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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Characterisation of the Mid-Cretaceous Mishrif Reservoir of the Southern Mesopotamian Basin, Iraq

Adnan A. Aqrawi1; T. A. Mahdi2; G. H. Sherwani3; A. D. Horbury4

(1) Program Manager, Technology Learning, People & Leadership Development, HRS, Global Business Services, Statoilhydro, Stavanger, Norway.

(2) Faculty of Geology, University of Bergen, Bergen, Norway.

(3) Ministry of Higher Education, Kurdistan, Iraq.

(4) Cambridge Carbonates Ltd, Solihull, United Kingdom.

The Cenomanian-early Turonian Mishrif Formation reservoir of the Mesopotamian Basin accommodates more than one third of the proven Iraqi oil reserves within rudist-bearing stratigraphic units. Difficulty in predicting the presence of reservoir units is due to the complex palaeogeography. Extensive accumulation of rudist banks occurred along an exterior shelf margin of the basin along an axis that runs from Hamrin to Badra and southeast of that, with interior margins around an intrashelf basin. Buildups were stacked or sometimes shingled as thicker shallowing-up cycles of several smaller-scale accommodation cycles. As a result, each field shows different combinations of pay zones, barriers and seal geometries.

The sequence stratigraphic analysis led to three complete 3rd order sequences being distinguished. Eustatic sea level changes controlled development of the sequence stratigraphy. Tectonism primarily defined the sites of platform development that complicated the architectural heterogeneity of the depositional sequences.

A porosity-predictive model, employing sequence stratigraphic concepts, shows porosity increasing beneath sequence boundaries due to meteoric dissolution and karstification, whilst rising sealevel induces dolomitization on the platform, causing porosity enhancement at early TST. Porous rudist facies usually coincide with the crestal areas of many fields in the region, particularly in those anticlines which show evidence of synsedimentary structural growth. However, other structures have also proven to be non-productive their crests because of the presence of tight or microporous offshore facies instead of rudist-bearing reservoir facies. Presences of interconnected-vug pores of grain-dominated fabric in the grainy facies make them the best reservoir units. Dissolution of the aragonitic components of rudist shells was the most important diagenetic process that enhanced reservoir characteristics. Presence of rudist-bearing facies with their diagenetic effects within highstand systems tracts is considered the primary factor in effective porosity development and distribution.

Predicted facies relationships indicates prograding and pinch out of rudist-bearing facies, including lowstand shelf systems, into shallow open facies that can form stratigraphic traps. However, exploring such trap types will require 3D seismic to fine-tune the positions of the external and internal shelf margins via application of high-resolution sequence