Syn-Depositional Tectonics Control Three-Dimensional Channel Organization at Multiple Scales: The Arro Turbidite System, Aínsa Basin, Spanish Pyrenees

Abstract

Axial submarine channels within structurally complex, elongate deepwater basins bounded by compressional structures are becoming increasingly viable exploration targets. Such channel systems have the potential to form prolific plays, with high net-to-gross sandstone reservoirs sealed by encasing fine-grained sediments and trapped by thrust-related structures. However, intense deformation and the common dominance of fine-grained heterolithic extra-channel sediment often degrade the signal-to-noise ratio of conventional exploration seismic data, making outcrop analogs crucial in the development of predictive models.

The Arro turbidite system, Aínsa Basin (Spanish Pyrenees), contains discrete channels, up to 30 m thick, comprising primarily sandstone (ranging from fine- to very coarse-grained). These channels traverse a basin dominated by finer grained (mudstone to fine sandstone), thin-bedded (typically less than 10 cm) sediments. This study integrates stratigraphic, petrographic and ichnofacies analysis to allow the mapping of channels from their proximal feeder canyon, to their most distal outcrop expression within the basin. The channels exhibit a dominantly incisional character. Regional stratigraphic correlation reveals two scales of organization: (i) small scale stacking, with lateral (up to 50 m) and vertical (up to 20 m) offset, whereby the secondary channel incises into or near the margin of its precursor forming a channel complex; (ii) Sediment fairway diversions which lead to large (up to 1 km) lateral shifts in channel complex axes. The position of protruding growth structures exerts a first order control on channel complex diversions, with sediment re-directed towards more efficient pathways once the original conduit was blocked by frontal topography, and on channel stacking, with the direction of lateral stacking controlled by the development of gradients perpendicular to channel axes. The latter was also locally augmented by emplacement of channel-plugging mass transport deposits, derived from the same structurally-induced topography.

Understanding the extent to which growing structures control coarse-grained sediment routing within compressional basins will help inform predictive models and reduce uncertainty in areas with poor seismic data resolution. Hence, this work will help de-risk frontier exploration of deepwater channel systems in structurally complex settings.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019