Development of intra-basaltic lava-field drainage systems within the Faroe–Shetland Basin

Nick Schofield and David W. Jolley
Petroleum Geoscience, 19, 273-288, 16 July 2013, https://doi.org/10.1144/petgeo2012-061

Abstract

The Faroe–Shetland Basin, located on the NW continental shelf of the UK, represents arguably the last frontier area of hydrocarbon exploration of the UK territorial waters. The basin contains an aerially extensive lava sequence, which forms part of the Palaeogene-aged North Atlantic Igneous Province (NAIP). In 2004, a major oil and gas discovery was made within the Palaeocene–Eocene lavas in the form of the Rosebank Field. Unusually, the reservoir intervals are a series of intra-basaltic fluvial clastic sequences separated by basalt lava flows, hyaloclastites and volcaniclastic sediments, giving rise to a new hydrocarbon play concept. The discovery has led to an increased emphasis on trying to understand the detailed stratigraphy of the offshore lava fields within the Faroe–Shetland Basin, as well as the nature of the regional intra-basaltic drainage systems and sediments that form the reservoir intervals in Rosebank.

In this paper we show a regional integrated three-dimensional (3D) seismic and well analysis of the lava sequences within the area of the Corona Ridge, exploring the age relationships and spatial distribution of the sequence T40 and T45 Palaeocene lava field. In particular, we note the occurrence of an intra-basaltic (sequence T40, Colsay Member) drainage network that appears to represent a major drainage pathway within the Faroe–Shetland Basin lava fields.

Few published studies have dealt with drainage system development on lava but our preliminary analysis appears to indicate that lava flow-field morphology plays a dominant role in controlling the development and evolution of a drainage system.

Introduction

The Faroe–Shetland Basin, located on the NW continental shelf of the UK, represents arguably the last frontier area of hydrocarbon exploration within the UK’s offshore territorial waters. During exploratory drilling in 2004, Chevron with partners Statoil, OMV and Dong made an oil and gas discovery on the eastern margin of the Palaeogene volcanic sequence in the Faroe–Shetland Basin (Figs 1 and 2). Reservoir sandstones of the Flett Formation, Colsay Sandstone Member (Fig. 3), are preserved between basaltic volcanic rocks (Duncan et al. 2009; Helland-Hansen 2009). The ‘Rosebank’ discovery (well 213/27-1) highlighted the potential for intra-volcanic hydrocarbon plays in areas where only subvolcanic plays had been considered. The discovery also demonstrated that within areas of the Faroe–Shetland Basin that have experienced substantial volcanic activity, a working petroleum system is still present.

Fig. 1.
Fig. 1.

Map of the Faroe–Shetland Basin (modified from Moy 2009), showing the main structural elements, the extent of extrusive basalt flows and the area covered within this paper.

Fig. 2.
Fig. 2.

Schematic geological cross-section through the Faroe–Shetland Basin (modified from Gallagher & Dromgoole 2007). The intra-basaltic Rosebank Field is located in lava sequences above the Corona Ridge.

Fig. 3.
Fig. 3.

Palaeocene stratigraphy west of Shetland, with BGS (Ritchie et al. 2011) and BP T-sequence framework (after Ebdon et al. 1995). The stratigraphical position of the FIBG is also shown (after Passey & Jolley 2009). Note the stratigraphical position of the Colsay Sandstone Member, which forms the main reservoir intervals within the Rosebank Field.

Sediments interbedded within the lavas can be examined onshore within the North Atlantic Igneous Province (NAIP), particularly in the Hebridean Islands, Faroe Islands and Greenland. These sediments are comprised of debris flows, and lacustrine and fluvial beds (Passey & Jolley 2009). While both volcaniclastic and siliciclastic fluvial sediments are recorded in the Hebridean Islands and Greenland (Jolley & Whitham 2004; Jolley et al. 2009), the spatial and temporal development of their drainage systems remains poorly documented. Within this paper we integrate regional well data on stratigraphy and depositional environment with three-dimensional (3D) seismic interpretation in order to identify the geometry of regional intra-basaltic drainage system with the Palaeocene lava sequences.

