Abstract
The Paleogene Avanah and Khurmala formations were investigated from a reservoir characterization and lithology determination point of view in the three selected wells (KX, KY, and KZ) in the Khurmala Oilfield, northern Iraq. The conventional wireline log data, core test measurements, and available rock samples were used to determine the petrophysical properties and reservoir quality of the studied formations. The common lithology of both formations is identified based on porosity log data and utilizing density versus neutron and M-N cross-plots. The upper part of the Avanah Formation, informally known as Avanah Dense, is composed of limestone and dolomitic limestone, whereas the lower part of the formation, known informally as Avanah Porous, is composed mainly of dolostone, Dolomitic limestone, and limestone with different ratios of shale content.
Thirty-five cutting rock samples from well KX depths 812–948 m were studied optically alongside twenty photomicrographs from depths 865.16 to 972.22 m, representing the Avanah and Khurmala formations, which revealed various facies, with Avanah Porous containing more diverse and porosity-enhancing facies (e.g., Grainstone, Packstone), while the Avanah Dense and the Khurmala formation showed more compact Wackestones. Diagenetic processes like dolomitization, dissolution, cementation, and compaction played a major role in porosity development. Log analysis confirmed lithology variations; the Avanah Dense is mainly composed of limestone with low shale content, while the Avanah Porous and the Khurmala formation are of dolomitic lithology with increasing shale content. Porosity and permeability peaked in Avanah Porous, Ranging between (20-35%) and (100-10000mD) especially in Well KZ, where secondary porosity likely boosted reservoir quality. Fracture porosity analysis reinforced the importance of secondary porosity which was about 0-10% from total porosity. Gas-bearing zones were found mainly in the Avanah Dense (wells KX and KY). Resistivity logs suggested hydrocarbon saturation with limited water invasion except in deeper intervals of the Khurmala Formation.
Water and hydrocarbon saturations showed movable hydrocarbons concentrated in upper Avanah, with low bulk water volume supporting reservoir quality. Based on the variations in the shale content, porosity, permeability, and water saturations, a number of main and secondary reservoir units were identified in the Avanah and Khurmala formations 6 units for wells KX and KY, and 7 units for well KZ. In Well KY, the Avanah Porous reservoir units showed effective hydrocarbon potential despite some higher water saturation. Four hydraulic flow units were identified in well KX that showed fair to good (FZI: 3.28–29 µm), six in well KY (FZI: 0.29–178 µm), and six in well KZ (FZI: 0.1–4.1 µm), which was mostly poor to fair with some good intervals.
Movable Hydrocarbon Index (MHI) confirmed hydrocarbon movability in several zones from the studied sections. Overall, the Avanah Porous interval shows the best reservoir potential due to enhanced porosity, Net reservoir and net pay intervals inside the Avanah and Khurmala formations in wells of the study were determined using lithological and petrophysical cutoffs, specifically for shale volume, porosity, permeability, and water saturation. Cross-plots were used to determine the porosity and saturation thresholds. N/G ratios vary significantly amongst formations and wells, according to the results, with well KX having the highest net pay and hydrocarbon content. However the Avanah Formation showed varied reservoir and pay characteristics throughout its units, the Khurmala Formation showed more constant productivity.
