Shale gas accumulation conditions and gas-bearing characteristics of the Lower Cambrian Niutitang Formation in Well Changye-1 in northwestern Hunan Province

To evaluate the accumulation conditions and gas-bearing properties of shale gas in the Lower Cambrian Niutitang Formation, northwestern Hunan Province, the first shale gas parameter well (Well Changye-1) that takes the Niutitang Formation as the target horizon in the Hunan Province was selected preferably and drilled, cumulatively revealing the thickest dark shale horizon of the Niutitang Formation among the single-well drillings in China, with a true vertical thickness of 674.5m. Through analyzing the core samples in terms of organic geochemistry, rock mineralogy and reservoir properties, the black shale horizons in the Niutitang Formation of Well Changye-1 have high organic carbon content (average 3.9%), moderate maturity (equivalent Ro average 2.6%), high brittle mineral content (quartz content average 50.1%), low clay mineral content (average 32.4%), low porosity (1.7%) and low permeability (1.32×10-3mD), and well-developed meso to micro-pores and fractures, indicating good conditions for shale gas accumulation. Field desorption and laboratory isothermal adsorption measurements on core samples show that Well Changye-1 has good gas-bearing properties, and gas content generally increases with depth. The black shale horizons at the depth of 1100-1250m have an average organic carbon content up to 10.1%, total gas content of 0.5-2.1m3/t and 3.7-6.4m3/t, and this is the most favorable depth for shale gas development, indicating that the Niutitang Formation has good a prospect for shale gas exploration. Due to huge sedimentary thickness, the black shale in the middle-lower part of the Niutitang Formation should be given priority for exploration.