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The AAPG/Datapages Combined Publications Database

Tulsa Geological Society


Tulsa Geological Society Digest
Vol. 31 (1963), Pages 146-176

General Geology and Oil Possibilities of the Amazonas Basin in Brazil

Luis G. Morales


The Amazonas Basin covers approximately 1,250.000 Km2 or about 400.000 square miles. Physiographically, this vast area is subdivided into the Upper, Middle and Lower Amazonas regions. The first two of these are filled primarily by Paleozoic sediments totaling about 4000 meters in thickness. The deltaic portion of the Marajo Island to the east contains in its deepest part about the same thickness of essentially Tertiary rocks. Structurally, this vast sedimentary area is bounded in the west by the Iquitos Arch, in the north and south by the Guyana and Brazilian Shields, and in the east by the Gurupa Arch. The divide between the Upper and Middle Amazonas, is generally set at the purus river which is also a geological boundary.

The sedimentary section is composed of soft, unconsolidated sediments at the top, underlain by a diabase flow, followed by a sequence of evaporites, limestones and by bituminous shales grading to indurated sands at the bottom of the lithologic column. The densities of these strata range, in the Paleozoic, from 2.5 to 2.7 - which added to the presence of diabase, anhydrite, salt and other high velocity beds - render the geophysical problem singularly difficult. In order to offset the absorption and decay of seismic impulse through the dense layers - methods have been devised for the mapping of the basement surface through the VLF (Very Low Frequency) refraction method. In addition, Carboniferous structure is being mapped by the reflection method which has succeeded in cancelling the interference and increasing the signal by the usage of multiple geophones, multiple holes and magnetic tape recording.

Geophysical methods, guided by isopachous, clastic ratios and biofacies maps - are spear-heading the exploration search for commercial oil production in the Amazonas. It is concluded that oil reserves exist on the flanks of major structural arches and in association with Devonian faulting and possible reef growths in the Carboniferous.

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