Compound seismic modelling of the Ainsa II turbidite system, Spain: Application to deep-water channel systems offshore Angola

Abstract

Seismic models of outcrops are essential for qualifying petroleum targets, because they bridge a critical gap in both resolution and scale between architectural geometries observed in outcrops and in seismic data. The paper describes how a seismic model of the deep-marine Ainsa II turbidite system of the St. Vicente Formation in the Southern Pyrenees, Spain is developed and used as an analogue for exploration prospects in the Quifangondo Formation, Lower Congo Basin offshore Angola. First, geometry and lithology information from the Ainsa II outcrop were utilised to construct a detailed earth model. The Ainsa II turbidite system consists of at least five clastic turbidite units bounded by erosional surfaces, where only the youngest unit is steep-sided. The input geo-model (earth model) includes stratigraphic information, such as truncations, onlaps, thickening/thinning of beds, condensed sections and detailed lateral changes in lithology. Second, subsurface petrophysical properties (velocity and density logs) necessary to build the forward seismic models were extracted from an offshore well in Angola. This allowed us to utilise the outcrop as a seismic analogue for a relatively narrow, laterally aggrading turbidite system offshore Angola. The seismic models showed that it is difficult to image and identify amalgamated beds inside a larger unit, particularly if beds have predominantly the same facies associations and weakly dipping basal boundaries (as in the lower part of the Ainsa II system). Second, seismic modelling helped to highlight areas where it is difficult to distinguish between seismic onlap and true stratigraphic onlap. Information which decides when evaluating stratigraphic traps. We also found that lateral degradation in amplitude may represent a lateral change in lithology (e.g. from sandy channel deposits to levee-overbank heterolithics). Integration of outcrop mapping, well data, 3D seismic interpretation and forward seismic modelling, has enabled us to study deep-water depositional elements at a sub-seismic scale. This has resulted in enhanced prediction of the lithology distribution and architecture of the turbidite system offshore Angola, and gives new insights into the 3D geometry of the Ainsa II system.