AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Outcrop Analogue of Pre-salt Microbial Series from South Atlantic: The Yacoraite Formation, Salta Rift System (NW Argentina)
(1) IFP Energies nouvelles, Rueil-Malmaison, France.
The pre-drift geological history of the South Atlantic ocean is attracting geoscientists attention since exploration in the Brazilian pre-salt has resulted in the discovery of an estimated 8-20 billion BOR over the past 4 years. The main reservoirs correspond to stromatolitic and bioclastic carbonate facies which were deposited in lacustrine to shallow marine depositional environments during the Sag phase. As only few subsurface data are available, the reservoir distribution as well as the reservoir heterogeneities are poorly constrained in such peculiar setting. We present hereafter new results in terms of reservoir architecture from an analogue of these reservoirs exposed in spectacular 3D outcrops in the Salta rift system, NW Argentina.
The sedimentary fill of the Salta basin (Early Cretaceous-Middle Palaeogene, NW Argentina) can reach a thickness of 5,000m including syn-rift to post-rift sequences: the syn-rift Pirgua subgroup which consists of alluvial to fluvial deposits, the transitional Balbuena subgroup which consists of shallow marine to lacustrine mixed deposits, and the post-rift Santa Barbara subgroup which consists of fluvio-lacustrine deposits. The Yacoraite Fm, which has been deposited during the Sag phase, records an overall transgressive trend from fluvio-eolian sandstones passing upward to shallow marine/lacustrine carbonates (oolitic grainstones to stromatolitic limestones) in turn overlaid by lacustrine dark shales, halite, anhydrite and gypsum.
At rift scale, the oolitic grainstones to stromatolitic limestones facies of the Yacoraite Fm are located on a structural paleohigh along the rift axis, 50 to 100km away from both sides of the rift margin. The carbonate-dominated Yacoraite Fm progressively evolves toward the edge of the rift to mixed environments. The best reservoir facies (stacked highly porous stromatolitic limestones) occur within an area of 30-50 km long and 20-25km wide on the structural paleohigh, and rapidly pass to muddy-dominated facies laterally.
At reservoir scale, the main heterogeneities correspond to laterally continuous 10 to 50cm thick mudstones layers acting as permeability barriers. To the north, in the Lomas de Olmedo sub-basin where the Yacoraite Fm is one of the main reservoir, the subsurface data indicate that diagenesis processes have also overprinted the sedimentological heterogeneities, with for example a primary porosity reduction by quartz overgrowth and anhydrite cement.