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A Royal Flush in the Great Campos (Brazil's Santos-Campos) Basin

Abstract

18 months prior to the Tupi discovery, a multi-disciplinary study predicted Santos Basin pre-salt sourcing working from non-exclusive data: key piston cores, a satellite SAR interpretation, 2D basin modeling, source rock screening, and a few crude oils. With detailed geologic reasoning a nearly busted flush in the Santos Basin became a winning hand. One face card was a piston core macro-seepage, extensively degraded, that still correlated to lacustrine oils from the Cabo Frio area, proving an active pre-salt source. Concurrently, a 2D compositional model predicted oil generation from a pre-salt Guaritiba lacustrine source in the basin center. With an impervious salt layer (as seen on a regional seismic composite profile) providing a barrier to vertical migration, the model predicted long-distance lateral migration to the basin margins before oil escaped vertically. A SAR study confidently identified seepage within the basin and, at lower confidence, dispersed seepage along the margins. Localized maturity of the post-salt Itajai-Acu source meant no expulsion paths existed from mature Itajai-Acu to the distal basin margin. That left lateral migration below the salt to reach the margins, hence filling any pre-salt traps before excess hydrocarbons escaped. From proving a working pre-salt source to inferring source interval depositional environments took more time and fluid samples. Current work is based on a much-expanded set of ~500 fluid samples from which four main oil families can be distinguished across the Campos and Santos sub-basins. A multivariate statistical comparison first separated a group of pre-salt oils derived from source rocks composed of a mixed (lacustrine algal + marine algal) kerogen assemblage deposited in sag basinal settings. Strong chemical similarities suggested a genetic relationship among oils from wells dotting the ANP “pre-salt polygon” area. This family of oils often overlaps expulsion area of another family from an older Syn Rift II lacustrine brackish setting (Lagoa Feia). Two further types derive from mature source rocks deposited in either earlier Syn-Rift I lacustrine freshwater environments or younger, post-salt Late Cretaceous marine settings. We discuss resulting oil mixing plus geochemical, tectono-structural and inferred environment of deposition settings of these families to illuminate the charge story of the Santos and Campos super-giant fields, discoveries and YTF (yet-to-find)!