Shale reservoir characteristics of the Lower Cambrian Qiongzhusi Formation in the southern Sichuan Basin

Experimental source-reservoir data from three wells in the Mianyang-Changning intracratonic sag in the southern Sichuan Basin showed that the Qiongzhusi Formation in the study area features high TOC, thick black shale, high maturity, and high brittleness. Although the southern Sichuan Basin has experienced multiple episodes of tectonic movement, the B1 well block still has favorable preservation conditions. Micro-reservoir space includes organic pores, inorganic pores (intercrystal pores in clay minerals, intergranular pores and intragranular dissolution pores in brittle minerals, and intergranular pores in pyrites), and micro-cracks (micro-structural seams, diagenetic shrinkage joints and dissolution seams), and can be further subdivided into 9 types. Most of the pores have narrow necks, and some are slit type holes. Fractures are tablet or wedge shaped. Mesopores were abundant, and there also exist micropores and macropores. High quality shales have high entrance and BET specific surface area, which is favorable for the preservation of free and adsorbed gas. TOC, maturity, diagenesis and pore fluid pressure determined the difference of pore type and distribution.