Sand body types of clastic shore deposits with delta backgrounds and their control over reservoir quality

Abstract

The “Donghe sandstones” of the Tarim Basin are not simple shore deposits. They show distinct sedimentary characteristics and different types of sand bodies that are controlled by modes developed in different areas. This study provides an example from the Bachu Formation, which consists of the “Donghe sandstones” deposits located in the Donghetang Oilfield in the Tarim Basin, China. Based on analyses, cores, logs, and seismic data, a new depositional system is proposed for the Donghetang section as a wave-dominated shore deposit with delta backgrounds. The sedimentary characteristics of the transitional shoreland, called “Three types of bar, two types of beach and two types of sands in the transitional zone,” were summarized by the further classification of the shoreface bars, beaches, and transitional sands on the basis of their origin types and varying degrees of wave reworking. The classifications are bars from the reworked distributary channels and mouth bars, normal bars, beaches from the delta sands, normal beaches, sands from reworked delta sheet sands, and normal transitional zone sands. Meanwhile, the Donghetang section in the study area is interpreted as wave-dominated shore deposits with delta backgrounds rather than normal shore deposits, which has significant implications for the distribution of the favorable reservoirs. The shoreface bars, formed from reworked distributary channels and mouth bars, and the shoreface beaches, formed from delta sands, have better connectivity and constitute the additional important reservoirs.