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The Genesis and Controlling Factors of Large Mesozoic Granite Weathering Crust Reservoir in Penglai 9-1 Oil Field, Bohai Bay Basin


Penglai9-1 buried-hill oil field locates at Miao Xi Uplift, and has proved reserves of more than 2×108 m3. It is so far the single largest and the only Mesozoic granite oil field in Bohai Bay Basin. The high-quality reserve stratum locates at the middle-upper part of the weathering crust of granite, with the distribution area of more than 80 km2 and the maximum reservoir thickness of more than 200 meters. The reservoir space is mainly pore type and fissure-pore type, and secondly pore-fissure type including intercrystalline pores, dissolving pores in the lithic fragments and hornblende and micro fissures. Reservoir show as loose sand in core, resulted in sand production during testing. Based on the analysis of petrology and outcrop, we proposed that weathering is the major factor responsible for the forming of the high-quality reservoir, and the dissolving of feldspar is the main genetic mechanism. Leaching result in the apparent decrease of mechanical strength of granite, then the tectonic stress, the force of gravity, and heat-expansion and cold-contraction of crystals may be the important factors for the formation of intercrystalline pores and micro fissures. High quality reservoir lateral variation is governed by the followed factors. First of all, shallow bury and low thermal evolution degree is a key factor to keep reservoir in a close condition to the initial weathering crust. Rock joints, faults zone and palaeotopography have important role in controlling the high-quality reservoir lateral variation. Joint fissures and fault fissures form an efficient channel for atmospheric water permeating, as a result, thick weathering crust developed. This effect in Penglai9-1oil field showing as high-quality reservoir at joint shaped reflection developed area is thick, and on the contrary, reservoir at joint shaped reflection underdeveloped area is thin. In concentrating belt of faults, the thickness of reservoir is obviously thicker. Palaeotopography control the preserving of the strongly or moderately weathered belt for which is always loose and easily being removed. This effect in Penglai9-1 oil field showing as thick high-quality reservoir mainly distribute at flat belt around high center of the granite. The reservoirs at the flat ground, the slope toe and the platform at the top of slope in micro-palaeotopography are always thicher than those at upper-middle part of the steep slope.