Early Post-Salt Differential Topography and Its Impact on Source Rock and Shallow-Water Carbonate Facies Distributions: Examples From the Jurassic of the North Atlantic and the Southern Gulf of Mexico

Abstract

A major risk to successful exploration efforts is the accurate prediction of source rock and reservoir presence. In many Mesozoic passive margin systems, differential subsidence related to early salt movement exerts a primary control on setting up both small, restricted basin centers and local highs allowing for both source rock and shallow water carbonate deposition. This depositional motif is common in the Jurassic along much of the North Atlantic and Gulf of Mexico (GoM) margins. An integrated scheme utilizing organic geochemical, geophysical, and geological methods clarifies these relationships.

Organic geochemical characterization of samples from early post-salt restricted basin settings yields similar characteristics. Pristane/phytane ratios are usually very low (often <0.5, likely related to hypersaline conditions), C₃₅/C₃₄ ratios are high (related to stratified waters), and C₂₉ steranes are abundant (common in carbonate-prone SR). In addition, unique diamondoid distributions likely relate to diamondoid formation within evaporite-prone settings.

Geometries observed in reflection seismic often include locally faulted basins linked to basement or salt-related movement. Infill is typically in the form of a sedimentary wedge that thickens towards the basin center and is rapidly overlapped by subsequent sedimentation. Bright reflections associated with rapidly changing composition of fill may include increasing negative amplitude toward the center of each sub-basin, often including AVO class 4 anomalies. Bounding edges of sub-basins may exhibit mounded or shingled geometries related to deposition of shallow water carbonates. These geometries are indicative of a genetic link between sub-basin formation and fill by a complex facies mosaic- from restricted, organic rich facies in the deepest centers to shallow water carbonates along the rims.

Conceptual stratigraphic models for these intervals must include limited, but predictable geographic extent for facies deposited in early post-salt systems. Source rock must be considered to be locally ponded and linked to timing of salt movement. Carbonate facies distribution is controlled by local topography provided by both basement and salt-related structures. Dynamic salt-related topography also provides both barriers and catchments for sediment routing in mixed carbonate/siliciclastic systems.

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