Takutu Basin, Guyana: Case History of a Frontier Exploration Project

Robert E. Webster, Richard G. Murdock

The Takutu basin, Guyana, is a Jurassic-Early cretaceous continental rift basin that cuts the Guyana shield in southwest Guyana and northern Brazil. Prior exploration had documented a stratigraphic section dominated by mudstone, including Jurassic lacustrine source shale, siltstone, evaporites, and basalt, shown the presence of numerous anticlinal and tilted fault block structures, and resulted in a noncommercial oil discovery in a fractured basalt reservoir.

In late 1988 Hunt Oil Co. began operations in the basin, with initial focus on the previously unexplored eastern part where better sandstone development was expected. A three year exploration program included field geology, photogeologic mapping, surface geochemical prospecting, reprocessing old data, and acquisition of SAR, aeromagnetics, and 1,331 km of new seismic. Unfortunately, longitudinal fault patterns in the eastern basin proved unfavorable for creation of large structural traps, so exploration efforts focused on the large central basin Savannah arch. An anticline mapped at the south end, near the present basin margin fault system, was interpreted from seismic stratigraphic work as a drape feature above a thick lacustrine fan delta complex, with sands derived from granulite a d granitic rocks in the Kanuku highlands to the south.

The exploration well Turantsink 1 was drilled on this attractive target, but the deep structure proved rooted in a thickened salt section deposited near the paleocenter of the basin. Minor oil shows were observed at several horizons, but the predicted sandstone reservoirs were not present. This part of the basin was also affected by a Tertiary hydrothermal event that locally overmatured the thick source shales and destroyed porosity in all potential reservoir units. Despite this failure, large volumes of oil have apparently been generated in the Takutu basin, and a number of large structural features remain untested. Future exploration must locate reservoir rocks in a part of the basin unaffected by hydrothermal activity.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995