Regional Geological History

The Faroe–Shetland Basin (FSB) is the collective term for a series of smaller rift basins affected by volcanism that extend along the NE Atlantic Margin (Fig. 1). The Faroe–Shetland Basin itself experienced a complex multiple-rifting history, with several rift episodes occurring between the Permo-Triassic and Palaeocene (Smallwood et al. 2004). This was followed by Late Palaeocene and Mid-Miocene basin inversion events (Smallwood & Maresh 2002). The complex rift history experienced by the basin has led to a highly structured configuration consisting of a series of SW–NE-trending basement blocks capped with Mesozoic sediments, with fault-bounded basins forming depocentres for Cretaceous and Palaeocene sediments (Figs 1 and 2). A series of ‘transfer lineaments’ are thought to transect the basin in a NW–SE direction (Ellis et al. 2009), roughly perpendicular to the structural basin trend (Fig. 1). Although the presence and origin of the lineaments is still debated (Moy & Imber 2009), they are thought to have operated some control on sediment routing and provenance within the Faroe–Shetland Basin (Jolley et al. 2005; Jolley & Morton 2007; Ellis et al. 2009).

The NE Atlantic experienced considerable igneous activity prior to, and associated with, the onset of sea-floor spreading in the Late Palaeocene–Early Eocene (Naylor et al. 1999). The earliest volcanism in the North Atlantic is thought to have occurred around 63 Ma (Hamilton et al. 1998), exposed as the Eigg Lava Formation, Rum, with the beginning of the Faroes flood basalt eruption occurring around 57 Ma ago (Passey & Jolley 2009) (Fig. 3). Within the Faroe–Shetland Basin, volcanism resulted in deposition of thick flood basalt sequences covering an area of at least 40 000 km2. Despite varying thicknesses of extrusive basaltic rocks within the various sub-basins forming the Faroe–Shetland Basin, all of the basins contain a suite of intrusive dolerite sills and dykes (Gibb & Kanaris-Sotiriou 1988; Stoker et al. 1993; Thomson & Schofield 2008; Schofield et al. 2012), which are thought to have intruded between 55 and 53 Ma (Ritchie & Hitchen 1996). The sills appear to preferentially intrude the upper Cretaceous shales and the lower part of the Palaeocene, forming an aerially extensive suite of intrusions, extending outwith of the basalt cover as far as the Shetland Platform (Naylor et al. 1999).

Within this paper, for ease of comparison with previous work within the Palaeocene west of Shetland, the T-sequence stratigraphical framework of Ebdon et al. (1995) is adopted (Fig. 3).

North Atlantic Igneous Province and the Faroe Islands Basalt Group

Well penetration of the NAIP volcanic subcrop is limited to a few exploration wells, and is poorly documented. Because of this, the rift centre proximal Faroe Islands Basalt Group (FIBG) is of primary importance in preserving sedimentary, extrusive and intrusive igneous strata that illustrate the range of depositional systems and eruptive styles that operated during the eruptive period of the NAIP. The FIBG lava pile has a gross stratigraphical thickness of about 6.6 km, dominated by sub-aerial basalt lava flows, and is subdivided into several volcanic and sedimentary formations (Passey & Jolley, 2009) (Figs 2 and 3). The Lopra Formation, seen in the basal approximately 1.1 km of the Lopra-1/1A borehole, is dominantly composed of hyaloclastites, volcaniclastic sandstones and invasive basaltic lavas/sills (Passey & Jolley 2009). It is overlain by the approximately 3.25 km-thick Beinisvørð Formation, dominated by laterally extensive basalt sheet lobes, associated with effusive volcanism, separated by minor volcaniclastic lithologies. The Beinisvørð Formation is overlain by the <15 m-thick, inter-eruption, coal-bearing facies of the Prestfjall Formation, and the <50 m-thick, syn-eruption, pyroclastic and sedimentary facies of the Hvannhagi Formation (Fig. 4). Eruption resumed, yielding the <1.4 km-thick largely compound flow sequences of the Malinstindur Formation. The final phase of volcanism recorded on the Faroe Islands consists of the >900 m-thick Enni Formation composed of basalt sheet lobes and compound flows with frequent volcaniclastic units. The complex nature of the flow-field morphologies and compositional variation of the FIBG are highlighted by the identification of an underlying magma plumbing system cyclicity that drove both eruption rate and the formation of sedimentary interbeds (Jolley et al. 2012).

Fig. 4.
Fig. 4.

Photograph of approximately 800 m-high coastal cliff-section from the Faroe Islands (courtesy of Dr Simon Passey, CASP), showing a visible change in the nature of lava facies within the FIBG across the sequence T40–T45 boundary. The Beinisvørð Formation is characterized by high effusion rate laterally extensive sheet flows, shown here with thin palaeosol horizons. The Malinstindur Formation marks a change in relative eruption intensity and tempo to a series of generally locally sourced compound-braided lava flows (after Passey & Bell 2007).

