Abstract
Shallow water carbonate rocks of the Lower Cretaceous Qamchuqa Formation host a significant reserve of oil in the NNE Arabian plate-Southwestern Zagros Sutured Belt in the Kurdistan Region, Iraq. Such carbonates consist of several dolostone bodies with unique and complex arrays of saddle and zebra dolomite textures especially in fractured intervals. Integration of field, core description, microstructural analysis, petrographic, stable C, O and Sr isotopes, and fluid inclusion analyses reveals two main phases of multiple fluxes of hydrothermal fluid circulation during the Zagros Orogeny at the end of Cretaceous and Cenozoic times. An early regional non-focused hydrothermal dolomitization and a subsequent fault- and fracture- controlled hydrothermal fluid flow, which led to replacement and cementation by a range of saddle dolomite and zebra textures. Several generations of matrix dolomite, saddle dolomite and calcite cement were identified. The host dolostone has been affected by shortening, folding, fracturing, and thrust faulting. Saddle dolomite pipes, in particular, were found to be associated with en-échelon folding. The morphology and areal extent of the zebra dolomite are controlled by the pore geometry of the host dolostone in relationship to fracturing and faulting.Both matrix and saddle dolomite cements have comparable, overlapping stable isotopic values ranging from 0.39‰ to 3.57‰ and −0.12 to 3.11‰ VPDB, respectively for δ13C and δ18O values range from −10.53‰ to −4.47‰ and −12.87 to −7.79‰ VPDB, respectively. 87Sr/86Sr ratios of pervasive matrix dolomite range from 0.707702 to 0.707839, fall close to late Cretaceous seawater values. Slightly more radiogenic values of saddle dolomites ranging from 0.707759 to 0.708024 resulted from mixing of basinal brines with connate waters. Isotopic signatures of later calcite cements show more radiogenic Sr ratios from 0.707755 to 0.708198 and negative δ13C values 1.76 to −7.45‰, possibly related to late diagenetic processes due to incursion of meteoric waters during subaerial exposure.Fluid inclusion data of pervasive matrix dolomite and saddle dolomites show homogenization temperatures (Th) with a wide range from 71.3 to 169.7 °C and 82.3 to 188.2 °C, and salinity between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These values demonstrate the effect of hydrothermal fluid flow of the alteration of host dolomite and formation of saddle dolomite in fractures and faults.
Published Version
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