An investigation into pore structure and petrophysical property in tight sandstones: A case of the Yanchang Formation in the southern Ordos Basin, China

Abstract

Abstract The complex and heterogeneous pore structures, caused by complex diagenesis and sediment properties (such as rock compositions and grain size), play a critical role in macroscopic evaluating tight sandstone reservoir performance. In order to gain insight into the genetic mechanism of the pore structure and evaluate reservoir quality in tight sandstone reservoirs, this study investigated the impacts of diagenesis and sediment properties on pore structures and the relationship between microscopic pore structure parameters and macroscopic petrophysical performance, based on the analysis of pore structure features (including the morphology, pore throat types, sizes, combinations, distribution of combinations and pore connectivity), sedimentary and diagenetic features, and petrophysical properties. Samples from tight sandstones of the Yanchang Formation in southern Ordos Basin (in China) were evaluated by integrating scanning electron microscopy (SEM), thin sections, mercury injection porosimetry (MIP), nuclear magnetic resonance (NMR) and gas (N2, He) porosity and permeability measurements. Three pore-throat combinations were identified which represent the full-range distribution of pores: MP (micropores-dominanted pore-throat combination), IAP (intragranular pores-dominanted pore-throat combination) and IEP (intergranular pores-dominanted pore-throat combinations). The type and intensity of diagenesis and sediment properties (grain size and ductile component content) were found to result in differences in pore throat size and the distribution of pore throat combinations. Sandstones with coarser grain-size and lower concentrations of ductile components have experienced weak compaction, thick chlorite rim and minor kaolinite/carbonate cementation, were found to retain large IEP pore space and wide strip shaped throat, formed the intergranular pore - throat (Inter-T) and intergranular pore - intragranular pore - throat (Inter-Intra-T). The Intra-Intst-T (intragranular pore – group of interstitial cement pores - throat) sandstones were characterized by intense dissolution and kaolinite cementation which resulted in small IEP pore space and large MP/IAP pore space. The Intst-T (group of interstitial cement pores - throat) sandstones were characterized by intense compaction and kaolinite/carbonate cementation. Compaction and cements destroyed IEP pore space and formed MP pore space. Effectively connected pore space (volume of IEP) and throat radius were positively correlated to porosity and permeability. Four pore structures are distinguished according to the proportion of effectively connected pore space and the radii of strip shaped throats, the macroscopic behaviors and the geological genesis of the four pore structures are investigated.