LAW, BEN E., U.S. Geological Survey, Denver, CO
Mean random vitrinite reflectance (R<o>) profiles of Tertiary throughJurassic rocks from onshore and offshore wells in Alabama and Mississippi are typically nonlinear. These nonlinear R<o> well profiles provide new perspectives into the pressure and thermal history of the eastern Gulf Coast. Non linear profiles have shallow, intermediate, and deep segments. The shallow segment has a relatively high gradient and extends to depths as great as 13,000 ft (3,960 m). The intermediate segment has a very low gradient and extends to depths of 15,000-19,000 ft (4,570-5,790 m). The deep segment has a gradient similar to the shallow segment. The R<o> at the intersection of the shallow and intermediate segments ranges from 0.9 to 1.0%, and the R<o> at the intersection of th intermediate and deep segments ranges from 1.0 to 1.35%. In the Tenneco, Viosca Knoll No. 117 well, located about 25 miles (40 km) offshore, the intersection of the intermediate and deep segments is nearly coincident with the top of overpressuring at a depth of about 19,000 ft (5,790 m) and an R<o> of about 1.35%. There is insufficient pressure data for the other wells examined to determine the depth to the top of overpressuring. However, in wells that were not examined, but where the top of overpressuring may be determined, the intersection of the intermediate and deep segments most likely will not coincide with the present-day top of overpressuring because of temporal changes in the stratigraphic position of the top of overpressuring.
I interpret these nonlinear profiles to have resulted from temporal perturbations of the thermal gradient and contrasting heat transfer processes associated with the development of overpressuring. In this hypothesis, the intermediate segment represents a zone immediately above the top of overpressured rocks that is characterized by pressure-induced, vertically flowing, relatively hot fluids. Passage of hot fluids through the zone has altered the levels of thermal maturity to nearly uniform levels.
AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.