RESUMEN
Although abundant unconventional oil resources have been discovered in conglomerate and sandstone reservoirs in rift basins, the mechanism of differential pore evolution in conglomerates and sandstone reservoirs within different secondary structural zones of rift basins is not yet clear. The pore structures of conglomerate and sandstone reservoirs in the distinct secondary structural zones in the Chezhen Sag were quantified in three dimensions using high-resolution microcomputed tomography (micro-CT). Thin section and scanning electron microscopy observations were used to investigate the differential evolution mechanisms of conglomerate and sandstone reservoirs. Micro-CT analysis of the pore structures of conglomerate and sandstone reservoirs revealed that sandstone reservoirs are superior to conglomerate reservoirs with regard to the pore number and pore connectivity and that sandstone reservoirs are more heterogeneous than conglomerate reservoirs. Triangles dominate the pore and pore throat geometries of sandstone and conglomerate reservoirs, while the sandstone reservoir pores are more regular than conglomerate reservoir pores. The depositional environment, mineral composition, and diagenetic intensity jointly control the quality of the reservoirs. Because of the lengthy transportation distance of their parent rocks, the compositional maturity and sorting behavior of sandstone reservoirs in depression and gentle slope zones are better than those of conglomerate reservoirs in steep slope zones, and thus sandstone reservoirs have a higher initial porosity than conglomerate reservoirs. The rapid compaction experienced by the conglomerate reservoirs in steep slope zones in their early stages creates a closed diagenetic environment, making it difficult to effectively improve reservoir porosity through dissolution. However, the widely developed microfractures in the reservoirs provide channels for fluid migration, promote the development of dissolution pores, and form a tight reservoir dominated by secondary pores. With weak compaction and an open diagenetic environment, the primary pores in sandstone reservoirs in the gentle slope zone are preserved in large quantities. Meanwhile, dissolution expands the secondary pores of the reservoir, resulting in a high-quality reservoir having both primary and secondary pores. In addition, an approach based on primary, secondary, and total porosity was proposed in the study to efficiently evaluate reservoir quality and identify reservoir evolution mechanisms.
RESUMEN
The fracture distribution and internal control factors after the fracturing of unconventional oil and gas reservoirs determine the reservoir reforming effect to a large extent. Based on the research of global scholars on the influencing factors of fracture propagation, comprehensive theoretical model, and numerical simulation, this Review systematically discusses the influence of internal geological factors and external engineering factors of unconventional oil and gas reservoir on fracture propagation behavior and summarizes the current problems and development trends in fracture research. The results show the following: (1) The fracture propagation is a comprehensive process constrained by lithology and mineral composition, water saturation, nonhomogeneity, natural weak surface, and ground stress. (2) External engineering factors have a meaningful control effect on fracture propagation; the type and temperature of fracturing fluids can also change the mechanical properties of different rocks, thus affecting the fracture propagation pattern. (3) The existing fracture propagation models have certain limitations, and their computational reliability still needs to be further verified. (4) Numerical simulation can break through the limitations of physical simulation, but different simulation methods have different shortcomings and applicability. In the future, we should focus on: (1) finding parameters to quantitatively characterize heterogeneity at the 3D level, which is an important direction to study the effect of heterogeneity on fracture propagation; (2) introducing computerized methods to establish a geological model that considers multiple factors and combining it with numerical simulation software to study fracture propagation; (3) considering the characteristics of fluid-liquid-solid phase comprehensively, establishing a suitable THL coupling equation; (4) how the interaction mode of fracturing fracture is combined with the natural fracture geometry, and how the fracture is affected by fracturing engineering parameters such as fluid injection rate and viscosity of fracturing fluid; and (5) geology-engineering dynamic integration, which is an important direction to be carried out in the future.
