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).