柴达木盆地西部地区断裂构造与油气聚集

Faulted structure and hydrocarbon accumulation in western Qaidam basin

柴达木盆地西部地区油气藏常与断裂构造相伴生。柴西地区南部沿断裂带分布有砂西-尕斯-油砂山油气聚集带、跃进2号-跃东油气聚集带、乌南-绿草滩油气聚集带;西部阿尔金山前带沿柴西等断裂分布有断鼻构造控制的构造-地层-岩性复合型油气藏;北部沿着英北断裂、南翼山断裂、尖顶山断裂分布有油泉子、开特米立特、南翼山、尖顶山等构造-裂缝型油气藏。柴西地区断裂的形成与阿尔金山和昆仑山在中新生代的剧烈挤压和走滑活动有关。断裂以逆断层和走滑断层为主,还有少量的同沉积正断层、滑脱断层、反转断层等。断裂主要发育北东向、北西向和近南北向,平面上呈平行状,斜列状、斜交状、正交状,反“S”形,透镜状、发散状等组合类型,剖面组合型式主要有叠瓦状、冲断式、花式3 种主要类型。其中,以冲断型及正交状、斜交状断层组合形成的构造油气藏最为发育。断裂不仅控制了生烃凹陷的展布及构造圈闭的形成,在一定程度上也控制了河流三角洲、扇三角洲有利储集相带的展布。断裂活动形成的大量裂缝,改善了储层的物性条件。柴西南区普遍存在以断裂为油气运移通道形成的侧接式上生下储式油藏。

Oil and gas reservoirs in western Qaidam basin are often associated with faulted structures. In the southern part of western Qaidam basin, several hydrocarbon accumulation zones are distributed along the faulted zones, such as Shaxi-Gas-Youshashan, Yuejin No. 2-Yuedong and Wunan-Lucaotan hydrocarbon accumulation zones. Structural-stratigraphic-lithologic composite oil and gas reservoirs controlled by faulted nose-like structures are distributed along Chaixi and other faults, in the foothill belt of the Altun Mountain in the west. Structural-fractured reservoirs, including Youquanzi, Kaitemilite, Nanyishan and Jiandingshan, are distributed along Yingbei, Nanyishan and Jiandingshan faults in the north. The formation of faults in western Qaidam basin has been related to the Meso-Cenozoic intense compressional and strike slip movements in Ahun and Kunlun mountains. The faults are mainly of reverse and strike slip faults, with a few synsedimentary normal, decollement and reversed faults. The fault trends include mainly NE, NW and nearly N -S. Horizontally, the faults appear as parallel, en echelon, oblique-crossing, orthogonal, reverse ＂S＂, lenticular and divergent fault complexes. Vertically, they are combined in the form of imbricated, thrusted and flower-like fault complexes. The structural reservoirs formed by the thrust type and orthogonal and oblique-crossing fault complexes are best developed. Faults have not only controlled the distribution of hydrocarbon-generating sags and the formation of structural traps, but have also controlled, to a certain extent, the distribution of favorable reservoir facies belts, such as river delta and fan delta facies. Large amount of fractures formed by faulting improve the poroperm characteristics of reservoirs. Laterally connected oil pools, above and migrated through faults and accumulated in in which hydrocarbons were generated in the source rocks the reservoirs below, are common in the southern part of western Qaidam basin.