Genetic Mechanism and Pore Evolution of High-Quality Glutenite Reservoirs of Deep Kongdian Formation in BZ19-6, Bohai Sea

In recent years, a large amount of oil and gas was discovered for the first time above the Archean large metamorphic granite buried hill condensate gas reservoir in the Bozhong 19-6 structure in the deep (buried depth greater than 3 500 m) Kongdian Formation glutenite reservoir. But it is characterized with larger reservoir difference.In order to identify the causes of deep conglomerate reservoir quality, using analyses of thin section casting and test data, particle size, conventional physical properties, such as inclusion, combined with burial history analysis, quantitative analysis of the physical properties of deep glutenite reservoirs in Bozhong 19-6 and its surroundings of the Bohai Sea was carried out, and the main controlling factors for the development of high-quality glutenite reservoirs in the Kongdian Formation were clarified. It is found that: (1) The deep Kongdian Formation in the study area has developed fan delta glutenite reservoirs with a buried depth greater than 3 500-4 200 m. It is a deep glutenite reservoir with poor physical properties, mainly low porosity and low permeability, and the diagenesis stage is in the middle diagenesis from A₂ to B stage, and dissolution pores are developed. (2) Potassium-rich feldspar framework particles, atmospheric fresh water leaching, organic acid dissolution, overpressure and crack are the main controlling factors for the development of high-quality glutenite reservoirs in the Kongdian Formation in the study area. (3) The Kongdian Formation glutenite reservoirs have mainly experienced atmospheric freshwater enhancement-early diagenesis long-term, rapid burial, rapid compaction-reduction-medium diagenesis A₁ mid-to-long-term, rapid burial, slow compaction-reduction, organic acid and ultra pressure massive dissolution to increase pores, first filling of oil and gas-mid-diagenetic A₂ or B period, short-term, rapid burial, slow cementation, pore reduction, (or formation of corrosion cracks) and large-scale oil and gas filling. (4) However, the diagenetic evolution stages and pore evolution strength experienced by different structures are obviously different. The Kongdian Formation glutenite reservoir in Bozhong 19-6 structure has experienced early diagenesis → middle diagenesis A₁ → middle diagenesis A₂ to B, corresponding The porosity of Bozhong has undergone an evolution process of 32.6%→17.3%→12.6%→10.6%, the reduction in porosity due to compaction and cementation is 22.0%, and the total reduction in porosity is 67.6%, respectively; while Bozhong 25-1 tectonic glutenite reservoirs mainly experienced early diagenesis → middle diagenesis A₁ → middle diagenetic A₂, and the corresponding porosity experienced an evolution process of 32.8%→19.9%→14.2%→13.2%, which suffered compaction and cementation. The porosity reduction is 19.6%, and the total porosity reduction rate is 59.7%. (5) Compared with the BZ 19-6 structure, the Kongdian Formation glutenite reservoir compaction in the BZ 25-1 structure is relatively weak, the overpressure is stronger, and the dissolution is stronger, which are the main factors for the relatively good porosity of the structure. And the coarse-grained and intragranular dissolution pores-fractures formed by the late strong compaction are the main factors for the relatively high permeability of the BZ 19-6 structural glutenite reservoir.