Black shale reservoirs from the Wufeng and Longmaxi Formations in northeastern Chongqing

The Wufeng and Longmaxi Formations in northeastern Chongqing are primarily made up of carbonaceous shales, mudstones, muddy siltstones, siliceous rocks, marls and bentonite, of which the black shales occur as the important shale gas reservoirs on a wide range of scales in the study area. The petrography, mineralogy and reservoir spaces of the black shales are treated on the basis of thin-section examination, mineral compositions, SEM, physical properties and organic geochemical signatures. These black shales are characterized by type I organic matter, higher total organic carbon(TOC)contents ranging between 2% and 7.56% with an average of 3.09%, and higher maturity levels of organic matter up to maturity to overmaturity, with Ro values ranging between 1.65% and 2.33%. The mineral compositions contain quartz and clay minerals dominated by illite and interstratified illite-montmorillonite. The porosity comprises intergranular pores, intragranular pores, micro-fissure and micropores, with an average porosity of 2.90% and an average permeability of 0.005×10^-3μm². The results of research may provide geological basis for future exploration of the shale gas in the study area.