Sub-salt and Pre-salt Plays – How Much Are Left To Be Discovered?
Arbouille, Didier1; Andrus, Vlad; Piperi, Theodhora; and Xu, Tianguang
The discovery of sub-salt and pre-salt fields has resulted in the addition of significant reserves in different parts of the globe. However, these may represent only a fraction of the play reserves. The question is how much are left? One way of evaluating the remaining reserves is to use geological models; another way is to use Field Size and Yet-To-Find (YTF) analyses. We focus here on the US Gulf of Mexico (GoM), offshore Brazil and offshore West Africa regions. We summarize the main hydrocarbon exploration events and give a description of the play characteristics. Creaming curves and field size graphs are presented and parabolic fractal distributions allow us to propose a low and a high estimate of the remaining recoverable reserves.
The US GoM has been extremely prospective with over 80 sub-salt/pre-salt discoveries. In the 1970s and 1980s, many wells were drilled into salt but the play exploration really took off during the 1990s. In 1995-1997, companies stepped up leasing into ultra-deep water; more recently the deepest US wells have been drilled in shallow waters identifying a major new sub-salt trend. Over 1.5 Bboe are estimated for these plays only and our analysis indicates that several fields of 0.1 Bboe or more can still be discovered, with total recoverable reserves between 2 and 9 Bboe.
First evidences of oil in pre-salt deposits of the Brazilian offshore were perceived on the shelf of the Campos Basin from Lagoa Feia carbonates (Badejo) in 1975, followed by about 10 discoveries in 1980s-1990s. In the mid-2000s, giant oil reserves were discovered in the Guaratiba pre-salt carbonates in the Santos Basin. The total recoverable reserves of the pre-salt accumulations exceed 50 Bboe. Our YTF analysis suggests that remaining recoverable reserves could be between 18 and 30 Bboe.
The first pre-salt discovery of offshore West Africa was made in 1966 in Angola. Since then, about 50 discoveries have been made with Malongo West being the largest. In late 2011, a large oil accumulation (Cameia 1) was found in the deep water Kwanza Basin. Approximately 3.1 Bboe have been discovered. The YTF method suggests that up to 1 Bboe are still to be found. However, if we consider the possibility to discover giant accumulations like offshore Brazil, this potential increases to 18 Bboe.
Other salt basins around the world (E. Mediterranean Sea, E. Canada, etc) are carefully revisited and sub-salt/pre-salt plays are now becoming part of many oil companies’ portfolios.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013