鄂尔多斯湖盆西北部延长组下部石油成藏机理及主控因素

Oil Accumulation Mechanism and Controlling Factors of the Lower Part of Yanchang Formation of Northwest Ordos Basin

长7优质烃源岩在大量生烃时产生较强的异常高压，研究区长7与下部长8地层之间存在10MPa以上的压力差，过剩压力差成为石油向下运移的主要动力。超压导致烃源岩及其下部地层发生流体压裂，在岩心、显微镜下和成像测井中发现了大量的流体压裂缝，流体以混相通过裂隙快速排出。在长7烃源岩和长8、长9储层中发现了沸腾油气包裹体，是油气幕式排烃、幕式充注的直接证据。包裹体均一温度表明，研究区主要发生了两次大规模的幕式充注过程，主要成藏时期为旱白垩世早期-早白垩世中期。鄂尔多斯湖盆西北部延长组下部油藏形成及分布主要受“近源、低压、优相”三大因素控制；主要发育4种成藏类型：储层与烃源岩直接接触成藏、通过叠置渗砂体运聚成藏、通过流体压裂缝运聚成藏、石油通过叠置渗砂体和流体压裂缝运聚成藏。

When a great deal of hydrocarbon was generated from the high-quality source rocks of the Cbang7 reservoirs, a strong abnormal high pressure would be produced. There were more than 10 MPa pressure differences between Chang7 and lower part of Chang8 formations of the study area. Excess pressure differences became a major driving power for downward migration of the oil. The ovetpressure caused the fluid fracturing happened in the source rock and its lower strata. A great deal of fractures by fluid fracturing was found in the cores, under the microscope and in the imaging logging. The fluid was speedily discharged through the fractures in miscible facies. Boiling oil and gas inclusions were found in Chang7 hydrocarbon souree rocks and Chang8, Chang9 reservoirs, being direct evidence of“screen-mode”hydrocarbon expulsion and “screen- mode” filling and injection of oil and gas. Homogenization temperatures of fluid inclusions indicate that two large-scale “screen-mode” filling process occurred mainly in the study area, with the main reservoir-forming period of early stage - mid- stage of Early Cretaceous. The formation and distribution of reservoirs of the lower Yanchang Formation in northwestern Ordos Basin is mainly controlled by three major factors of “a close source rocks, low-pressure, and high-quality facies-tract”. There were mainly four types of reservoir formation developed： reservoir formation ①by direct contact of reservoir and hydrocarbon source rocks; ②by oil migration and accumulation through the overlapped permeability sand body; ③by oil migration and ac- cumulation through the fluid-fractured cracks, and ④by oil migration and accumulation through overlapped permeability sand body and through the fluid-fractured cracks.