Thermo-Chronological Approach to Reservoir Diagenesis in the Offshore Angola Basin: a Fluid Inclusion, {40}Ar-{39}Ar and K-Ar Investigation (1)

Abstract

Microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in Neocomian sandstones and Albian dolomitic reservoirs from the offshore Angola basin have allowed determination of the age and the physical-chemical conditions for fluid migration in a late diagenetic stage. The {40}Ar-{39}Ar laser probe dating of adularia overgrowths from the Albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early Miocene (25.6 + or - 3.2 Ma). The K-Ar age of authigenic illites from the Neocomian sandstones is slightly younger (19 + or - 1 Ma), and the crystallization of regular illite-smectite mixed layers is related to a previous Eocene to Paleocene diagenetic stage (51-58 Ma). There appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the Miocene. The fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids, assuming that pressures during the Miocene were close to the present reservoir pressures, and by intersecting the isochores of aqueous and oil inclusions, assuming they were trapped at strictly similar pressure and temperature conditions. The fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content (AlIV) of authigenic chlorites: 168-206 degrees C in the Albian reservoirs at depths between 2350 and 3580 m, and 175-237 degrees C in the Neocomian sandstones at depths between 2770 and 4470 m. In both reservoir formations, Miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient.