An Alternative Model for Shelf-margin Depositional Systems in the Sureste Basin, Southern Mexico and Implications for Deepwater Reservoir Distribution

Abstract

Summary Historical paleogeographic maps and sedimentary models of the Upper Miocene to Pliocene Sureste shelf margin describe middle to outer shelf, wave-dominated deltaic systems, inboard of their coeval shelf-edges and updip of significant deepwater slope reservoirs. The mechanism of sedimentary delivery from the shelf to the slope is shown to be shelf-incising submarine canyons, which tap into perched shelf deltas that do not prograde to the margin/upper slope. While a geological model of sediment delivery from the shelf to deepwater via shelf-indenting submarine canyons is sound, the absence of such canyons in the offshore seismic datasets, presence of laterally persistent train of growth-faults and linked counter-regional faults and anomalously thick deltaic deposits in offshore wells suggest that the Sureste deltas did indeed frequently make it to the shelf-edge and the paralic reservoirs in the Southeastern Basin are in fact shelf-edge delta systems, similar to sedimentary systems in the US Gulf Coast offshore Louisiana and the Orinoco prism offshore Trinidad. Amplitude extractions just below the present-day seafloor (a proxy for the last sea-level lowstand) on the Sureste margin show shore-parallel amplitudes inboard of the modern shelf edge and a distinct change to regional, shore-normal channel geometries beyond the margin. This supports the geologic model of shelf-edge delta systems nourishing a regional upper slope with turbiditic sands. This alternative model has two main consequences: 1) significantly thickened paralic reservoirs in the Sureste shallow water, and 2) a more ubiquitous sediment delivery mechanism to the regional slope, where underlying salt withdrawal provides a structured slope onto which turbidite deposits can be ponded. A key feature of shelf-margin deltas that is important in predicting deepwater reservoir distribution is a highly-unstable delta front to upper slope region. An interesting observation on the present-day seafloor amplitude maps in Sureste is that the observed slope channel systems do not occur directly beyond the shelf edge, but rather 5–10km further downdip. Seismic profiles in the relatively unstructured Veracruz Basin to the west show a prograding shelf-margin system with clinothems. Directly downdip of the shelf edges are highly chaotic seismic packages, interpreted as mass transport deposits (MTD's) associated with instability on the margin. These deposits are thickest on the uppermost slope and thin downdip, over a distance of 10's of km. Eventually they transition to more conformable seismic reflections, suggesting the MTD's erode much of the uppermost slope stratigraphy. This quiet zone just outboard of the shelf-edge that transitions downdip to channelized geometries in map view is also observed on the Orinoco shelf-margin. Seismic sections and piston coring of this profile confirm an upper slope, MTD-dominated setting that transitions downslope to a sandstone-prone channelized setting. These observations on the shelf to upper slope margin offshore Sureste and resulting alternative interpretation of the depositional system and sedimentary routing significantly reduces the risk of encountering both prolific, high-quality shelf-margin paralic reservoirs inboard of the shelf edge, as well as a more uniform and regional delivery of sediments to the slope. On the slope margin, there may also exist an updip zone of MTD dominated sedimentation which merges downdip with the more desirable turbidite channel and fan systems on the structured middle slope. Sedimentary models derived from less deformed parts of the Southern Mexican margin and other analogous basins offer insights for geoscientists exploring for reservoir systems in the structurally deformed and seismically-challenged offshore Sureste basins.