--> Abstract: Stratigraphic Analysis of Cretaceous Sag Phase Sequences of Southern San Jorge Basin, Argentina, by E. Figari, P. Lafourcade, G. Conforto, M. Cevallos, A. Silveyra, and C. Martinez; #90933 (1998).

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Abstract: Stratigraphic Analysis of Cretaceous Sag Phase Sequences of Southern San Jorge Basin, Argentina

Figari, Eduardo; Lafourcade, Pedro; Conforto, Gustavo; Cevallos, Martin; and Silveyra, Anibal - YPF and Claudia Martinez - Geoquest

The sedimentary record of the sag phase stage in the intracratonic San Jorge basin, central part of Patagonia Terrain, is mainly continental and provides the main oil reservoirs of the whole stratigraphic column that reaches more than 8000m thick and evolved since Jurassic to Tertiary times. During this stage of thermal subsidence, fault activity decreased, sedimentation expanded beyond basin boundaries, pyroclastic supply increased, and depositional systems varied from coarse alluvial deposits in the basin flanks to high sinuosity meander belts and shallow ephemeral lakes in the central area. This sag phase section, more than 2000 m thick, was divided by formal stratigraphy into a small group of formations such as Castillo, Bajo Barreal and lateral equivalents. These divisions resulted insufficient to describe several smaller sections which can be identified with a more detailed analysis.

Starting from a complete well log data set, lithological cross plots, cores and well images a group of units in the order of parasequence sets and sequences were defined. Merging them with 3D seismic data these units were correlated and ranked. Even though the majority of reflectors have poor lateral continuity, some of these sections have particularly reflection arrays and seismic character that allow them to be differentiated. Sequence boundaries are gently unconformable and eroded thickness can be recognized only on the borders of the basin. Internally, some of these smaller units with a variable thickness in the order of tens of meters, are made of bed load sandstone packages, laterally amalgamated , finning or coarsening upward, and with poor paleosoil developing in a restricted floodplain. These levels belong to braided or low sinuosity channel belt systems developed during a relative lowstand of a regional base level. On the other hand , another kind of units have a lower sand shale ratio, with more isolated sandstone bodies, finning upward arrangement and lateral accretion surfaces belonging to high sinuosity rivers, and a good and more frequent paleosoil development in overbank deposits ending in thick shaly sections. They represent major flooding and highstand stages of a regional base level. These shaly sections turn out to be the best correlation levels as well in sonic log correlation as in seismic sections and they seem to represent the best regional seal to main oil accumulations. Changes in pyroclastic supply could be generating an additional element in these theoretical relationships in the western part of the basin closer to a forming arc.

In conclusion, in the Cretaceous sag phase stage of San Jorge Basin several units smaller than classic Formations have been identified. Sonic log correlation and seismic tie of main discontinuities allowed them to be followed in a regional scale. The presence of a higher or lower sand shale ratio depends mainly on accommodation space variations during low and high local base level stages related to an ephemeral playa lake and or a shallow lake in the centre of the basin . Tectonic control on facies changes is only important close to main faults or old basement highs.

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