Effects of types and content of clay minerals on reservoir effectiveness for lacustrine organic matter rich shale

As a promising supplement of China's petroleum production, the lacustrine shale, compared with the marine shale, is characterized by higher clay mineral content, which has significant impacts on the hydrocarbon reservoir quality. Controlled by the paleo-lacustrine background and thermal maturity, the clay mineral contents vary tremendously in different shale systems. This study targets on three shale series formed in different basin types, namely the Lucaogou Formation shale (Junggar Basin), the Chang 7 Member shale (Ordos Basin), and the Qingshankou Formation shale (Songliao Basin), where the Junggar Basin was a saline lacustrine basin and the latter two were freshwater-brackish lacustrine basins. We find that the composition and content of clay minerals are considerably varied among the three shales that the clay mineral content of the Lucaogou formation shale is smaller than that of the Chang 7 Member and Qingshankou Formation, and the content of illite in the Chang 7 shale is lower than that in the Qingshankou shale. The occurrence of clay minerals has crucial effects on pore structures, that is the pore-filling clays has indiscernible contribution to the storage space, like in the Lucaogou shale, but those developing along the shale laminae can improve the storage capacities to form inter-granular pores and micro-fractures such as in Chang 7 Member and Qingshankou Formation. The development of clay mineral-related pores and fractures is controlled by thermal maturity (Ro) and affected by the content of terrigenous minerals. This pore structure alteration derived by the clay mineral occurrence types further affects the hydrocarbon contents. In the Lucaogou shale, the oil content is inversely proportional to clay mineral con-tents, while in Qingshankou Formation, the oil content is even higher than that of the adjacent felsic silt laminae due to the favorable development of micro-fractures and inter-granular pores in clay mineral laminae. The high clay mineral content in lacustrine shales in China make it necessary to investigate the clay mineral evolution to evaluate the hydrocarbon generation potential, pore structures, hydrocarbon contents, and hydrocarbon accu-mulation mechanisms in shale systems.

Automated summary

The lacustrine shale in China has higher clay mineral content compared to marine shale, which greatly impacts the quality of hydrocarbon reservoirs. The composition and content of clay minerals vary among different shale systems, resulting in differences in pore structures and hydrocarbon contents. The development of clay mineral-related pores and fractures is controlled by thermal maturity and the content of terrigenous minerals. It is important to investigate the evolution of clay minerals in lacustrine shales to evaluate their hydrocarbon generation potential and accumulation mechanisms.