Development mechanism of metamorphic fractured reservoirs in the Bozhong area, Bohai Bay Basin: Implications from tectonic and magmatic hydrothermal activities

The BZ19-6 condensate gas field is the largest metamorphic gas field of the Bohai Bay Basin in China and contains typical fractured reservoirs within the inner parts of an Archean metamorphic buried hill. However, the mechanisms driving its creation and evolution are not well explored. Based on core and sidewall samples, thin sections, X-ray diffraction (XRD) analysis, U–Pb age dating, fluid inclusions, and imaging logging and seismic data, the lithologies, mineral contents and reservoir characteristics of the inner parts of a metamorphic buried hill were evaluated and then the main factors controlling the fractured reservoir were further explored in the Bozhong area. The results show that the lithology mainly includes granitic gneiss, cataclastic rocks and Mesozoic intrusions, among which the granitic gneiss is widely distributed and contains a large number of light-colored minerals. The reservoir spaces are mainly composed of tectonic fractures (TFs), cleavage fractures (CFs), introduced fractures (IFs) and dissolved pores (DPs). Numerous tectonic fractures provide the main reservoir spaces and can be divided into two groups, namely those with nearly WE- and NS-oriented strikes, which are closely related to the stress fields produced by Indosinian and Yanshan movements, respectively. IFs and DPs were developed by magmatic activities, which occurred during the Mesozoic, while magmatic hydrothermal fluid had an obvious negative impact on reservoir quality, resulting in fracture fillings with minerals such as ankerite and calcite. In this study area, reservoir spaces underwent various changes, from creation to development and from filling to dissolution. Finally, we proposed an integrated reservoir model to describe the distribution and evolution of high-quality reservoirs. This study explores the formation mechanisms of fractured reservoirs within metamorphic rocks and makes practical contributions to oil and gas exploration of buried hills.