The Internal Seismic Character of the Gamtoos Basin's Late Valanginian to Late Hauterivian Syncline Fill Sequences and the Potential for a Hydrocarbon Play
The Gamtoos Basin is located within the greater Outeniqua Basin off the southern coast of South Africa. Initial rifting commenced during the Late Jurassic and continued up until the Early Valanginian. There are two distinct tectonic regimes that have influenced the formation of the Synrift phases within the Gamtoos Basin. Synrift I and II described by Broad et al. (2006), focused on the tectonic regimes that influenced the sedimentation. Paton and Underhill (2004) refer to these phases as Principal Synrift and Late Synrift and distinguish them as mega-sequences governed by crustal anisotropy. Accommodation space developed in response to movement on the Agulhas-Falkland Fracture Zone. As a result of this movement the Gamtoos Fault, in plan-view, trends SE-NW from the onshore portion of the basin and makes an almost 90° bend offshore continuing in a general north-south trend (Paton and Underhill, 2004). The main Gamtoos depocentre is thus not only as a consequence of the growth of the Gamtoos Fault but also due to the occurrence of folding (Paton and Underhill, 2004). This fold-fault system trending across the basin has produced various concave structures giving rise to mini depocentres. A depocentre along the Gamtoos Fault, also referred to as the Late Valanginian to Late Hauterivian canyon by Davids et al. 2018, created a “mini basin” whereby several features as well as possible source rocks and seals have been interpreted using seismic stratigraphy methods. In this study potential reservoir and trap configurations have been identified using seismic / sequence stratigraphy methods by observing stratal terminations and lapouts (Catuneanu, 2006). Then seismic facies were examined using external geometries, the internal seismic character and seismic amplitude and frequency identification. Structures within the synclinal fill packages have been interpreted throughout the basin in variable extents and provide insights into sediment-transfer zones prior to being sediment-filled. Several leads have been identified analysing the petroleum system elements. The Gamtoos Basin, being under-explored, holds many key elements for an active petroleum system justified by oil and gas shows / fluorescence in many wells. Channels and canyons have fed valuable accretion packages, as mapped during this study, to the deeper reaches of the basin. This emphasises the hydrocarbon potential with leads identified in this study thus enhancing the prospectivity of this subbasin.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018