Abstract

Recently available well data from the northern part of the Danish Central Graben have been analysed to further understand the basin development, biostratigraphy, depositional models and palaeogeography of Upper Jurassic reservoir sandstones, which are the primary exploration targets in this basin. Notably, the discovery of the Hejre accumulation in 2001, where oil has been encountered in Upper Jurassic good reservoir quality sandstones at a depth of more than 5000 m, triggered renewed interest in the Upper Jurassic High Temperature–High Pressure sandstone play in the area.

Overall the Danish Central Graben was transgressed from east to west during the Late Jurassic. During the Late Kimmeridgian, marginal and shallow marine sandstones assigned to the Heno Formation were deposited at the margin of the Feda Graben, and on the Gertrud and Heno Plateaus and constitute the reservoirs in the Freja and Hejre discoveries. The sandstones are analogues to the UK Fulmar and Norwegian Ula Formations encountered in several hydrocarbon fields.

During the Early Volgian, the transgression continued westwards across the Outer Rough Basin along the margin of the Mid North Sea High, where shoreface sandstones with excellent porosities and permeabilities were deposited close to similar sandstones of the Fulmar Formation in the British Fergus, Fife and Angus fields.

During this overall westward transgression, the eastern and central parts of the Danish Central Graben continued to subside and offshore mudstones accumulated, locally intercalated with gravity-flow sandstones. In the easternmost Danish Central Graben, in the Tail End Graben, Upper Kimmeridgian gravity-flow sandstones of the Svane-1 well have proved the presence of gas at c. 6 km depth.

Hydrocarbon-bearing Upper Jurassic sandstone reservoirs at significant depths (deeper than 5 km) may form the future exploration targets in the northern part of the Danish Central Graben.