Differential Genetic Mechanisms of Deep High-Quality Reservoirs in the Paleogene Wenchang Formation in the Zhu-1 Depression, Pearl River Mouth Basin

Abstract

The Lufeng Sag and Huizhou Sag, both located in the Zhu-1 Depression, have similar geographical locations, but their reservoir characteristics in the Paleogene Wenchang Formation show obvious differences. Primary intergranular pores are mainly developed in the Lufeng Sag. However, secondary pores are the main reservoir space in the Huizhou depression. Overall, the reservoir properties of the Lufeng Sag are better than those of the Huizhou Sag. To analyse the differences between the Paleogene reservoirs in these two areas, this study mainly uses assay data, such as rock thin sections, scanning electron microscope images, drilling, and logging, to analyse the differential development mechanisms of high-quality reservoirs, and two types of reservoir development models were concluded. The results show that the anti-compaction primary porosity preservation mode is mainly developed in the Lufeng Sag. High compositional maturity quartz sandstone is the congenital condition of primary porosity development. The top and bottom calcareous cementation formed of the large set of thick sand bodies increases the rock’s anti-compaction ability. The early shallow burial slows down the compaction action of overlying strata. Under the low geothermal temperature, it can delay the time for deep reservoirs to enter the middle diagenetic stage. The reservoirs in the Huizhou Sag are developed with the secondary dissolution pore development model. The Wenchang Formation reservoir in the Huizhou Sag has a large area of contact with source rocks, and organic acids can migrate to sandstone reservoirs for dissolution. Additionally, the secondary dissolution pores are more developed because the Wenchang Formation reservoirs in the Huizhou Sag contain more easily dissolved substances.

Formation Mechanism of NW-Trending Faults and Their Significance on Basin Evolution in Zhu III Depression of the Pearl River Mouth Basin, SE China

“…The South China Sea (SCS) is at the convergence of the Pacific Plate (Philippines Sea Plate), the Indo-Australian Plate, and the Eurasian Plate. It records the interaction among multiple surrounding plates and has been considered a natural laboratory for studying the tectonic evolution of marginal seas [28][29][30][31][32]. However, the origin of the SCS remains obscure, and four principal tectonic models have been proposed for SCS opening: (1) slab pull from subduction of a proto-South China Sea; (2) extrusion tectonics from the India-Asia collision; (3) basal drag from a mantle plume; and (4) backarc rifting [33][34][35][36][37][38].…”

“…Additionally, a NE-striking depression-controlling fault, known as the Zhu III South Fault, governed the formation of the Zhu III Depression as a half-graben, with faulting in the south and overlapping in the north. Given the development of en echelon folds, horsetail faults, and oblique steps in the Wenchang Sag, it is revealed that the Zhu III South Fault has undergone dextral strike-slip deformation [10,15,25,26,32]. Except the boundary fault, it has also been revealed that strike-slip activity occurred along the faults within the depression.…”

“…Additionally, the NW-oriented faults, which cut through the Enping Formation, function as synsedimentary normal faults and contribute to the evolution of the depocenter. 6 for locations; the seismic data are from [22,32]).…”

“…Figure 7. Fault system and structural style shown in the seismic profiles of the Wenchang Sag, with corresponding unexplained seismic profiles (see Figure6for locations; the seismic data are from[22,32]). …”