A Review of Non-Marine Carbonates with Seismic Examples from the Santos Basin, Brazil
The South Atlantic rift system consists of a number of sedimentary basins, most having already proved the presence of hydrocarbons in commercial volumes. In the Santos Basin, following the discovery of the Lula oil field, exploration targets are now focussed on the pre-salt with particular interest for non-marine carbonate reservoirs.
Diagenesis, tectonics and the depositional environments in which carbonates are formed have a major control on the potential reservoir quality. Due to recent success within non-marine hydrocarbon reservoirs, new research has been initiated into lacustrine microbial carbonates in an attempt to understand how they form and develop. Carbonate build-ups tend to accumulate in a range of settings, from shallow waters to sub-lacustrine groundwater springs. These carbonates are produced in basins with a limited supply of clastic input and a certain amount of evaporation.
Analogues such as Lake Tanganyika in East Africa and the Green River Formation in Colorado have been extensively documented. They can be used as a reference when attempting to interpret the type of environment in which the Santos Basin carbonates are found. Lake Tanganyika is an ideal example to show the range of characteristics to be expected for non-marine deposits in a rift setting, with carbonate development controlled around basement highs. The Green River Formation is a key analogue when looking at lacustrine carbonates capped by evaporites; similar to the sediments seen in the deep waters offshore Brazil.
Within the Santos Basin, the pre-salt stratigraphy is the main area of focus, looking in particular at the non-marine sag sequence. Lacustrine deposits and carbonate build ups within this section accumulate predominantly along the edges and adjacent to faulted basement highs. The Lula oil field located in the north-eastern part of the basin, demonstrates a proven hydrocarbon reservoir of microbial carbonates within titled fault block geometry. Using well and seismic correlation, distribution of facies can be identified and geometries of the reservoirs established. Using seismic data from the Santos Basin, a potential depositional model has been applied to these non-marine carbonate deposits with reference to the analogues used.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013