Use of Sequence Boundaries to Map Siliciclastic Depositional Patterns Across North America

Abstract

Sequences are discrete packages of sedimentary rock bounded by interregional erosional surfaces that are traceable on a continental scale. They are the ideal stratal units for regional stratigraphic study, but few continent-wide studies of the Sloss-defined sequences have been published. We present a new, comprehensive and continental-scale study of the six sequences across North America, with special emphasis on siliciclastic architecture. Details of the siliciclastic rocks across North America, including offshore shelf regions, were compiled using the AAPG COSUNA stratigraphic columns and supplemented with the Geological Atlas of Western Canada Sedimentary Basin and numerous other published sources and wells. Rockworks 16 software was used to track individual sequence boundaries, lithologic data and stratigraphic column locations. The raw data were processed into 16 subset areas which were sampled using a 10-km spacing grid to create comprehensive, three-dimensional models of the lithology on a sequence-by-sequence basis. Isopach maps and basal sequence lithology maps were also created for each of the six sequences. Results show siliciclastics in the first three sequences (Sauk, Tippecanoe and Kaskaskia) comprise a significantly lower volume compared to the latter three sequences (Absaroka, Zuni and Tejas). Siliciclastics total 13.2 million km³ in the first three sequences combined, whereas siliciclastics within the individual Absaroka, Zuni and Tejas sequences total 9.4 million km³, 19.9 million km³ and 12.0 million km³, respectively. In addition to the highest volume, the overall percentage of siliciclastics reaches a maximum in the Zuni sequence, with 36% of all sandstone and 36% of all shale deposited as part of the Middle Jurassic to Late Cretaceous systems across North America. Isopach maps show most of the siliciclastics in these later sequences were deposited across the western portion of North America and offshore in the East and Gulf of Mexico. Three-dimensional diagrams of lithology within each sequence allow visualization of shale architecture across the continent through time. These data provide insight into potential exploration targets for shale oil and gas. Identification of areas of thickest shale in the individual Absaroka, Zuni and Tejas sequences, in combination with geochemical data, can focus renewed exploration efforts into previously overlooked regions.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016