断陷盆地斜坡带砂体古致密期与源岩排烃期耦合关系研究-以吐哈盆地为例

Tight sandstone gas is the most potential and the most realistic field of oil and gas exploration on land in our country, but tight sandstone gas in Faulted Basin has its own particularity, performance characteristics of reservoir structure large angle and continuous densification. which reservoir-forming conditions and distribution characteristics are differed distinctively from cratonic basins and sag basins like Albert , Ordos and Sichuan Basin.The existing studies are lack of formation mechanism and pattern of reservoir.The projects selected Tuha Basin Shuixigou Group tight sandstone as target area, and density sampled in reservoir and source rock of slope zone, on the basis of experimental analysis about reservoir porosity and permeability, cast thin section, scanning electron microscopy, controlled mercury, X-diffraction, particle size analysis, isotopic dating, the determination of the temperature of the fluid inclusions, compared with petrological characteristics of the sand body of slope zone and established refined diagenetic sequence, according to the analysis of tight reservoir genesis, determined reservoir dense period with experiment proper method, and quantitatively recovered porosity evolution. On the basis of conventional geochemical analysis and hydrocarbon thermal simulation of hydrocarbon source rocks, we quantitatively evaluated gas generation and expulsion process from the point of view of the chemical kinetics, combined with burial history and thermal history, and accurately evaluated the history of expulsion and filling of source rocks of slope zone. Combination of dense process of reservoir porosity evolution and the history of hydrocarbon expulsion evolution, we divided the type of tight sandstone gas reservoirs, and then established rift basin slope belt tight sandstone gas reservoir forming mechanism and distribution mode.

致密砂岩气是我国陆上最有潜力、最现实的油气勘探领域，而断陷盆地致密砂岩气成藏具有特殊性，表现为储层构造倾角大及连续致密化等特点，其成藏条件与阿尔伯达、鄂尔多斯、四川等克拉通及坳陷型盆地存在较大差异，成藏机理及模式缺乏针对性研究。本项目选取吐哈盆地水西沟群致密砂岩为靶区，对斜坡带储层及源岩密集取样，在储层孔渗、铸体薄片、扫描电镜、恒速压汞、X衍射、粒度分析、同位素定年、流体包裹体均一温度测定等实验分析基础上，对比斜坡带砂体岩石学特征并建立精细成岩序列，针对储层致密成因,选择相应的实验分析手段确定古致密期，并定量恢复孔隙演化。在烃源岩常规地化分析及生烃热模拟基础上，从化学动力学的角度对天然气的生排烃过程进行定量评价，结合埋藏史、热史，准确评价斜坡带源岩排烃史。结合储层孔隙演化致密进程及排烃演化史，划分斜坡带致密砂岩气藏类型，并建立断陷盆地斜坡带致密砂岩气藏成藏机理及分布模式。

课题以吐哈盆地水西沟群致密砂岩为靶区，采集大量的烃源岩样品和致密储层样品，开展了有机碳、热解、生烃动力学模拟（Rock-Eval、PY-GC）、储层物性、扫描电镜、恒速压汞、X衍射等测试，在此基础上分析了源岩及储层的有效性，利用二者的匹配关系划分了斜坡带砂体的气藏类型。代表性研究成果为：（1）西山窑组二段是重要的有效烃源岩发育层系，煤系烃源岩全区分布稳定、厚度大，是主力烃源岩；（2）炭质泥岩的成烃转化率最高，其有效源岩的下限标准为IH=150mg/g，TOC最小=6.0%；（3）水西沟群储层已普遍致密化，储集空间以溶蚀孔隙为主，还有各类晶间孔、微裂缝等微米—纳米级储集空间类型；储层正处于早成岩B期到晚成岩A或B期；（4）首次从致密油气的赋存和流动能力角度出发，由孔隙度、渗透率、含油气饱和度构建储能评价参数，实现了从地质意义上对致密储层分级评价；（5）首次利用水膜厚度理论确定了靶区致密储层的物性下限，并利用水膜厚度与孔隙度、束缚水饱和度等参数之间的关系，并把由临界喉道半径转化为孔隙度；（6）斜坡带不同构造位置砂体的致密时间差别较大，面向凹陷的砂岩致密时间早，沿斜坡上倾方向储层致密时间依次变晚。斜坡下倾方向的砂岩致密时间早于油气充注时间，形成“先致密后成藏”油气藏。先致密的砂体在剖面上好比“致密墙”，充分接受下倾方向主力源岩的供烃，并以“活塞式”缓慢向上排替孔隙水，形成气水倒置的致密砂岩气藏；由于纵向上储层致密的时间是一个连续的过程，中间必然存在一个致密时间与油气充注时间大致相当的过渡区域，在这个区域形成“边成藏边致密”油气藏；当烃源岩供烃充足，油气必然突破这个过渡区域，在当时为常规储层的构造高部位聚集成藏，形成“先成藏后致密”；（7）储层致密化与烃源岩大量生排烃期的时空匹配关系是不同类型致密砂岩油气藏形成的关键，该方法可推广到其他非常规油气资源的类型划分。