陆相湖盆深水重力流混合事件层沉积及沉积学意义

重力流混合事件层在陆相湖盆广泛发育,其形成和分布对理解重力流沉积演化过程及重力流沉积常规与非常规油气勘探开发意义重大.以涠西南凹陷流沙港组一段和鄂尔多斯盆地延长组7段重力流沉积为研究对象,分析湖盆重力流混合事件层的沉积特征、类型、成因及沉积模式,并进一步探讨其地质意义.湖盆主要发育滑动、滑塌重力驱动块体搬运沉积和砂质碎屑流、泥质碎屑流、高密度浊流和低密度浊流等重力流流体沉积,同时广泛发育重力流混合事件层沉积.湖盆重力流混合事件层包含多层结构、双层结构和频繁互层三种大的类型;其中,双层结构的重力流混合事件层进一步根据上下两个沉积单元厚度的差异可细分为两个亚类.多层结构的混合事件层主要为流体侵蚀或砂体液化成因,多发育于混合事件层沉积近端;双层结构与频繁互层结构的混合事件层主要为流体减速膨胀、泥质碎屑流中碎屑颗粒的差异沉降成因,多发育于混合事件层沉积远端.相同沉积单元组成的沉积层在垂向上的规律叠置是岩芯中识别重力流混合事件层沉积的可靠依据;在未明确其沉积过程的情况下可能会导致沉积信息的错误解读.同时,重力流混合事件层的发育会导致重力流沉积非均质性增强,不利于常规油气的储集;但是,重力流混合事件层形成的细粒沉积物是非常规油气"甜点"区发育的优势沉积岩相组合类型.

Deposition of deep- water gravity- flow hybrid event beds in lacustrine basins and their sedimentological significance

The formation and distribution of hybrid event beds, common in lacustrine basins, is significant for understanding the evolution processes of gravity- flow, conventional and unconventional oil and gas exploration, and development of deposits within. Considering the gravity- flow deposits in the first member of the Liushagang Formation in the Weixinan sag, and the seventh member of Yanchang Formation in the Ordos basin as examples, the sedimentary characteristics, types, formation mechanisms, depositional model as well as the oil and gas geological significance of hybrid event beds in lacustrine basin are discussed. The gravity- flow deposits in the lacustrine basin include slide and slump deposits, sandy and muddy debris flow deposits, high and low density turbidity- current deposits, and the common hybrid event beds. Three types of hybrid event beds are identified including sandwich structure, bipartite structure, and banded structure. The bipartite structured hybrid event beds can further subdivided based on the thickness difference between the upper and lower division. Sandwich structured hybrid event beds are mainly caused by flow erosion or liquefaction and located in the proximity of hybrid event beds. Bipartite structured and banded structured hybrid event beds, which are located in the distal part of the hybrid event beds, are mainly attributed to flow deceleration and expansion, and differential settling of detrital grains in a muddy debris flow. The vertical stacking of different beds with the same internal depositional unit composition in the core is a reliable identification mark for hybrid event beds. A lack of knowledge of the formation processes of these hybrid event beds may lead to a misunderstanding of the depositional processes. The development of hybrid event beds adds to the depositional heterogeneity of gravity- flow deposits, which is bad for conventional oil and gas preservation. However, the fine- grained deposits caused by hybrid event beds are good lithofacies associations, which is the sweet spot for unconventional oil and gas development.