The rate of eruption of the FIBG is difficult to calculate but possibly represents as much as 2.6 Ma (Passey & Jolley 2009). However, the Malinstindur and Enni formation-equivalent 5 km-thick lava pile in East Greenland is thought to have been erupted in 0.3 Ma (Larsen & Tegner 2006), suggesting that significantly more rapid eruption of the FIBG, or parts of, may have occurred.

Correlation of the FIBG stratigraphy to the time-equivalent sedimentary sequences of the Faroe–Shetland Basin has demonstrated stratigraphical continuity, which was constrained within the regional sequence stratigraphy (Ebdon et al. 1995; Passey & Jolley 2009). However, it cannot be presumed that UK and Faroes waters volcanic units observed on seismic data, or penetrated by exploration wells, are physically linked to FIBG flow fields (i.e. that all lava sequences are sourced from the Faroes). As our geophysical interpretations and well-based stratigraphical data demonstrate, lava sequences of the Corona Ridge (including the Rosebank Field), although coeval/time-equivalent with the FIBG, are not part of the FIBG flow fields.

Summary of Drainage System Development on Volcanic Terrains

In the majority of fluvial systems, drainage development is often inherited from older systems. However, drainage networks developed on volcanic flows do so tens to thousands of years after eruption, and are therefore not as heavily influenced by previous drainage heritage (Wells et al. 1985). Despite this, several key deductions can be made. Within clastic systems topographical morphology plays a dominant role in controlling the spatial occurrence of where a drainage system develops and how it evolves through time (Gawthorpe & Hurst 1993). Topographical morphology also plays a key role in controlling the distribution of lava flows, as flowing lava will naturally seek out existing topographical lows (Branca 2003), either pre-existing topographical lows exploited by drainage or topographical lows created between two separate lava flows that have inflated, creating topographical highs (Fig. 5).

Fig. 5.
Fig. 5.

Illustration of the controlling effect of the lava on fluvial drainage system development. (A) A drainage system developed on lava sequences within the Afar Region, West Africa (image courtesy of Google Earth). (B) Sketch overlay of (A), illustrating how the drainage system exploits lows within the lava field. Note the creation of isolated mini-highs of basalt within the drainage system Schematic illustration showing the development of a drainage system on a lava terrain (topography is exaggerated).

It is important to note that the establishment of a drainage system (and intra-basaltic sedimentation) is directly linked to the nature of the preceding eruptions. During periods of highly effusive volcanism, such as those that produced the inflated sheet flows of the sequence T40 Beinisvørð Formation of the FIBG, the rapid eruption tempo resulted in little time for the establishment of a drainage system. Under these conditions, intra-basaltic sedimentation was derived from accumulations of volcanic ash and scoria, and from the in situ weathering of flow tops (Passey & Jolley 2009).

Volcanic terrains are highly variable landscapes, changing morphology after eruptive periods. Images of immature drainage systems (Figs 5 and 6a) developed on basaltic lava fields show a drainage system controlled by the geometry and topography of the flow field itself, with the drainage system developing between flow lobes and in lows of the lava flow field (Figs 5 and 6a) (Dohrenwend et al. 1987). Subsequent eruptions of lava into a developed drainage system will fill or modify pre-existing topographical lows and valleys, diverting any fluvio-lacustrine drainage system (Fig. 6b). In summary, during the initial stages of a drainage system on a volcanic terrain, the topography of the pre-volcanic landscape will play a significant role in controlling the distribution of lava flow and the aerial development of any drainage system. With continued eruption, lava flow-field morphology exerts increasing control over topography (Fig. 6c).

Fig. 6.
Fig. 6.

Figure exploring the possible controls on short- and long-term development of a drainage system on a lava field. (A) A considerable hiatus in eruption needs to occur before a fluvial drainage system can establish. If rapid eruptions continue, intra-basaltic horizons will be mainly the result of in situ weathering of flow tops. If a fluvial system does develop, this is likely to occur in topographical lows of the lava field (e.g. in-between lava lobes and at the sides of lava flows; see Fig. 5). (B) Once a fluvial system has developed, lava from subsequent eruptions is likely to exploit the same topographical low as occupied by the drainage system. This has the potential to dam and divert the drainage system. (C) Over a longer term, cycles of sedimentation punctuated by periods of lava eruption will occur.

Methodology

In order to constrain the age and depositional environment of the drainage systems within the Corona Ridge area, a stratigraphical framework based on lithological, wireline-log and palynological evidence was erected. Data for this framework were derived from key wells orientated approximately along basin strike SW–NE. Well-data analysis utilized both dinocyst and pollen and spore palynology, allowing correlation between marine, marginal and terrestrial facies within the Corona Ridge lava fields. Correlation of the individual well stratigraphies has highlighted patterns in the regional distribution of lava flow fields within the Faroe–Shetland Basin (Fig. 7).

