Abstract
The Eocene Avanah Formation is one of the most productive carbonate reservoirs in the northern Zagros Basin, which is located within the fold-thrust Belt of Kurdistan Region, Iraq. Nevertheless, reservoir heterogeneity, particularly the noticeable difference in porosity–permeability between crest (oil-bearing) and flank (water-bearing) zones of the Khurmala Anticline, remains poorly understood. This thesis investigates the conditions of diagenesis and the related impact on reservoir quality creation and degradation within the dolostone-dominated Avanah Porous (the lower part of Avanah Formation) by integrating petrography, petrophysics, stable isotopes, and fluid-inclusion microthermometry of the formation in these two zones. Petrographic analysis reveals that the precursor wackestone–packstone–grainstone textures were extensively overprinted by pervasive dolomitization, producing intercrystalline, moldic, and vuggy pore systems with widely varying connectivity. Dolomitization, interpreted to be caused by seepage-reflux of mesohaline to penesaline brines during relative sea-level fall, selectively affected the lower part of the formation, forming the Avanah Porous unit. The lack of extensive dolomitization in the upper Avanah Dense unit is attributed to deposition during prolonged marine transgression. Subsequent burial, oil emplacement, and tectonically driven flow of aggressive hypogenic fluids during the Zagros Orogeny caused dolostone dissolution and localized cementation of the formation in the flank well by calcite, dolomite, and anhydrite, accounting for the markedly higher porosity and permeability than the crest well. The crest well exhibits moderate porosity (average ~20%) and relatively low permeability, dominated by intercrystalline and moldic pores with limited cementation. In contrast, the flank well displays higher and more variable porosity (average ~30%) and significantly higher permeability (up to >1000 mD). The absence of late intercrystalline calcite cement in the crest well is attributed to oil emplacement before the flow of hypogenic fluids. Thus, oil emplacement reduced water–rock interaction, inhibited late calcite precipitation, and preserved a significant portion of the dolostone pore system. Stable carbon and oxygen isotopes (δ¹³C = –2.0‰ to +4.1‰; δ¹⁸O = +0.1‰ to +2.8‰) indicate that the dolomitizing fluids were modified Eocene seawater affected by limited evaporation. Fluid-inclusion microthermometry of the coarse-crystalline dolomite and calcite cements suggests trapping of hot (~95–125 °C) NaCl-dominated basinal brines with salinities of approximately 5–12 wt.% NaCl equivalent, during multiple phases of fluid flow. Integration of petrography, isotope ,fluid inclusion and burial history supports a model in which early seepage-reflux dolomitization created the primary reservoir framework, while later flow of hypogenic fluids caused dissolution of the dolostones, particularly on the flanks. After oil emplacement, the crest zone of the Avanah Porous was, to a large extent, protected from additional late diagenetic modifications, thereby preserving its porosity and permeability.
These findings demonstrate that reservoir quality in the Avanah Porous is the result of the interplay between depositional facies, early dolomitization, structural position, hypogenic fluid flow, and hydrocarbon emplacement. The study provides a refined conceptual model for crest-to-flank porosity and permeability variation in Tertiary carbonate reservoirs and highlights the importance of timing of oil filling in preserving reservoir quality. The results hold significant implications for reservoir characterization, development planning, and secondary-recovery strategies in structurally controlled Middle Eastern carbonate reservoirs.
