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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
Suzanne D. Weedman, Albert L. Guber, and Terry Engelder
Methodology and Concepts
Published 1994 as
part of Memoir 61
Copyright © 1994 The American Association of Petroleum
Geologists. All Rights Reserved
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Chapter 6
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Deep Pressure
Seal in the Lower Tuscaloosa Formation, Louisiana Gulf CoastSuzanne D. Weedman
Albert L. Guber
Terry Engelder
Penn State University
University Park, Pennsylvania,
U.S.A.
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ABSTRACT
Repeat formation tester (RFT) pore pressure
measurements spanning a depth range of 5500-6060 m in the lower Tuscaloosa
Formation (Upper Cretaceous) document a pressure discontinuity of >20 MPa
at ~5680 m forming a pressure seal in two natural gas fields in the Tuscaloosa
trend, Louisiana. In the Morganza field the depth to the top of overpressure
varies by less than 30 m across two adjacent fault blocks, though equivalent
strata are downthrown by 100 to 120 m. In contrast, the depth to the top
of overpressure in the nearby Moore-Sams field rises slightly across the
same fault. Therefore, the nearly horizontal top of overpressure does not
appear to coincide with time- or lithostratigraphic boundaries.
The overpressures in all of the Moore-Sams
and some of the Morganza fields wells follow a local hydrostatic gradient
with increasing depth indicating that pore fluids below the pressure seal
are in communication, and demonstrating that sandstone connectivity occurs
below the pressure seal as well as above. In the remaining Morganza wells,
overpressure increases with depth in a stair-step manner that may comprise
offset local hydrostatic gradients, to  magnitudes of 117 MPa at depths
of 5.9 km. The occurrence of the pressure seal within interbedded sandstones
and shales, where high sandstone connectivity is expected, suggests that
the sandstones of the seal zone are unusually tight.
The above observations coupled with a petrographic
study of sandstones from the vicinity of the pressure seal suggest that
extreme compaction of the sandstones after dissolution of carbonate cements
may have contributed to the low permeability indicated by the pressure
data, and that the seal formed a kilometer or more shallower than it is
today.
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