FRANSSEN, R. C. M. W., and J. F. BRINT, KSEPL/Shell Research, Rijswijk, Netherlands, T. J. SLEESWIJK VlSSER, Shell Expro Lowestoft, Suffolk, England, and A. BEECHAM, Shell Expro, London, England

ABSTRACT: Fracture Characterization and Diagenesis in the Clipper Field, Sole Pit Basin, Southern North Sea

The Clipper field in the Sole Pit basin produces from tight Leman sandstones of the Rotliegende Group (Lower Permian). The reservoir consists of aeolian sediments. Gas production comes from open natural fractures and from dune slipface sands, with rates that are highly variable. The effects of fractures and diagenesis on reservoir quality were investigated.

Three fracture networks have been observed in two highly deviated cored wells. Fault-related fractures occur close to, and parallel with, seismically mapped faults. Fold-related fractures occur as two sets of conjugate fractures, with the local maximum compressive stresses (s1) trending northeast-southwest and northwest-southeast, respectively. The dominant fracture types are cataclastic and dilational shear fractures. The cataclastic shear fractures were reopened and both fracture types are partially filled by silica, carbonate, and anhydrite cements.

The main cement types within the sandstone matrix include dolomite, silica, anhydrite, illite, and ferroan carbonates. Early carbonate cements precipitated during initial burial from a mixture of Rotliegende groundwater and marine pore-fluids from the overlying Zechstein evaporites. Anhydrite cements formed at higher temperatures from Zechstein-derived pore fluids. Pore-filling and fracture-related ferroan carbonate and silica cement precipitated between temperatures of 100-150 degrees C from isotopically evolved pore fluid. The cements in the fault- and fold-related fractures have different isotopic signatures.

Integration of these data with the burial history and regional geological data reveal that the fault-related fractures formed during the formation of the Sole Pit rift basin in the Middle to Late Jurassic. The fold-related fractures formed during the Late Cretaceous inversion. The open fractures that contribute to production are associated with the inversion-related deformation. Modeling of these fracture networks, calibrated against available well data, can be used to define areas with high shear fracture density and hence assist development of fields in the Sole Pit basin, which occur in a similar structural setting.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.