Sedimentology, Sequence Stratigraphy and Reservoir Characterisation of Middle Cretaceous Carbonates of the Middle East

Ian R. Sharp, Paul Gillespie, David Hunt, and Jean-Christophe Embry
Programe Exploration, Statoilhydro Research Centre, Bergen, Norway

Aptian-Turonian carbonates contain some of the most prolific reservoirs of the Middle East. These units outcrop extensively in the Oman and Zagros Mts, affording a unique opportunity to document the relationship between structure, sequence stratigraphy, facies and diagenesis. These data have been used as a semi-quantative framework to predicte subsurface reservoir distribution.

Aptian-Albian systems are dominated by muddy ramps, whilst Late Albian-Turonian systems are characterized by both ramps and shelfs. A reccuring lateral facies association of clastics-intrashelf basin-carbonate shelf-open marine basin is characteristic for both systems moving from the Arabian Shield towards the Tethys. Volumetrically HST deposits dominate, characterized by strongly progradational successions. Margin facies comprise skeletal (rudistic) sands. TST deposits are volumetrically subordinate, dominated by ooid-pelloid sands.

Reservoir development is related to primary facies, sequence architecture and late stage fracture controlled diagenesis. HST skeletal sands form the best primary reservoirs (late HST leaching and karstification) and are sealed by overlying TST-MFS deposits. TST sands are often early cemented. Late stage fracture controlled diagenesis (dolomitisation, leached microporous Lst) shows a close relationship to depositional facies and sequence architecture. Vertical barriers to fracture fed diagenetic fluids are cycle capping low perm mudstones, below which fluids moved laterally. Where these barriers are cut by fractures dolomitisation/Lst leaching advanced upwards, indicating that faults/joints were fluid migration conduits. Dolomite bodies are 100m-5km in width, have plume-like geometry, with both fracture and diagenetic contacts to Lst country rock. Leached Lst's are favourably delevoped in HST skeletal sands and regionally extensive (10’s of Km’s).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas