Genesis and evolution of pore-fractures in deep sandy conglomerate reservoirs in Bohai Bay Basin: Taking the Paleogene Kongdian Formation in Bozhong 19-6 structure as an example

Mapping of quality reservoirs is critical to the successful exploration and discovery of deep-to ultra-deep clastic oil and gas reservoirs. This paper studies the genesis and evolution of storage space in high-quality and deep sandy conglomerate reservoirs in the Paleogene Kongdian Formation, Bozhong 19-6 structure, and establishes pore-fracture evolution models through physical simulation experiments and other means. The results show that: (1) pores formed by dissolution of feldspar particles are usually high-quality reservoir space. The dissolution of potassium feldspar intensifies or even exceeds that of plagioclase with rising temperature, indicating that potassium feldspar has greater potential for increasing porosity of deep layers. (2) Gravel-sized particles, felsic composition and low matrix content are more conducive to the generation of induced fractures, which are more highly developed in feldspar than in quartz. Vertically, induce fractures of the first stage appear at a burial depth of about 2500 m, that of the second stage appear at a depth larger than 3000 m with their number peaking at about 4000 m deep, and that of the third stage appear at a depth greater than 4500 m with their amount increasing with the increasing depth. (3) Vertically, porosity is damaged by early compaction and late cementation but improved by dissolution occurring in between: the early compaction reduced the porosity by 17.38 % and the late ankerite cementation reduced the porosity by 7.76 %, while organic acid dissolution between the two processes increased the porosity by 5.45 %. (4) Hydrocarbon charging is well timed relative to the second-stage dissolution with porosity enhancement, and the first-and second-stage induced fracture development, thus, hydrocarbons enrich in reservoirs at about 3000 m deep. In addition, high-quality reservoirs may occur in the deep sandy conglomerate at 4000 m deep, where the mass development of induced fractures, together with the dissolution such as potassium feldspar, greatly enhance reservoir quality.