鄂尔多斯盆地陇东地区延长组低渗透砂岩油藏成藏机理与成藏模式

低渗透油藏在国内外分布十分广泛，逐渐成为未来石油勘探开发的重要接替领域。与常规油藏不同，低渗透砂岩油藏具有以下特征：(1)含油饱和度低，与储层物性关系复杂；(2)油水关系复杂，无明显的油水界面；(3)大面积广泛分布，受构造高低控制不明显；(4)距离源岩越远，石油富集程度越差。陇东地区延长组低渗透砂岩油藏的成藏机理研究表明：低渗透砂岩储层排替压力较大，浮力很难驱动石油发生明显的运移，石油在优质烃源岩生烃作用产生的剩余压力驱动下，向上、向下连续充注进入邻近砂体，近源聚集，形成原生油藏；裂缝发育条件下，已聚集石油可在浮力驱动下沿裂缝进行垂向、侧向运移调整，远源成藏，形成次生油藏。综合石油成藏期次、裂缝特征及形成时间、储层孔隙演化史以及成藏动力演化等特征，建立了陇东地区延长组油藏的成藏模式：早期低熟油小规模充注模式、中期成熟油大规模充注成藏模式、晚期构造抬升调整成藏模式。其主要控制因素为源储大面积广泛接触奠定了低渗透砂岩储层石油富集的基础，优质烃源岩生排烃范围控制了原生油藏分布范围，裂缝发育特征控制了次生油藏分布部位。

The accumulation mechanism and accumulation models of oil in low permeability reservoir of Yanchang Formation in Longdong Area, Ordos Basin

Low permeability oil reservoirs are widely distributed all over the world,which are important for petroleum exploration and development in the future.The characteristics of low permeability sandstone oil reservoirs are as follows: the oil saturation is low and its relationship with reservoir physical properties is complex;the oil-water relationship is very complicated with unobvious oil-water interface;the reservoirs are widely distributed without control of structures;and the farther from the sources,the poorer is the oil enrichment.The accumulation mechanism of low permeability reservoirs was analyzed through the study of accumulation process of Yanchang Formation reservoirs in Longdong Area.It is showed that the displacement pressure of low permeability reservoirs is high and the buoyancy force is very limited in low permeability sandstones without fractures,and hydrocarbons charge into adjacent sandstones forming primary reservoirs driven by the excess pressure from hydrocarbon generation of high-quality source rocks.When fractures develop,hydrocarbons migrate vertically and laterally driven by the buoyancy force and accumulate at the sandstones far away from the source rocks,forming secondary reservoirs.Three models of hydrocarbon accumulation were established based on the analysis of hydrocarbon accumulation periods,characteristics and forming time of fractures,pore evolution history and reservoir forming dynamics.They are low mature oil charge in a small scale at the early stage,mature oil charge and accumulation in a large scale at the middle stage,and adjusting accumulation of tectonic uplift at the late stage.Furthermore,the main controlling factors of low permeability oil reservoirs were expounded on the basis of the above studies.The source rocks are in close proximity to sandstone reservoirs,which lay the foundation of oil enrichment.The distribution of primary reservoirs is controlled by the generation and expulsion of high-quality source rocks.And the distribution of secondary reservoirs is controlled by the characteristics of fractures.