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Abstract:
Basement reserve is an important target in oil and gas exploration. Basing on high resolution 3D seismic data that cover main buried-hill structures, the core data and petrophysical interpretation of nearly one hundred basement wells, this paper aims to study basement reservoir characteristics, vertical and lateral distribution and influence factors of basement reservoir property. The direct basement core observation and thin section microscopic study show that the basement is a complex of igneous and metamorphic rocks which are mainly composed of granite, syenite, monzonitic granite, migmatitic granite and gneiss. The main spaces in basement reservoir are inter-grain pore, cracks and cavities. The reservoir study of the wells drilled in the main buried-hill structures indicate that basement reservoirs are vertically zoning. The reservoir property in crust and broken zone on top of buried-hill is better than that in the bottom and it gets worse as going down into the buried-hill. The basement reservoir is widely and heterogeneously distributed on the buried-hills. The thickness and property are different at different structural areas. Integrated study of structural restoration, seismic, well logging and core data indicate that the basement reservoir property is influenced by paleostructural location, lithology and mineral component and structural movements. The buried-hills which located at high paleotectonic location usually have thicker basement reservoir than the buried-hills located in low paleotectonic location. Felsic minerals are more brittle than mafic minerals and easier to develop fractures under pressure. Dissolutions usually happen with pores and caves developing along the fractures. Comparing to the mafic minerals dominant intervals, felsic minerals dominant intervals usually have better reservoirs. The fractures are well developed nearby the main faults because of structural movements. The directions of present maximum principle stress field and main fractures are very close and it is favorable for the fracture opening and preservation. The study of characteristics and influence factors of basement reservoir provides good possibility for the buried-hill target evaluation and basement reservoir pre-drilling prediction.
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