ORIGINAL PAPER Depositional environments and porosity distribution in regressive limestone reservoirs of the Mishrif Formation, Southern Iraq Moutaz Al-Dabbas & Jassim Al-Jassim & Saad Al-Jumaily Received: 27 February 2009 / Accepted: 30 April 2009 / Published online: 9 June 2009 # Saudi Society for Geosciences 2009 Abstract Eight subsurface sections and a large number of thin sections of the Mishrif Limestone were studied to unravel the depositional facies and environments. The allochems in the Mishrif Formation are dominated by bioclasts, whereas peloids, ooids, and intraclasts are less abundant. The sedimentary microfacies of the Mishrif Formation includes mudstone, wackestone, packstone, grainstone, floatstone, and rudstone, which have been deposited in basinal, outer shelf, slop followed by shoal reef and lagoonal environments. The formation displays various extents of dolomitization and is cemented by calcite and dolomite. The formation has gradational contact with the underlying Rumaila Formation but is unconformably overlain by the Khasib Formation. The unconformity is recognized because the skeletal grains are dominated by Chaophyta (algae), which denotes the change of environ- ment from fully marine to lacustrine environment. Thus, the vertical bioclast analysis indicates that the Mishrif Forma- tion is characterized by two regressive cycles, which control the distribution of reservoir quality as well as the patterns of calcite and dolomite cement distribution. Mishrif Formation gradationally overlies Rumaila Formation. This was indicated by the presence of the green parts of Chaophyta (algae) as main skeletal grains at the uppermost part of well Zb-47, which refer to lacustrine or fresh water environment. Petrographical study shows that the fossils, peloids, oolitis, and intraclasts represent the main allochem. Calcite and dolomite (as diagenetic products) are the predominant mineral components of Mishrif Formation. Fossils were studied as an environmental age and facial boundaries indicators, which are located in a chart using personal computer programs depending on their distribu- tions on the first appearance of species. Fifteen principal sedimentary microfacies have been identified in the Mishrif Formation, which includes lime mudstone, mudstone– wackestone, wackestone, wackestone–packstone, pack- stone, packstone – grainstone, grainstone – floatstone, packstone–floatstone, packstone–rudstone, and wacke- stone–floatstone. Markov chain analysis has been used to study the transitional pattern of different microfacies types vertically in each well and laterally in all wells as a composite section. The vertical analysis indicates that the Mishrif Formation characterized by two regressive cycles, the main one started with basinal or outer shelf environ- ment, slop environment followed by shoal or reefal environment, and ended with a lagoonal environment. The lateral analysis shows the same regressive cycle, and by using the lithofacies association concepts, we built the depositional model of the Mishrif Formation environment. Keywords Mishrif Formation . Porosity . Facies analysis . Depositional environment Introduction The Mishrif Formation (Cenomanian–Early Turonian age) is one of the important carbonate reservoirs in middle, southern Iraq and throughout the Middle East (Videtich et al. 1988; Alsharhan and Nairn 1993; Alsharhan 1995; Aqrawi et al. 1998). In southern Iraq, the formation provides the reservoir in oilfields such as Tuba, Zubair, West Qurnah, and Nasryia (Fig. 1). The formation contains up to 40% of Cretaceous oil reserves in Iraq and about 30% of total Iraqi oil reserves (Aqrawi et al. 1998).The Mid- Arab J Geosci (2010) 3:67–78 DOI 10.1007/s12517-009-0057-x M. Al-Dabbas (*) : J. Al-Jassim : S. Al-Jumaily College of Science, University of Baghdad, P.O. Box 47138, Baghdad, Iraq e-mail: prof_aldabbas@yahoo.com