2D Modeling of overpressure in a salt withdraw basin, Gulf of Mexico, USA

Jeff Alwardt, Eric Michael, and Chuck Shearer
ConocoPhillips, Houston

This study represents an illustration of how 2D basin models can be utilized to address the overpressure developed due to compaction disequilibrium in a basin with very high sedimentation rates and highly mobile salt bodies. Full 3D basin models that include a robust salt restoration through time are often impractical either due to a lack of data to constrain salt movement or the evaluation time required to conduct this restoration. By using a partial calibration and the proper 2D line selection from a grid stack, it is possible to obtain reasonable timing of overpressure development and approximate present-day overpressures. In the best case, a model using a single shale-sand lithology (low resolution) with default porosity and permeability compaction curves reproduced pressure measurements to an average of about 0.4 ppg with a maximum error of 1.0 ppg. This suggests that knowing the age, depth, and the approximate lithology (sand net-to-gross) is adequate to approximate pressure in highly overpressured basins to address issues around trap containment and first pass well planning purposes. Forward modeling pressure for purposes of well planning may require more localized information on rock properties and stratigraphy; albeit these data are not always available in exploration. In addition, this presentation shows the quality of overpressure prediction utilizing different model stratigraphic resolution, available calibration data and sensitivity of overpressure modeling to 2D line selection.

Three models were also developed to model how the permeability of salt welds affects the overpressure development in the basin. These consisted of two end-member models and an intermediate, where the salt welds of the end-member models contained either salt (impermeable) or a shale-sand mixture (permeable). The intermediate case was intended to imitate a fault gouge and consisted of a shale-sand lithology with a reduced permeability. This shows that a small permeability reduction of 1 to 2 orders of magnitude results in pressure calculations that are very similar to the results of the impermeable model. Additionally, this intermediate model qualitatively reproduced the seismic velocity differences between neighboring basins, suggesting that the salt welds in the actual basin are at least partially sealing as required for hydrocarbon trapping.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands