Geological Model for Oil Gravity Variations in Oriente Basin, Ecuador

Martin F. Dashwood, Ian L. Abbotts

The Oriente basin is one of the major productive Subandean basins. Most of the fields produce 29°-33° API paraffinic oils, but oils have been discovered with gravities ranging from 10° to 35° API. All the oils have been recovered from multiple middle to Late Cretaceous sandstone reservoirs (Hollin and Napo Formations). Wells display a variety of oil gravities by reservoir.

The origin of the Oriente oils is problematical and controversial, but structural, geochemical, and well evidence suggest a vast oil kitchen west of the present Andean foothills that was mature for oil generation by at least early Tertiary. Oil analyses indicate a single family of oils is present. Oil gravity variations can be explained systematically in terms of the various alteration processes suffered by the oil in each reservoir.

Intermittent early Andean uplift (latest Cretaceous to mid-Eocene) resulted in biodegradation and water-washing of oils, particularly in the uppermost Napo reservoirs. The main Andean orogeny (Pliocene) uplifted the Hollin reservoir to outcrop in the west, and tilted the basin down to the south. This movement resulted in water washing or flushing of the Hollin aquifer and a phase of northward remigration of oil. Late Andean structures postdated primary oil migration.

Almost all structures displaying growth during the Late Cretaceous to early Eocene have been oil bearing, but some, particularly those located on the present-day basin flanks, were later severely biodegraded or breached.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.