深层背斜圈闭中泥岩盖层完整性评价方法及其应用——以四川盆地川中地区震旦系气藏为例

An evaluation method for the integrity of mudstone caprock in deep anticlinal traps and its application:a case study of the Sinian gas reservoirs in the central Sichuan Basin

深层致密泥岩盖层由于成岩程度高、脆性强,易发生脆性破裂并造成盖层泄漏,因此,盖层的完整性评价十分关键。泥岩盖层的变形特征主要受泥岩密度和围压控制,随着埋深增大,泥岩会经历塑性—脆性—塑性的复杂转变,但在沉积盆地埋深范围内总体以发育脆性变形为主。深层泥岩盖层在抬升过程中由于脆性增强,更易发生脆性破裂。在同样的抬升背景下,浅层泥岩盖层比深层泥岩盖层会优先破裂;对于同一套泥岩盖层,抬升量大的地区,其泥岩盖层会优先破裂。综合考虑泥岩的脆性程度以及泥岩盖层所经受的构造应变大小,建立了泥岩盖层完整性评价方法,利用该方法对四川盆地川中地区震旦系气藏中的寒武系筇竹寺组泥岩盖层进行了实例研究。结果表明,威远构造在喜马拉雅期的抬升量和应变大,由于寒武系筇竹寺组泥岩盖层发生破裂形成泄漏窗口,导致威远气田现今为残留气藏,其圈闭充满度仅为25%;安岳气田晚期构造稳定、应变小、埋深大,尽管其寒武系筇竹寺组泥岩盖层保持完整,但天然气可从圈闭溢出点沿着震旦系—寒武系不整合面向威远构造的泄漏窗口发生侧向泄漏,造成安岳气田震旦系灯影组的古超压气藏在现今转变为常压气藏。在四川盆地腹部寒武系筇竹寺组泥岩盖层埋深大于4km且断裂不发育的地区,震旦系灯影组天然气藏的保存条件好,勘探潜力大。该方法适用于对断裂不发育的背斜圈闭进行泥岩盖层完整性评价,可推广应用到深层勘探目标的评价和优选中。

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.