Incised Valleys and Shoal Water Deltas: Depositional Model for the Middle Jurassic of the Danish Central Graben

Abstract

Hydocarbons are found in M. Jurassic sands of the Danish Central Graben (DCG). Production occurs in the Søgne Basin, where abundant core, log and seismic data are available. Well logs and cores are sparse in other parts of the DCG. By combining the sparse data from these areas with data and the author's depositional model from the Søgne Basin a regional depositional model is established that explains and predicts the distribution of DCG reservoirs.

Deposition that began after Toarcian-Aalenian regional uplift, was dominated by sands with fluvial, wave and tidal signatures. The deposition is subdivided into 4 phases.

Phase 1 deposition took place in a transgressive fluvial to estuarine system with large channels. Transgression was from the south via the Dutch Central Graben. Channel units were stacked in the Søgne Basin. In the centre and south of the DCG estuarine deposition was interrupted by local progradation of shoal water deltas. Phase 2 saw widespread progradation of tidal shoal water deltas. Phase 3 was associated with structural rearrangement. Uplift focused in the Dutch Central Graben affected the southwestern part of the DCG, closed the marine connection to the south, and caused a change to fluvial deposition on the flanks of the Rosa Basin. This uplift, and simultaneous rotational fault movements in the north, enhanced by a regional fall in sea-level, caused widespread erosion. In the Søgne Basin large incised valleys were cut and filled with fluvial and estuarine deposits. Incision in the southern part of the DCG was shallower. A new marine connection opened towards the north, west of the Mandal High. In phase 4 a low-energy, vegetated delta plain dominated the south, with thick coals and rare sand. Sand deposition was focused in the Søgne Basin. Here repeated rotational fault movements caused transgressive estuarine wedges to interfinger with wedges of regressive wave-influenced delta deposits. The top wedge was truncated by a ravinement surface and overlain by marine mud of Oxfordian age.

The existing depositional model from the Søgne Basin is extended into the central and southern parts of the DCG. In the Tail End Graben and Rosa Basin it shows that phase 1 channel sands are present deep in the section, but phase 1 and 2 distributary channels higher in the section may also provide reservoir sands. In the Salt Dome Province sand distribution is difficult to predict due to the strong influence of salt tectonics on depositional patterns.

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