叠覆式三角洲——一种特殊的浅水三角洲

Overlapping Delta:A New Special Type of Delta Formed by Overlapped Lobes

不同于常规三角洲以分流河道体系所形成的分流河道、河口坝、席状砂等微相为三角洲朵体的基本单元,叠覆式三角洲以内部结构简单的朵体为基本构成单元,朵体相互叠置,形成复合叠合体,进而构成三角洲骨架。单个朵体由河道扫描或扩展而成,复合朵体则是由单朵体侧向迁移或前(退)积而成。三角洲因大量朵体叠置而形成厚层状、内部结构复杂的复合砂体。不同朵体形成于不同时期,因而不存在统一的分流体系,单一沉积体具有层状特征,但不同期朵体受可容空间和地貌控制,呈三维叠置,而非简单的层状叠加,从而使得三角洲内部呈现出拼合式、立体式特点。单朵体是结构的基本单元,发育范围有限,与相邻朵体发育于不同时间单元,因而只能在复合体约束下小范围追踪。单一朵体接触关系及接触界面的渗流能力决定了油气富集和注水开发响应特征。朵体迁移、叠置造成大面积、巨厚的砂层可形成大型油气藏,而同时朵体间泥岩的不均匀分布也造就了砂体局部不连通或朵体间连通性变化,为岩性油气藏形成创造了条件,并且影响了注水开发中的注采对应性,进而影响水驱采油效果。

Different from common deltas which are made of distributary channels, distributary mouthbars and sheet-like sand sheet, the primary filement of overlapping delta are lobes. Different lobes can form complex lobe combinations, which eventually form the delta. Single lobe is deposited through the expansion or laterally sweeping, while the complex lobe combination is formed through the progradation （retrogradation） or laterally moving of single lobes. Delta architecture is very complex because of the overlapping pattern of the lobes. There is no united distributary channel system which can string all the lobes for they are formed in the lower place of the delta front in different time and space, and controlled by the micro geomorphology. The lobes are overlapped in 3D space with a very complex relations to each other. Single lobe,as the primary element,is very small and can be correlate between wells within a small area in the frame of complex lobe combination. The contact of lobes and the characteristics of the surface between lobes control the connectivity of lobes and eventually make deep influence of the fluid flow in the reservoir. The overlapping of lobes can form continuous sandsheet which made it possible to form gaint fields,while the distribution of shale between lobes may compart the lobes to form litho traps and bypass oil will also be formed in the isolated lobes.