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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 31 (1981), Pages 97-103

Abnormal Formation Pressures: Recognition, Distribution, and Implications for Geophysical Prospecting, Brazoria County, Texas

Edward T. Flanigan (1)

ABSTRACT

The Frio and Anahuac Formations in east-central Brazoria County, Texas, were deposited in deltaic to prodeltaic environments during the active and waning stages of growth of a piercement-style salt diapir, Danbury dome. Growth faulting is prominent in the area and abnormal pore-fluid pressure is present in much of the section.

Borehole shut-in pressure measurements, drilling mud density records, and shale transit times are used to interpret distribution of subsurface pressures. Three pressure regimes are defined in terms of vertical pressure gradients: normal pressure (0.465 psi/ft), soft overpressure (0.465 to 0.70 psi/ft), and hard overpressure (>0.70 psi/ft). Distribution of these pressure regimes is controlled by the distribution of sands in the sedimentary section and the extent of flow continuity within them. Flow continuity can be cut off by stratigraphic pinch-out or by faulting.

Comparisons of stratigraphic thicknesses measured in boreholes with those derived from seismic reflection data show significant mis-ties if a single velocity function is used for time-depth conversion throughout the area. These mis-ties result from lateral variations in acoustic velocity which can be related to the distribution of normally and abnormally pressured zones in the subsurface.

Abnormally pressured zones have lower acoustic velocities than the normally pressured zones above and below them. Where these abnormal zones exist and dip significantly, lateral velocity variations should be expected.


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