Abstract
An abnormally high-porosity zone (AHPZ) is beneficial for petroleum exploration, especially for the deep tight reservoirs in a petroliferous basin. Because of lacking effective research methods, it is hard to analyze the formation process of AHPZs in different geological periods. From the perspective of the diagenetic fluid type and activity history, geochemical characteristics and fluid inclusions of diagenetic minerals were utilized to reconstruct the diagenetic fluid type and dynamic evolution. The ultimate goal is to study the genetic process of AHPZs in the Songtao–Baodao region of the Qiongdongnan basin, South China Sea. It was found that there are three sections of AHPZs at different burial depths, which are generally favorable for high-quality reservoirs. Moreover, it can be concluded that the AHPZs are closely related to multiple actions of various diagenetic fluids. The meteoric waters, organic acid, and thermal fluids facilitated the enlargement of porosity by dissolving minerals to form secondary pore spaces. The hydrocarbon fluids have positive effects on the preservation of pores by preventing cement from filling the pore space.
Highlights
The development of abnormally high-porosity zones (AHPZs) in sedimentary basins has always been an important topic, which is of great significance for high-quality reservoir predictions and commercial oil-gas exploration [1]
The result was that the diagenetic process in the west of the Qiongdongnan basin can be divided into two stages, as shown in Table 1, the early diagenetic stage and the middle diagenetic stage; the corresponding depth is about 1600, 2400, 3200, 4500, and more than 4500 m (Table 1 and Figure 3)
AHPZ refers to the reservoir porosity exceeding that induced by the normal compaction curve with the increase in burial depth, and it is not completely equivalent to secondary porosity
Summary
The development of abnormally high-porosity zones (AHPZs) in sedimentary basins has always been an important topic, which is of great significance for high-quality reservoir predictions and commercial oil-gas exploration [1]. Main mechanisms for the formation of AHPZs commonly include sedimentation, grain coat, hydrocarbon filling, overpressure, and dissolution [2, 3]. Sedimentation can control original physical properties of the sandstone reservoir. Grain coat grows outward from the surface of skeleton grains. It can hinder the development of quartz cements and enable the pores to be effectively preserved in the middle-deep reservoirs [5]. An important function of fluid overpressure is to reduce the effective stress on the formation. Effective stress is an important factor controlling the compaction of Geofluids clastic rock, so fluid overpressure can inhibit the compaction, which is beneficial to the preservation of reservoir space [7, 8]
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