Shallow Water Delta and Beach Bar Mixed Deposition Model during Lake Level Fluctuation in E1f1 of Gaoyou Sag, East China

Lu, Huan; Ji, Youliang; Liu, Yurui; Shang, Chao; Li, Qingshan; Wang, Yanyan; Li, Meng

The previous research suggests that delta and fan delta sediments were deposited in the E1f1 formation in Gaoyou sag of Subei basin whose provenances are from north, west and south directions. However, based on the rock core, thin slice, seismic and well logging response analysis of 38 wells drilled through the E1f1 formation in Gaoyou sag combined with contours of stratigraphy strata thickness, sandstone thickness and sandstone/strata, it is discovered that the sedimentary facies of E1f1 is characterized by shallow water delta and beach bar sedimentary features, which is different from previous study.

This paper summarizes the facies mark of mixed depositional system of shallow water delta and beach bar from different aspects. 1)Lithology: characterized by mudstone with color transiting from oxidation to reduction, sandstone interbedded with mudstone and oolitic limestone or biological clastic limestone which contains depositional debris; 2)Texture: well-sorted grains, probability analysis indicating one bouncing population, one suspension population and three-segment population; 3)Bedding: flaser, wave and lenticular beddings, intraclast formed by channel downcutting and developed bioturbate structure; 4)Well-logging curves: frequently changed from coarsing upward to fining upward.

In study area, the shallow water delta can be divided into sub-facies of delta plain and delta front in which micro facies, such as frontal sand sheet, mouth bar, subaqueous distributary channel, bay between subaqueous distributary channels and subaqueous natural levee are included. Shoals are deposited in lateral area of shallow water delta, along the lake shoreline corner, or on the Jianhu subaqueous paleo-uplift.

As lake level waves fluctuation frequently changes the shoreline, the distributary channel of shallow water can stretch into the center of the basin. Adjacent to shallow water delta, shoals are distributed in ribbon shape along the shoreline. Additionally, since the fault terrace zone in the south is relatively steeper than the slope zone in the north, the scale of shallow water delta and shoal bar is smaller but thicker compared with that in the slope zone.

Consequently, a new concept of sedimentation named 'mixed depositional model of shallow water delta and beach bar' has been proposed to explain the depositional system of study area.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013