Characteristics of conventional and unconventional shale gas displays of the Cambrian strata in Yichang area, West Hubei, and their exploration significance

The Cambrian strata in west Hubei have favorable hydrocarbon accumulation conditions, but it is difficult to obtain substantial breakthrough in exploration due to multiple tectonic movements and old strata.Recently, industrial gas flow was tested in the Cambrian Shuijingtuo Shale and a large quantity of natural gas was discovered in multiple formations, which represent an enormous potential gas resource in Yichang area.The proved and possible reserves of Cambrian formations include Shuijingtuo shale and Shilongdong-Tianheban carbonate, which represent the shale gas and conventional gas system respectively.Black rock series of the Shuijingtuo Formation has high hydrocarbon potential and shale gas content.Micropores in organic matter and microcracks in shale are reservoir space, which resulted in a self-generation and accumulation for shale gas.The test of Shuijingtuo shale revealed an industrial gas flow after horizontal well fracturing.Methane is the main constituent of gas sample with concentration exceeding 90%.Considering the well blowout with methane and formation water in the carbonates of the Tianheban strata, the carbonate reservoir must be semi-filled fracture and dissolved pores.The formation water is of magnesium chloride type and the storage conditions are medium.Sand-clastic dolomite of the Shilongdong Formation is characterized by large cumulative thickness, low porosity and low permeability with partly high reservoir property.The sedimentation of Cambrian strata in Yichang slope developed the gas play with organic shale of the Shuijingtuo Formation as the source rock, and the fractures inlimestone of Tianheban Formation and dissolved pore in dolomite of Shilongdong Formation serve as the main reservoirs.Gypsum dolomite and shale constitute the main cap rock of the conventional gas system.All this constitutes a vertical gas system.An analysis of the evolution history of oil and gas accumulation shows that the Cambrian source rocks began to generate oil in the early Silurian, generated gas in the Early Permian, and reached the gas peak in the Late Triassic.After undergoing transformation of multiple stages of tectonic movement, all the petroleum reservoirs have been converted into gaseous hydrocarbons.All this shows thatthe study areamight be the favorable area for shale gas accumulation in the Middle Yangtze region, and it is a new area for shale gas exploration outside the Sichuan Basin.The above understanding will play an important guiding role in the exploration of Cambrian shale gas and natural gas in west Hubei.