Controls on Deepwater Sandstone Deposition during the Divergent Margin Phase of Campos Basin, Offshore Brazil: From the Grain to the Plate

Campos Basin, with about 10 billion bbl proven hydrocarbon reserves, was filled by four second-order sequences (S2): the taphrogenic S2 (Barremian to Aptian); the transitional S2, with a thick Aptian evaporite section; the marine transgressive S2 (Albian to Paleocene), with an Albian carbonate shelf, and a Turonian MFS; and the regressive marine S2 (Paleocene to Recent). Most of the hydrocarbon reserves are hosted in deepwater sandstone systems (DSS) from the marine transgressive-regressive cycle (Albian to Miocene). This work describes the main controls on the deposition of such huge DSS, based on quantitative petrography, facies architecture, sequence stratigraphy, structural analysis and geodynamic evolution of the basin. A complex paleogeographic evolution was interpreted, rather than the previously supposed scenario of a steady state passive margin (thin skinned gravitational salt tectonics plus eustatic base level fluctuations), which imply the lowstand paradigm for deepwater sandstones. The evolution of Campos Basin is actually prone of geodynamic reactivations, which imprint clear evidence on sandstone composition, facies architecture, and stratigraphic pattern of the main DSS: the Albian-Cenomanian DSS, petrographically mature, with high quartz content and fine grained sand; the Campanipan-Santonian DSS, immature, with high feldspar content and medium grained sand; the Maastrichtian DSS, immature, with high volcanic contribution; the Paleogen System, still immature; and the Neogen DSS, again mature (high quartz and fine grained sand). The main DSS depocenters in Campos Basin were controlled by basement reactivation events that triggered succeeding episodes of salt tectonics, which in turn control sand distribution and facies architecture. The first event caused the complete destruction of the Albian shelf, and the formation of a mega slope during Upper Cretaceous. Subsequently, the main deepwater sand systems, petrographically immature, correspond to basement tectonic reactivations (upstream control), as they systematically filled and spilled out the recurrent relief of the Campos Basin offshore high. The high volcanic contribution in the Maastrichtian DSS is related to the magmatism caused by the movement of the basin above a mantle thermal anomaly. The classic lowstand tract approach can only be applied to the mature sand systems, deposited when the shelf was well established, during the Albian-Cenomanian, and from the Oligocene to the Recent.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California