Fig. 7.
Fig. 7.

Well correlation through five wells from SW to NE in the Faroe–Shetland Basin. Note the thinning of the Flett F2a sequence (and sequence T45 lava flows) away from well 213/27-1 located on the Corona Ridge. This implies that the sequence T45 lava flows were erupted from sources close to or on the Corona Ridge.

In conjunction with the regional well analysis, a regional 3D seismic interpretation was carried out to try to image any sequence T40 Colsay Sandstone Member intra-basaltic drainage pathways. The 3D seismic database consists of Faroe–Shetland PGS MegaSurvey, covering an area of about 24 000 km2 (Fig. 8). The data have a 25 m-line spacing and a positive reflection coefficient represented by positive amplitude and a black peak. Regional seismic surfaces were correlated back to well penetrations in the area.

Fig. 8.
Fig. 8.

(A) Figure showing the two-way time (TWT) map of the intra-basaltic near-Base Colsay 3 surface, showing the main features and areas referred to in this paper. (B) Figure showing relevant wells drilled within the area and the orientation of the seismic lines in Figures 10 and 11. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

Constraining the Volcanic Succession – Well Analysis

Much of the study area is covered by flow fields and associated volcaniclastic facies that occur within sequence T40 (Fig. 7), and are age-equivalent to the Beinisvørð Formation, FIBG. These rest either unconformably on older Lamba Formation (sequence T36–T38), or early sequence T40, sedimentary rocks penetrated in wells from the Foinaven sub-basin up to the NE Faroe–Shetland Basin close to the Erlend volcanic centres, (Fig. 7). Flows within the lower part of this interval are generally thick (up to 100 m), indicating a high effusion rate and topographical constraint on their lateral extent. This high effusion rate is also indicated in the Beinisvørð Formation, FIBG (Passey & Jolley 2009) (Fig. 4), although the palynostratigraphy indicates that the basal part of the FIBG (Cycles I: Jolley et al. 2012) is apparently not represented within the Corona Ridge sequence T40 volcanic succession.

Wells drilled in the vicinity of the Corona Ridge, including the Rosebank discovery well 213/27-1, intersected both sequence T40 volcanics and lower sequence T45 volcanics (Figs 9–14). These lower sequence T45 volcanics comprise both of lava flows and volcaniclastic sedimentary units, and are stratigraphically equivalent to the Malinstindur Formation, FIBG. Their distribution is restricted to wells in the Corona Ridge–Cambo area (Fig. 7), although they exhibit significant lithological variation between areas. The implication of this is that the lower sequence T45 volcanics were erupted from localized fissure systems close to or on the Corona Ridge. The comparability of age and morphology suggests that these flows may have been sourced from similar low-angle shield volcanoes to those which sourced the stratigraphically equivalent Malinstindur Formation, FIBG (Passey & Jolley 2009) (Fig. 4). The lower sequence T45 volcanics are restricted to the NE–SW-orientated Corona Ridge in the Rosebank–Cambo area. This suggests that the cyclic evolution of the FIBG identified by Jolley et al. (2012) is reflected across the region in the architecture and facies of flow fields across the Faroe–Shetland Basin. This relationship also suggests a highly interconnected sub-surface magma network. Significantly, the sequence T40–T45 boundary marks a major volcanic hiatus in the Flett and Judd sub-basins (Underhill pers. comms) (Fig. 4). Marked by thick carbonaceous sediments in the FIBG (Prestfjall Formation) and similar deposits in East Greenland (Jolley & Whitham 2004), the same eruption hiatus is reflected as thick fluvial and marine siliciclastic sediments in the Rosebank Field (‘Colsay 1’).

Fig. 9.
Fig. 9.

Well log showing the relationship between lavas and sediments in well 213/27-1Z and their stratigraphical equivalent within the FIBG. Within the well, both upper sequence T40 and lower sequence T45 flows were encountered. Upper sequence T45 lavas (equivalent to the Enni Formation within the FIBG) were not found in the well. The Colsay Sandstone Member (Colsays 1, 2 and 3) forms the interbedded units within the sequence T40 lavas.

Fig. 10.
Fig. 10.

Figure showing the regional seismic line, and the interpretation of T40 and T45 volcanic sequences over the area. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

Fig. 11.
Fig. 11.

Lithostratigraphy, sequence stratigraphy and local assemblage palynofloral zones (LAZ) of the latest Palaeocene–earliest Eocene of the Corona Ridge. The lithology ornament is also used in Figures 12 and 13.

