Fluid Dynamics, Petrography and Depositional Environments of the Valanginian Age Gas Bearing Sandstone, Bredasdorp Basin, South Africa

Fadipe, Oluwaseun A.¹; Carey, Paul P.¹
(1) Earth Science, University of the Western Cape, Cape Town, South Africa.

The salinity of pore water in sedimentary basins worldwide varies by orders of magnitude and is controlled primarily by physical processes of mixing meteoric, marine and subaerially evaporated waters with fluids derived by subsurface dehydration reactions and dissolution of evaporites. Bredasdorp basin which belongs to the passive continental margins of South Africa is virtually unexplored although some potential is assumed and even proven by the F-O gas field (study area) offshore South Africa.

The geochemistry and mineralogy of selected samples within the reservoir zone of the F-O1 well from Bredasdorp basin was carried out to gain insight into the effects of diagenesis, ingressed oxygen and CO2 and infiltrating rain water on the mobility of the different chemical species in different physico-chemical forms entrapped within the reservoir zones. The sandstone samples were subjected to petrographic and geochemical characterizations using XRD, IC, ICP-OES and ICP-MS, HR-TEM, FTIR and stable isotope analysis to understand the fluid dynamics and depositional environments within the Valanginian age.

Eight facies are recognized during core examination while the integration of core examination with the geochemistry of the analyzed samples reveals that the environment of deposition is subaqueous (i.e. lake or sea). Petrography results show the alteration and/or dissolution of feldspar and mica, silicification (syntaxial overgrowth) with partial dissolution/precipitation of calcite. The quasitabular bands observed during core examination depict most of the reservoir zones are mineralized fault damage zones (granulation seam).

Stable isotope data obtained show two different trends: In the reservoir A sandstones as the cements become more enriched in the 12C component, there is a proportional enrichment in the 160 component. In contrast, the cements in the reservoir B sandstones show that with an enrichment of the 12C component (from -5% to -13%), there is a trend towards 180 enrichment (from -9% to -5%). These trends reflect the evolution of the calcite types. The low 13C content observed in the C1 calcite of the reservoir B sandstones is related to an organic origin of the carbon which probably comes from the oxidation of CH4 in the sulphur reduction zone.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.