Stratigraphy and depositional model of the Namorado Sandstone in the Albacora Fild, Campos Basin

Abstract

This paper concerns the Albian turbiditie sandstones of lhe Albacora Field (NamoradoSandstone), aiming to geologically
charaeterize the reservoirs of this field, and to establish its depositional model. Facies and facies associations were described, and stratigraphie correlations were made using gamma rayand resistivity welllogs. It was possible to subdivide the reservoir, from top to base, in four stratigraphic intervals: Namorado Sandstone Zero, NamoradoSandstone I, Namorado Sandstone IIa and Namorado Sandstone IIb, separated by diamictite and calcilutite layers. A geological model for these units is also presented. Using 457 m of core from nine wells, seven sedimentary faeies were individualized based mainly on grain size, sedimentary structures and vertical lithological association. The Massive Sansdstone Facies (Am) is characterized by well sorted very fine/fine-grained sandstones and subordinatelyb y medium-grained sandstones. Anhough massiva, it may show gradational bedding and fuidization features. The Laminated Sandstone Facies (AI) is composed by well sorted very fine/fine-grained sandstones with parallel laminations and climbing ripples.The Conglomeratic Sandstone Facies (Ac) is composed of granule sand pebbles of granitoid rocks and abundant bioelasts. This facies is typical of the base of lhe Am facies packages, with an average thickness of 20 cm.The Intraformational Conglomerate (Cgl) is composed of shale, marls and calcilutites fragments. The Interlaminated Facies (I) is formed by thin layers of very fine/flne-grained sandstones, intercalated with shales and calcilutites. The Diamietite Facies (Dia) is formed of sandstone, calcilutite and shale fragments scattered in a sandy-argillaeeous and argillaceous-sandy matrix. The
Calcilutite, Shale and Marl Facies is represented by an association of  lithologies (Clu,Flh, Mrg), with great continuity in lhe Albacora Field. The Massive Sandstone Faeies(Am) and Interfaminated Facies (I) represent 84 % in volume of the total eored intervals. Based on the facies association, and the stratigraphie correlation, three types of deposits were identified: a) Deposits of the axis of confined lobes, represented mainly by Am Facies, generated by the amalgamation of several superimposed sandy flows. The individual lobes make up packages up to20 m thick, where the sansdtone/shalerate is 90 %. The external geometry is tabular, and the estimated area ot the lobes, in the central portion ot the field is of 6 km'; b) Deposits of the margins of confined lobes, represented mainly by the Interlaminated Facies, and present in the lateral and/or distal portions of the lobes, where fine sediments can be preserved. The sandstone/shale ratio is 70 %, lhe sheel-like external geometry is typical of this deposits, and they may be up to 15m thick; c) Residual coarse-grained deposits, formed by Conglomerate Facies and  of local occurence, wilh a maximum thiekness of 50 em, represent the most coarse-grained fraction transported by lhe turbidity currents. lsopach and facies maps, besides general data collected in the basin, indicate that the source-area was preferential located to the NW of lhe field, with a NW/SE deposilional axis.