Guillermo Jalfin¹, Eduardo Bellosi², Mariano Marzo³, Ron Steel⁴, Javier Sanagua⁵

(1) REPSOL-YPF, Buenos Aires, Argentina
(2) CONICET, N/A
(3) University of Barcelona, Barcelona, Spain
(4) University of Wyoming, Laramie, WY
(5) REPSOL-YPF, N/A

ABSTRACT: Mega-Architecture of the Alluvial Lower Bajo Barreal Fm. (Cenomanian-Coniacian), San Jorge Basin Argentina

4000 wells and 1200 selected E-logs in the Las Heras area of the San Jorge Basin were analysed and correlated, linking the oil pools in fields across the region. The mega-architecture of the alluvial Lower Bajo Barreal Fm. was reconstructed from 10 regional stratigraphic transects. The large data set was handled in three stages: 1) 1-D evaluation of the vertical facies changes, including the fluid type, up through the alluvial succession in each well, (2) 2-D stratigraphic correlation along each of the 10 transects to delineate the apparent dimensions, distribution and connectedness of fluvial channel belts, and (3) 3-D cross correlation between transects, allowing visualization of dimensions, orientation and density of the channel belts, as well as of the development of the drainage network across the region.

Analysis of the channel-belt stacking pattern in wells through the Bajo Barreal Fm. shows a hierarchy of channel/bar, channel belts and possible valley-fill systems. The major concentrations of multistory channel belts plotted in 3-D define a series of spectacular fluvial 'avenues', up to 7km in width, separated by broader belts that are sand-poor. The principal sand-prone avenues show a low-sinuosity braided pattern. The inter-avenue areas are formed of thick tuffaceous shales, paleosols, and minor sandstones. Many of the latter originated in crevasse fans, possibly related to the initial stages of avulsion that led to new channel belts. The rivers consistently flowed to the SE (range SW to ESE), except in the southeast of the region, where flow was to the east.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado