鄂尔多斯盆地西南部中侏罗统直罗组储集砂体特征及聚砂成藏模式

鄂尔多斯盆地西南部中侏罗统直罗组是潜在的新勘探层系,勘探研究程度极低。在野外露头观测、岩芯观察、测井数据解释和显微薄片鉴定的基础上,通过古流向分析和古地貌恢复,确定古水系分布; 在古地貌和古水系的约束下,基于500余口钻井的测井解释结果,确定8个亚段的砂体宏观分散体系和沉积相分布。结果表明:鄂尔多斯盆地西南部中侏罗统直罗组物源主要来自西南侧的西秦岭造山带; 早期物源较强,沉积环境为辫状河-辫状河三角洲,形成了灰色厚层砂岩夹泥岩; 晚期物源较弱,沉积环境转变为曲流河三角洲,以发育紫红色泥岩与砂岩互层为特征; 直罗组砂岩储层以原生粒间孔为主,溶蚀孔次之,孔隙度为0.87%～31.20%(平均值为16.34%),渗透率为(0.13～53.70)×10^-3 μm²(平均值为17.83×10^-3 μm²),有效储层的孔隙度下限高于15%; 直罗组沉积初期古地貌既控制了砂体的发育分布,又制约了有利储集砂体发育的优势部位及后来的油气运移的有利指向; 潜在的油藏勘探目标包括古河道上游高地、中游汇水坡咀和下游河间丘等3种古地貌聚砂成藏模式。

Favorable Sandstone Reservoir Characteristics and Hydrocarbon Accumulation Models of Middle Jurassic Zhiluo Formation in the Southwestern Ordos Basin,China

Middle Jurassic Zhiluo Formation in the southwestern Ordos Basin is a new potential exploration target with low degree of exploration. Based on outcrop and core observation, interpretation of logging data and thin sections identification, the paleocurrent and paleogeomorphology were reconstructed to reveal the distribution of paleo-drainage system; based on the logging interpretation of more than 500 boreholes, combined the constraint of paleogeomorphology and paleo-drainage, the macroscopic dispersion system of sandbody and the distribution of sedimentary facies were proposed. The results show that the source of Zhiluo Formation in the southwestern Ordos Basin is mainly from West Qinling orogenic belt in the southwest; the supply of sand is abundant in early, and the sedimentary environment is braided river-braided river delta, which builds grey thick sandstone intercalated with mudstone; then the supply of source changes to fine grains dominated, and the sedimentary environment turns into meandering river delta characterized by the development of purple mudstone with sandstone interbed; sandstone reservoir of Zhiluo Formation is dominated by primary intergranular pores and dissolution pores, and the porosity is 0.87%-31.20% with the average of 16.34%, the permeability is(0.13-53.70)×10^-3 μm² with the average of 17.83×10^-3 μm², the lowest porosity-limit of effective reservoir is 15%; in the early deposition of Zhiluo Formation, the paleogeomorphology not only controls the development and distribution of sandbody, but also influences the advantageous position for the development of sandbody and the direction of hydrocarbon migration subsequently; potential reservoir targets include three types of hydrocarbon accumulation models related with paleogeomorphology, that is, highlands in upstream, curved slop toe in midstream and interriver mound in downstream.