TITLE:
Reservoir Characterization and Stratigraphic Relationships of Mishrif Formation in Gharraf Oil Field
AUTHORS:
Nabaa Al-Itbi, Xuri Huang
KEYWORDS:
Mishrif Formation, Gharraf Oil Field, Mudstone, Wackestone, Packstone
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.14 No.4,
April
30,
2026
ABSTRACT: Mishrif Formation (Cenomanian-Early Turonian) is considered a main reservoir in the Gharraf oil field, south of Iraq. Three wells, namely Ga-1, Ga-2, and Ga-3, are selected to study the reservoir properties and stratigraphic relationships for the Mishrif succession. Six main microfacies are identified, which are lime mudstone, bioclastic wackestone, bioclastic packstone-wackestone, bioclastic wackestone-mudstone, pelagic mudstone-wackestone, and bioclastic packstone-grainstone microfacies in addition to their associated depositional environment. The diagenesis process has affected the Mishrif rocks and played a role in deteriorating reservoir porosity in well Ga-2 and enhancing it in wells Ga-1 and Ga-3. These processes include cementation, micritization, recrystallization, dissolution, compaction pressure solution, and dolomitization. The formation is subdivided into three main lithological units separated by an impermeable unit depending on the log’s response. Middle Mishrif has been divided into MB1, MB2, and MB4 in Ga-1 and Ga-3, and MB3 and MB4 in Ga-2 according to log data and microfacies analysis. Reservoir properties are determined by using different well logs. Porosity is calculated via (sonic, neutron, and density) logs, and its value is compared with porosity derived from core analysis. The primary porosity represents the main porosity in the formation, while the secondary is rare. The log response shows that the Mishrif Formation has different oil contacts. In Ga-1 and Ga-3, it occurs at 2360 m, while in Ga-2, it is at 2344 m. The results of water and hydrocarbon saturation show it also has different quantities of water, residual oil, and mobile oil. The main hydrocarbon accumulation is found in well Ga-3, then Ga-1, and less accumulation appears in Ga-2. The microfacies analysis shows that the Mishrif Formation forms a shallowing upwards succession starting with a deep shelf environment and ending with a restricted environment and consists of two third-order. Two maximum flooding surfaces are determined within the formation. The seismic section shows the downlap termination that forms the stratigraphic trap in the western side of the field, dividing the field into two sectors: an eastern sector where Ga1 and Ga3 are located and a western sector where Ga-2 is located.