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Influence of Tectonic Stress Regime on Fracture Porosity of Tight Carbonate Reservoirs


Fracture zones are usually characterized by an increased fracture density and thus a higher secondary porosity. In the studied area (Potwar Basin, Northern Pakistan), the porosities derived from a neutron log and fracture apertures (obtained via high-resolution resistivity images) are very low, even if the resistivity images show a high fracture density cataclastic zone. This analysis refers to a tight carbonate reservoir located close to a major fault zone, with a high density of fractures that do not contribute to fluid flow due to their small apertures. The compressive, reverse faulting regime with active hanging walls led to the formation of the main hydrocarbon traps, these being laterally delimited by such faults. Borehole breakouts and drilling-induced fractures were used for tectonic stress evaluation. The dominant minimum stress direction identified in the analyzed wells is WSW - ENE, normal to the direction of drilling-induced fractures, as indicators of the maximum stress direction, which are oriented NNW - SSE. The orientation of drilling-enhanced fractures confirms the maximum stress direction as determined via drilling-induced fractures. The alignment of the major fault and the natural fractures, parallel to the minimum horizontal stress direction, suggests a compressional tectonic regime at the moment of their formation. The maximum tectonic stress, normal to the fractures direction, has contributed to apertures closure and secondary porosity reduction. This explains why in an area characterized by intense cataclasis, resulted from the fault proximity, porosity is less than 2% and fracture apertures are, generally, less than 0.5 mm. But the simultaneous presence of breakouts and drilling-induced fractures in the same well is an indicator of a strike-slip stress regime. One may conclude that a change in the tectonic stress regime took place, from a compressive to a strike-slip one. The major fault, which initially had a compressive character (dominantly vertical movement), later became a strike-slip one, with a dominantly horizontal component. Knowledge of the fractures' propagation direction with respect to maximum tectonic stress is extremely important for the potential design of horizontal wells. Experience has shown that production is highly correlated to fracture volume and connectivity in tight formations, implying the future successful economic development of this particular field lies in knowledge of the fracture propagation direction.