川西坳陷深层叠复连续型致密砂岩气藏成因及形成过程

川西坳陷深层上三叠统须家河组发育典型的致密砂岩气藏，由于该气藏多期构造演化、多期生排烃过程、多期成藏及储层致密化过程复杂的特点，其成因类型和成因机制经过多年的勘探和研究仍然存在争议。文章在剖析典型气藏的基础上，结合研究区构造演化、烃源岩生排烃史和储层致密化过程的研究，综合判识了致密气藏的成因类型，并分析其成因机制。研究结果表明：川西坳陷深层须家河组致密砂岩气藏具有“叠复连续资源潜力大、构造高点低点富气共存、高孔低孔储层含气共存、高压低压气层共存、气水分布复杂”的地质特征。目前气藏的成因类型为“后成”型致密气藏与“先成”型致密气藏的叠加复合型，属叠复连续型致密气藏。川西坳陷深层须家河组演化过程中存在浮力成藏下限和成藏底限，致密砂岩气藏形成和演化受控于2个界限控制下的3个流体动力场的分布和演化。叠复连续型致密砂岩气藏成藏过程可划分为4个阶段：①三叠纪沉积初至三叠纪末，初期天然气成藏条件准备阶段；②三叠纪末至中侏罗世末，早期常规气藏形成阶段；③中侏罗世末至晚侏罗世末，“先成”型致密气藏与“后成”型致密气藏形成阶段；④早白垩世至今，叠复连续型致密油气藏形成及改造阶段。

Genetic Mechanism and Formation of Superimposed Continuous Tight Sandstone Reservoir in Deep Xujiahe Formation in Western Sichuan Depression

Typical tight sandstone reservoir are well developed in the deep Triassic Xujiahe Formation in
western Sichuan depression. It experienced multi-stages of tectonic evolution, hydrocarbon generation and expulsion and reservoir formation,and also experienced complicated processes of reservoir densification,thus, its genetic types and formation mechanisms of gas reservoir are difficult to be recognized.The genetic types and its formation mechanisms have been discussed,through dissecting the typical gas reservoir,combining the tectonic evolution, history of hydrocarbon generation and expulsion and the process of sandstone reservoir densification.The comprehensive study determined the genetic type of tight sandstone reservoir and analyzed its formation mechanism.The results show that the tight sandstone reservoir in deep Triassic Xujiahe Formation in the western Sichuan depression has complex geological characteristics.It characterizes as both high structural and low structural enriched with gas,coexistence of high porosity and low porosity reservoir,coexistence of high pressure and low pressure reservoir,complicated gas-water distribution etc.The present gas reservoir is the recombination and superimposition of two types of reservoirs:pre-existing tight sand gas reservoir and subsequent ones.There are two limits to identify the formation of tight gases, the lower limit of buoyancy-controlled accumulation and the lower limit of hydrocarbon accumulation during the evolution of deep Xujiahe Formation. These two lower limits can divide the hydrocarbon-bearing basin into three kinds of fluid dynamic fields and control the formation and distribution of tight sandstone reservoir.Based on the history of hydrocarbon generation and expulsion,reservoir densification history,reservoir formation and the evolution of structures,the genetic model of the formation and distribution of superimposed continuous hydrocarbon reservoirs has been established, which can be divided into four stages.① From the Early Triassic to the end of Triassic,a stage of gas accumulation preparation;②From the end of the Late Triassic to the Middle Jurassic,an early stage of formation of conventional gas reservoir;③From the end of the Middle Jurassic to the end of Jurassic,a stage of formation of subsequent tight sand gas reservoir and pre-existing tight sand gas reservoir;④ From the end of the Late Jurassic to the Middle Cretaceous,a stage of formation of superimposed continuous gas reservoir.