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Controls on organic carbon content of the Lower Cambrian black shale in southeastern margin of Upper Yangtze


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. Trace element ratios (V/Cr and Ni/Co) and the concentrations of Cu+Zn imply the lower Cambrian shale was deposited under suboxic-anoxic bottom water with high primary productivity. The average values of total organic carbon in Qiongzhusi Formation are 1.43 % in lower organic-rich shale and 1.75 % in upper organic-rich shale in form of sapropelic (TypeIkerogen), and Ro values range from 2.65 % to 2.97 % in the overmature stage.. The mineral components in this shale are dominated by brittle minerals. The averaging porosity is 1.62 % in lower organic-rich shale and is 3.82 % in upper organic-rich shale, and the permeability in the shale varies between 0.003 mD and 0.09 mD, which means lower Cambrian shale in Sichuan Basin has extremely low porosity and low permeability. Pores are dominated by organic pores in lower organic-rich shale and dominated by inorganic intragranular and intergranular pores in upper organic-rich shale, respectively. In the meantime, the increasing of thermal maturity in a certain range would elevate the porosity. Shale gas in Qiongzhusi Formation consists of absorbed gas and free gas, and free gas contributes the majority of total gas content in the upper organic-rich shale. Shale gas content increases with TOC, porosity, and thermal maturity even for this overmature Lower Cambrian shale with maturity up to 3%, which are the main controlling factors for shale gas accumulation.