--> Abstract: The Silurian-Devonian Chaco Basin: a Review for Reservoir Correlation in Argentina and Bolivia, by F. Fernandez-Seveso, M. C. Vlstalli, and R. F. Viñes; #90933 (1998).

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Abstract: The Silurian-Devonian Chaco Basin: a Review for Reservoir Correlation in Argentina and Bolivia

Fernandez-Seveso, F.; M. C. Vlstalli; R. F. Viñes - YPF

The Ordov.-Early Silurian Cancañiri Fm. and its equivalents (100-2000m), which are tectonically inverted in the Altiplano region, were deposited in fault- controlled, narrow depocenters. An Important sea level transgressive+highstand episode was responsible for deposition of the Early-to-Late Silurian Kirusillas+Tarabuco sequence-set (SSt.). The basin received the highest volume of sand during an E. Devonian global lowstand episode (Santa Rosa SSt.). The lcla, Huamampampa, and Lower and Upper Los Monos SSts were deposited from the end of E. Devonian through the Late Devonian and represent an overall transg.+highstand episode with several lesser order transgressive-regressive cycles.

A huge volume of Huamampampa sands were constrained to a coastal ramp setting as "lowstand prograding shoreface" deposits along the SW border. These sandstones were subsequently folded and fractured, and are the reservoirs in the largest gas and condensate fields (i.e., Aguarague and Ramos fields). Overlapping the ramp to the west, smaller fields developed mainly in relation to substantially thinner fluvial-deltaic sand intervals (i.e., Aguas Blancas and Bermejo fields). Towards the east, the offshore counterparts received bypassed sands; thus, lower sand volumes, and a diminished folding-fracturing intensity, could have only resulted in fields of lesser magnitude. Consequently, the location and delineation of the ramp is one of the main components involved in prospecting large volumes of reservoir rocks. A low sedimentary supply from the Brazilian shield was responsible for a condensed stratigraphic record along the NE border, composed of E-W elongated sand belts, equivalent to Santa Rosa, lcla and Huamampampa SSts. Based on regional seismic interpretation, the upper portion of Roboré Fm. seem to be equivalent to the base of Huamampampa SSt., while the lower Roboré Fm. (= Yapacanl, 4750m. TD-SJN x-2) and Sara sandstones seem to he equivalent of the Santa Rosa and Tarabuco SSts respectively. This portion of the basin is assumed to be highly productive base on the proven structural plays, as well as on the potential plays related to stratigraphic-diagenetic pinch-outs. The Los Monos-lquiri interval is subdivided in two prograding SSts of around 1000 m. thick each. An important thickness reduction - up to 1000 m for the Loa Monos-lquiri interval is shown in the composite seismic section from Bolivia to Argentina. The Lower Los Monos SSt. is characterized by downdip terminations towards the south that become considerably thinner in Argentina, like a condensed section. The sandstones present at the top of Lower Los Monos or at the base of Upper Los Monos have often been misinterpreted from well logs as belonging to the top of Huamampampa SSt (the Miraflores, SJN x-2).

A structural restoration was attempted, in order to aid the paleogeographic interpretation. Though these preliminary results are regional in nature and much more input of data will be required, this work has allowed us to develop an alternative basin-generating mechanism, which involved asymmetrical lithospheric extension by simple shear. This hypothesis accounts for any spatial offset between the fault-controlled depocenters in the Altiplano region and the zone of interpreted maximum lithosphere thinning to the east in the Subandean and Chaco regions, where the L. Silurian-Devonian basin axis was driven by thermal subsidence.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil