HOW DO THE DEPOSITIONAL MICROFACIES CONTROL THE PORE CHARACTERISTICS OF TIGHT RESERVOIRS? THE CASE OF THE UPPER TRIASSIC YANCHANG FORMATION IN THE SOUTHWESTERN ORDOS BASIN, CHINA

The tight reservoir of Upper Triassic Yanchang Formation in the Ordos Basin, China are used as a case study to reveal the controlling effect of depositional microfacies on pore characteristics including pore types, pore-size distribution and pore fractal characteristics, which has been achieved using a combination of petrographic, geophysical logging, FE-SEM and high-pressure mercury intrusion methods. The results identified six microfacies including distributary channels (DC), interdistributary bay (IB), mouth bar (MB), shallow lacustrine mudstones (SLM), semi-deep lacustrine mudstones (SDLM) and turbidite fan (TF). The reservoir pores were classified into three categories according to pore radius distribution: micropores (1.5 ~ 10 nm), mesopores (10 nm ~ 1 μm) and macropores (> 1 μm).

Characterized by low pore-throat sorting coefficient and broad skewness, DC and MB tight reservoirs abundant in rigid silicate grains is dominated by residual intergranular pores, microfractures and feldspar intragranular dissolved pores. The mesopores demonstrate a major contribution to the total pore volume of arenaceous microfacies (DC, MB and TF) which show better reservoir capacity. Displaying high pore-throat sorting coefficient and narrow skewness behaviors, the argillaceous microfacies (IB, SLM and SDLM) tight reservoirs rich in clay minerals are dominated by diagenetic shrinkage microfractures, clay intercrystalline pores and residual intergranular pores. The total pore volume is mainly provided by micropores. The calculations of pore fractal dimensions show that the macropore and micropore structure of arenaceous microfacies is more heterogenous than that of argillaceous microfacies. Whereas the mesoporous structure of arenaceous microfacies is more homogeneous than that of argillaceous microfacies. So the dominant reservoir spaces of different depositional microfacies are distributed over diverse pore radius ranges, and the pore structural heterogeneity gives rise to the different pore characteristics between various depositional microfacies. Mesoporous development of varying degrees is a crucial factor affecting the reservoir capacity of different microfacies, which is caused by the differences in depositional texture and mineral composition fundamentally.