Sedimentological and Geochemical Characterization of the Lagoa Feia Formation, Rift Phase of the Campos Basin, Brazil

Abstract

The Barremian rift-stage lacustrine sequence of the Lagoa Feia Formation comprises the source rocks f all petroleum so far discovered in the Campos basin, the most prolific oil province in Brazil. The Lagoa Feia Fm. is characterized by a complexity of terrigenous and carbonate sedimentary rocks which are subdivided into four distinct brackish to saline lacustrine sequences, with increasing upward salinity. Calcareous black shales rich in bacterial and algal-derived type I kerogen (up to 9% TOC) were deposited under anoxic conditions in brackish to saline alkaline waters. These source shales reach a net thickness of up to 200 m. Diagnostic biomarker features include low concentrations of steranes, presence of β-carotane, gammacerane, 28,30-bisnorhopane, and 25,28,30-trisnorhopane, high concentrations of hopanes and methylsteranes and high relative abundances of extended tricyclic terpanes up to C45. The main source beds are identified in the Upper Barremian and are mature throughout most of the basin. A close association between source rock depocenters and oil accumulations is observed. Migration was controlled by unconformities and porous layers within the Lagoa Feia Formation and was dominantly vertical, constrained by windows in the salt layer and listric faults and unconformities in the shallower marine sequence.