A 3-D Fracture Model of the Kuyumba Oil Field (Eastern Siberia) Reflecting the Clay and Bed Thickness-related Fracture-density Variations of its Dolomite Reservoir

Abstract

A geological and structural approach aimed to analyze and model fractures in subsurface-fractured reservoirs is proposed using the complete geological record of well data such as cores, borehole images, and wireline logs. This methodology is applied to an exploratory vertical well drilled in the hinge zone of a large asymmetric anticline fold in the central part of the Kuyumba oil field in eastern Siberia, Russia. Here, the reservoir succession is mainly made up of fractured tight dolomites of Precambrian age, with apparently no matrix contribution. Results from core descriptions and analysis of borehole images showed that the studied reservoir formation is highly fractured, and the fracturing is not homogeneously distributed throughout the well. Fractures are subvertical, bed bounded, with a roughly planar geometry and a regular arrangement. We documented the presence of three well-developed sets of closely spaced fractures: N30, N50, and N90. The first two are predominant and are interpreted to be conjugated shear fractures possibly generated prior to and during initial anticline growth. The comparison between the fracture distribution and the wireline logs allowed us to observe that fracture spacing within the studied reservoir succession is a direct function of the lithology (expressed by the volume of shale) and thickness of the fractured beds. In fact, an increase of the shale content corresponds to an increase of the fracture spacing. Fracture spacing in clean dolomites (volume of clay [Vcl] 20%) is much smaller than in shaly dolomites. By contrast, no fractures occur in pure shale intervals. Within the same lithology, we observed that fracture spacing is strongly dependent upon bed thickness. In fact, we found that, in pure dolomites (Vcl 5%), fracture spacing (S) scales with bed thickness (B) according to the equation: S = 0.11B0.9. Within rocks with 5% Vcl 15%, fracture spacing and bed thickness are related by the following relation: S = 1.52B4.3. As a result of this structural analysis using well data, a three-dimensional discrete fracture network (DFN) model was set up around the studied well. Using this both statistical and deterministic approach, the vertical lithologic variation at the well(s) and the interrelationships between fracturing at well scale, petrophysical heterogeneity and reservoir deformation can be quantified, thus allowing the building of a well-constrained and more realistic DFN. This latter could be used in the future to assess the hydraulic properties of fractures and to simulate the flow conditions during different production operations.