MECHANISM FOR OVERPRESSURE DEVELOPMENT SUBSTANTIATED BY MODELING OF THE REGIONAL GEOPRESSURE GRADIENT IN SOUTHERN LOUISIANA AND ADJACENT AREAS

A geopressure-gradient model developed by the U.S. Geological Survey (USGS) characterizes the subsurface pressure system of the onshore and offshore Gulf of Mexico basin, USA. 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 surface downward to approximately 30,000 ft in depth. This study is the first published, quantitative investigation of the regional, subsurface pressure system of one of the most important petroleum provinces in the United States.

Fifteen maps were generated from the USGS geopressure-gradient model and include (1) five contour maps that characterize the subsurface pressure system on a regional scale, (2) five supporting maps that provide the data distribution used to construct the regional-scale contour maps, and (3) five contour maps that describe the subsurface pressure system of southern Louisiana and adjacent areas.

The maps, which encompass one of the most densely drilled regions of southern Louisiana and adjacent areas, 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 anomalies located subparallel to the present day coastline of southern Louisiana. These anomalies represent depth depressions of the isopressure-gradient surfaces and indicate that the pressure transition zone is located deeper in the stratigraphic section. These anomalous depressions coincide with Cretaceous-age stratigraphic sequences that prograded over the underlying, basinward-stepping shelf margins. The mechanism of overpressure development in these locations can be attributed, in part, to disequilibrium compaction of alternating shale and sand deposits which created barriers to pore-water dissipation.