相-势耦合控制鄂尔多斯盆地中生界石油聚集

鄂尔多斯盆地的石油主要聚集在中生界,因此研究盆地中生界石油聚集规律具有重要意义。在回顾油气勘探理论发展历程的基础上,通过鄂尔多斯盆地中生界烃源岩、沉积相带、运聚与油藏关系的系统分析,来探讨鄂尔多斯盆地中生界石油聚集规律。研究认为沉积相决定了烃源岩及储层的展布及发育特征,成岩相决定了优质储层的分布及储层的储集空间,压力势或流体势决定了油气运移方向和路径,相-势耦合决定了油气聚集圈闭位置。由于鄂尔多斯盆地含油层系延长组及延安组沉积期为大型内陆陆相坳陷盆地,内部构造相对简单,地层平缓,断裂较不发育,在6大成藏条件中盖层、保存、圈闭一般不存在主要问题,在烃源岩明确的条件下,主要需要考虑的是储层和运聚的问题,因而相-势耦合控制鄂尔多斯盆地中生界石油聚集,体现了"源控区、相控带、势定位"的石油聚集规律,具体表现为盆地中生界长7段、长9段生油岩的生烃范围控制了油藏分布的区域;含油区带的分布受控于盆地周缘的11大河流-三角洲体系;对受浮力控制的浅层油藏,鼻状构造凸起背景对油藏的形成和分布具有重要控制作用;对于受异常压力控制延长组下组合油藏,源-储压力差或者流体势决定了油气运移方向及聚集位置;在平面上及垂向上相-势耦合控制鄂尔多斯盆地中生界成藏的差异性,具体体现为盆地东部浅层呈现"补偿成藏"、盆地西部呈现"垂向串珠式成藏"、延长组深层呈现"异常压力控藏"的规律。这一认识同样适用于勘探程度较高的、以"定藏"或"定位"为目的其他大型坳陷含油气盆地的目标勘探。

Controls of facies-potential coupling on oil accumulation in the Mesozoic Ordos Basin

Oil in Ordos Basin mainly accumulates in the Mesozoic, so it is of great significance to study the oil accumulation patterns of the Mesozoic basin.This study provided an overview of the history of hydrocarbon exploration theories and discussed the oil accumulation patterns in the Mesozoic Ordos Basin based on the systematic analysis of the relationships between the Mesozoic source rocks, sedimentary facies belts, hydrocarbon migration and accumulation, and the oil reservoirs.The results show that the sedimentary facies control the distribution and development characteristics of hydrocarbon source rocks and reservoirs, whereas the diagenetic facies control the distribution and reservoir space of quality reservoirs.Pressure or fluid potentials determine the migration directions and pathways of oil and gas, thereby the coupling of facies and potentials controls the locations of hydrocarbon accumulations and traps.The Ordos Basin was a large inland down-warped basin during the deposition of the Yanchang and Yan'an Formations as the main oil-bearing layer series, featuring relatively simple structures, relatively flat-lying strata, and poorly developed fractures.Therefore, among the six major requirements for hydrocarbon accumulation, risks associated with cap rocks, traps and preservation are almost negligible.When the source rocks are ascertained in the Mesozoic Ordos Basin, the main risks are related with reservoirs, as well as hydrocarbon migration and accumulation.Thus, facies-potential coupling controls oil accumulation in the Mesozoic Ordos Basin, revealing the accumulation pattern as follows:"source rocks control the distribution of play, sedimentary facies control the occurrence of play fairways, and potentials control the location of oil reservoirs".To be specific, the source rocks in Chang 7 and Chang 9 in the Mesozoic basin control the distribution of play, and the 11 fluvial-delta systems around the basin control the distribution of play fairways.For shallow reservoirs controlled by buoyancy, the nose structural uplift plays a significant role in controlling the formation and distribution of oil reservoirs, while for the lower assemblage reservoirs controlled by abnormal pressures, the migration directions and accumulation locations are determined by the diffe-rential pressures or fluid potentials between source rock and reservoir.In addition, facies-potential coupling controls the diversity of hydrocarbon accumulation in the Mesozoic Ordos Basin seen from both planar and vertical directions, which is embodied by the "compensatory accumulation" in the shallow layers of the eastern Ordos Basin, "vertical bead-like accumulation" in the western Ordos Basin, and "abnormal pressure-controlled accumulation" in the deep Yanchang Formation.This understanding is also applicable to the exploration targets in other large down-warped petroliferous basins with higher exploration maturity.