Depositional Facies, Sequence Stratigraphy, and Diagenesis of Lower Cretaceous Carbonate Reservoir, Southern Iraq: Implications for Petroleum Exploration

Abstract

The Lower Cretaceous Yamama Formation of southern Iraq represents a key carbonate reservoir within Iraq and the Middle East, yet its complex depositional facies architecture and diagenetic alterations present challenges for predicting reservoir quality. This study integrates well-log interpretation, detailed description of about 400 m of core intervals, petrographic analysis, fluid-inclusion microthermometry, and stable-isotope (δ¹³C, δ¹⁸O) geochemistry from several oilfields to determine the controls on reservoir heterogeneity. Fourteen depositional facies are recognized across a W–E-trending, slightly steepening homoclinal ramp. These vary into four reservoir (YRA–YRD) and four nonreservoir (B0–B3) units. Reservoir intervals consist mainly of shoal grain-supported peloids and ooids and Lithocodium–Bacinella reefal facies, whereas mud-supported lagoonal and middle-ramp facies form permeability barriers. Diagenetic processes exerted strong control on reservoir quality evolution. Early marine rim and scattered calcite cements preserved interparticle pores in the grain-supported limestone facies, whereas burial equant calcite sourced by stylolitization of the host limestones reduced porosity. The precipitation of pore-filling kaolin and saponite in the mud-supported limestone facies limited porosity. Saponite is suggested to have been formed from alkaline brines developed under restricted, evaporative conditions. The formation of kaolin is attributed to the migration of Al³⁺-bearing brines charged with organic acids along stylolites, during tectonic compression of the basin. The presence of framboidal and euhedral pyrite indicates a diagenetic shift from microbial to thermochemical sulfate reduction. TSR suggests that brines were thermally evolved and migrated upward from the underlying, hotter evaporitic formation, such as the Upper Jurassic Gotnia. Fluid-inclusion microthermometry in blocky calcite in moldic/vuggy pores indicates homogenization temperatures of 85°C–140°C (exceeding the maximum burial temperature, ∼116°C) from brines composed of NaCl–KCl–H₂O, with total salinity of 10 wt.% NaCl eq. and δ¹⁸O_VSMOW values of +4‰ to +8‰. These values, along with depleted δ¹⁸O_VPDB (−6.9‰ to −5.6‰), suggest precipitation from evolved basinal fluids related to stylolitization. The occurrence of saddle dolomite in close association with euhedral pyrite suggests precipitation from brines that were affected by TSR. Four third-order depositional sequences bounded by Type 2 sequence boundaries are recognized; regressive shoal and reefal facies formed the reservoir units, whereas transgressive, mud-dominated facies are impervious. The integration of depositional facies, diagenesis, and sequence stratigraphy indicates that depositional facies architecture and fluid migration allow better understanding of the reservoir heterogeneity in the Yamama Formation.