The Nature and Diversity of Pressure Transition Zones

Richard E. Swarbrick and Mark J. Osborne

Pressure transition zones occur where the rate of pressure increase or decrease exceeds a fluid gradient, and departs from the hydrostatic pressure. Transition zones are found between intervals of permeable rocks, and in each case fluid movement is impeded by a "seal", which in almost all cases acts as a temporary barrier over geological time The composite profile of pressure vs. depth is a function of three phenomena: (1) the mechanism responsible for abnormal pressure, (2) redistribution of pressure due to fluid movement during and after the mechanism is occurring, and (3) the lithological profile of the rock succession. Pressure profiles in overpressured systems can reveal which of the mechanisms is causing the overpressure. For example, transition zones which plot subparallel to the lithostatic (overburden) gradient are likely to be caused by fluid retention during rapid loading. intervals where mineral transformation (e.g. smectite-illite; kerogen maturation) is in progress are likely to be characterized by transition zones which are spatially linked to these intervals. Once the overpressure has been created, pressure decay and transference away from the interval of generation can strongly influence the nature of transition zones. in pa ticular vertical and lateral transference in permeable units can rapidly modify the initial pressure profile. Finally transition zones are controlled in a fundamental way by the lithology, or more specifically the permeability, of the rocks in which the abnormal pressure is found.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California