Online Journal for E&P Geoscientists

Kesh, Subhasis ^*1
(1) Petroleum Engineering and Earth Science, University of Petroleum and Energy Studies, Dehradun, India.

Fractures play a noteworthy role in both the porosity and migration pathways of Middle East carbonate reservoirs. Fracture distribution in folded carbonate reservoirs is controlled by a blend of the tectonic evolution and the mechanical properties of the strata. In order to improve prediction of fracture distributions in Middle East carbonate reservoirs, we are performing studies of fracture development in folded carbonates in the Zagros fold and thrust belt in Iran, Iraq, and the Adam Foothills of Oman. Scales of observation range from satellite imagery studies, through fieldwork, to micro-structural analysis. The orientations and spatial distribution of fracture sets mapped around fractured/faulted anticlines are allied to the deformation history, particularly with respect to folding mechanisms and the likely evolution of stresses during folding. Such genetic controls are thought to be important in ballpark figuring likely reservoir fracture geometries from large-scale, regional information such as tectonic deformation history and evolution of stress regime. Also important in controlling fracture development during folding are the presence of pre-fold and syn-fold faults, at the same time as

late or post-fold faulting commonly develops independent fracture clusters. The relationship between lithofacies and fracture properties is discussed, from a number of points of view. Firstly, strain partitioning along weak beds during folding may affect the vertical continuity of fractures. Secondly, fracture characteristics (e.g. density and aperture) depend on the mechanical properties of a given lithology. Thirdly, diagenetically altered units along key surfaces such as hard-ground surfaces or paleokarstified units exert a Geo-mechanical control affecting the development of later fracture arrays. An important issue in fractured carbonate reservoirs is whether much of the fracture porosity has been shattered by calcite cementation. On the other hand, micro-structural observations show that even cemented fractures can resurrect, under certain stress conditions, to act as conduits to fluid flow or as surface cracks held open during reburial by porous sediment infill. Such diagenetic controls highlight the sensitivity of final fracture array properties to the burial and uplift history in relation to folding and fracture cohort.