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.

One way to simplify the characterization of pore pressure, P_p, 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), P_p has a hydrostatic gradient, whereas in the uppermost overpressured portions of these fields (i.e., zones two and three), P_pis 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), P_p 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 P_p reaches a limit that is 85%-90% of the lithostatic (i.e., vertical) stress. Because a P_p gradient of ~0.9 psi/ft (20.3 MPa/km) is maintained throughout pressure zone FOUR in several South Texas fields, the P_p limit is a regional phenomenon. Two conditions leading to a P_p limit involve a cyclic leakage of pore fluid through zone FOUR. In both cases, leakage is governed by a balance between P_p and the minimum horizontal total stress, S_h. 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 P_p at a constant value through repeated cycles, whereas leakage by fault slip leads to an ever-increasing P_p as S_h increases through repeated cycles.