Quantitative characterization and formation mechanism of the pore system heterogeneity: Examples from organic-rich laminated and organic-poor layered shales of the upper triassic chang 7 member in the southern Ordos Basin, China

Abstract

The heterogeneity of shale pore structure is a vital problem that cannot be ignored in the process of shale oil exploration and development. The organic-rich laminated shale (ORLS) and organic-poor layered shale (OPLS) show significantly different pore system heterogeneity. In this study area, the porosity and permeability of ORLS (2.7% and 0.00053mD) are higher than those of OPLS (2.29% and 0.00041mD). The heterogeneity of the pore system in ORLS is reflected by the wider pore size range of intercrystalline pores (pore size of 0.002–5.5 μm), the stronger holistic distribution heterogeneity of intergranular pores (average △α of 1.61) and organic pores (average △f of 2.65) at local locations. The wider range and discontinuity of the pore size distribution for intergranular pores and organic pores, and the instability of the morphological structure for intercrystalline pores and microfractures, all lead to the heterogeneity of the pore system in OPLS. The micropore (<2 nm) volume and macropore (>50 nm) volume of ORLS are higher than that of OPLS, while the surface area and volume of mesopores (2–50 nm) are lower than those of OPLS. The macropore has the most obvious fractal characteristics and three fractal dimensions. The macropore volume of ORLS is positively correlated with reservoir physical and pore multifractal properties, while the mesopore volume of OPLS is positively correlated with reservoir properties and negatively correlated with fractal properties. Pore system evolution patterns of shale reservoirs are obviously controlled by the mineral composition, laminae, TOC content, thermal evolution degree and fluid pathways. The macropore volume in ORLS shows a clear positive correlation with the pyrite content. The high TOC content and high thermal evolution degree are more likely to produce erosive fluids. Laminar interfaces, microfractures and primary pores provide channels for the fluid migration. The development degree of secondary dissolution may be an important factor leading to the difference of reservoir qualities between ORLS and OPLS.