Predicting Reservoir Variation through Simulation of Turbidite Deposition over Restored Paleotopography: a Case Study from the Western Atwater Fold Belt, Gulf of Mexico
Lower Miocene turbidite reservoirs drilled in the Western Atwater Fold Belt, Gulf of Mexico, exhibit significant variations in gross reservoir thickness, character, and quality within individual fields on salt-cored structural highs. Three-dimensional structural restorations based on seismic and well data indicate the presence of significant paleotopography during deposition of these turbidite reservoirs and the observed reservoir variations appear to be systematically related to position on paleotopography. The primary driver of this paleotopography is autochthonous salt diapirism. Understanding the interaction between substrate topography and turbidite sedimentation is critical in attempting to quantitatively predict reservoir variation within these fields.
A new technique used at BHPBilliton Petroleum to help predict these turbidite reservoir variations is to first perform structural restoration to infer paleotopography, and then simulate deposition on that paleotopography. The depositional algorithm is based on the work by Waltham (2004) for two-dimensional, depth-averaged gravity currents. One specific geographic area in particular, with three prominent Lower Miocene turbidite reservoirs deposited around a paleostructure estimated to have relief on the order of a few hundred meters, has been modeled. Estimates of the absolute magnitudes of paleotopographic relief have been tested by the degree of similarity of drilled well penetrations and simulation predictions. Reasonable matches to well penetrations were achieved through step-wise modification of the number and character of turbidite flows making up each reservoir.
The critical factors that produce variations in the character of turbidite flow deposits around obstructing topography are relative flow height with respect to paleorelief and flow trajectory relative to paleotopography. Given the uncertainty in multiple parameters (e.g. number of flows, flow width, flow height, flow volume, grain size distribution, and parameter variation from flow to flow) it is important that multiple screening scenarios be simulated and multiple well penetrations be matched successfully before having reasonable confidence in additional inter-well predictions. The rapid simulation capability available with depth-averaged simulations enables this testing of multiple scenarios in a reasonable time frame.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009