Evaluation of lacustrine mudstone top sealing capacity in the Lenghu-5 area, Qaidam Basin, Tibetan Plateau

Abstract

Top seal capacity is one of the fundamental geological factors that must be considered during petroleum exploration. Based on thin section, low-pressure N2 adsorption, bulk mineralogy, X-ray diffraction (XRD), mercury injection experiment and pressure data, the sealing capacity, lithofacies, petrophysics, diagenesis, capillary breakthrough pressure, overpressure, and brittleness index characteristics of a lacustrine mudstone top seal from the Lenghu-5 area, Qaidam Basin, Tibetan Plateau was investigated. The bulk rock composition of the top seal was dominated by clay minerals with carbonate cements and has a gross mudstone to sandy mudstone lithofacies. The isotherms from low-pressure nitrogen adsorption tests which presents as reversed S-shaped, belong to type B and the characteristics of hysteresis loops indicating the silt-like pores in the samples. The pores of the mudstone top seals are predominantly mesopores according to the N2 adsorption and mercury injection experiment results. The cumulative mudstone thickness varies from 100 to 1400 m. The overpressured mudstone top seal in the Lenghu-5 area has three potential hydrocarbon preservation implications: overpressure cannot seal oil and gas, overpressure seals oil and gas directly, and overpressure seals oil and gas indirectly. The brittleness index (BI) values calculated by four methods based on the mineralogical composition of the mudstone top seals and the BI is higher at the top and bottom of the studied interval than in the middle mudstone layer. Based on the mud content, pore diameter, thickness, diagenetic stage, capillary breakthrough pressure, overpressure coefficient and brittleness index of the lacustrine mudstone, the top seal in the Lenghu-5 area is classified into four types: type I, II, III and IV, representing good to poor sealing capacities. The method for evaluation of the sealing capacity in this study can not only provide useful guidance for hydrocarbon exploration in Lenghu-5 area, Qaidam Basin, but also can be used in basins around the world by selecting the main influencing factors of sealing capacity in the basin.