Journal of Petroleum Geology, Vol.2, No.1, pp. 55-74, 1979

©Copyright 2000 Scientific Press, Ltd.

PETROLEUM GEOLOGY IN THE PALEOZOIC CLASTICS OF THE MIDDLE AMAZON BASIN, BRAZIL

Albert V. Carozzi*

* Professor of Geology, University of Illinois at Champaign--Urbana, Urbana. Illinois. 61801, USA, and International Oil Exploration Consultant.

Abstract

Paleoenvironmental maps of all the stratigraphic units of the Middle Amazon Basin from Ordovician to Early Carboniferous are analyzed in terms of their source-rock and reservoir potentials. These maps were obtained by applying the following techniques: detailed petrography of diamictites, sandstones and siltstones including measurements of index of clasticity; analysis of early and late diagenetic cementation processes; environmental interpretation of sedimentary structures; geochemistry of trace elements and lateral variations of clay mineral assemblages based on X-ray diffraction data. Five distinct sedimentary models were recognized. Fluvio-deltaic model consisting of fluvial, deltaic, distal deltaic and offshore subenvironments. Tidal flat model consisting of marshes, intertidal flat, shoreface and offshore subenvironments. Beach model consisting of dunes (inferred), beach, shoreface and offshore subenvironments. Glacial model / Glacial model (Curiri time, Late Devonian) consisting of glacial (tillites with subglacial sandstone channels and interglacial braided streams), shoreface and offshore subenvironments. From the association of these depositional models, the direction and intensity of the supply of coarse clastics was determined in a semi-quantitative manner for each stratigraphic unit. The time and space distribution of the detrital supplies and of the areas of coarse clastic sedimentation indicates a continuous synsedimentary tectonic control of the depocenters by the differential subsidence of the underlying Precambrian basement in the shape of large blocks limited by a system of NW-SE and NE-SW lineaments. By using an index of synsedimentary tectonic activity it is possible to define three major episodes of coarse clastic sedimentation which have the best reservoir potential: Early Silurian fluvio-glacial systems, Early Devonian prograding deltas and Late Devonian fluvio-glacial systems. The traps for hydrocarbons are mainly stratigraphic but enhanced by the weak structures of the basin and result from the relationship between the coarse clastics and two intervening sequences of mature black shales.