3D high resolution stratigraphy of early rift deltaic deposits in the Sergipe-Alagoas Basin: impact on the reservoir compartmentalization

Abstract

A high resolution stratigraphic study applied to reservoir characterization, based on well logs, cuts and cores from the Furado Field, a mature oilfield of the Alagoas Sub-basin, was carried out on deltaic strata of the lower portion of the Barra de Itiúba Formation (early rifting of the Sergipe-Alagoas Basin). Three lacustrine systems tracts were recognized within a 3rd order sequence: the lowstand systems tract (tectonic pulse initiation) that includes extensive, medium- to coarse-grained fluvial-deltaic sandstones that display good reservoir quality; the transgressive systems tract (tectonic climax), which comprises lacustrine and prodelta shale and distal delta front, fine- to very fine-grained sandstone; and the highstand system tract (tectonic quiescence) that encompasses several cycles of delta front progradation and related poor- quality reservoirs relative to those of the lowstand system tract. The 4th order sequences are represented by climate-driven transgressive-regressive cycles that constitute independent reservoirs, equivalent to a reservoir zones. This high resolution stratigraphic framework was then used on the building of the 3D geologic model, which honored the systems tracts geometry, and related facies, as well as the paleos-tructure, including a syn-depositional fault propagation anticline. Several normal faults split the reservoirs at different scales. Their potential impact on reservoir fragmentation was approached through the use of fault juxtaposition and shale gouge ratio diagrams.