Quantitative characterization of critical reservoir physical properties of tight oil charging in the third member of the Shahejie Formation in the Gaobei Slope of Nanpu Sag, Bohai Bay Basin

Abstract

Unconventional petroleum occupies an important position in the world energy structure. Tight oil is an important field of unconventional oil and gas exploration and development, and the physical property condition of a tight oil reservoir is an important determinant of its commercial production. The application of different geological and engineering strategies to quantitative characterization of the aspect of hydrocarbon charge vis-à-vis reservoir physical properties is critical and essential to thoroughly understand the formation and accumulation mechanism and help in achieving the optimum producibility of tight oil resources. This study focuses on the tight oil of the third member of the Shahejie Formation (Es3) region in the Gaobei Slope of the Nanpu Sag. The upper and lower physical limits of charging were quantitatively deduced and explained characterized by a combination of geological analysis, analytical testing, and numerical simulation of force balance; the formation mechanism of the tight oil reservoir is primarily constrained by its permeability. The inversion phenomenon of oil and water in the tight oil Es3 reservoir occurs at a burial depth of 3900 m. The pore type of the reservoir is mainly primary in nature, with a positive correlation between the porosity and the permeability. The upper limit of tight oil charging is quantitatively characterized by porosity approximately equal to 10%, the corresponding permeability being 1 × 10−3 μm2. When the reservoir's physical properties exceed the upper limit of tight oil charging, the reservoir mainly develops a water layer. This is mainly caused by the boundary effect of tight reservoirs, where the pore throat radius corresponding to the critical physical property is equivalent to the boundary layer thickness of oil in the reservoir pore, making it difficult for boundary fluids to flow in the pore space at this time. On the other hand, based on comprehensive analysis of the minimum flow hole roar radius, the irreducible water saturation, and drilling result statistical methods, the lower limit of the tight oil charging in Es3 reservoir is characterized quantitatively by the porosity of about 2%–4%, wherein the pore throat radius is approximately equal to 0.04 μm. The lower limit of tight oil charging varies from one place to another, mainly attributed to factors such as interfacial tension, hydrocarbon generation dynamics, kerogen type, and basin temperature. The charging process of tight oil complies with the principle of force balance, whereas the pressure exerted the generated hydrocarbon is counterbalanced by the capillary force of the pore space.