Characteristics of Nano-Micro Pore Networks and Petroleum Microscopic Occurrence State in Ultra-Low Permeability (Tight) Sandstone Reservoir

Tight reservoirs have unique petroleum accumulation and distribution characteristics due to developed nano- micro pore networks, thus the characterization of nano-micro pore networks (NMPN) has been brought into attention in current studies. Combining micro-CT, casting thin section and scanning electron microscopy techniques, a systematic procedure is proposed to study the characteristics of NMPN and microscopic petroleum occurrence state in tight sandstone reservoirs of Chang 6-7 members of Yanchang formation, Heshui area, Ordos basin. Analysis indicates that pores and throats are continuously distributed in nanometer to micrometer scale, and three main pore types and two major classes of throats in size have been identified. Pores of different types vary in genesis, morphology, and connectivity. Generally, pores and throats in the same spectrum are well connected due to genesis, therefore small-pore with medium-throat and micro-nano pore with fine-throat become two primary combinations in NMPN. Heterogeneity exists in the composition of pore networks developed in different sedimentary micro-facies of same member attributed to depositional environment and diagenesis: the small pore with medium throat combination occupies the main body of pore networks in reservoirs of braided channel in turbidite fan, and reservoirs in the front fan gets a major pore configuration in which nano-micro pores matches with fine throat. Influenced by pore structures and genesis, the microscopic petroleum occurrence states fall into three categories: stripped intergranular, thin film intergranular, and spherical intra-granular forms. The characteristics of pore networks have huge impacts on oil accumulation and distribution.