坳陷湖盆浅水三角洲的沉积特征——以松辽盆地南部姚一段为例

Forming Conditions and Depositional Characteristics of Shallow-water Deltas in Depression Basins:A case study of K_2y^1 in the south of Songliao Basin

坳陷湖盆浅水三角洲的形成古地形背景、水动力学特征、平面形态、微相类型及三角洲内部结构等多个方面与正常三角洲有着极大的差异。姚一段沉积时期,松辽盆地古地形平坦并不具备明显的坡折带,水体浅、面积小并且湖泊能量较弱,湖岸线不稳定并常常发生大范围的迁移,为典型的浅水沉积湖盆。浅水湖盆的沉积充填主要以浅水三角洲和滨浅湖沉积为主,深湖、半深湖相对不发育,浅水三角洲砂体是湖泊砂体的主要类型。根据供源水系的不同,松南的浅水三角洲可以分为浅水型扇三角洲、浅水型辫状河三角洲和浅水曲流河三角洲等。浅水型三角洲主要有以下特点:1.具高建设性河控三角洲的沉积特点,平面上呈鸟足状,前缘相带延伸较远,形成了大规模的湖盆中心砂体;2.浅水三角洲前缘主要以分流河道为主,河口坝极不发育;3.前缘分流河道呈网状展布,河道遭受不同程度的席状砂化,依次划分为弱席状化、中等席状化、强席状化等3种类型;4.不具备典型三角洲的三层式沉积结构及向上变粗的反序列特征。松南姚一段沉积时期,总体表现为基准面上升的湖侵过程,I、II砂组低位和湖侵域的砂体为成藏提供了的有利储集空间,III砂组的湖侵为区域性成藏提供了良好的盖层。

Shallow-water delta in large lacustrine depression basins is different from conventional delta in aspects of framework, microfacies, sandbody, scale and shape. In this article, forming conditions and deposition characteristics of K2y1 shallow-water deltas in the southern Songliao basin has been analyzed. Songliao basin are characterized by sta- ble structure, slow subsidence, flat palaeotopography, dry climate and sufficient material source during Putaohua For- mation, thus formed a distinctive shallow-water delta deposition. Unlike conventional terrigenous basins, its shoreline often migrated in large scale and few deep lacustrine and semi-deep lacustrine facies developed . The shallow-water delta can be distinguished by following characteristics ： （ 1 ） The channel of shallow-water delta extended far away, up to 160 km from delta plain to lake center; Divaricating and confluence were often happen in delta . Sand bodies show obvious direction and birds foot-like distribution on the isopach map. （2）Mouth bar in the front of delta was not developed as restricted by accommodation and washing action. The sandstone reservoirs were ob- viously controlled by fluvial system. （3）Channel on the delta front were reformed to sheetlike sandstone by lake water and can be divided into three styles which include poorly sheetlike, moderately sheetlike and intensely sheetlike styles ; （4） The sedimentary models of shallow-water delta differ from Gilbert delta style which have topset bed, bot- tomset bed and foreset bed. During K2yldeposition period, the base level increased gradually, I and II sand group provided a favorable reservoir storage space, III sand group transgress provided a good seal for Putaohua reservoir.