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Multi-Attributes Applied in Mapping of Fracture Zones and Reservoir Evaluation of a Granite Buried Hill, Bohai Bay, China


The study area is located in the east of Bohai bay, China. The buried hill is a large semi-anticline, and controlled by a large growth fault that trends north-east and has throws of up 1.5km, and induced faults are well developed. The buried-hill was raised and eroded in a long period before Neocene sediments overlapped directly, and the structural feature of the top is very rounded with North and South two highs. The lithology of buried hill is Proterozoic metamorphic rocks, and the mainly oil-bearing formation is a Mesozoic granite intrusive body with fracture-pore reservoir type. This is mainly affected by fractures development and the effect of weathering and denudation. The principal objective of this study is to outline the areal extent of granite intrusions and predict the fracture-pore reservoir characteristics. Structural fractures are the primary factor forming the reservoir of buried hill based on the integrated analysis of imaging logging, core and thin section analysis. The fractures are likely to be better developed in the vicinity of faults, local flexures and folds. Curvature attributes have been proved to be very useful in the interpretation of micro faults and fractures based on the understanding of the variety of curvatures. All the attributes extraction is based on the top surface of buried hill, especially the geometrical attributes, such as dip, azimuth and curvature. Curvature is very sensitive to the noise as the 2nd derivate of target surface, so iterative median filter is applied first on the accurate auto-tracking top horizon of the buried hill. Based on the filtered surface, coherency and horizon-based edge detect attributes is extracted and used to map the areal extent of the granite intrusive body. Then the maximum positive curvature is calculated to describe the tectonic stress concentration on the plane, and an obvious zonality paralleling to boundary fault is discovered by contouring map of the curvature attribute. The spectral decomposition and negative amplitude attributes present and confirm the lateral zonality. Cross plot of the maximum trough amplitude and reservoir thickness from well log interpretation is carried out based on the understanding above. The good correlation gives an effective prediction of reservoir thickness trends. The results have been verified effective to character the development of granite reservoir, and have been used in finalizing well locations and calculation of reserves.