Critical conditions of tight oil charging and determination of the lower limits of petrophysical properties for effective tight reservoirs: A case study from the Fengcheng Formation in the Fengcheng area, Junggar Basin

Abstract

Studying the lower limits of petrophysical properties is important to evaluate the exploration and development potentials of tight oil reservoirs. At present, the evaluation criteria are based on a large number of cores and logging data, and rules are based on collections of empirical statistics and displacement pressure experiments. However, there have been few studies on the lower limits of the petrophysical properties of tight reservoirs based on force balance conditions for the hydrocarbon charging process. In this paper, the critical condition for hydrocarbon charging is determined using the oil expansion pressure as the main driving force, with the capillary pressure as the major resistance, via an analysis of the dynamic mechanism of tight oil reservoirs. A force-balance model for judging the lower limits of the petrophysical properties of tight reservoirs can be established by assuming that the oil expansion pressure is equal to the capillary force. This discriminant method is applied to calculate the critical pore-throat radius of the effective reservoirs of the Lower Permian Fengcheng Formation in the Fengcheng area, Junggar Basin, which are 0.310 μm, 0.248 μm, 0.208 μm, and 0.135 μm, when the buried depths are 2500–3000 m, 3000–3500 m, 3500–4000 m, and 4000–4500 m, respectively. The corresponding critical porosities are 6.15%, 4.84%, 3.81%, and 1.20%, respectively. The lower limits of the petrophysical properties of effective tight reservoirs gradually decrease with increasing burial depth. This method is of great significance for resource evaluations of tight oil reservoirs and the selection of exploration zones.