Application of U-Pb detrital zircon geochronology to drill cuttings for age control in hydrocarbon exploration wells: a case example from the Rukwa Rift Basin, Tanzania

Abstract

Precise dating and correlation of drilled wells through continental successions is challenging for hydrocarbon exploration, especially where preservation and recovery of age-diagnostic fossils is poor. As a complement or alternative to biostratigraphic dating we demonstrate the effectiveness of U–Pb geochronology via laser ablation–inductively coupled plasma–mass spectrometry on detrital zircon from well cuttings. In basins with syndepositional volcanic input, the youngest zircons in a stratigraphic interval can refine and serve as a proxy for the age of deposition. We demonstrate the reliability of this technique when applied to hydrocarbon exploration wells by analyzing drill cuttings through a continental interval of the Galula-1 well in the Rukwa Rift Basin, East African rift system, Tanzania, which previously yielded conflicting biostratigraphy results. The lower third of the well section reveals a late Miocene to Pliocene up-hole younging trend in the youngest detrital zircon populations, which matches new radioisotopic ages on volcanic tuffs from a correlative outcrop section. This is followed by an interval with recycled young zircons, followed by a zircon-free interval, interpreted to correspond to changes in magma composition of the nearby Rungwe volcanic province. This study provides the first radioisotopic age constraints for the Lake Beds in the Rukwa rift and demonstrates that sedimentation in the basin began by 8.7 Ma, critical for burial and thermal history modeling and establishing the probability of a working hydrocarbon system. Correspondence in age and zircon preservation between well and outcrop samples from the same intervals provides strong support for applying U–Pb detrital zircon geochronology to well cuttings, as a rapid, inexpensive approach for hydrocarbon exploration