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Petroleum Systems Analysis in the Santos Basin: New Risks in the Extension of the Pre-Salt Play?

Abstract

The Pre-Salt petroleum system of the Santos basin, offshore Brazil, hosts several of the largest global oil discoveries made in the past decade. Exploration in the inboard region of the basin has focused on the Aptian carbonate four-way play, with proven reserves in excess of 50 billion barrels of oil. This oil is sourced from organic-rich mudstones that were deposted in evolving rift basins following the breakup of the Gondwana supercontinent in the Lower Cretaceous. Charge of the four-way traps in this efficient and simple petroleum system occurs through lateral migration of hydrocarbons into time equivalent lacustrine carbonate reservoirs of the Barra Velha Fm. that are sealed by late Aptian evaporites.

While the Aptian carbonate play is well established in the inboard area, future exploration efforts will focus on the outboard regions where there is significant uncertainty surrounding the timing and distribution of key play elements. For example, while the Barra Velha Fm. was deposited as a relatively thick package inboard of the outer basin high, this appears to thin in the outboard region on potentially younger crust. Despite this being such a prolific petroleum system in the proximal environment, could crustal age limit the time for adaquate source deposition, raising concerns for charge risk to traps identified in the outer-basin? Similarly, the timing of deposition and initial distribution of the Ariri evaporites play a critical role in driving maturation of the source rock and in acting as the top seal for the Aptian carbonate four-way play. However, the age and original depositional thickness of the salt is not well constrained. Could this represent a risk to maturity and evaporite seal presence outboard of the outer-basin high? Finally, while fluids provide a useful guide to maturity of the source interval in the established part of the basin, there is a paucity of absolute age control and thermochronology data available to constrain the thermal history of this key interval. What can be done to place time-temperature constraints on the thermal history in the inboard Santos basin to improve the confidence of maturity predictions in the outer-basin far from well calibration? We present an integrated geochemical, sequence and tectono-stratigraphic characterization of the Santos basin to address some of these key questions, and how these can be interrogated to reduce the uncertainty in the unexplored part of this prolific petroleum system.