Upper Limits of the Physical Properties and Classification of Tight Conglomerate Reservoir of Baikouquan Formation in Mahu Sag

The conglomerate reservoir of Baikouquan Formation, located in Mahu depression, Junggar Basin, Northwest China, is of low porosity and permeability and varies in formation properties, oil content and oil production. An effective reservoir classification method is lacked to guide formation evaluation, well placement and development parameters optimization Based on the dynamic mechanical balance principle of tight oil charging, the upper limit of physical property of tight conglomerate reservoir is determined. Futher based on the pore-throat distribution and capillary pressure curve obtained from high pressure mercury injection test, the classification and evaluation criteria for reservoir physical properties were established, and the development degree of different grades of reservoirs in different blocks was compared. The results show that the upper throat radius limit of the tight conglomerate reservoir is 3.7 μm in this area, and the corresponding overburden permeability is 0.6 mD. The densified samples can be further divided into five grades, from Grade I to Grade V, where the throat size of the sample decreases, the starting pressure increases, the corresponding mercury intake saturation decreases, and the quality deteriorates gradually. The mercury intake curves of Grade I to Grade IV samples all showed the characteristics of platform or weak platform, and the mercury intake saturation generally exceeded 50%, which could be classified as reservoir. The pore throat radius of Class V samples is less than 1.0 μm, and the mercury intake saturation is less than 40%, showing an upward convex mercury intake curve, which is classified as non-reservoir. The porosity and permeability of different grades of samples reveal that the permeability limits are 0.2 mD、0.1 mD、0.03 mD and 0.006 mD, respectively, and the porosity has no clear limits, which lower limits are 9%, 8%, 7% and 5%, respectively. There are significant differences in the development degree of samples of different quality in Baikouquan Formation in the five blocks of the study area. As a whole, the M18 and FN4 well block are the best, followed by the AH2 and M131 well block, and the M2 well block is relatively poor.