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.

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Pore space of tight sandstone samples exhibits fractal characteristics. Nuclear magnetic resonance is an effective method for pore size characterization. This paper focuses on fractal characteristics of pore size from nuclear magnetic resonance (NMR) of tight sandstone samples. The relationship between the fractal dimension from NMR with pore structure and water saturation is parameterized by analyzing experimental data. Based on it, a pore structure characterization and classification method for water-saturated tight sandstone and a water saturation prediction method in a gas-bearing sandstone reservoir have been proposed. To verify the models, the fractal dimension from NMR of 19 tight sandstone samples selected from the gas-bearing tight sandstone reservoir of Shahejie Formation in Nanpu Sag and that of 16 of them under different water saturation states are analyzed. The application result of new methods in the gas-bearing tight sandstone reservoir of Shahejie Formation in Nanpu Sag shows consistency with experimental data. This paper has facilitated the development of the NMR application by providing a non-electrical logging idea in reservoir quality evaluation and water saturation prediction. It provides a valuable scientific resource for reservoir engineering and petrophysics of unconventional reservoir types, such as tight sandstone, low porosity, and low permeability sandstone, shale, and carbonate rock reservoirs.

Fracture detection and in‐situ stress determination via well logs are important for exploration and exploitation of subsurface hydrocarbons. Cores, thin sections, and image logs are used to describe and interpret the subsurface fractures and in‐situ stress in the Paleogene Dongying sandstones in Nanpu Sag, Bohai Bay Basin, China. The maximum horizontal stress (SHmax) indicates a nearly east‐west trend according to the borehole breakouts and drilling‐induced fractures. Natural fractures in the Dongying sandstones are classified into: (1) open fractures and (2) closed fractures. The cement‐filled (closed) fractures are commonly detected by image logs as bright discontinuous sinusoidal waves due to resistive filling materials, while the open fractures are evident on image logs appearing as dark sine waves. The open fractures are of dominantly high dip angles, and the rose diagrams confirm the presences of two sets of fractures: northwest‐southeast and northwest‐southeast orientation. Fractures with strikes approximately parallel to the SHmax have good connectedness, making a significant contribution in hydrocarbon production. In addition, the oil‐bearing layers are mainly associated with the intervals with open fractures. The presence of natural fractures provides important pore spaces and fluid flow conduits. Insights can be provided into the subsurface fracture (natural and induced) detection and characterization using image logs.