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The Ecuadorian Oriente, a sparsely inhabited and badly accessible jungle area east of the Andes, covering nearly 100,000 sq. km., is part of the broad foreland belt which spreads between the Andean Cordilleras and the Brazilian shield. The Ecuadorian part of this foreland represents an asymmetric sedimentary basin filled with 324-1,000 meters of mostly marine Cretaceous and up to 4,500 meters Tertiary deposits in brackish- and fresh-water facies. The line of greatest basinal depth extends 80-100 km. east of the Andes. East of this line the thickness of the Cretaceous-Tertiary column decreases gradually toward the rising pre-Cretaceous basement and in the same time important facies changes take place. The basin deepens from north to south.
The pre-Cretaceous basement is well exposed in the Cutucu Mountains of the southwestern Oriente and consists there of at least 1,400 meters of marine Paleozoic rocks, 1,500 meters of marine Lower Jurassic, and 2,300 meters brackish to continental Middle to Upper Jurassic.
The line of greatest basinal depth separates two structural provinces: on the west the sub-Andean zone of foothill folds culminating in the Napo and Cutucu uplifts; on the east the Yasuni-Lorocachi trend of low basement ridges on the eastward rising Brazilian shield. The pre-Cretaceous substratum has been affected by Paleozoic and Jurassic folding (the latter along north-south trends, ancestral Andes), as evidenced by overlaps and angular unconformities at the base of the Pennsylvanian, of the Middle to Upper Jurassic, and of the Cretaceous. The folds and uplifts in the sub-Andean zone of the Cretaceous-Tertiary basin are the result of the post-Miocene Andean orogeny with a slight precursory warping near the end of Cretaceous time and weak posthumous movements in Quaternary time. The arious terraces of Mesa fan deposits, contemporaneous with a climax of volcanism in the Andes, are post-orogenic, but suggest a possible epeirogenic uplift of the Andes to the extent of 1,000 meters or more.
The occurrence of the bituminous Napo formation (Albian-Coniacian) throughout the whole sub-Andean foreland, together with favorable reservoir rock and structural conditions, led to drilling on five foothill anticlines and on one of the fault structures on the Yasuni-Lorocachi trend. The tests resulted in negligible quantities of heavy oil or produced only water with or without a scum of tarry oil. Most of the reservoir rocks had been flushed by fresh water.
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