Controlling effect of pore-throat structures on tight oil accumulation effectiveness in the upper Cretaceous Qingshankou formation, Songliao Basin

Abstract

The Gaotaizi reservoir in the Qingshankou Formation, northern Songliao Basin is one of the most potential tight oil targets in China and it is underlain directly by the OM-rich shales of the Qingshankou Formation. Previous studies indicated a strong oil-bearing heterogeneity in the Gaotaizi reservoir, while the relationship between pore-throat system and oil-bearing property is poorly understood. Thin section, Soxhlet extraction, MICP, fluid inclusion PVTX simulation and basin modelling were combined to investigate the pore-throat structure, and its effect on oil-bearing abundance and the tight oil accumulation effectiveness. Three pore-throat structures in the Gaotaizi reservoir were identified. The pore-throat size distribution (PSD), pore-throat configuration, and the corresponding physical properties became worser from Type I to Type III, as well as the pore types. A close relationship between pore-throat structures and sedimentary facies was established. Type I and II were dominated in mouth bar sand, while Type II, and Type III were mainly distributed in distal sandbars and sheet sand, respectively. The fluid properties and oil-bearing grades in the Gaotaizi tight sandstones were significantly controlled by the pore-throat structures. There were few fluids produced from the tight sandstones with Type III pore-throat structures, whose pore-throat size was almost half of that in tight sandstones with oil production. The dynamic model and ratio of pore containing oil (RPO) parameter were proposed. The residual pressure difference between the source rock and reservoir was 4.1 MPa, and the corresponding lower limit of pore-throat radius for tight oil accumulation was around 180 nm considering the limited buoyancy. Under the same driving force, the RPO of the Type I, II, III pore-throat structure reservoir was 74.4%, 58.0%, 23.3%, respectively, caused by pore-throat structure differences. It shows that an optimal tight oil “sweet spot” formed in the Type I reservoir, and favourable tight oil zones typically developed in the Type II reservoir, whereas only poor tight oil or dry zones developed in the Type III reservoir. This research improves the understanding of continental tight oil enrichment mechanism and provides a reference for optimizing favourable area of tight oil exploration.