Facies Association, Depositional Systems, and Paleophysiografic Models of the Barra Velha Formation, Pre-Salt Sequence – Santos Basin, Brazil

Abstract

This study shows the facies, facies associations and the stacking pattern of the Barra Velha Formation of the pre-salt Aptian sucession (average thickness of 300 m). Sedimentation evolved onto distinct structural styles (ramp, horst and platform) on the External High, Santos Basin. The research was based upon a rock (cores and sidewall core samples) - well-logs seismic integrated data, in order to identify the depositional systems and their evolution through time.

The main facies are: a) in situ: shrub-like limestones; bedded travertines; laminated microbial boundstones; Mg-rich silicate argillites; b) high to low energy reworked facies: rudstones, grainstones and silty mudstone; c) diagenetic facies: commonly showing intense replacement/cementation by dolomite and quartz and a varied degree of dissolution. Grainstones and rudstones are mainly composed by intrabasinal particles yielded from the erosion of previously deposited in situ and diagenetic facies.

The main carbonate and non-carbonate depositional systems are: carbonate build-up (shrub-like limestone and travertine); coastal lacustrine systems (shoreface, foreshore, berm/crest, washover fan/reflux back flow - storm processes) to distal lacustrine system; microbial flat system; alluvial fans and fandelta system; magnesium argillaceous evaporitic system, and palustrine system.

Facies definitions and associations are the key tasks for identifying cycles in different hierarchy, depositional systems, and stratigraphic surfaces regionally mapped.

Stratigraphic correlations and their integration to seismic data enabled us to build geological models and paleophysiographic maps for each stage of evolution of the Barra Velha Formation, showing the relationship among depositional systems, geometries and reservoir characteristics. Moreover, the study suggests that each of the recognized sequences has a different architectural style, due to factors such as variation on substrate morphology (structural control), environment energy, water chemistry and water depth.

The understanding of the facies, their associations and depositional systems of each sequence of the studied pre-salt section allow us to create a predictive stratigraphic framework, which can support new well locations, increasing both the index of success of discoveries and the oil production rate.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018