Russell W. Spears1,
Jeffrey A. Nunn1
(1) Louisiana State University, Baton Rouge, LA
Abstract: Relationship between overpressured compartments and spatial variations in pore fluid salinity in sediments of South Marsh Island OCS 310, offshore Louisiana
Middle to late Miocene to Pleistocene overpressured sediments with pore fluid pressures higher than hydrostatic have been identified in the South Marsh Island, offshore Louisiana area using wireline logs to derive parameters such as porosity, shale weight fraction, pore fluid pressure, and pore fluid salinity. Petrophysical data from the area show undercompaction trends (porosities higher than expected from normal compaction processes) with increased pore fluid pressures correlating with lower relative pore fluid salinities. This trend is most notable in sands which are typically capped by shales. Excess fluid pressures in the sands are inferred from increased porosity with observed corresponding decreases in gamma ray and spontaneous potential response. Wireline data indicate a quick transition from pore fluids with a brine composition (greater than 100 grams per liter dissolved salts) in the shales above the sands to pore fluids closer to seawater salinity (approximately 35 grams per liter dissolved salts) within the sands. Pore fluids with near sea water salinity indicate that these geopressured sands are not in hydrologic contact with dissolving salt structures. Therefore, the shales overlying these sands are acting as barriers to fluid flow.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana