Static and Dynamic Characterization of a Major Transtensional Fault Zone: Its Effect on Reservoir Quality and Deliverability From an Offshore Thamama Oil-Field in Abu Dhabi
Authors: Edwards Ewart 
Published: December 2007

Interpretation of recent seismic data has enabled identification and characterization of major fault zones in the Thamama reservoirs of offshore fields in the UAE, many of which correspond with porosity and permeability anomalies previously mapped from well data. Detailed analysis of static, well and dynamic data (seismic, core, image logs, wireline data, PLT, MDT, Ktest results, interference tests, water cut/paths) have been used to confirm and illustrate the effects on fluid flow around the example fault zone, whose lateral extent covers several 10's of kilometers and that displays components of weak extensional and, predominantly, strike-slip activity: the fault zone cuts the crest of the field and is referred to as the Crestal Graben Fault Zone (CGFZ).

Key effects observed within the CGFZ and its proximal areas include the following:

Reservoir properties maps consistently show decreased quality in the same general locations around specific fault zones within the layers of a stacked Thamama sequence. Reservoir degradation (porosity reduction by cementation) extends for several hundreds of meters laterally from the fault zone, being most severe within tens of meters around individual faults splaying from the parent zone. Wireline logs highlight the fault-accessed diagenetic modification zones around the faults. Increased fracture density and connectivity within specific reservoir and dense limestone zones in an aureole around specific fracture-fault damage zones. A combination of enhanced mechanical layer fracturing in thicker reservoirs (&gt;30ft) contrasts the development of cemented zones within the thinner reservoirs (&lt;15ft). Fault splays, with up to 15ft vertical offset, have been cored and show reactivation effects with complex oil, bitumen, cement-filled and open fractures within the tighter fault zones. Producing sequences adjacent to faults are locally rendered non-reservoir whilst local re-distribution of hydrocarbons is observed. Dead oil is found within many fractures and minor faults within the main damage zones, often directly linked to tar mats within specific reservoir layers. Fault-associated wells display consistent marked tar mat development in almost all reservoirs but these are absent from wells outside the fault zones. Production is broadly consistent with that from field-wide layers but in some reservoirs enhanced production and water cut occurs from fractured tighter reservoir zones as well. Cored fault characteristics mirror the kinematics, activity and style of seismic scale faults within the zone. Late reactivation with predominantly tensile to weak strike-slip open fractures is critical in developing conduit paths, often locally constrained within fault damage zones. Inter-reservoir connectivity via water cut and pressure response display staircase trajectories of fluid communication within the main fault zone and along minor seismic scale and sub-seismic faults, along broadly NW-SE (fault-zone parallel) and NE-SW (cross-fault) directions.

In general, the CGFZ displays the complexity of fluid flow behavior possible within fault zones in the offshore UAE fields. A multitude of behaviors in this specific zone highlights the need to gather static and dynamic data from faulted areas and to understand that apparently contrasting and non-uniform flow impact is to be expected in some of these complex damage zones. This reflects both a response to reservoir development and the properties of the fault-fracture zones as developed through geological time.