鄂尔多斯盆地西缘中部微小断层的成因机制与控藏特征

Forming mechanism of dmall-cale faults and their control for the reservoir formation in the middle of western margin of the Ordos Basin

鄂尔多斯盆地西缘作为南北构造带的一部分，不仅是研究该盆地形成、演化及油气成藏的重要窗口，也是探讨中生代特提斯洋关闭、新生代青藏高原隆升的热点地区。本文将盆地西缘中部及以东的新生界覆盖区作为一个整体，利用二维和三维地震资料进行精细构造解释，在天环坳陷中生界发现大量微小断层。地震解释表明：地震剖面上，微小断层具有正、逆断层并存，切穿层位差异大，与挤压褶皱相伴生的特点；平面上，微小断层呈北西西-南东东向规律性展布，与西部前缘带的大型断层共同构成扫帚状展布的断层发育带。晚侏罗世燕山运动主幕，南南西-北北东向的构造挤压应力南南向北传递，由于受东西向横向构造转换带的阻挡，在本区产生压扭作用，微小断层的形成受此压扭作用控制。微小断层形成后，主要经历了早白垩世伸展作用和晚白垩世-至今的后期改造。依据断层切穿的层位及改造阶段，将微小断层划分为3个期次，井震联合分析认为：Ⅰ期断层主要改善了延长组致密储层的物性，一般在延长组内部成藏；Ⅱ期断层主要促进了石油的运移，在延长组和延安组均能成藏；Ⅲ期断层对早期油藏具有一定的破坏作用。

The western margin of the Ordos Basin as a part of the north-south tectonic belt, is not only an important window for studying the basin formation, evolution and hydrocarbon accumulation, but also the hot spot for discussing evolution of the Mesozoic Tethys Ocean and uplifting of the Qinghai-Tibet Plateau in Cenozoic. In this study, we make a high quality structural interpretation with large amount of 2D and 3D seismic data. Numerous small-scale faults have been found within Tianhuan depression where the Cenozoic strata covered. The seismic interpretation shows that small-scale faults have the characteristics of coexistence of normal and reverse faults separating,obvious differences in cutting layers and co-existed with compression folds. The small-scale faults are distributed mainly in NWW-SEE direction along horizon, and together with the large-scale faults in the western front belt, showing a broom-shaped faults arrangement. The Yanshanian Movement in the Late Jurassic induced the NEE-direction compressional stress propagated from south to north. Due to the block of E-W transverse tectonic zone, the tranpressional motion is produced in this area, inducing formation of small-scale faults. After then, the small-scale faults experienced extension in the Early Cretaceous and modification from Late Cretaceous to present. The evolutionary history of small-scale faults are divided into three stages according to the difference in cutting layers and modification. Well and seismic data show that fault movement during stage Ⅰ mainly improves the physical property of the tight reservoir in the Yanchang Formation, leading to oil accumulation in the Yanchang Formation; fault activity during stage Ⅱ mainly promotes the migration of oil, forming reservoirs both in Yanchang Formation and Yah＇an Formation; fault activity during stage Ⅲ has a destructive effect on the reservoir preservation.