Abstract: The Calibration of Borehole Images to Core for Detailed Sedimentological, Structural and Reservoir Studies

Denis Haller, Benoit Mathis, Eric Standen

Detailed sedimentary and tectonic analysis of a reservoir requires the comparison and analysis of numerous data sets of differing source and scale. Core analysis, when available, is of prime importance in defining the sedimentary structures, facies and understanding of depositional environments within the reservoir. Borehole images are vital for defining the orientation of bedding, structures and fractures. Through two case studies, this paper describes a technique for much improved accuracy in the correlation and orientation of core features using wellbore images. The system developed, allows the building of a detailed, composite document for a comparison, at the same scale, of all the data., in order to define the reservoir evolution. This data can include full circumfe ence core images, core sedimentological analysis, standard core photos, high resolution borehole images, petrophysical formation evaluations and well test data. With this complete analysis, a detailed stratigraphic model of the reservoir is easily produced to improve the understanding of the relationships between porosity, permeability and hydrocarbon saturation, to sedimentary and diagenetic features.

The first case study is a carbonate reservoir in which porosity results from both primary and diagenetic processes. The main productive layers are strongly heterogeneous and alternate between porous networks, with better oil saturation and tight zones containing calcite nodules. The porosity is ascribed to the top of each shallowing-up sedimentary cycle of a sequence in which vadose, solution leaching has occurred. The permeability barriers correspond to stylolites or cementation at the base of the individual cycles. The second case study, from the Brent reservoir in the North Sea, Frigg area, is a cross-bedded sandstone reservoir with structurally developed fracturing. The workstation allows to display, side-by-side, the electrical borehole images and the unrolled core photographs. W th a direct correlation between both data sets it is easy to understand the features observed on the borehole images where natural, fractures can be differentiated from drilling-induced fractures, giving a procedure for analysing fracture types on the images in non-cored sections. For the geometry of the sand-bodies, reliable and detailed sedimentary dips are obtained from both sets of data. The classical description of sedimentary features in core is complemented by their orientation from the borehole images, and leads to a better definition of geometry and potential extent of the sandbodies.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France