Shallow and deep subsurface sediment remobilization and intrusion in the Middle Jurassic to Lower Cretaceous Agardhfjellet Formation (Svalbard)

Abstract

Sedimentary injectites are increasingly documented in many hydrocarbon plays at various scales, either interpreted as potential risks (e.g., top-seal bypass, a drilling hazard) or benefits (e.g., reservoir interconnection, increased hydrocarbon volumes) for production operations. As such, they have potential critical implications for the assessment of suitability for CO2 injection and sequestration. Detailed characterization of such units, especially in terms of diagenesis and (paleo) fluid flow, is directly achievable at outcrop scale, overcoming dimensional and time constraints otherwise unresolvable at seismic scale. Two sedimentary injection complexes have been recognized in the succession of the Middle Jurassic–Lower Cretaceous Agardhfjellet Formation exposed at Deltaneset, central Spitsbergen, Norway, at different stratigraphic levels. The upper complex comprises two main clastic dikes characterized by different orientation and consolidation, tapering out vertically (upward and downward) within a stratigraphic thickness and lateral extent of more than 50 m and 200 m, respectively. The lower complex is coarser grained, made up by a network of interconnected dikes and sills, shooting off from isolated lenticular and morphologically articulated bodies, interpreted as sedimentary intrusions linked to seafloor extrusion (sand volcano). Petrographic and micromorphological analyses were used to identify the underlying lithologies of the Late Triassic to Middle Jurassic Wilhelmøya Subgroup as the possible source of this remobilized material for both the upper and lower complexes. This subsurface remobilization and consequent intrusion were first achieved in the lower complex during the Late Jurassic at shallow burial conditions, and then at higher confinement pressure for the upper complex, probably during the Late Cretaceous. These results highlight how field data can be used to constrain long-lived spatiotemporal relationships of sedimentary intrusions, allowing a finely tuned upscaling of seismic data and interpretations.