Diagenesis and low-permeability tightening mechanisms of the deep Paleogene reservoirs under high temperature and highly variable geothermal gradients in the Baiyun Sag, Pearl River Mouth Basin

In the Baiyun Sag of the Pearl River Mouth Basin(PRMB), the oil and gas exploration targets have gradually shifted from the conventional reservoirs in the shallow to moderately deep Hanjiang-Zhujiang formations to the low-permeability, tight reservoirs in the deep to ultra-deep Zhuhai-Enping formations. Due to their distinct geological setting of highly variable geothermal gradients, the low-permeability, tight reservoirs exhibit significantly different diagenesis and tightening mechanisms from the conventional reservoirs. Using techniques such as casting thin section observation, scanning electron microscopy(SEM), physical property tests, diagenetic reconstruction, and physical property restoration, we systematically analyze the diagenetic processes of the Paleogene sandstone reservoirs from the Zhuhai Formation’s lower member to the Enping Formation in the area from the northwestern low uplift to the central sub-sag zone in the Baiyun Sag and their disparities. Considering the tectonic evolution setting, stratigraphic burial history, and current physical property data, we investigate the major factors influencing the relationships among the reservoirs’physical properties and explore their tightening processes and mechanisms. The results suggest that the reservoirs from the Zhuhai Formation’s lower member to the Enping Formation have experienced intense compaction, two-stage carbonate cementation, three-stage siliceous cementation, and three-stage feldspar dissolution. During their diagenetic processes, the reservoirs exhibited varying compaction rates due to changes in geothermal gradients and underwent water-rock interactions in different open-closed systems. These are major reasons for the different physical properties of the reservoirs across various tectonic zones in the Baiyun Sag. Compaction emerged as the primary factor leading to the reservoir tightness, which was further enhanced by siliceous and carbonate cementation. In contrast, dissolution improved the physical properties of the reservoirs. From the low uplift to the sub-sag zone, strata from the Zhuhai Formation’s lower member to the Enping Formation exhibited increasing geothermal gradients and burial depths. Accordingly, their reservoirs in the low uplift, slope zone, and sub-sag zone are in the middle diagenetic stage A2, middle diagenetic stage B, and late diagenetic stage, respectively, with diagenetic intensity gradually increasing. The diagenetic variations significantly impacted the evolution of the reservoirs’physical properties. Specifically, the reservoirs in the sub-sag zone had become tight prior to the late-stage hydrocarbon charging, while those in the slope zone underwent a gradually tightening process during this period. In contrast, the reservoirs in the low uplift still feature low permeability currently. The slope zone, with burial depths of generally less than 5 km and overlying strata exhibiting excellent sealing capacity, emerges as a focus of deep oil and gas exploration in the Baiyun Sag. © 2024 Editorial Department of Oil and Gas Geology. All rights reserved.