Turbidite Termination Against Confining Basin Slopes: Implications for Stratigraphic Trapping and Reservoir Heterogeneity

Abstract

Deep-marine clastic deposition is often affected by complex seafloor topography. This can result in reservoirs with significant heterogeneity and variable stratigraphic trapping potential, with facies-dependant onlap geometries and flow transformation common. The Eocene-Oligocene Grès d’Annot of SE France is an example of a clastic deep-marine system deposited in such a setting. The Grès d’Annot was deposited within a series of steep and interconnected minibasins that comprised the western Alpine foreland basin. Denudation of an uplifting granitic massif resulted in the progradation of a coarse deep-marine clastic system into the minibasins, which terminated hemipelagic carbonate deposition. This study uses outcrop data to characterize the earliest clastic fill of the Annot minibasin, particularly focussing on the understudied and poorly-constrained eastern margin of the basin, where excellent exposure allows detailed sedimentological observations to be made at the onlap surface. The earliest clastic stratigraphy is marked by a thick sequence of thin beds followed by organic-rich hybrid beds and debrites. These are overlain by a sequence of coarsening-upward low- (LDT) and high-density (HDT) turbidites. Each of these facies interacts differently with the up to 13^o, highly rugose, lateral onlap surface. The rugosity of the slope is suggested to be caused by frequent slope failure in a tectonically active setting. Incoming flows are shown to deposit rapidly at the base of slope, forming flat-topped, coarse HDT wedges, while the finer-grained upper parts of the flow are able to travel further up the slope, forming a genetically-related LDT slope drape. Individual flows are also shown to transform from turbulent to cohesive approaching the basin margin, represented by the transition of clean turbidites to hybrid-beds over tens of metres. These hybrid-beds show internal stratifications with debritic components commonly pinching-out prior to their encasing turbidites. The high occurrence of HEBs is suggested to be caused by both erosion of the onlap surface and distal fan flow transformation of high concentration flows rapidly decelerating against a slope. This study further provides quantitative estimations of turbidite thinning rates, insights into grain-scale reservoir heterogeneities at onlap surfaces and integrated models for deep-marine depositional architecture and stratigraphic trap characterization at steep basin margins.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018