关键词: 花岗岩, 潜山, 风化壳, 储层成因, 储层演化, 油气藏

Abstract: Globally, granite buried-hill oil and gas reservoirs have been revealed in 25 basins across 17 countries, with proven reserves exceeding 3 billion tons of oil equivalent. In China’s basins, proven reserves exceed 1.5 billion tons of oil equivalent. Granite has become an important target for oil and gas exploration in China, and rich achievements have been made in reservoir space types, rock physical properties, reservoir distribution patterns and genesis. Studies show that there are 7 categories and 12 types of reservoir spaces, mostly secondary pores and secondary fractures, with a small number of vesicles. The characteristics of reservoir space types vary among different oil and gas reservoirs. Granite buried-hill reservoirs are mainly of low to ultra-low porosity and medium to low permeability. The weathering crust thickness mostly ranges between 40 and 280 meters. The physical properties of granite buried-hill reservoirs decrease with increasing distance from the top of the weathering crust, though in rare cases, high porosity zones also exist at depth. Most oil layers/suspected oil layers/gas layers/oil-bearing water layers/gas-bearing water layers are distributed within about 200 meters to the top of the weathering crust, mostly concentrated within 150 meters. Granite buried-hill reservoirs exhibit zonation: The residual zone and weathering dissolution zone are mainly porous, the weathering fracture zone is mainly porous-fractured, and the bedrock zone is mainly fractured. Structural fractures exhibit a “sweet spot” distribution pattern. Parameters such as uniaxial compressive strength, permeability, and chemical alteration index can be used to establish quantitative relationships for the vertical zonation of the weathering crust. The reservoir formation mechanisms are mainly tectonic activity, weathering, erosion, and burial dissolution. Rock mechanics test results indicate that favorable internal conditions for fracture formation include coarse-grained crystals, high quartz and potassium feldspar content, etc.. Favorable external conditions include rapid cooling processes of the rock, and strong stress transformation during surface exposure. Weathering can form abundant weathering intergranular pores, weathering fractures, mold pores, sieve-like pores, intragranular micropores, and dissolution fractures in the residual zone and weathering dissolution zone. Erosion affects the preservation of the residual zone and weathering dissolution zone. A gentle slope on the top surface of the buried-hill favors the preservation of the residual zone and weathering dissolution zone. Burial dissolution can further expand pre-existing dissolution pores and fractures, promoting the development of inner reservoir zones in the bedrock zone. It is pointed out that inner reservoirs are more developed in the basin subsidence center areas. The porosity evolution of the reservoir at the top of the residual zone and weathering dissolution layer under compaction conditions follows a modified Athy formula. The average particle size and plagioclase content are negatively correlated with the porosity reduction amount while the chemical alteration index is positively correlated with the porosity reduction amount. Differences in rock fabric and the coupling relationship between the buried-hill uplift process and tectonic stress action result in differential fracture effects in buried-hills. In summary, favorable exploration areas for granite buried-hills are identified as regions with coarse grains, high quartz content, high potassium feldspar content, rapid exhumation, gentle slope surface, proximity to the basin subsidence center, and strong late tectonic modification. 

Key words: granite, buried-hill, weathering crust, reservoir genesis, reservoir evolution, oil and gas reservoirs