Factors Affecting Shale Gas Accumulation in Overmature Shales Case Study from Lower Cambrian Shale in Western Sichuan Basin, South China

A series of comprehensive and systematic experimental data and log data of lower Cambrian Qiongzhusi Formation, from the first successful Well Jin-1 in western Sichuan Basin, were analyzed to investigate the properties of lower Cambrian organic-rich shale and to illustrate its shale gas potential, so as to investigate the factors affecting shale gas potential in overmature and ultratight shale. Trace element ratios (V/Cr and Ni/Co) and the concentrations of Cu + Zn imply the lower Cambrian shale deposited under suboxic–anoxic bottom water with high primary productivity. The organic matter is enriched in Qiongzhusi Formation as sapropelic (type I) mainly, and R o (2.65–2.97%) values indicate the organic matter is in the overmature stage. Pores are dominated by organic pores in lower organic-rich shale and by inorganic intragranular and intergranular pores in upper organic-rich shale. Generally, organic-rich shale was deposited in anoxic shelf (high ratios of V/Cr and Ni/Co) with high productivity (high concentration of Cu + Zn), and shale gas has accumulated and been preserved in the region with stable tectonic setting. Comprehensive analysis of the properties in overmature shales (Marcellus, Longmaxi, and Qiongzhusi) and their effect on shale gas shows that (a) organic matter formed a material basis for shale gas generation, (b) porosity provided the capacity for shale gas accumulation, and (c) moderately high thermal maturity ( R o up to 3%) improved the storage space through elevating porosity. This indicates that total organic carbon, porosity, and thermal maturity in overmature shales are the main factors affecting shale gas accumulation.