Shale Gas Accumulation Conditions and Target Optimization of Lower Cambrian Shuijingtuo Formation in Yichang Area, West Hubei

Good gas shows in the Cambrian Shuijingtuo Formation indicate the resources potential in Middle Yangtze region. The weakness of basic research on the shale gas accumulation constrains gas exploration and development in this area. In this paper,it focuses on the geological characteristics of Shuijingtuo shale gas in Yichang area by integrating geological and geochemical data from latest exploration well and outcrop. The gas potential is discussed based on preliminary evaluation of lithofacies,geochemical characteristics,reservoir features,and shale gas content. These organic-rich intervals are concentrated along and between continental shelf and slope. Thermally mature strata of the Shuijingtuo shale attain thickness between 28 and 41 m,with a total organic carbon (TOC) exceeding 2%. Organic matter within Shuijingtuo shale is high,which contains an average of 2.65% TOC,with present-day thermal maturity between 2.4% and 3.2% equivalent vitrinite reflectance (R_o),which is in the high mature of dry thermogenic gases dominated stage. The Shuijingtuo shale is noted for having generally high silica content. Mineral composition of shale consists of 29.33% quartz,25.94% carbonatite calcite/dolomite,35.06% clay minerals. Illite smectite mixed layer is the major clay mineral. The gas shale reservoir has an extremely low porosity and permeability. Porosity ranges from 0.96% to 3.22%,with an average value of 2.08% at the bottom of Shuijingtuo Formation. The effective permeability in gas shale is (0.01-3.05)×10^-3 μm². Through scanning electron microscope observation and isothermal adsorption,pores in shale also show multiple genetic types and small size of multi-pore diameter. Fractal dimensions of the organic pores vary from 10 to 50 nm. Overlying formations and underlying strata are favorable for shale gas preserved. The tectonic stress field is regionally very stable and short of fault activity,which all indicates favorable preservation conditions for Shuijingtuo shale gas in gentle slope of Yichang area. Measurement of total hydrocarbon and methane through mud logging indicates anomaly. The measured gas content of the Shuijingtuo shale in Well Yiye-1 ranges between 0.31 to 5.48 m³/t. Thermally mature strata of the Shuijingtuo Formation attain thickness of 44.05 m,with a continuous gas content exceeding 2 m³/t. Well Yiye-1 tested an industrial gas flow after horizontal well fractured. Methane is the main constituent of gas sample and varies in concentration from 81.17% to 92.75%. Ethane ranges from 0.83% to 0.94%,nitrogen in non-hydrocarbon gas ranges from 5.86% to 9.37%,and carbon dioxide ranges from 0.05% to 2.25%. It is concluded that it could be the favorable area for shale gas accumulation in the Middle Yangtze region,and it's a new area for shale gas exploration outside Sichuan basin.