Paleofluid Flow, Fracturing and Reservoir Implications in Foreland basins: an example from the Cretaceous Qamchuqa Formation, Kurdistan Iraq

Gali Ali Bag Gorge is located in W-NW Zagros Fault Thrust Belt, Kurdistan Region, northern Iraq. The gorge exposes shallow marine Lower Cretaceous Qamchuqa carbonates along more than 5km. The investigated outcropped carbonate strata and equivalent subsurface sections consist of several dolostone bodies with unique and complex arrays of saddle and zebra dolomite textures especially in fractured intervals. Such carbonates are structurally controlled and affected by hydrothermal fluid flow and represent one of the major and important reservoir rocks in the High Folded oilfields. An integrated study combining field observations, core descriptions, microstructural analysis, petrographic study, stable C, O and Sr isotopes, fluid inclusion data, mineralogical and geochemical investigations reveals at least two main phases of multiple fluxes of hydrothermal fluid circulation during the Zagros Orogeny at the end of the Cretaceous and in Miocene to Pliocene time. These fluids were responsible for the formation of early regional non-focused, strata-bound hydrothermal dolomitization followed by localized fault- and fracture-focused hydrothermal dolomitization. These hydrothermal fluid flows caused variable textural and compositional alteration, overgrowth and zonation in dolomite occurred in a fairly open diagenetic system. The host dolostone was affected by shortening, folding, fracturing, and thrust faulting. Saddle dolomite pipes are associated with enéchelon folding. The morphology and areal extent of the zebra dolomite was controlled by the pore geometry of the host dolostone in relationship to fracturing and faulting. Five types of pervasive dolomites, three types of saddle dolomites and four types of calcite cements have been identified. Compositionally, the pervasive and saddle dolomites are dominantly poorly ordered, non-ferroan and non-stoichiometric. Both matrix and saddle dolomite cements have comparable, overlapping stable isotopic values and slightly radiogenic 87Sr/86Sr ratios. The primary fluid inclusion microthermometry of pervasive and saddle dolomites reveal saline and moderate to high homogenization temperatures. Pervasive dolomites moderately display comparable saddle dolomite REE patterns, light REE (LREE) enrichment, and moderate to low (∑REEs) content. These geochemical attributes suggest hydrothermal saline brines mixed with connate marine fluids caused the dolomitization and precipitation of dolomite cements. These processes modified the original pore system and, in combination with fracturing, generated areas of excellent porosity and permeability, as exemplified by the presence of porous saddle dolomite and variety of zebra textures. Both host and cement dolomite fabrics show a wide variability of diagenetic alteration that modified the original pore system. These alterations resulted from the incursion of multiple, earlier hydrothermal and saline fluid fluxes and later meteoric fluids during uplift and exposure. These alteration processes include dissolution, cementation and karst formation. Excellent intervals of reservoir facies and well-developed porous and permeable intervals are found in association with hydrothermal dolomites. In the outcropped section, the best reservoir properties encountered in the central gorge around thrust faults. In these zones zebra textures crossed cut by late extensional semi-vertical joints, especially when dissolution along these fractures led to the development and enlargement of the vuggy pores and fractures. This study demonstrates the linkage between fluid flux history and related diagenesis during the tectonic evolution of Zagros Basin, and provides an ideal example of the role of thrusting and tectonics in controlling diagenetic fluids and modifying reservoir characteristics. The improved understanding of porous saddle dolomite and HZDs may aid future exploration for hydrocarbon reservoir in the Zagros Fold Thrust Belt Basin