Role of facies diversity and cyclicity on the reservoir quality of the mid-Cretaceous Mishrif Formation in the southern Mesopotamian Basin, Iraq

Abstract

An integrated sedimentological and petrophysical approach was implemented to define the role of facies diversity and cyclicity on the reservoir quality of the Mishrif Formation in several oil fields in southern Iraq. The reservoir quality in most regressive cycles was enhanced upwards from deep-marine facies towards the shallower shelf-margin facies. The change in reservoir quality could be detected in the facies stacked systematically within the regressive cycles, which was also easily recognized using the porosity logs. The impact of early diagenetic overprints was quite obvious in developing both reservoir and non-reservoir rock types within the Mishrif Formation in the study area. A simple rock-typing nomenclature was proposed based on the available data in order to classify the existing reservoir (R) and non-reservoir (S) rock types.The best-recognized reservoir rock types were rudistid microfacies with grain-dominated fabrics (including both grainstone (R1) and grain-dominated packstone (R2)), which were subjected to an early diagenetic dissolution process, usually located beneath discontinuity surfaces. Such reservoir units or rock types have a regional extent within the southern Mesopotamian Basin, as they have often developed during the Mishrif shelf-margin progradation. In addition, the other important reservoir rock type was a microbialite (i.e. peloidal mud-dominated packstone (R3)), which was additionally characterized by micropores within the mud-dominated portion of the facies. However, owing to the variable intensity of the diagenetic effects and differences in the depositional texture components, the reservoir quality in this rock type could vary regionally.The regional distribution of the rudistid grainstone and grain-dominated packstone reservoir rock types (R1 and R2) was mostly related to the palaeogeographical highs that existed during deposition. However, such reservoir rock types could pinch out within the depositional sequences, showing their potential to become stratigraphic traps outside the structural crest of the field. The delineation of the reservoir rock types within a sequence-stratigraphic framework can be quite beneficial for reservoir prediction and exploration within and outside of the field.

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…2); this is a process whereby dolomite is replaced by calcite during uplift, especially in aquifers (Jacobson et al 2010). Calcite in carbonates and sandstones can be dissolved leaving enhanced porosity (Mahdi & Aqrawi 2017). Anhydrite can be replaced by gypsum if there is sufficient water available (Fig.…”

Reservoir Modeling of Mishrif Formation in Majnoon Oil Field, Southern Iraq

“…The petrophysical calculations can be performed for an entire reservoir or individually for any layers or fault segments that have been defined during model construction (Bjørlykke, 2015). The petrophysical heterogeneity of the reservoir units within the Mishrif Formation is due to variations in porosity, water saturation and fluid volume (Mahdi and Aqrawi, 2017). 2D models of facies and petrophysical properties are constructed for the Mishrif Formation in Majnoon Oilfield.…”

“…The Mishrif Formation is the most important carbonate reservoir unit in the south of Iraq and it contains up to 30% of the total Iraqi oil reserves (Aqrawi et al, 2010). The reservoir units of the Mishrif Formation in southern Iraq occur at locations where the rudistid facies with appropriate porosity and permeability are developed at different structural elevations of Mesopotamian basin's Oilfields (Mahdi and Aqrawi, 2017). The task of the reservoir geologists is to identify such reservoir units.…”

Biostratigraphy and Sequence Stratigraphy of the Tarbur Formation (Maastrichtian) from Iranian Zagros Foreland Basin, Southwest of Iran

“…The input of clastic sediments (shale facies) influenced Loftusia and rudistids in the Tang-e Shabi Khoon area. Simone et al (2003), Pomar and Kendall (2008), and Mahdi and Aqrawi (2018) concluded that microfacies such as rudist grainstones (MF 7) were deposited in high energy environments. Microfacies MF 7 was formed in the shoal environment.…”