Effect of Pore Structure on Reservoir Quality and Oiliness in Paleogene Dongying Formation Sandstones in Nanpu Sag, Bohai Bay Basin, Eastern Chinae

Routine core analysis, thin sections under transited and ultraviolet light, scanning electron microscope (SEM) analysis, and nuclear magnetic resonance (NMR) measurements were used to investigate the pore structure of the Paleogene Dongying Formation sandstones in the Nanpu Sag, Bohai Bay Basin. Regression analysis was performed to build up the relationships between microscopic pore structures and macroscopic behaviors. The pore systems mainly include primary intergranular pores, intergranular and intragranular dissolution pores, and clay-dominated micropores. Most of the intergranular pore bodies are connected by effective necking and sheet like pore throats, while the intragranular pores and micropores are connected by bending-flake and narrow cluster pore throat. Unimodal, bimodal, or multimodal T-2 (transverse relaxation time) distributions can be observed due to the wide ranges of pore bodies and throats in Dongying sandstones. Consequently, wide variations of NMR parameters are encountered, and high-quality reservoirs contain abundant intergranular pores connected by large pore throats, and therefore have high T-2gm, (the geometric mean of the T-2 distribution) but low BVI (bulk volume of immovable fluid) values. The intragranular dissolution pores and clay-dominated micropores display strong fluorescences, while only the edge of the intergranular pores emits weak fluorescence, and no fluorescence was identified in the intergranular pores. The sandstones generally lack fluorescence when they are tightly compacted or cemented by carbonate cements. NMR T-2 distributions also reveal that the main oil-bearing pore size distribution is in the small pore realm (1-100 ms), which is in good agreement with fluorescent thin sections. The results help improve the knowledge of the effects of pore structure on reservoir quality and oil-bearing property in sandstones, and could provide insights into enhancing oil recovery.