2. SEDIMENTOLOGY
Review and background information
Reservoir geology requires a good basic knowledge of sedimentological and stratigraphic principles. By sedimentology, one means understanding the nature (lithologies, sedimentary structures/textures, etc) and the origin (depositional environments) of the sediments making up the different reservoirs. This is addressed below for the Statfjord Fm., the Dunlin Gp. and the Brent Gp.
The Statfjord Formation is predominantly a fluvial reservoir, but with some shallow marine influence in some intervals (confirmed by core observation). The formation comprises three main lithostratigraphic units : the Raude Member, the Eiriksson Member, and the Nansen Member (Fig. 2.1). There is a marked upward change in the reservoir architecture / sandstone geometry : from isolated sandbodies with high shale content in the lower part of Raude (straight to meandering channel sandstones, and estuarine deposits), to more continuous (amalgamated) sandbodies in the upper part of Raude and higher sand-shale ratio (depositional environment = braided rivers, with some estuarine deposits). The Eiriksson is even more sand-rich than the upper Raude (braided rivers + estuarine), and the Nansen is entirely sandstone (shallow marine). The Raude Member is subdivided into five sequences or reservoir zones, each of them being sand-rich at the base and shale-rich at the top. The Eiriksson is subdivided into two sequences / reservoir zones.
Figure 2.2 shows how the Statfjord Formation was deposited on the Statfjord Field: the Raude Member is a prograding unit shed towards the south / southwest, the Eiriksson is retrogradational towards the north / northwest, and the Nansen is related to a marine transgression with the sea coming in from the south. The smaller-scale sequences (five in Raude, two in Eiriksson) represent smaller pulses of retrogradation and progradation.
A 3D geological model of the Statfjord Formation of the Statfjord Field also exists. Figure 2.3 illustrates the sandstone-shale geometries typical of lower Raude (isolated rivers, Raude 1) and upper Raude (braided rivers, Raude 5). The estuarine deposits are not represented in this model, since this interpretation is relatively new and will need to be incorporated into the 3D model at a later stage.
The Cook Formation of the Dunlin Group is related to the gradual infill of the ”proto North Sea”. Sandstones were deposited during a short time interval in the Middle Jurassic, and are believed to have originated from a lateral delta system to the east of the Statfjord Field (Fig. 2.4). The Cook Formation on the Statfjord Field was deposited in the shallow marine, tidal-influenced environments lying in front of this delta. After deposition of the Cook, a new transgression occurred and resulted in the deep marine Drake Formation of the upper part of the Dunlin Group. This event was to be followed by the dramatic northward progradation of the Brent delta, later in the Middle Jurassic.
The Brent Group consists of five formations that are all reservoirs: the Broom, Rannoch, Etive, Ness and Tarbert formations (Fig. 2.5). The Broom Formation is an equivalent to the Oseberg Formation : the former name is used on the Norwegian sector, the latter on the British sector. On the Statfjord Field, the Broom is thin (less than 5 meters), basically non-reservoir, and represents sandy/gravelly mudstones deposited in a fan-delta environment (a fan-delta is a submarine / deep marine delta). The Broom / Oseberg Formation deltas were oriented east-west and are likely related to a minor phase of rifting. The Rannoch Formation is related to the main phase of the Brent delta progradation from the south. It was deposited in shoreface (lower to upper) environments. The Etive Formation lies above the Rannoch, and accumulated in nearshore marine environments, most likely as a sand-rich, estuarine tidal bar and / or barrier island system. Above the Etive is the Ness Formation, a fluvial / lagoonal / shallow marine unit deposited on the delta plain. On the Statfjord Field, the middle part of the Ness marks the time when the Brent delta started to retrograde towards the south. The upper Ness is characterized by mainly open bay sand and shale environments. The Tarbert Formation is an overall retrogradational unit deposited in a variety of environments (largely estuarine, but also bay, shoreface, and alluvial). Above the Tarbert Formation lies the deep marine shales of the Viking Group.
The paleogeography during Brent Group deposition is shown in Figures 2.6 to 2.9.
Well log interpretation / Facies interpolation and correlation between wells
Well logs can be used for interpreting formation boundaries, establishing a reservoir zonation, and to a certain point interpret depositional environments. Figure 2.10 shows the main log patterns that occur and can be used to interpret environments. Relevant to the Statfjord Field are the following patterns: 1) upward increasing gamma ray values (fining-upward grain size trends) represent most likely either a fluvial environment or transgressive marine environment; 2) upward decreasing gamma ray values (coarsening-upward grain size trends) represent most likely delta progradation or shoreface progradation. Delta progradation can occur both at the large scale (entire Brent delta) and the small scale (small delta within a Ness lagoon or bay) ; 3) blocky / uniform and low gamma ray values: often represent fluvial or tidal bar sandstones.
In order to have sufficient information to do one of the exercises below, details about the Ness Formation are here presented. The Ness, as seen in Figure 2.11, is subdivided into several small-scale zones (right-hand side of figure). Each zone separates sands from shale / siltstone / coal-bearing intervals. Each zone has good lateral continuity on the field. The so-called E1 sand has a thickness distribution as shown in Figure 2.12. It has three separate thickness depocenters, reaching 10-15 meters at the thickest. These sandstone depocenters are each related to different environments in the Ness: a fluvial environment in the south (near B platform), a lagoonal environment in the middle part of the field (A platform area), and a tidal / shallow marine environment in the north (C platform area) (Fig. 2.13). Each sand in the zonation can also be similarly interpreted, giving a range of different environments at any time during Ness deposition. These environments have been interpreted from cored wells, and extrapolated to uncored wells using log patterns. Figure 2.14 illustrates how the individual sand maps can be related to the main paleogeography presented earlier. The depositional model for the Ness and Etive formations is further shown in Figure 2.15.
Exercise #1 (Fig. 2.16)
On the well log for A-31, where the stratigraphic column has been omitted / removed, set some horizontal lines at formation boundaries such that you delineate the five formations of the Brent Group.
Exercise #2 (Fig. 2.17)
On the A-31 well log containing the stratigraphic column, identify the sand intervals in the Ness Formation based on the SAND log / GR log / Neutron-Density log. Then, try to interpret the depositional environment for these sands, based on the different paleogeographic maps, and also on log patterns.
Do the same exercise for well B-26 (Fig. 2.18). Remember that this well was drilled from the B-platform, which is in the southern part of the field (the A platform is central on the field; the C platform is in the north)
Both of these wells are cored in the Ness, and the course plan was such that we should have made a visit to the core room at Statoil at this point, and interpret the facies / environments based on core observations. But this ideal timing was not possible because of logistics.
Exercise #3 (Fig. 2.19)
Based on your understanding of individual sandstones in wells A-31 and B-26, attempt to correlate them on the correlation panel which comprises six wells in total. You will need to first identify the sandstones in the other four wells prior to correlating. When correlating, use the coal layers as stratigraphic markers: the coals in Ness are continuous laterally, and are relatively easy to correlate between wells. Using log patterns in both sandstones and shales/siltstones is also useful. You should end up with a drawing were all the sandstones (at least the thicker ones) are bounded in all wells by horizontal lines, and correlated with one another between the wells (P.S. Some wells may not contain some of the sandstones. In these cases, make the two bounding lines join in the well that doesn’t contain the given sand). You may want to colour the sandstones yellow for better visualizing, and try to give an interpretation of depositional environment(s) for each sandstone.
Such a correlation framework in Ness is fundamental when looking after remaining oil, and planning new injectors or producers. Knowledge of depositional environments can also be useful, since different environments often lead to different sand quality and geometry.