Coarse-grained Lacustrine Slope Apron Deposits in Shaobo Area, Gaoyou Sag, Southern China

Zhou Yigang,Chen Panpan,Yan Yunbiao,Li Cunlei

doi:10.11648/j.jenr.20200901.18

Abstract

The Dainan formation reservoirs in Shaobo area, Gaoyou sag, Southern China, sedimented mass conglomerates dominated by lacustrine slope apron system which fended by multiple flood channels with massive terrigenous clasts. In the early days, offshore fan-shaped sediments in the Shaobo area of the Gaoyou Sag were considered to be the main sedimentary system. However, a new lacustrine sedimentary model is presented in this paper. By careful core description and interpretation, four facies unions are recognized: (1) Facies union 1: Downslope clastic pathways that composed of massive conglomerates up to 6-25m thick, alternating with brown-dark gray mudstone and the etching structures can be often observed; (2) Facies union 2: Debris-flow Lobe is composed of massive conglomarates, matrix-supported to clast-supported polymictic conglomerates up to 10-50m thick; (3) Facies union 3: Underwater distributary channel is composed of trough cross-stratified sandstone, horizontally stratified sandstone and ripple cross-stratified fine to medium-grained sandstone, up to 5-20m thick; (4) Facies union 4: mudstone is dominates by thick grey black mudstone interbeding thin sandstone, the etching features are rare and bedding is typically parallel. In our study area, the traction current deposits constitute the producing petroleum reservoirs, and the conglomerates deposits are poor reservoirs. The sedimentary model may be applicable to other fault depression for predicting reservoir distribution.

Highlights

Slope apron was first proposed as a gravity apron sedimentary accumulation body with multiple supply channels while Cooks studied the allochthonous carbonate blocks [1]
Russell et al deemed the Northwest African continental magrin to a typical fine-grained calstic slope apron, with pelagic & hemipelagic ‘background’ sedimentation overprinted by downslope gravity flows and modified by alongslope bottom currents [3]
The significance of our study is that we propose a new lacustrine sedimentary model to explain this phenomenon and guide the oil exploration and development

Summary

Introduction

Slope apron was first proposed as a gravity apron sedimentary accumulation body with multiple supply channels while Cooks studied the allochthonous carbonate blocks [1]. Sedimentary facies maps show the nearshore subaquous fan is the main depositional system of Shaobo area in Gaoyou sag [4, 5], but the exploration and development practices show the main oil reservoirs are the sandy layers (caused by traction flow) which distribute between or the front of the conglomerate layers. The significance of our study is that we propose a new lacustrine sedimentary model (lacustrine slope apron) to explain this phenomenon and guide the oil exploration and development. The establishment of this model is benefit by the vast core data which make it possible to do the distribution research of lithofacies

Methods

Results

Conclusion