The early to middle Eocene [ahrurn Formation constitutes a major gas reservoir In the southeastern Zagros Basin of Iran. The depositional and diagenetic history of the Formation is being investigated with the aim of better understanding the controls on reservoir quality and their spatial context. The data base consists of a combination of a 636m thick surface outcrop (north flank of the Faraghun anticline) and subsurface well sections (Sarkhun gas-field) of the [ahrum Formation. At outcrop the Formation can be divided into a lower part (246 m) comprising gray massive dolomites and an upper part (390 m) of variably dolomitised limestones with a great diversity and abundance of larger benthic foraminifera (LBF), especially those with porcelaneous calcareous walls (e.g., miliolids, Somalina and Alveolina). The [ahrurn Formation in the Sarkhun gas-field is dominated by a uniform package of shallow marine limestones with abundant hyaline larger benthic foraminifera (e.g., Nummulites). Cenozoic formainiferal biostratigraphy utilizes a shallow benthic zonal (SBZ) scheme (SerraKiel et al., 1998). Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the formation is interpreted to range between the Middle Cuisim (SBZ11 defined by abundant specimens of Alveolina cremae) and Early Bartonian (SBZ17 defined by the biostratigraphic range of Nummulites perforates). Detailed facies analysis of surface exposures and core, augmented by thin section petrography, has identified 12 principal shallow marine carbonate facies. An absence of calciturbidites, frame-building organisms capable of building steep-rimmed platforms and a preponderance of gradual facies changes suggests that the [ahrum Formation was deposited on a gentle carbonate ramp. Analogous platform types are commonly inferred for other Eocene LBF-dominated limestones in circum-Tethyan settings (Buxton and Pedley, 1989; Beavington-Penney et al., 2005; 2006). [ahrum carbonates display a wide spectra of diagenetic features, includes micritization, dissolution, calcite cementation, mechanical and chemical compaction, fracturing and significant dolomitisation. The last four of these processes, and in particular dolomitisation, have resulted in the most significant changes in texture and reservoir quality. Dolomite is abundant in the [ahrum Formation, and is the most important factor enhancing reservoir quality either by preserving or increasing porosity (approximately 15%) and permeability, and/or by making these reservoir rocks more brittle and thus more prone to fracturing. This does not occur within limestones (with average porosity 3% and permeability 0.005 to 0.06 mD). In general, dolomite intervals have higher average porosity and permeability than limestones, and inter-crystalline porosity and fracturing due to dolomitisation is an important factor controlling reservoir quality in the [ahrum Formation. The lower part of the [ahrum Formation has been subjected to both selective and pervasive dolomitisation. Replacement dolomite constitutes more than 90% of the rock volume, while the remaining 10% is occupied by calcite (100 - 500 11m) and dolomite (100 - 150 11m) cements and a minor residual porosity. Dolomite textures are predominantly \"microsucrosic\" non planar to planar-s and crystal sizes range from 10 11m to 60 11m and 60 - 110 11m respectively. This finely crystalline dolomite partly preserves original limestone fabrics such that grains and matrix can be distinguished. Where dolomitisation was less pervasive micrite matrix and some skeletal grains are selectively replaced by planar-e crystals of about 120 11m. 12

Keywords