A computer procedure extracts geological information from wireline logs and generates a well summary document similar to a core description. Lithology is determined by reference to a lithofacies database of responses from open-hole logs, comprising at least density, neutron, and gamma-ray logs, and preferably also sonic, photoelectric cross section, thorium, uranium, and potassium logs. Texture and structure are derived by analyzing dipmeter microresistivity curves. The various descriptive elements then are analyzed and combined using geological information encoded as rules or knowledge bases.
The results of this technique provide a lithological zonation of the well showing bedding type (massive, laminated, etc.), parallelism of bed boundaries, presence and nature of heterogeneities, and sand/shale ratio. The geometry of sand/shale layers is defined with high precision. Resistivity trends and sedimentary dip distributions are displayed and provide elements for a sedimentological interpretation.
The log-derived description must be calibrated with core data and can be extended to geologically similar uncored intervals. It can also serve to prepare and enhance the core description itself.
Applications of this procedure to the appraisal of reservoir potential and the interpretation of borehole imagery illustrate how the method helps in understanding reservoir heterogeneity and shale distribution, which largely control fluid flow in the reservoir.