 Member, the layered anaerobic environment was beneficial to the preservation of organic matter. Lower grade aquatic lives were the main biological precursor of organic matter. Black shale and laminar mudstone developed in deep lacustrine environment which was quiet and not influenced by terrigenous coarse grains. However, silty mudstone developed in the environment that was influenced frequently by terrigenous coarse grains. As a result, organic matter types of silty mudstone are diverse and complicate. With the combination of fine description of core and logs, the vertical development characteristics of each kind of lithofacies were uncovered. Influenced by baselevel changing and sediment supply, the third section of Chang7 Member (C73) is composed of black shale and laminar mudstone, however, the second section and first section of Chang7 Member are mainly composed of laminar mudstone and silty mudstone. Black mudstone and shale in C73 have better lithofacies that bear lamellation fracture and lamina facies, better organic matter type and high abundance. C73 is the best prospect target for shale oil and shale gas.

Key words: organicrich shale, lithofacies, organic facies, shale gas and shale oil, Yanchang Formation, Ordos Basin