Abstract: Stratigraphy and Sedimentology of the Upper Cretaceous and Lower Tertiary at the Northern Flank of San Jorge Basin, Argentina

Courtade, S.; Rodriguez, J. C.; Merlo, E.; Russo, M. - YPF; Bett, J. C. - Halliburton

The Golfo San Jorge Basin is located on the south east of Argentina. It is a Jurassic rift basin that evolved to a Cretaceous and tertiary sag basin. The Upper Cretaceous and Lower Tertiary record at the northern flank of Golfo San Jorge Basin involves different reservoirs that are collectively known as Complex I. In this paper we expose a sedimentologic and stratigraphic model for these reservoirs.

The lower unit of Complex I is the Member Valle C of Yacimiento El Trébol Fm, it is composed of a 50 m. thick shale overlaid by a 150 m. thick coarsening up and thickening up sandy package. Valle C SP patterns are predominantly funnel shape in the lower half of the unit and bell shape in the upper half. Core data from the upper part show facies that were interpreted as fluvial and deltaic. The net sand map of this succession shows a digitated distributary pattern advancing from a NNE direction. Seismic sections show at the base of Valle C interval a group of reflections that onlap from the south to the north of the basin and that are overlaid by another group characterized by downlap and offlap terminations showing progradation from N. to S. The Valle C record is interpreted as a current dominated deltaic to fluvial depositional system that prograded in a SSW direction into a lacustrine or estuarine depositional system. Valle C sandstones are overlaid by a succession of sandstones and shales known as the Glauconitic Member of the Salamanca Formation. The succession begins with a 30 m. thick, net based interval of gray quartz sandstones and conglomerates with bell type SP pattern overlaid by a shale interval called Horizonte Madre. Immediately above there is a 60 m. thick succession of gray shales, rich in plant fossils that interbed with grey quartz and green glauconitic sandstone beds in a coarsening up and thickening up arrangement with funnel shape SP pattern known as Horizonte Lignitífero . This succession can be assigned to fluvial, lagoon and estuarine deltaic environments. The succession continues with a shaly glauconitic sandstone interval, 20 meters thick with a net base and barrel shape followed by a 50 m. section of platform shales known as Sección Fragmentosa. The sand isopach map of the glauconitic sandstones shows a belt trending in E-W direction. These glauconitic sandstones are interpreted as marine coastal deposits, possibly sand bars that heralded the marine transgression over the basin. Seismic sections show at the top of Valle C sequence a group of reflections that onlap from S to N. Existing stratigraphy in the area can be interpreted as two main depositional cycles related to basin scale processes. The Valle C cycle shows a progressive filling of the basin and it involves a progressive reduction of the accommodation space as a result of the waning subsidence of the late sag stage. The Salamanca cycle records the progressive transgression of the Atlantic Ocean over the basin; this transgression is related to an episode of basin scale subsidence. Five lower scale units were identified in the studied record.

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