Abstract: Petroleum Systems of the Southwest Tertiary Basin and Marbella Area, Southwest Mexico

Pacheco, Luis Francisco Fuentes - Pemex

Molecular characterization of select oil samples indicate most oils have been generated from several facies. Examination of oil molecular characteristics, oil-rock correlation?s, molecular characteristics of key stratigraphic horizons, paleofacies maps, maturation and potential migration pathways suggests the Tithonian shaly carbonates as major source rocks. Moreover, there is good evidence for other intervals of high quality source rocks in Oxfordian, Kimmeridgian, Middle-Upper Cretaceous and Paleogene (Principally in Eocene).

The hydrocarbon accumulations are contained within several reservoir rocks identified, based on identification of plays. Plays were located in Upper Jurassic oolitic sequences, in Early to Middle Cretaceous carbonate platform rocks and breccias, and in Late Cretaceous fractured carbonates of basin. Also has plays in Cenozoic rocks from Paleogene carbonates and breccias; in Early to Middle Miocene lobs, submarine fans and isolated carbonate platforms with reefs and post reefs ( 1); and finally, in Middle Miocene to Recent submarine canyon turbidites associated to compressional structures, compressional salt tops, normal faults derived from salt withdraw and pinchouts of turbidite sands in rollovers.

The most important seal rocks are shaly carbonates and laminated anhydrates from Tithonian for Upper Jurassic plays. For Early - Middle Cretaceous plays, seals are in a basin?s carbonates and anhydrites from Middle - Upper Cretaceous. Upper Cretaceous plays have seals corresponding to the basin?s carbonates and marls from Upper Cretaceous and Paleogene shale?s. In the case of Paleogene plays, it´s seals are basin shales from the same Paleogene sequences. The Early Miocene to Recent plays present seals comprising intraformational batial and basin shales.

Models for the hydrocarbon emplacement including structural traps formation have been based on distribution and type of hydrocarbons, tectonics development, hydrocarbon generation/expel models, hydrocarbon alterations and probable migration pathways. The first phase of oil emplacement from Upper Jurassic ? Early Cretaceous source rocks occurred in the Early - Middle Cretaceous.

In the Upper Cretaceous, the Chortis block collided with the Chiapas which resulted in a gentle folding. Some hydrocarbons were emplaced to the structural tops. The main phase of structuration and folding in the Sierra de Chiapas started in the Miocene and generated well defined structural traps and additional vertical pathways of migration along faults. Finally, in the Plio - Pleistocene time, the Chortis block completed major compression phase and the south portion of Sierra de Chiapas suffered an isostatic uplift. The result was a big subsidence in the north giving origin to the principal phase of salt withdrawal and increased burial for the generation of liquid hydrocarbons and also, oil and gas from the Paleogene marine organic rich shales. The last tectonic activity caused extensive redistribution of hydrocarbon accumulations. This later stage played an important role in forming the oil and gas accumulations. Fig 2 shows the distribution of geologic events which formed the basin.

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