Northern limit of the “Brent delta” at the Tampen Spur —a sequence stratigraphic approach for sandstone prediction

Abstract

Approximately 70 wells from the Tampen Spur area in the Northern North Sea were studied within the framework of sequence stratigraphy to predict the lithofacies and thicknesses within the “Brent delta” at its northern limit. The “Brent delta” (comprising Rannoch, Etive, Ness and Tarbert Formations) was deposited during pre-rift and initial syn-rift phases. Four 2nd-order, tectonically induced, transgressive/regressive sequences (T-R sequences) were identified. Stacking pattern of facies associations and stratigraphic key surfaces such as subaerial unconformities, transgressive surfaces and flooding surfaces (including MFS) were correlated and mapped. The Rannoch, Etive, Ness and Tarbert Formations are lithostratigraphic and diachronous units. Within the Statfjord and Gullfaks Fields, the Rannoch and Etive Formations and the lower/middle parts of the Ness Formation constitute Sequence 2. The upper parts of the Ness Formation and most of the Tarbert Formation are within Sequence 3. Second-order Sequence 1 (Aalenian in age, including Oseberg and Broom Formations) forms mainly coarse-grained fan-delta and gravity flow sandstones deposited in response to tectonic activity along the basin margins. Second-order Sequence 2 (Early to Late Bajocian) was deposited during a period of relatively uniform subsidence (pre-rift phase) and is divided into three 3rd-order sequences (2A, 2B and 2C) and several 4th-order sequences. The 3rd-order T-R sequences were deposited during long-term base-level rise, as 4th-order landward-stepping sequences (transgressive systems tract, TST), and during long term base-level fall, as 4th-order seaward-stepping sequences (regressive systems tract, RST). Systems tract correlations and the stacking pattern of facies associations in Sequence 2 indicate stepwise northward progradation of the “Brent delta”. Second-order Sequence 3 (Late Bajocian to Early Bathonian) was deposited during differential subsidence, causing great thickness variations. The stacking pattern of facies associations, the coarse grain size and the occurrence of reworked palynomorphs indicate a strongly regressive development of systems tract 3-RST. The occurrences of pronounced erosion surfaces on palaeostructural highs and basin margins, together with wedge-shaped stratal geometries in downflank and hanging-wall positions (seen on seismic profiles), suggest that Sequence 3 was deposited during an initial syn-rift phase. Second-order Sequence 4 (starting in Early Bathonian) comprises thin landward-stepping sandstones overlain by thicker mudstones (lower Heather Formation). The base of systems tract 4-TST is picked at the lowest conglomerate near the top of the Brent Group. The thin conglomeratic beds at the base of 4-TST represent erosion and bypass surfaces formed during the onset of the rifting phase. During the pre-rift phase (Sequence 2) a fluvial-to wave-dominated delta prograded stepwise northwards, interrupted only by two 3rd-order transgressions. The stacking pattern and sediment volume partitioning of facies associations for the two lower 3rd-order sequences (2A and 2B) are used to predict the facies and sandstone volumes of the succeeding Sequence 2C. Thus, reservoir sandstones can be predicted to occur in undrilled areas to the north of areas where earlier interpretations suggested the shorelines of the Brent delta to be located. The 2nd-order Sequence 3 was deposited during the initial syn-rift phase. The change from pre-rift to syn-rift was accompanied by a change in the drainage system. Drainage was axial, parallel to the Viking Graben, during the pre-rift phase but was transverse, with dominant sediment supply from the eastern and western flanks of the graben, during the initial syn-rift phase. During the initial tectonic phase, local highs were subaerially exposed and eroded. Contemporary deposition occurred in local grabens and within the axial parts of the Viking Graben. The strongly regressive nature of this sequence and the occurrence of erosion surfaces at the boundary between 2nd-order sequences 3 and 4 suggest that reservoir sandstone may be developed downflank of structural highs in the northern Tampen Spur area.