Assessment of Subsurface Risk Uncertainties Associated With a Typical Near-Field Exploration Prospects, Block 9, Bredasorp Basin, Offshore South Africa

Abstract

The Bredasdorp Basin is an offshore sedimentary Basin that forms part of the Great Outeniqua Mesozoic sub-basins located in the South Coast of South Africa. The Great Outeniqua Basin comprises a number of northwest-southeast trending en-echelon sub-basins and interbasinal highs. The sub-basins are (from west to east): the Bredasdorp, Pletmos, Gamtoos and Algoa sub-basins. The Outeniqua Basin is characterized by northwest-southeast trending half-grabens developed during the initial breakup of Gondwana in the Middle to Late Jurassic. Further rifting took place in the Valanginian, as South America separated from Africa. Tectonic activity persisted until the Aptian along the Agulhas-Falkland Fracture Zone. The syn-rift grabens are infilled with continental clastic sediments and red beds, becoming lagoonal to shallow marine at the top of the succession. Major stratigraphic units for Bredasorp basin are controlled by rifting on-set (Valanginian) characterised by a regional unconformity – 1At1 and drift on-set (Aptian). Block 9 is located in the Bredasdorp basin, and to date, Block 9 is the most mature in exploration drilling, development of discovered oil and gas fields , and as well as production of oil and gas hydrocarbons with well-established infrastructure and gas market. The study will focus on one of the exploration prospects identified in block 9, with the emphasis in the evaluation of the sub-surface risk uncertainty. In general, in petroleum geology, risking of the prospect considers five petroleum systems elements namely Reservoir, Source, Seal, Closure Components, Timing and Migration pathways. Analysis of these risking elements as applied to the prospect will be demonstrated. The prospect is near a producing asset of which the producing reservoirs in the Valanginian Upper-Shallow Marine (USM) sandstone sequence and fluvial sequences is an analogue to the objectives of the prospect. Production from the producing field commenced in 1992 and the Development wells have produced significant volumes of gas from the Upper Shallow Marine (USM) reservoir rocks. The exploration prospect is targeting similar stratigraphic reservoir and exploration drilling of the target could potentially un-lock a new play type in the basin, close to existing production infrastucture. The exploration prospect trap style is combined stratigraphic –structural trap. Hydrocarbon presence is more certain in the prospect area while the highest risk element is reservoir presence.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018