Foraminiferal biostratigraphic studies from Mesozoic succession of selected wells from the Orange Basin, western offshore, South Africa

Abstract

Located on the western offshore on the Atlantic Ocean margin of Southern Africa, the Orange Basin is the youngest and largest of the South Africa’s seven sedimentary basins. This passive margin basin in known for its hydrocarbon potential and therefore is the focus of attraction of several oil exploration companies. The study area lies near the continental margin in which four exploratory wells were drilled. An attempt has been made in this work to understand the depositional settings of these reservoirs and their biostratigraphy. Distribution of important planktonic index foraminifera helps in dating the reservoir sections. Paleoecological studies of benthic foraminifera were used for understanding the prevailing environment during the Cretaceous period. The study indicates that most of the reservoirs are distributed in the Albian (Early Cretaceous) and a few in the Cenomanian age sediments. Relatively shallow shelf sedimentation prevailed in the Late Aptian to middle part of Albian with deposition of arenaceous units. There were periodic localised deepening as well as very shallow depositional condition leading to exposure (diastem) as indicated by lithology and faunal composition. Gradual rise in sea level started in Late Albian and the entire area was under bathyal environment till the end of Cenomanian stage. This is indicated by deposition of claystone rich units and the associated fossil benthics indicates deposition in slope area. The few relatively minor argillaceous sandstone and siltstone units are with poor reservoir quality.