河控浅水三角洲前缘席状砂沉积特征及沉积模式探讨--以黄河口凹陷渤中34地区明下段为例

黄河口凹陷渤中34地区明化镇组下段下部河控浅水三角洲前缘席状砂较发育。为进一步认识前缘席状砂体的发育规律,通过岩芯、钻井、生产动态、黏土矿物等分析对席状砂体沉积特征、控制因素及沉积模式进行综合研究。席状砂主要表现为薄层或薄互层特征,根据发育位置可以划分为水下分流河道边部及河道末端席状砂两种类型。基于高分辨率层序地层学原理,以基准面短期旋回为时间区间,将席状砂体的沉积模式划分为三种类型:湖水上升初期的河道弱席状化模式、湖水上升期至最大湖平面时期河道末端面状流增长模式以及最大湖平面时期席状化改造模式。随着基准面的下降,浅水三角洲前缘沉积类型亦从席状砂体向水下分流河道演化。暖湿气候、一定的水体规模及地形坡度条件是前缘席状砂体大面积分布的控制因素。研究成果为进一步加快明下段河控浅水三角洲前缘席状砂的勘探开发提供了依据。

Characteristics and Sedimentary Models of Sheet Sand in Shallow Lacustrine Fluvial-dominated Delta Front: A case study from lower member of Minghuazhen Formation in BZ34 area,Huanghekou Sag

A well-developed sand sheet occurs in the fluvial-dominated delta front at the bottom of the lower Minghuazhen Formation in the BZ34 area, Huanghekou Sag. The data from cores, production dynamics, clay minerals and so on were analyzed to gain a better understanding of the sedimentary development. This enabled construction of models of the sheet sand, which consisted of a thin layer, or interbed, of two kinds: one located at the edge of the underwater distributary channel, and the other at the end of the channel. High-resolution sequence stratigraphy for the period of short-term base-level cycles recognized three sedimentary models: poorly sheet-like initially, then sheet-like sand formation at the end of the distributary channel due to laminar flow as the water level rose, eventually becoming strongly sheet-like at the high-water level. As the water level fell, the sand in the delta front changed from sheet to distributary channel. It is concluded that the conditions of a warm/wet climate, a deepening-water lacustrine environment and slightly greater slope favored the development of a broad sheet of sand. This finding provides a basis for further exploration and development of sheet sand at the front edge of the fluvial delta in the lower member of the Minghuazhen Formation.