Seismic tectono-stratigraphic analysis of the Aptian pre-salt marginal system of Espírito Santo Basin, Brazil

Abstract

The Aptian sedimentary sequence of Mucuri Member, lower Mariricu Formation, constitutes the pre-salt of Espírito Santo Basin. Proximal successions are dominantly clastic consisting of fluvial and coastal sediments deposited within the margins of a wide lake, thus corresponding to the marginal deposits of lacustrine carbonate reservoirs located in distal portions of the basin. Economic interest for oil is centered on such distal reservoirs, consequently proximal sections lack detailed studies. The Espírito Santo Basin is located near pre-salt official polygon, with oil exploration active for over six decades. The main objective of this study is the tectono-stratigraphic analysis of the clastic Mucuri Member, Aptian sag succession of the Espírito Santo Basin. Leading methodology is a seismostratigraphic characterization based on 220 2D lines, 1 3D volume, 103 well log data and 1 cored well. Four seismostratigraphic units were identified, delimited by three conformable horizons. Unit-bounding reflectors coincide with gamma ray maxima or minima interpreted to reflect shale or anhydrites deposited in deep water environments, corresponding to lacustrine maximum flooding surfaces. The structural framework is characterized by minor faults in the north onto shallow basement platforms, and in the south by half-graben bounded by normal faults developed during the rift phase. The Mucuri Member records an enlargement in depositional area from its base to top unit witnessing an overall lacustrine base level rise during depositon. The geometry of depositional area was initially influenced by the paleorelief of the preceding rift basins. The seismic units record a decrease in thickness as remnant topography was gradually filled and the upper units extended beyond the rift boundaries, covering a larger area. The Early Cretaceous Mucuri Member composes the first post-rift sequence of Espírito Santo Basin marked by the cessation of fault activity and the onset of thermal subsidence.