论致密大油气田成藏模式

Hydrocarbon accumulation patterns of large tight oil and gas fields

根据对国内外致密油气聚集成藏特征的分析,提出致密大油气田存在3种成藏模式,即连续型(深盆气型)、准连续型和不连续型(常规圈闭型)。与连续型油气藏相似,准连续型油气聚集也表现为:油气分布面积较大,无明确边界,也无边底水;源、储邻近,广覆式分布;油气运移主要为非浮力驱动,运移动力主要为异常压力、扩散作用力和毛细管压力,浮力作用受限;运移的方式主要为非达西流,以涌流和扩散流为主。所不同的是:准连续型油气聚集由多个彼此相邻的中小型油气藏组成,油气呈准连续分布;油、气、水分布比较复杂,无显著油、气、水倒置;油气充注以大面积弥漫式垂向排驱为主,初次运移直接成藏或短距离二次运移成藏;储层先致密后成藏或边致密边成藏,且非均质性较强;圈闭对油气聚集成藏具有一定控制作用。研究认为,以深盆气或盆地中心气为代表的连续型油气藏与典型的不连续型常规圈闭油气藏,分别代表了复杂地质环境中致密油气藏形成序列中的两种端元类型,二者之间应存在不同的过渡类型。准连续型油气藏就是这样一种过渡类型的致密油气聚集,并且可能是致密储层中大油气田形成的主要方式。事实上,典型的连续型油气聚集应是那些形成于烃源岩内的油气聚集(如页岩气和煤层气),典型的不连续型油气聚集则是那些形成于烃源岩外近源-远源的常规储层中、受常规圈闭严格控制并且具有边底水的油气聚集;而形成于烃源岩外并且近源的致密油气藏则主要为准连续型油气聚集,其次为非典型的不连续型(常规圈闭型)油气聚集,而像盆地中心气或深盆气那样的连续型聚集则较为少见。

Three patterns of hydrocarbon accumulation in large tight oil /gas fields are proposed ,namely ,continuous accu-mulation （or basin-centered gas type or deep-basin gas type）,quasi-continuous accumulation,and discontinuous （con-ventional trap type ） accumulation .Like the continuous accumulation ,the quasi-continuous accumulation also features in large area,without clear boundaries and water lags ,reservoir being adjacent to source rocks and pervasive distribution .In addition,non-buoyancy-driven hydrocarbon migration is dominant .The major driving forcers of migration are overpres-sure,diffusion and capillary pressure ,while buoyancy is limited .The migration is largely in the form of non-Darcy surge flow and diffusion flow .However , the quasi-continuous accumulation is different from the continuous accumulation in many aspects.The quasi-continuous accumulation commonly consists of many adjacent medium to small oil /gas reservoirs and the hydrocarbons are in quasi-continuous distribution .Oil/gas and water contact is complex and no regional updip water present .Hydrocarbon charging is mainly vertical and pervasive expulsion .Hydrocarbon accumulation is characte-rized by direct accumulation via primary migration or short-distance secondary migration .The reservoirs get tight prior to or contemporaneous with the hydrocarbon charging and are highly heterogeneous .The traps have certain controlling effect on hydrocarbon accumulation .The continuous unconventional accumulation represented by the basin-centred gas or deep-basin gas and the typical discontinuous conventional accumulation are representative of two end -member types of tight hy-drocarbon accumulations in complicated geological settings .Hence some transitional varieties must exist between them and the quasi-continuous accumulation is just such a transitional type that might be a universal and predominated pattern＆nbsp;for large oil/gas accumulation to occur in tight formations .In reality,a typical continuous accumulation ought to be that occurs within a source rock interval like shale gas or coalbed methane ,while a typical discontinuous accumulation is that occurs within the conventional reservoirs outside or even far away from source rocks ,and it is not only strictly confined by conventional traps but also possesses bottom or edge water .The hydrocarbon accumulations in tight reservoirs out of but close to source rocks are predominantly quasi-continuous accumulation ,followed by atypical discontinuous accumulations . In contrast,continuous accumulations such as basin-centered gas or deep-basin gas are,if any,uncommon.