Characteristics of shale gas reservoirs in Wufeng Formation and Longmaxi Formation in Well WX2, northeast Chongqing

To further understand shale reservoir characteristics of Wufeng Formation and Longmaxi Formation in the Wuxi area, northeast Chongqing, based on drilling data of Well WX2, and taking the graptolite biostratigraphy as the standard marker of stratigraphic division and comparison, the geochemistry, petrology, reservoir space and properties of organic-rich black shale were well investigated, and its gas-bearing capacity and controlling factors were also analyzed. The result shows that in the Wufeng Formation and Longmaxi Formation of Well WX2, the organic-rich shale is 89.8 m thick and is characterized by good kerogen type, high organic abundance, moderate maturity and favorable hydrocarbon-generation condition, and the graptolite sequence is developed completely and continuously; the organic abundance is influenced by depositional rate, and the slow depositional rate is favorable for accumulation of organic matter in the black graptolite shale; from top to bottom, content of siliceous minerals increases and content of clay minerals decrease, therefore the brittleness increases; the organic-rich siliceous shale and clay siliceous shale are favorable lithofacies for development of shale reservoirs; the nanopore is dominated by the parallel-plate pore with four open sides and has good connectivity; the pore size distribution curve has the multimodal characteristic, and the pore diameter mainly is in the range of 0.42–0.62 nm and the range of 3–5 nm; organic pores and interlayer pores of clay minerals make the greatest contribution to the total pore volume, while pores of brittle minerals have the least contribution; from top to bottom, organic pores gradually increase while interlayer pores of clay minerals gradually decrease; the on-site core gas content exceeds 8 m3/t, and the gas-bearing capacity is jointly controlled by hydrocarbon generation, reservoir and preservation conditions; and the WF2-LM6 biozone of the Katian to the early Aeronian is the high-quality shale reservoir, where the LM1 biozone of the Hirnantian was the best “sweet spot” which is the target of horizontal well drilling. Keywords: Shale gas reservoirs, Graptolite biozone, Micropore, Gas-bearing capacity, Wufeng Formation and Longmaxi Formation, Northeast Chongqing