A sequence stratigraphic study of the Cenomanian-early Turonian Mishrif Formation has been carried out in the SW of the United Arabian Emirates (UAE) in order to understand the evolution of the reservoir distribution and occurrence of potential stratigraphic and diagenetic traps in the UAE.
The study has used an integrated reservoir approach that includes regional geology, seismic stratigraphy together with well data. The use of the core data and logs has allowed identification of facies changes, key stratigraphic surfaces and a number of a few feet thick dense intervals that show a relatively high lateral correlatibility, which appear to be below the seismic resolution. The use of seismic attributes together with the well calibration has been crucial to define seismic facies and their lateral and vertical distribution.
Results from this study allowed recognition of several smaller scale 3rd order cycles, which are comprised in a larger 2nd order genetic sequence. Each small-scale cycle is bounded by a sequence boundary overlain by a flooding event made up of a thin dense interval which shows an increase in argillaceous content and locally chert. These sequences are characterised by an eastward progradation from muddy inner ramp facies passing upwards to rudist-rich patch reef/mixed skeletal shoal complexes developed in an inner to mid-ramp depositional setting. This latter setting appears to be best developed in the uppermost large-scale sequence, below the regional unconformity that separates the Mishrif from Ruwaydha and Tuwayil formations. Highly porous facies appear to decrease seismic velocity.
The integrated approach presented here captures the variety in depositional environments, architecture, geometry and reservoir heterogeneity, which results in higher likehood to identify stratigraphic and diagenetic traps.
Furthermore, the recognition of seismic characteristics of the Mishrif reservoir may be used as a tool to predict the distribution and evolution of units that create velocity variations in the Mishrif Formation and which affect the structural definition of deeper reservoirs as the Shuaiba Formation.
The Aptian Shuaiba Formation is among the most important reservoir units in the Middle East. Despite being extensively studied in recent years (Droste, 2016; Murris, 1980; Droste and van Steenwinkel, 2004; Yose et al., 2006;2010; van Buchem et al., 2002, 2010; Vahrenkamp, 1996), the interpretation of the Shuaiba depositional geometries, and specifically those associated with Shuaiba reservoirs, remains challenging in the seismic realm due to their intrinsic heterogeneity, variability and limited thickness.
An integrated and iterative analysis including regional geology, well and seismic data was conducted to unravel the internal depositional geometries and associated reservoir properties for the Shuaiba Formation in the western region of UAE. In the study area, spatial distribution and geometry of the Shuaiba seismic facies was mapped in detail using full-stack reflectivity, discontinuity, curvature and spectral decomposition seismic attributes and further integrated with available well data and analogues. The Shuaiba Formation is expressed on seismic by mounded facies characterized by an irregular appearance, with discontinuous outline of sub-transparent reflectors, crossing reflectors and a larger thickness than the surrounding facies. Reflectors may locally coalesce and form mounded features up to 70 ms thick. The areas between mounds are characterised by sub-parallel to inclined reflections. The latter could be interpreted as clinoforms, prograding from the isolated nucleation areas to their deeper surroundings. The base of the mounds is located just above the top Thamama B/base Shuaiba reflector and the top coincides with the top Shuaiba. Well data suggest that these mounds consist usually of Bacinella/Lithocodium facies and rudists. The detailed 3D seismic interpretation of the Shuaiba reservoir geometries indicates a complex depositional architecture, which results in a variety of potential stratigraphic trap geometries and a heterogeneous reservoir property distribution. The proposed integrated workflow can therefore be used as a predictive tool to unravel further prospectivity in the Shuaiba Formation and in similar complex carbonate reservoirs, as well as to significantly improve reservoir property distribution prediction in static models.
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