Subtle Faulting in Carbonates by Differential Compaction: Evidence and Effects in UAE Oil Fields
Moen-Maurel, Laure *1; Yin, Yahui 2; Sirat, Manhal 2; Al Hashemi, Tayba 2; Abou Ali, Mouynia 3; Pabian-Goyheneche, Cécile 4; Chenot, Dominique 4
(1) Total UAE, Abu Dhabi, United Arab Emirates. (2) ADCO, Abu Dhabi, United Arab Emirates. (3) ADMA-OPCO, Abu Dhabi, United Arab Emirates. (4) TOTAL EP, Paris, France.
Differential compaction in carbonates over shoal belts, clinoform belts, reefs and lagoons may generate numerous subtle faults, which display throws usually below the vertical resolution of the 3D seismic, but which can be evidenced by well correlations and image logs. Still, these faults can be highlighted on seismic by special attribute calculation (derived from coherency, curvature, gradient…) as they are associated to vertically pervasive local flexures, fracture corridors and damage zones. Examples from UAE fields are presented from Cretaceous reservoirs.
The common characteristics of these faults are:
Compaction faults are non-tectonic; their initiation requires some burial after deposition of the irregular palaeotopography and variable facies palaegeography in order to be initiated; but do not require the influence of a regional crustal stress field. Differential compaction may deform and fracture originally homogeneous and flat-deposited reservoirs. Resulting faults may display normal throws of up to 20 feet. In some instances polygonal patterns may result from differential compaction, with apparently random abuting relationships between faults.
Being the likely earliest faults in the deformation history they can be subjected to cementation by calcite or anhydrite, reactivated and joined into larger tectonic faults, and make pathways for hydrocarbon migration (if open), or be barriers (if cemented).
The segmented disposition of these compaction faults (both horizontally and vertically) favors relays which limit the barrier effect of cemented faults. During production, baffles are represented by transmissivity values for history match, and/or X-Y permeability anisotropy ratios. Both these parameters may have a significant impact on pressure evolution and breakthough, resulting in increased uncertainty in reservoir performance prediction. Monitoring plans linked with phased development become then essential to diagnose the influence of these compaction faults for production forecast.
Another concern is the fault throw for well planning and geosteering. As development plans require longer drains and more complex well architecture in thin reservoir layers, further improvement in seismic imaging (including acquisition design and processing to increase resolution and reduce multiples) will be needed to efficiently steer well drains across these faults and maximize reservoir contact.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain