Predicting the Mishrif Reservoir Quality in the Mesopotamian Basin, Southern Iraq

Adnan A.M. Aqrawi¹* and Andy D. Horbury^2
1StatoilHydro, International E&P, Iraq
²Cambridge Carbonates, UK
*[email protected]

The middle Cretaceous Mishrif Formation contains a widespread carbonate succession of Cenomanian-Early Turonian age in the Mesopotamian Basin of southern Iraq. More than one third of the proven Iraqi reserves are found within the rudist-bearing carbonate reservoirs of the Mishrif Formation. Rudist facies coincide with the crestal areas of many fields in the region, particularly in those anticlines that show evidence of synsedimentary structural growth. However, other structures have also proven to be non-productive at this level because of the presence of tight or microporous offshore facies instead of rudist-bearing reservoir facies. Difficulty in predicting reservoir distribution is due to the complex palaeogeography. It would appear that following drowning of the uniform and extensive Albian carbonate platform, the Mishrif Formation was established along a NW-oriented zone that is largely bounded by the East Baghdad and Hamrin-Makhul structures and their continuation towards Iran (as the Bala Rud zone). To the southwest of this zone, the intra-shelf basin carbonates of the Rumaila Formation were deposited. Whereas to the northeast, the sub-basinal carbonates of the Dokan Formation were deposited. Across the Mishrif Formation facies belt, sequences show stacked or sometimes shingled geometries. As a result, each field shows different combinations of pay zones, barriers and seal geometries. Detailed palaeogeographical reconstruction on a systems tract basis can be used to construct play fairway maps for exploring this carbonate system, particularly with respect to finding stratigraphic traps. Many existing anticlines, tested by one or more wells that are thought to be ‘dry’, may turn out to have productive facies along strike, or down-dip on the flanks. In addition, pure stratigraphic traps may be found in relatively undeformed ‘synclinal’ areas between the major anticlines. However, exploring such trap types will require 3-D seismic to pin-point tune the positions of the external and internal shelf margins via the application of high-resolution sequence stratigraphy.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain