--> Abstract: Structural Style and Timing of Hydrocarbon Entrapments in the Ecuadorian Oriente Basin, by P. Baby, M. Rivadeneira, C. Bernal, F. Christophoul, C. Dávila, M. Galárraga, R. Marocco, A. Valdez, J. Vega, and R. Barragán; #90933 (1998).

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Abstract: Structural Style and Timing of Hydrocarbon Entrapments in the Ecuadorian Oriente Basin

Baby, P.; Rivadeneira, M.; Bernal, C.; Christophoul, F.; Dávila, C.; Galárraga M.; Marocco, R.; Valdez, A.; Vega, J. - Petroproducción-Orstom; Barragán, R. - Oryx

The Ecuadorian Oriente basin is a foreland basin which has developed since Eocene times. The sedimentary column ranges in age from Paleozoic to Recent. It can be subdivided into three sequences: a pre-Cretaceous series, which is unconformably covered by a continental to shallow marine Cretaceous sedimentary cycle and a post Cretaceous continental foreland cover. Main oil fields correspond to transpressive structures developed along three NNE-SSW right-lateral convergent wrench-fault zones, which form the plays of the Oriente basin. (1) In the western part, the Subandean wrench-fault zone (Napo Uplift and Cutucú Uplift) is still seismically and volcanically active. The positive flower structure of Bermejo is not eroded and constitutes the unique productive oil field of the Subandean play. (2) In the center of the basin, the Shushufindi-Sacha wrench-fault-zone is the most productive play of Ecuador. It results from the inversion of a NNE-SSW trend of Upper Triassic to Lower Jurassic half-grabens which emerges actually in the Cutucú Uplift. Giant fields as Shushufindi correspond to positive flower structures too (Fig. 1). (3) To the east, the Capirón-Tiputini play is an inverted system of half-grabens (Fig. 2) probably Permo-Triassic in age. In the Sacha-Shushufindi play as in the Capirón-Tiputini play, half-grabens were eroded and sealed by Middle Jurassic to Basal Cretaceous volcano-clastic sediments (Chapiza-Yaupi Fm.). Reflection seismic data show syn-tectonic sedimentation which recorded 3 stages of transpression: Coniacian-Maastrichtian; Early Eocene; Pliocene-Quaternary. The Sacha-Shushufindi play formed in large part between the Coniacian and the Maastrichtian, while the Capirón-Tiputini play is principally Eocene in age. The Subandean Uplift developed during the Pliocene and Quaternary, but includes some Maastrichtian structures as the Bermejo field. The Napo Formation shales, marls and limestones (Albian-Campanian) are accepted as the source rocks of the Oriente basin oils, but were considered as immature in the northern part of the basin to early mature in its southern part. A new study of the Lower Napo shales shows that two other zones exist where these source rocks reached the oil window: the Napo Uplift and the central part of the Sacha-Shushufindi play (Auca zone). In the Nap o Uplift, source rocks maturity is due to subsidence of the Cenozoic foreland basin, while in the Auca zone it is produced by a continental mantle plume contemporaneous with the Coniacian-Maastrichtian tectonic crisis. The Napo Uplift is probably the remnant part of a largest petroleum system which developed towards the west. Two peaks of oil generation and expulsion are evidenced. The first one occurred in the Lower and Middle Eocene, in a foreland basin which extended to the west of the present basin limits. Upper Eocene to Oligocene times correspond to a period of erosion and poor subsidence, where oil expulsion stopped. During the Neogene, the second peak of oil expulsion occurred in the Napo Uplift and the Auca zone with the return of the foreland basin subsidence, and the western part of the petroleum system was progressively deformed and destroyed by the Andes uplift. The more productive oil fields correspond to Late Cretaceous and Eocene structural traps. Oil was accumulated in Aptian to Maastrichtian sandstones. Below the Sacha-Shushufindi play, in the Upper Triassic to Lower Jurassic half-grabens, the poorly studied Santiago-Sacha Formation is a thick shallow marine series with good source rock potential. In the eastern flank of the Cutucú Uplift, some source rock levels of the Santiago Formation are just maturing. Probably, some oils of the reservoir of the Cretaceous series base (Hollín Fm.) come from these pre-Cretaceous source rock levels.

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