Pore characteristic responses to categories of depositional microfacies of delta-lacustrine tight reservoirs in the Upper Triassic Yanchang Formation, Ordos Basin, NW China

Abstract

Primary depositional variations interact with diagenetic modifications to exert strong control on pore networks in tight hydrocarbon reservoirs. However, the linkage between pore characteristics and depositional microfacies remains enigmatic, which restricts the prediction of high-quality reservoir distributions. The continental tight reservoirs of the Yanchang Formation in the Ordos Basin are targeted to clarify and estimate the sensitivity of pore characteristics to depositional microfacies. A multi-methodical approach utilizing petrographic assessments, logging evaluations, digital imaging analyses and high-pressure mercury intrusion experiments enables us to reveal the high microheterogeneity of pore types, pore-size distribution and pore fractal features. The results identified the microfacies including distributary channels (DCs), mouth bars (MBs), interdistributary bays (IBs), sheet sands (SSs), turbidite fans (TFs), shallow lacustrine mudstones (SLMs) and semi-deep lacustrine mudstones (SDLMs) from the Chang 8 to Chang 6 members. Dominated by residual intergranular pores and microfractures, the main storage spaces of the DC and MB microfacies are within mesopores, which provide 62%–92% and 81% of the total pore volume, respectively. However, the micropores that are mainly composed of clay intercrystalline pores and feldspar dissolved pores provide 55%–70%, 52%–62% and 53%–80% of the total pore volume for the IB, SLM and SDLM microfacies, respectively. In addition, the pore fractal dimension results indicate that the mesoporous structure of the argillaceous microfacies (IB, SLM and SDLM) is more heterogeneous than that of the arenaceous microfacies (DC, MB and TF). Therefore, the dominant reservoir spaces of the different depositional microfacies are distributed over diverse pore-radius ranges, and the varying degrees of mesoporous development are a crucial factor affecting the reservoir capacity. Moreover, the fractal dimensions have a significant negative correlation with pore volume in the meso-micropore range. Characterized by lower pore sorting coefficients, broader skewness and more abundant rigid siliceous grains, the arenaceous microfacies with lower fractal dimensions and higher pore volume have better reservoir capacity than argillaceous microfacies that are rich in ductile minerals. Microscopic observations show that the muddier reservoirs of interdistributary bays and lacustrine mudstones are more susceptible to pore-filling chlorite and illite, which can deteriorate the reservoir quality. These results indicate that the controlling effect of depositional microfacies on pore characteristics is fundamentally caused by differences in mineralogical composition and depositional texture, and these results could provide new insights into the mechanisms of microfacies controls on reservoir quality.