A new technique of locating turbidite fans: Candeias formation (lower cretaceous), Recôncavo Basin, Brazil

Abstract

The reservoir package in the Candeias oil field, the largest stratigraphic accumulation of hydrocarbons in the Recôncavo Basin, has been reinterpreted as consisting of sublacustrine turbidite fans interbedded among Candeias shales, with the best quartzose arenite reservoirs corresponding to channeled mid-fan lobes. The occurrence in the turbidites of reworked lacustrine carbonate clasts with subaerial exposure features indicates that the Candeias submarine fans originated from an intrabasinal shallow platform and followed NNW-SSE fault-controlled channels on their way to final deposition. Thick and irregularly shaped lobes of massive fine sandstones (Pitanga Member) are always located immediately behind (that is, updip from) the relatively thin and coarse turbidite fans and overlap them diachronically. The petrographic composition of the massive sandstone expresses an extrabasinal origin from low-grade metamorphics of the Precambrian basement, and their sedimentary structures indicate large-scale fluidized sediment flow depositional processes. However, the admixture of a small amount of shallow water lacustrine carbonate clasts reveals that Pitanga sediments also crossed the carbonate platform before being deposited as isolated or coalescent lobes. Candeias turbidite fans had a stationary source area, the carbonate platform, and reached the distal portion of the shale slope. Pitanga fluidized flows were deposited in a proximal position of the shale slope and had a deltaic source that gradually prograded from NNW to SSE across the Tucano Basin. The constant geometric relationship between Pitanga lobes and Candeias turbidite fans originates from the fact that both depositional systems followed the same NNW-SEE fault-controlled channels; this becomes, therefore, an exploration tool for the prediction of oil-producing Candeias fans that are at the limit of resolution of seismic reflection techniques. Three operational steps are required: structurally analyzing the carbonate platform to locate potential transverse faults to which feeder channels might correspond; outlining the lobes of Pitanga massive sandstones related to such channels using seismic reflection profiles; and drilling exploratory wells for Candeias turbidite fans beneath the frontal portions of Pitanga lobes.