Reservoir evaluation of the Lower Turonian–Cenomanian successions in the Jambur Field, Zagros Basin, Iraq: insights from petrophysical analysis

This study evaluates the reservoir quality of the Lower Turonian-Albian succession (Gulneri, Dokan, and Qamchuqa formations) in Jambur Field, Zagros Basin, Iraq, utilizing integrated petrophysical data analysis from three wells (Ja-28, Ja-34, and Ja-38). The data includes environmentally corrected well logs, including gamma ray (GR), neutron (NPHI), density (RHOB), sonic (DT), and resistivity (LLD, MSFL) logs. These data were used in order to establish lithology/fabric, utilizing N-D and M-N crossplot, computer-processed interpretation (CPI) for porosity and saturation (PHIT, PHIE, Sw, BVW, BV _SXO ), and log-based permeability estimation (perm. calc.) calibrated to core measured permeability. Crossplots show dolomite and calcite dominance with secondary porosity in the Qamchuqa Formation, moderate but largely ineffective porosity in the Dokan Formation due to microporosity/argillaceous inclusions, and tight beds in the Gulneri Formation.The Qamchuqa Formation is the main reservoir with a PHIT of 10–30% (average 14–15%), PHIE of 7–8%, and very large net pay thicknesses: 116 m in Ja-28, 74 m in Ja-34, and 120.5 m in Ja-38. Sw values are always below 40%, BVW is ˂0.08, BV _SXO values are greater than BVW, which is evidence of movable hydrocarbons. Core permeability is up to 145 mD, and log derivations of calculated permeability (perm. cal.) are above the core results, which are systematically biased by the transform of empiricism. Conversely, the Dokan Formation exhibits moderate PHIT (10–18%) and low PHIE (< 8%) with net pay < 10 m, high average Sw (> 60–70%), BVW > 0.10, and overlapping BVW-BVSXO curves, subsequent to low reservoir quality. Gulneri Formation remains very tight, PHIE is close to zero, net pay is negligible, Sw average (> 80), BVW average (> 0.10), and BV _SXO = BVW, all of which verify that it is a seal.These results indicate that Qamchuqa intervals are the most favorable formation in terms of reservoir quality, in which diagenetic processes of dolomitization and dissolution increase porosity and permeability. Structurally favorable Qamchuqa intervals should be given priority in development and improved oil recovery with core-calibrated log interpretation in order to decrease the uncertainty in the prediction of permeability and movable hydrocarbon.