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

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.