Shale reservoir characterization and control factors on gas accumulation of the Lower Cambrian Niutitang shale, Sichuan Basin, South China

The Lower Cambrian Niutitang shale is reported to be the oldest strata for large-scale shale gas reserves. Based on outcrop observations and laboratory analyses, this study used a comprehensive approach that integrated sedimentology, mineralogy, petrology, organic geochemistry, and reservoir geology to estimate the prospects for exploring for shale gas in the Lower Cambrian Niutitang Formation, southern China. Whole rock and clay mineral X-ray diffraction, scanning electron microscope (SEM) observations, and petro-physical property measurements indicate that the Niutitang shale is dominated by quartz and clay minerals with minor amounts of plagioclase, potassium feldspar, calcite, dolomite, and pyrite. The organic matter in the Niutitang shale is dominated by type-1 kerogen, with TOC levels ranging from 1.35% to 14.3% (average 4.46%) and Ro values ranging from 1.86% to 4.18% (average 2.79%). The three primary types of pores in the shale are organic pores, microfractures, and micropores. The micropores are primarily found in the clay mineral crystals and pyrite, and the microfractures are found along the boundaries of quartz grains. Most of the pore diameters are less than 20nm, mainly concentrate at about 10nm, which is consistent with that the organic pores act as the main storage space. The pore volume and gas content are strongly controlled by the mineral composition and lithofacies, and TOC content. The factors favouring shale gas accumulation of the Niutitang shale in the study area include (1) an effective thickness and widespread distribution, (2) a high organic carbon content and appropriate organic matter maturity, and (3) a favourable depositional environment.