关键词: 泥岩盖层, 脆性, 塑性, 盖层完整性, 构造抬升, 盖层泄漏, 震旦系气藏, 四川盆地

Abstract: Due to the high diagenesis degree and strong brittleness, the deep tight mudstone caprock are prone to suffer from brittle fracture and leakage. Therefore, the evaluation of caprock integrity is very important. The deformation characteristics of mudstone caprocks are mainly controlled by mudstone density and confining pressure. As the buried depth increases, mudstone will undergo a complex transition from plasticity to brittleness and then to plasticity, but brittle deformation is dominant within the buried depth of sedimentary basins. Due to the increased brittleness during the uplift process, brittle fracture is more likely to occur to the deep mudstone caprock. Under the same uplifting geological setting, shallow mudstone caprocks will rupture more preferentially than deep mudstone caprocks. For the same set of mudstone caprocks, the mudstone caprocks in the area with large uplifting amount will have the priority to fracture. Comprehensively considering the brittleness of mudstone and the structural strain experienced by the mudstone caprock, this paper establishes a method for evaluating the integrity of mudstone caprock, and this method is used to study the mudstone caprock of Cambrian Qiongzhusi Formation in the Sinian gas reservoirs of the central Sichuan Basin. The results show that the Weiyuan structure had large uplifting amount and strain during the Himalayan epoch. The mudstone caprock fracture of Cambrian Qiongzhusi Formation results in a leakage window in Weiyuan structure. As a result, the Weiyuan gas field is now a residual gas reservoir, with the trap filling degree of only 25%. The Anyue gas field had stable structure, small strain, and large burial depth in the later period. Although the mudstone caprock of Cambrian Qiongzhusi Formation remains intact in the Anyue gas field, gas can migrate from the spill point of a trap to the leakage window of the Weiyuan structure along the Sinian-Cambrian unconformity, so lateral leakage occurs. Thus, the paleo-overpressure gas reservoir of Sinian Dengying Formation in the Anyue gas field has been transformed into a normal pressure gas reservoir. In the area where the burial depth of Cambrian Qiongzhusi Formation mudstone caprock is more than 4 km and the faults are not developed in the hinterland of Sichuan Basin, the gas reservoir of Sinian Dengying Formation has good preservation conditions and great exploration potential. This method is suitable for evaluating the integrity of mudstone caprock in anticlinal traps with underdeveloped faults, and can be popularized and applied to the evaluation and optimization of deep exploration targets.

Key words: mudstone caprock, brittleness, plasticity, caprock integrity, tectonic uplift, caprock leakage, Sinian gas reservoirs, Sichuan Basin