ABSTRACT

The Devonian carbonate platform (Obj-2) of the Kharyaga field (Russia) has been operated by Total since 1999 and produces 42° API oil, rich in H2S. A multidisciplinary (sedimentological, structural, stratigraphic, and geomechanical modeling) and multiscale study was made of the field, integrating seismic, micro-resistivity log, and core data to characterize the reservoir.

The Kharyaga carbonate platform is characterized by complex relationships among sea-level variations, subsidence, and sedimentation rates. The field is divided into a platform edge (barrier zone) in the southern area, and back barrier and lagoon environments to the north, each characterized by different facies distribution and dynamic behavior. The platform shelf and the reef flat show dome-shaped thrombolites; the back barrier is characterized by grainstone shoals associated with columnar stromatolites, whereas the lagoon comprises peloidal wackestones and packstones associated with laminated fenestral stromatolites. Significant and prolonged subsidence, in both the lagoon and the platform edge, is associated with partial emergence of the platform reef, as shown by karst development.

Seismic interpretation highlighted both underlying north-south structures and syndepositional east-west collapse faults along the southern barrier edge. Two-dimensional and three-dimensional sedimentological modeling of the Obj-2, calibrated by well data and core analyses, allowed the simulation of alternative scenarios to explain the evolution of the carbonate platform during the Devonian and suggested the observed facies distribution is compatible with the presence of a fixed outer platform edge represented by east-west faults.

The carbonate platform of the Kharyaga field exhibits complex behavior; hydrocarbon (HC) accumulation and flow is controlled by the interplay of primary porosity, karst distribution, and structural discontinuities (faults and fractures). Besides the role of the karstification, this paper mainly focuses on the role played by tectonic structures on the evolution of the carbonate platform and on the HC flow. From drill stem test data, east-west faults and karsts are considered as major permeability heterogeneities, but interference tests showed more complex fluid pathways at the smaller scale. Microresistivity images show dominant northeast-southwest and northwest-southeast fracture sets in the central barrier, forming an interconnected flow network with the seismic faults. Geomechanical modeling suggests that these fractures are subseismic structures linked to inherited, deep-seated faults active at the time of fracturing, in addition to the listric reservoir faults.

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