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Abstract
Chapter from:
AAPG Memoir 67: Seals, Traps, and the Petroleum System, Edited
by R. C. Surdam
(Publication Subject: Oil Methodology, Concepts)
AAPG Memoir 67: Seals, Traps, and the Petroleum System. Chapter 15: A Pore-Pressure Limit in Overpressured South Texas Oil and Gas Fields, by Terry Engelder and John T. Leftwich, Jr.,
Pages 255-267
Copyright © 1997 by The American Association of Petroleum Geologists. All rights
reserved.
Chapter 15
A Pore-Pressure Limit in Overpressured South
Texas Oil and Gas Fields
Terry Engelder
The Pennsylvania State University University Park,
Pennsylvania, U.S.A.
John T. Leftwich, Jr.
Old Dominion University, Norfolk, Virginia, U.S.A.
ABSTRACT
One way to simplify the characterization of pore pressure, Pp,
in deep, overpressured basins is to divide oil and gas fields into stratified zones, based
on average pressure-depth trends that are approximately linear. With this approximation,
each zone is assigned a constant pressure-depth gradient. In the shallow portion of South
Texas oil and gas fields (i.e., zone one), Pp has a hydrostatic
gradient, whereas in the uppermost overpressured portions of these fields (i.e., zones two
and three), Ppis characterized by gradients that exceed the lithostatic
trend of 1 psi/ft (22.6 MPa/km) (Leftwich and Engelder, 1995). At greater depth (i.e.,
zone four), Pp increases along a gradient of about 0.9 psi/ft (20.3
MPa/km). The transition between zones three and four defines the depth at which Pp
reaches a limit that is 85%-90% of the lithostatic (i.e., vertical) stress. Because a Pp
gradient of ~0.9 psi/ft (20.3 MPa/km) is maintained throughout pressure zone FOUR in
several South Texas fields, the Pp limit is a regional phenomenon. Two
conditions leading to a Pp limit involve a cyclic leakage of pore fluid
through zone FOUR. In both cases, leakage is governed by a balance between Pp
and the minimum horizontal total stress, Sh. One condition favors
leakage of pore fluid through zone FOUR upon the opening of existing joints or the
propagation of new joints by natural hydraulic fracturing. The other condition favors
leakage along faults following refracturing during slip events. The difference between
these conditions is that leakage through joints can regulate Pp at a
constant value through repeated cycles, whereas leakage by fault slip leads to an
ever-increasing Pp as Sh increases through repeated
cycles.
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