沉积盆地异常低压与低压油气藏成藏机理综述

SUMMARIZATION OF SUBNORMAL PRESSURES AND ACCUMULATION MECHANISMS OF SUBNORMALLY PRESSURED PETROLEUM RESERVOIRS

地下异常低压主要有两种成因:抬升—剥蚀反弹和在介质孔隙度、渗透率非均质性条件下的区域地下水稳态流动,而化学渗透与流体"冷却"在低压形成中只起次要作用。根据圈闭类型、储盖组合及成藏过程,将低压油气藏分为三种类型:1常规地层型(除砂岩透镜体外)低压油气藏,低渗透岩石通常起遮挡作用,底水与边水不发育;2砂岩透镜体低压油气藏,通常分布于盆地中心的深部,具有不含水、充满油气的特点,油气的充注和水的排出与构造抬升之前压实作用、超压引起的水驱裂缝和毛细管力的作用有关,抬升—剥蚀引起的异常低压导致水由砂岩向页岩的流动有助于油气藏中水的排出;3深盆区低渗透储层低压气藏,通常分布在含水层的下倾方向(气水倒置),异常低压是由于构造抬升致使超压向低压演化的结果。实例研究表明,构造抬升盆地中的低压系统是一个水动力相对封闭的体系,有利于油气的聚集与保存。

The synthesizing of a great number of references on origin of subnormal pressure and underpressured pools demonstraites that two mechanisms are predominantly responsible for subsurface underpressure. The first, probably the most important one, ascribes to erosional rebound; and the second is the effect of hydrostratigraphy and topography. Other causes, such as chemical osmosis and cooling of fluid, seem to play a minor role in underpressure development. Three typical underpressured pools are classified in terms of trap geometry, accumulating mechanism associated with subnormal pressure development. The first type of conventional stratigraphic pools (except lenticular sandstone body) is sealed by low permeability rocks, with little tendency for downdip and edge water to advance, as well as with petroleum charged frequently from downdip source rocks. The second type of isolated lenticular sandstone body reservoirs,abundant in the deepcentral parts of many basins, is characterized with the full presence of oil and gas and absence of water. Several processes, including compacting, hydrofracturing and capillary phenomena prior to uplifterosion, as well as subsequent inwardshale flow driven by erosional rebound, will be able to explain petroleum charged into and water expelled out of isolated porous sandstone bodies. The third type of gas accumulations in lowpermeability reservoirs commonly is located in a basincenter and is downdip from waterbearing rocks. Underpressured gas accumulations evolve from overpressured accumulations due to regional or local structural uplift. Two casestudies, southwestern Alberta Basin and Baise Basin (southwestern China),both experiencing regional structural uplifterosion, indicate that hydrodynamicallyclosed underpressured systems facilitate petroleum accumulation and preservation.