准噶尔盆地西北缘二叠纪原型盆地分析

Permian attributes and tectonic evolution of the west Junggar Basin

准噶尔盆地西北缘玛湖凹陷是准噶尔盆地重要的生烃凹陷,二叠纪原型盆地属性和构造演化,是制约盆地油气勘探的关键要素。玛湖凹陷二叠纪—早三叠世的陆相沉积序列是准噶尔盆地西北缘构造演化的沉积记录。本文在综合分析构造、沉积、物源及古地理重建的基础上,对准噶尔盆地西北缘二叠纪—早三叠世构造和沉积过程进行解剖。二维地震剖面显示早二叠世受控于边界正断层,具半地堑结构,而晚二叠世—早三叠世的右旋走滑断裂带为构造反转。区域内下二叠统软沉积物变形构造是边界正断层活动的沉积响应。玛湖凹陷二叠系—三叠系包含两个二级湖侵—湖退层序。早二叠世湖侵体系域1的沉积环境由扇三角洲逐渐过渡到了深湖,沉积中心沿边界正断层分布,构造机械沉降速率大于沉积速率。中二叠世湖退体系域1沉积环境由湖泊向扇三角洲过渡,沉积中心向玛湖凹陷中心迁移,以热沉降为主。晚二叠世—早三叠世,扇三角洲沿红车断裂带、克百断裂带和乌夏断裂带分布。二叠纪—早三叠世玛湖凹陷经历了早二叠世同断陷机械沉降阶段,中二叠世后断陷热沉降阶段,及晚二叠世—早三叠世构造反转阶段,其构造演化受控于中亚造山带西南缘晚古生代增生造山后的伸展作用。

The Mahu sag in the northwestern margin of the Junggar Basin is one of the most important hydrocarbon-rich sags in the Junggar Basin. The understanding of basin attributes and tectonic evolution of the Junggar Basin during the Permian-early Triassic is significant for oil and gas exploration and development. The Permian to early Triassic terrestrial successions in the Mahu sag record the tectonic evolution of the northwestern margin of the Junggar Basin. Comprehensive studies of structures, sequence stratigraphy, sedimentary infill, sediment dispersal and paleogeographic construction were performed on the Permian-early Triassic successions. 2 D seismic profiles show the occurrences of basin-bounding normal faults and half-graben structures during the Early Permian and the tectonic inversion during the late Permian to early Triassic. Widespread soft-sediment deformation structures in the lower Permian Fengcheng Formation with lateral continuity correspond with the main activity on basin-bounding normal faults. The Permian to Triassic strata are organized into two second-order transgressive-regressive(T-R) sequences. During the early Permian, sediments of transgressive systems tracts 1 recorded depositional system transition from fan delta to deep lacustrine setting, and the depocenter was distributed along the boundary normal fault. At this time, tectonic mechanical subsidence exceeded sediment supply. During the middle Permian, the depositional system of regressive system tracts 1 transitioned from lacustrine to fan delta setting, and the location of the depocenter shifted to the center of the Mahu sag, where thermal subsidence played a great role. During the late Permian-early Triassic, fan-deltas were distributed along the HongChe, Ke-Bai and Wu-Xia fault zones. Therefore, three stages of the tectonic evolution of the Permian-early Triassic Mahu in the northwestern margin of Junggar Basin are recognized: early Permian syn-rift mechanical subsidence, middle Permian post-rift thermal subsidence, and late Permian to early Triassic tectonic inversion. The dynamic mechanism of the tectonic evolution of the northwestern margin of the Junggar Basin is consistent with the evolutionary pattern of the late Paleozoic post-accretionary orogenic stage in the southwestern margin of the Central Asian Orogenic Belt.