川中古隆起北斜坡震旦系岩性气藏保存有效性分析

斜坡部位发育的岩性圈闭能否长期有效地保存油气，成为影响四川盆地川中古隆起北斜坡地区下一步天然气勘探目标优选和油气资源规模评价的关键因素。将基于背斜气藏剩余压力与顶部盖层排替压力建立的气藏保存能力评价方法，引入到碳酸盐岩斜坡区的岩性气藏研究，以蓬探1井灯二段气藏为解剖对象，结合地质背景分析与类比、单井资料分析与模拟，论证了川中古隆起北斜坡震旦系岩性圈闭在古油藏形成与裂解、气藏形成与调整等关键时期的油气保存能力。结果表明：古油藏形成后，灯二段储层的剩余压力逐渐增加但远低于顶部筇竹寺组页岩和侧向灯二段泥晶云岩的排替压力，保存条件较好。走滑性质断层的继承性活动，可能使北斜坡地区呈现纵向多层系古油藏分布的特征。裂解气藏形成阶段，气藏剩余压力升高但侧向围岩的封堵始终有效。天然气仅在顶部筇竹寺组盖层和不整合界面处发生部分逸散，剩余的天然气有效保存至今。由于北斜坡地区灯影组气藏侧向的封堵始终有效，其保存条件与高—磨地区灯影组的构造—地层气藏较为相似。基于与高—磨地区灯影组气藏保存条件和资源规模的类比，得益于北斜坡地区厚度更大且质量更高的筇竹寺组泥页岩，灯二段气藏理应具备更优的顶部保存能力，天然气勘探潜力巨大。

Analysis on preservation effectiveness of lithologic gas reservoirs in north slope of central Sichuan paleo-uplift: Case study of the second member of the Dengying Formation in Well Pengtan-1

The ability of lithologic traps to maintain long-term hydrocarbon preservation in the north slope of the central Sichuan paleo-uplift has a crucial effect on the selection of natural gas exploration targets and the scale evaluation of petroleum resources. In order to determine their capacity, the traditional relationship between the displacement pressure of mudstone surrounding rock and the residual pressure of gas reservoir was introduced into the carbonate lithologic trap of the slope area with lateral inclination angle. The results of this geological analysis, analogy, and simulation in the gas reservoir of the second member of the Dengying Formation （Z₂dn²） in Well Pengtan-1 proved the preservation ability of the lithologic traps. The results show that after the formation of paleo-reservoir, the residual pressure of the reservoir gradually increased, but it was far less than the displacement pressure in the vertical and lateral surrounding rocks. Therefore, the preservation conditions were good. The inherited activities of strike-slip faults may lead to the distribution of the multi-layer paleo-oil reservoirs. During the formation of the cracked gas reservoir, though the residual pressure of the gas reservoir increased rapidly, the lateral sealing remained effective. As a result, natural gas only escaped from the upper Qiongzhusi Formation （?₁q） and unconformity interface, and most of the remaining natural gas was effectively preserved up to now. Because the lateral sealing of the Z₂dn² gas reservoir in north slope area was consistently effective, its preservation effects were similar to those in the structure-stratigraphic gas reservoir in the Gaoshiti-Moxi area. Still, compared to the reservoir preservation conditions and resource scale in the Gaoshiti-Moxi area, the Z₂dn² gas reservoir in north slope area should have better top preservation abilities and greater potential for natural gas exploration owing to the greater thickness and higher quality of the ?₁q shale.