姬塬地区长8油层组浅水三角洲沉积特征

Sedimentary Characteristics of Shallow-water Deltas in Chang-8 Subsection of Yanchang Formation,Jiyuan area

姬塬地区位于鄂尔多斯盆地中偏西部地区,晚三叠世主要受北东、北西及南部三个方向的物源控制。通过岩心观察、薄片鉴定、测井曲线等资料的综合分析,对研究区上三叠统长8油层组岩石类型、沉积构造、物源方向、古生物和沉积相特征进行了详细研究,发现长8沉积时期姬塬地区地形平缓,河流作用较弱,主要发育一套浅水三角洲沉积体系,其沉积作用和砂体展布受湖平面升、降和物源方向控制明显,对油气成藏有着重要的意义,表现为:①湖平面上升期,湖岸线快速向物源方向迁移,以发育长条状和鸟足状水上—水下分流河道砂体为主;②下降期以发育三角洲平原为主,来自不同物源方向的前缘砂体在研究区中部交汇;③湖平面上升期水下分流河道砂体呈席状发育以及岩性遮挡条件更好,因而比湖平面下降期发育的砂体更有利于形成岩性油藏。

Jiyuan area is in the middle of the western Ordos Basin and its sedimentary facies is controlled by prove-nance from northeast, northwest and south directions in Late Triassic epoch. Detailed studies have been made on the rock types, sedimentary structures, and direction of provenance, paleontology and sedimentary facies characteristics of Chang-8 subsection of Yanchang Formation of Upper Triassic of Jiyuan area by an integrated analysis of cores, thin sections, well log and other data. During the deposition of the Chang-8 subsection of Yanchang Formation, the topog- raphy was gentle and stream actions were weak and the depositional system of shallow-water delta facies was devel- oped. The sedimentation and sandbodies distribution were controlled by the raise and fall of the lake level and the di- rection of provenance obviously which were significant for hydrocarbon accumulation. It can be summarized as follows：①The shoreline moved to provenance directions quickly and strip-shaped or birdfoot-shaped subaqueous distributary channel sandbodies developed in the periods of high water levels ;②The delta plain developed and the front sandbodies came from different provenance converged in the center of the study area in the periods of low water levels;③The sandbodies deposited in the periods of high water levels were favorable to develop lithologic reservoirs than those deposited in low water levels because the former distributed as sheet-shaped which formed lithologic barriers.