Abstract: Important Elements Of The Chubutiano Petroleum System, Offshore San Jorge Basin, Argentina

Courtade, Sergio - YPF; Curiale, Joseph; Sperry, Steven; Trevena, Arthur - Unocal Corporation; Chebli, Pablo and Petrelli, Gustavo - YPF

San Jorge Basin (Argentina) is a Jurassic and early Cretaceous rift basin that later evolved into a Cretaceous and Tertiary sag basin. Here we present a model for the Chubutiano petroleum system, offshore San Jorge Basin. This petroleum system is responsible for most of the oil discovered and produced in the basin. The source material is type II or type I/II kerogen in the lacustrine shales and oolitic limestones of the Pozo D129 Fm. Reservoir rocks include fluvial and deltaic sandstones in the Yacimiento El Trébol Fm., Comodoro Rivadavia Fm. and the Mina El Carmen Fm. and shallow-marine facies in the Salamanca Fm. The Mina El Carmen, middle Yacimiento El Trébol and upper Salamanca Formations are regional seals. The structural style indicates the fragmentation of the basin into extensional blocks (horsts, grabens and half-grabens) during the rifting stage. Later, during the sag stage, these structural blocks still controlled the thickness of the sag record. On the other hand, as a consequence of the more regional thermal subsidence at the sag stage, the basin was divided into three tectonic domains: Platform, Platform Rim and Basin Center; these domains controlled the development of all of the elements of the petroleum system. During the Tertiary, the basin continued to subside and experienced a tectonic reactivation event that in the offshore part of the basin was recorded as a transtensional Miocene pulse. This deformation reactivated many of the older structures. This complex tectonic history developed two basic types of traps: older traps, above the high rift blocks and new traps formed at Miocene at the edge of different structural blocks.

Oil expulsion in the offshore San Jorge Basin occurred progressively as different structural blocks moved into the oil window. When initial expulsion occurred in the Basin Center, the only seals were Mina El Carmen Formation shales, and it is unlikely that accumulations from this migration episode occur above this formation. Expulsion in the structural Platform Rim zone begun in the early Tertiary times and peaked in the Miocene, while the structural Platform zone was never mature enough to expel oil. During the Miocene, when tilting and fault reactivation occurred in most of the basin, significant amounts of oil migrated to the structural highs in the Platform Rim and the Platform. This second migration pulse was accompanied by biodegradation and degasification in the shallower traps, as well as enhanced gas generation from the source and from earlier oil accumulations. In addition to vertical migration along the faults, lateral migration occurred to some degree and two pathways were defined for it. The first one involves migration into deep reservoirs at Cretaceous time. This migration followed the structural pattern defined by rift blocks. The second migration scheme controlled tertiary migration in levels above Comodoro Rivadavia Formation and it followed a radial direction. This integrated model presented for the Chubutiano petroleum system is consistent with geological and geochemical data from wells drilled in the area.

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