Logging assessment of tight clastic rock reservoir fractures via the extraction of effective pore aspect ratios: A case study of lower Permian strata in the southern Qinshui Basin of eastern China

Research on tight gas enrichment in coal measure strata is contentious worldwide. Gas logging anomalies are very common in the upper Paleozoic coal-bearing strata in the Southern Qinshui Basin of eastern China. The distribution characteristics of these free gases are closely related to the degree of fracture development. The rock pore aspect ratio (α) can reflect the morphology of rock pores and fractures. In this paper, we use full wave logging data to propose a new logging assessment method for tight clastic rock reservoir fractures via the extraction of effective pore aspect ratios. The basic concept is as follows: (1) we extract the rock matrix mineral modulus and dry rock modulus of the rocks using an optimized algorithm, and (2) we invert the effective pore aspect ratio of coal measure strata using the theoretical model of a differential equivalent medium. The research results show that the studied clastic rocks in coal measure strata are very tight, with porosity levels that are generally less than 5%, and sections in which fractures have formed often correspond to relatively low α values of less than 0.1. The microfracture-bearing sections identified based on α inversions correspond well to the sections with fractures at the core scale. Thus, this method can be used to identify fractured sections at the core scale. For sandy mudstone, microfractures mainly exist in sandy intervals, and α is negatively related to rock porosity and positively related to shale content. Regardless of lithology, i.e., sandstone, mudstone or coal, the communication and transport capacity of fracture systems are greater than those of the pore system, and coal-derived gas reservoirs form when there are good gas sources, transportation systems, fracture systems and preservation conditions. Similar to the formation mechanism of fractures in rocks, α is also derived from mechanical mechanisms, and this study shows that the inversion process is effective in characterizing the degree of fracture development in tight clastic rocks in coal measure strata based on elastic rock properties.