Comparison of organic matter occurrence and organic nanopore structure within marine and terrestrial shale

Abstract

Samples from Cambrian Qiongzhusi Formation (Fm.), the Silurian Lungmachi Fm. of the middle-upper Yangtze, and the Triassic Yanchang Fm. of the Ordos Basin were comprehensively investigated by high-resolution scanning electron microscopy (SEM) and image analysis, to further elucidate the influence of preservation parameters on organic matter (OM) and intraparticle pores (intraP) within OM. OM preserved in shale is divided into three types: organic residues, bitumen particles, and amorphous remains. Development of organic nanopores is controlled by factors such as preservation of OM, biological sources of organic residues, and thermal evolution. The original biological structure and hydrocarbon generation control the nanopore structure within organic residues. Thus, the intraP of organic residues have the highest facial porosity and fractal dimension among the pore types examined. Nanopores associated with bitumen particles vary greatly within different shales and provide considerable pore volume. Amorphous remains are mixed with inorganic matrix, indicating the presence of OM–mineral matrix compound, while nanopores are rarely seen within these remains. The development of organic nanopores in the Yanchang Fm. appears dominated by maturity, indicating that appropriate maturity is critical for the formation of effective pore spaces. With increasing maturity, there is less dissimilarity among bitumen intraP spaces. The Qiongzhusi Fm. possessed the highest facial porosity among organic residues, but the isolated fossils could not form an effective pore network. Among the shales investigated, the thoroughly interconnected pores of the carbonaceous graptolite and bitumen particles from the Lungmachi Fm. are most promising for gas accumulation.