Stratigraphic Architecture of Pre-salt Microbial Series from the Salta Rift System (Yacoraite Formation, NW Argentina)

Rohais, Sebastien ¹; Hamon, Youri ¹; Deschamps, Remy ¹
(1)IFP Energies nouvelles, Rueil-Malmaison, France.

Pre-salt plays deposited during rift to drift tectonic phases attract attention since the Santos and Campos basin pre-salt discovery has been estimated up to 20 billion BOR over the past 4 years. To better constrain the reservoir distribution at rift scale as well as the heterogeneities of these reservoirs deposited during the sag phase, we studied an outcrop analogue 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 corresponding to alluvial to fluvial deposits and volcanic rocks, the transitional Balbuena subgroup corresponding to shallow marine to lacustrine mixed deposits, and the post-rift Santa Barbara subgroup deposited in fluvio-lacustrine environment. The present study is focused on the Yacoraite Fm (Balbuena subgroup) that has been deposited during the sag phase of the rift basin evolution.

The depositional model includes proximal carbonate-dominated facies (sebkha, mudflats) to lagoonal and back-barrier facies, that laterally pass to oolitic shoal facies associated with large stromatolites. Toward the edge of the rift, specially in hangingwall setting, clastic-dominated facies (fluvial, aeolian, tidal, shoreface) commonly occurred. In the distal setting, where the overlying salt are the thickest, volcanics from the Palmar Largo event are interbedded within the Yacoraite Fm and deeply influenced the facies distribution as well as the petrophysical properties of the Yacoraite Fm.

The Yacoraite Fm records an overall transgressive trend from fluvio-aeolian sandstones passing upward to shallow marine/lacustrine carbonates (oolitic grainstones to stromatolitic limestones) in turn overlaid by lacustrine dark shales, halite, anhydrite and gypsum. It can be subdivided into four third-order sequences. A major sequence boundary records a main intra-sag deformation phase.

At rift scale, the oolitic grainstones to stromatolitic limestones facies of the Yacoraite Fm are mainly located on a structural paleohigh along the rift axis, 50 to 100km away from both sides of the rift margin. The best reservoir facies (stacked highly porous stromatolitic limestones) occur preferentially within an area of 30-50 km long and 20-25km wide on the structural paleohigh, and rapidly pass to muddy-dominated facies laterally.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.