Regional Geopressure Gradient Modeling and Interpretation of Seismic Lines in Southern Louisiana and Adjacent Areas Reveal Mechanisms of Overpressure Development

Abstract

The subsurface pressure system of the onshore and offshore Gulf of Mexico Basin is characterized by a regional-scale, geopressure-gradient model developed by the U.S. Geological Survey (USGS) and published by the American Association of Petroleum Geologists. More than 300,000 mud-weight measurements from over 860,000 wells were examined, and 200,000 mud-weight measurements from over 70,000 wells were incorporated into the model. The stratigraphic section of investigation ranges from the land surface or seafloor down to a depth of approximately 30,000 ft. Fifteen maps generated from the USGS geopressure-gradient model include: (1) five contour maps that characterize the subsurface pressure system on a regional scale, (2) five associated maps that provide the data distribution used to construct the regional maps, and (3) five detailed contour maps that delineate the subsurface pressure system of southern Louisiana and adjacent areas. The detailed maps, which encompass one of the most densely drilled regions in the Gulf of Mexico Basin, show contours that represent the depths to surfaces defined by the first occurrence of isopressure gradients at magnitudes of 0.60, 0.70, 0.80, 0.90, and 1.00 pounds force per square inch per foot (psi/ft), respectively. These maps reveal the presence of two, east-west trending pressure-gradient anomalies located subparallel to the present-day coastline of southern Louisiana. These anomalies represent depth depressions of the isopressure-gradient surfaces, which indicate that the pressure transition zone, or top of overpressure, is located deeper in the stratigraphic section. New interpretation of seismic cross sections indicate that these anomalous depressions of the pressure gradient coincide with Cretaceous and Miocene stratigraphic sections that prograded over the underlying, basinward-stepping shelf margins. Therefore, the mechanism of overpressure development in these locations can be attributed, at least in part, to disequilibrium compaction from the massive, rapidly deposited, alternating sand and shale deposits which created barriers to pore-water dissipation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015