Chapter from:
AAPG Memoir 70: Abnormal Pressures in Hydrocarbon Environments
Edited by B.E. Law, G.F. Ulmishek, and V.I. Slavin
Copyright ©1998 by The American Association of Petroleum Geologists. All rights
reserved.
Memoir 70, Chapter 13: Abnormal Pressure and the
Occurrence of Hydrocarbons in Offshore Eastern Trinidad, West Indies, by P.D. Heppard, H.S. Cander, E.B.
Eggertson, Pages 215 - 246
Chapter 13
Abnormal Pressure and the Occurrence of Hydrocarbons in Offshore Eastern
Trinidad, West Indies
P.D. Heppard
H.S. Cander
E.B. Eggertson
Amoco Corporation, Houston, Texas, U.S.A.
Abstract
Abnormal pore pressure is widespread in the Tertiary through upper Mesozoic,
clastic-dominated section of the Eastern Venezuelan Basin and the eastern extension of the
basin into Trinidad. Some of the largest oil and gas columns are found within abnormally
pressured sandstones which account for 43 million bbl (6.8 million m3) of oil
in Poui field and 882 billion ft3 (24.98 billion m3) of gas in
Cassia field. Abnormal pressure within the Tertiary to Upper Cretaceous rocks resulted
from the transfer of overburden stress to the pore system during the rapid subsidence and
infilling of the foredeep basin during the Miocene and Pliocene. Primary migration from
thick, Upper Cretaceous source rocks and secondary migration through the thick Tertiary
clastics occurred principally through hydraulically induced fractures within a highly
overpressured section. Final migration out of the overpressured section and charging of
present-day reservoirs off the east coast of Trinidad occurred during the late Pliocene
to Pleistocene uplift and associated complex normal faulting. The multiple pressure
compartments within the six fields studied are separated by relatively thin, abnormally
pressured shale. The shale seals are most effective in trapping hydrocarbons when the
pressure difference across the shale is less than 4 psi/ft (90 kPa/m) regardless of the
shale thickness. Normal faults form effective pressure seals throughout the basin,
separating porous sandstone pressure compartments with pressure differences as great as
1,856 psi (12.8 MPa). The oil and gas fields of offshore Trinidad reveal a widely varying
depth to the top of abnormal pressure, large pressure differences across faults, pressure
reversals, and a narrow zone of transition from mild abnormal pressure (<11 PPG
[lb/gal] equivalent) to highly overpressured conditions (>14 PPG equivalent).
