Delineating storm- and wave-dominated deltaic systems to improve geothermal reservoir prediction, Bückeberg Group, Lower Saxony Basin, Germany

Deltaic sandstone deposits of the Berriasian Bückeberg Group in the Lower Saxony Basin are potential geothermal reservoirs. Detailed sedimentological and ichnological analysis of subsurface expressions to underpin the paleodepositional interpretation as deltaic have yet to be presented, but are crucial to understanding reservoir quality and distribution.

Deltaic intervals in two cores were logged with a focus on sedimentological and ichnological character, e.g., grain size, sedimentary structures, lithological accessories, bioturbation index (BI) and ichnological elements. Facies are grouped into facies associations.

Two types of deltaic successions are identified (1) wave-dominated deltaic deposits and (2) storm-dominated deltaic deposits. Wave- and storm-dominated deltaic facies associations display coarsening-upward successions, variable bioturbation intensities (BI 0–5) and host overall low-diversity, facies-crossing assemblages with marine ichnological elements. Wave- and storm-dominated deltaic successions differ in sedimentological and ichnological expression. Wave-dominated deltaic deposits display abundant wave-ripple lamination, normal and inverse graded mudstone beds, and overall moderate to elevated bioturbation intensities (BI 3–5). By contrast, storm-dominated deltaic deposits display abundant hummocky cross-stratification and fewer burrowed beds (BI 2–5).

Reservoir quality and distribution differs in studied wave- and storm-dominated deltaic successions. Wave-dominated deltaic successions display sandstone beds with abundant biogenically reworked mudstone beds. Sandstones in storm-dominated deltaic deposits display fewer and less burrowed mudstone beds. Reservoir distribution in purely wave-dominated deltas is expected to form relatively smooth shoreline protrusions, whereas storm-dominated reservoirs are associated with subaqueous channel deposits. Results highlight the importance of sedimentological and ichnological facies analysis to improve geothermal reservoir exploration in the Bückeberg Group.