Abstract:

Based on field geological and drilling data, the reservoir characteristics and gasbearing affecting factors of the Lower Paleozoic marine shale in the Northwestern Hunan were studied through the related experimental results. The results show as follows. The black shale of Niutitang Formation is formed in the deep water continental shelfmargin slope sedimentary environment, with thickness of 50250 m. The Longmaxi Formation is stranded bay deposit and the black shale is mainly developed at the bottom. The organic matter type in both formations is predominantly sapropelic. The Total Organic Carbon (TOC) content of the black shale from the two formations is averaged at 3.57% and 1.16%, respectively, with high equivalent vitrinite reflectance (Ro) of 2.61% and 2.08%, respectively. Black shale in both formations are characterized by high content of quartz, low content of clay minerals, and small amounts of carbonate minerals which is controlled by sedimentation and diagenesis. Shale reservoir spaces can be divided into three broad categories: mineral matrix pore, organic pore and microfracture. Influenced by factors such as organic matter and clay minerals, the pore structure parameters of shales are different. The micropores (< 2 nm) and mesopores (250 nm) are mainly developed, which have massive contribution to the pore specific surface area and pore volume where the shale gas adsorption mainly occur. The average methane adsorption capacities of shales in the Niutitang Formation are 1.98 cm3/g, a little higher than that of shales in the Longmaxi Formation (1.16 cm3/g).The organic matter and clay minerals both play a role in the methane adsorption, and high maturity and water content can reduce shale adsorption capacity.

Key words: Northwestern Hunan, marine shales, reservoir characteristics, porestructure, adsorption capacity