Sedimentological characterization of the mid-Cretaceous Mishrif reservoir in southern Mesopotamian Basin, Iraq

Abstract

The CenomanianÐEarly Turonian reservoirs of the Mishrif Formation of the Mesopotamian Basin hold more than one-third of the proven Iraqi oil reserves. Difficulty in predicting the presence of these mostly rudistic reservoir units is mainly due to the complex paleogeography of the Mishrif depositional basin, which has not been helped by numerous previous studies using differing facies schemes over local areas. Here we present a regional microfacies-based study that incorporates earlier data into a comprehensive facies model. This shows that extensive accumulation of rudist banks usually occurred along an exterior shelf margin of the basin along an axis that runs from Hamrin to Badra and southeast of that, with additional interior rudist margins around an intra-shelf basin to the southwest. Regional tectonism defined the accommodation sites during the platform development. Facies analysis allowed the recognition of 21 microfacies types and their transgressive-regressive cyclic stacking pattern. Sequence-stratigraphic analysis led to the recognition of three complete third-order sequences within the studied Mishrif succession. Eustatic sea-level changes were the primary control on this sequence development but local tectonics was important at the Cenomanian/Turonian boundary. Rudist biostromes are stacked as thicker shallowing-up cycles composed of several smaller-scale cycles. In places, smaller cycles are clearly shingled (stacked laterally). Iraq’s Mishrif sequences are thus analogous to coeval systems across the Arabian Plate in Oman, United Arab Emirates, offshore Saudi Arabia and Kuwait, southwest Iran and the Levant. Analysis of poroperm trends shows porosity increasing beneath sequence boundaries due to karstification and meteoric dissolution. The presence of interconnected vugs in grain-dominated fabric make the rudist biostromes the best reservoir units. Dissolution of aragonitic components of rudist shells was the most important diagenetic process that enhanced reservoir characteristics. The presence of rudist-bearing facies with their diagenetic overprint within regressive cycles is considered the primary factor in effective porosity development and distribution. As a result, because of depositional heterogeneities (facies type distribution and their 3-D geometries) and the influence of sequence boundaries on reservoir quality, each field shows unique geometrical combinations of pay zones, barriers and seals.