Control of basin-scale accommodation zones on syn-rift Aptian lacustrine carbonate reservoirs. Implications for reservoir quality estimates of Brazilian pre-salt

Abstract

This paper presents the tectono-sedimentary time and space evolution of the Sururu oil field (Brazilian Pre-Salt). The field is located at the well-known Santos External High (SEH), north of the Tupi oil field—the pioneer accumulation of the largest and most prolific hydrocarbon trend of the 21st century. In the central portion of the field, the regional NE-trending faults system that prevails in the Santos Basin bends to NNW and becomes diffused into several low-displacement normal faults. These faults control a series of segmented structural highs, with some holding significant amounts of oil within lower permeability mudstones. These are the reservoir rocks, controlled by an NW horst, segmented in three domains by 290° Az and 245° Az trending relay ramp systems. The structural domains control discontinuous depocenters that evolved during the late syn-rift phase (Mid Aptian, c. 117 Ma). In this work, the aim is to show how the evolution through time of these segmented domains influenced variations of sedimentary facies, affecting vertical connectivity of the oil within the production zones. The work was based on interpreting 3D multi-azimuthal high-resolution seismic data, eight wells, and cross-section restoration techniques. We interpret multi-stage fault growth along the NW structural trend as a mechanism for distributing reservoir rocks, promoting localized syn-rift footwall uplift of isolated normal faults. The intervening domains between interacting faults induce the nucleation of accommodation zones, which in turn, controls the distribution of sediment catchment areas. Facies quality degradation along syn-rift structural highs impacts the distribution of preferential fluid flow. Therefore, the association of structural elements described in this study demonstrates that shallow water facies were controlled by uplift at the footwall of low displacement normal faults, inducing local lake-level retraction. Such evolution is strongly influenced by displacement transfer along local transfer zones dissecting the structural highs, which in turn controls local structural highs and lows, hence the distribution of high- and low-quality reservoir rocks during later phases of rifting.