GEOLOGY AND EXPLORATION IN THE TAKUTU GRABEN OF GUYANA BRAZIL

Abstract

The Takutu basin is a Mesozoic graben 280 km long and 40 km wide in northern Brazil and adjoining Guyana. It occupies an ancient Pre Cambrian shear zone within the heart of the Archean Guyana shield. Metavolcanic rocks and thick Proterozoic quartzites crop out north of the basin. Granitic rocks border the graben on the south and east. Mesozoic basalt forms a band of outcrop along the southern and eastern margins of the modern‐day valley. The basin is filled with up to 5,400m (18,000 ft) of Mesozoic sedimentary rocks, underlain by more than 1,700m (5,600 ft) of mafic volcanics of Jurassic and possibly older (i.e, Proterozoic) ages. Six regional unconformities were recognized within the basin fill.The geologic history of the graben is characterized by one volcanicphase and three depositional phases. Rifting began in Late Triassic to Early Jurassic time with the extrusion of thick Apoteri flow basalts. Lower Jurassic non‐marine clastics of the Manari Formation overlie the eroded basalt. They are overlain by the thick Lower and Middle Jurassic non‐marine salt and gray shale of the Pirara Formation. These sediments were deposited over most of the basin and contain good oil source‐rocks. Middle and Upper Jurassic non‐marine clastics of the Takutu Formation overlie the evaporites. They were deposited in lacustrine and deltaic environments, as interpreted from seismic and well data.There are two contrasting structural styles, of Late Triassic to Early Jurassic, and Cretaceous to Tertiary ages. Block‐faults are common in the pre‐salt section, and non‐piercement halokinetic structures in thepost‐salt rocks. The graben contains a cross‐basin arch, evidence of wrench faults and minor salt solution.Four exploratory wells have been drilled to date. Home Karanambo No. 1 discovered highquality crude oil in the fractured volcanic basement. Seismic stratigraphic interpretations indicate that alluvial fan and deltaic reservoirs may overlie undrilled structures elsewhere in the basin.