Fig. 12.
Fig. 12.

An example of assemblage biostratigraphy, well 205/9-1. The Flett Formation in this well spans a transition between marginal-marine and terrestrial sedimentation. It also penetrated lava flows that are of equivalent age to some of those recorded in the Rosebank Field but are locally sourced and are of different thicknesses. The local assemblage zones for the Corona Ridge area are shown against a raw data frequency plot of selected pollen and spores. Because of cavings, the zonation is defined using the first downhole occurrence of associations. The positions of boundaries of zones are therefore influenced by sample spacings (e.g. the base of Rc1–Rc2 is within the upper sequence T38, not at the base of the Flett Formation). One characteristic signature of this interval is the rapid turnover of terrestrial palynofloras associated with climatic change around the Palaeocene–Eocene Thermal Maximum (PETM). Being present in all except the lowermost part of sequence T40, the influx of the freshwater fern massule of Azolla cretacea, the mildly thermophylic wingnut relative Caryapollenties veripties and the hickory relative Platycaryapollenties platycaryoides are all proxies for this short-lived (80–120 ka) global hyperthermal event (Cohen et al. 2007).

Fig. 13.
Fig. 13.

Showing the microplankton raw data record for the same interval as Figure 12, the marginal-marine–terrestrial depositional environment of this succession is marked by the low frequencies of microplankton and their sporadic distribution. Pulses of inner neritic marine facies are recorded in the Hildasay Member section and at the base of sequence T45. These are characterized by the autotropic gonyaulacoid dinocysts C. comatum, I ligospinosum and S, ramosus subsp ramosus. A similar association is seen in the middle neritic sediments of the Lamba Formation at the base of the figure. Here, the dinocyst A. gippingensis is significant both in age, not occurring in sediments younger than sequence T38, and also as an indicator of eutrophic turbid shelf conditions. Further flooding surfaces are marked by brackish to freshwater marginal-marine taxa. A flooding surface at the base of LAZ Rc3 is marked by a brackish water flora containing B braunii (a chrolophycean algae) and the acritach Leiosphera spp, A second dominantly freshwater flooding event is seen within LAZ Rc5, marked by the freshwater taxa B braunii, S. parvus (a Spirogyra type) and the turbid water, arenaceous facies acritarch P. indentata. It is of interest to integrate this microplankton palynoflora with the pollen and spore record for the same interval (Fig. 12). This is dominated by floating freshwater ferns (Azolla), bulrushes (S. magnoides), and the mire/transitional swamp taxa P. platycaryroides and C veripites. This interval can be interpreted as broadly freshwater estuarine with a flanking complex floodplain overbank zone of lakes, mire and transitional swamps. Understanding of the ecology and the seral successional composition of Palaeogene microplankton and higher plant communities is essential for the appreciation of environmental controls on biostratigraphical events, and hence their stratigraphical utility.

Fig. 14.
Fig. 14.

Lithostratigraphy, sequence stratigraphy and local assemblage palynofloral zones (LAZ) of Rosebank well 205/1-1. The position of the seismic well-tie (see Fig. 15) is also shown.

Regional Distribution and Seismic Character of Lava Sequences

Sequence T40 volcanics: seismic character

Regionally, and in agreement with the well analysis, the most prominent and extensive lava facies mapped are a suite of sequence T40 lavas (Fig. 10), confident tie-backs of these sequences can be made to the sequence T40 lava in the Rosebank wells (205/1-1 and 213/27-1) at the base of the Colsay 3 Sandstone Member sequence (Figs 14 and 15). Seismically, the flows are represented by a fairly continuous, often high-amplitude, event whose seismic facies is laterally continuous (Fig. 15a, b). The generally easily identifiable and well-defined nature of the lava flows forming the Base Colsay 3 reflector is possibly not unexpected, as the age-equivalent Beinisvørð Formation of the FIBG is characterized by a series of thick, laterally continuous inflated sheet flows with thin interbeds (Passey & Jolley 2009), making their seismic character less laterally variable than one may expect when compared against a series of anastomosing braided lava flows (e.g. that of the FIBG Malinstindur or Enni Formation). However, it should be noted that taking the individual average thickness of Beinisvørð Formation flows within the FIBG, which is around 25 m (Passey & Jolley 2009), it is likely that the picked surface in Figure 15 actually represents several stacked lava flows as a single flow would be below the vertical seismic resolution (Ellefsen et al. 2010).

Fig. 15.
Fig. 15.

(A) Seismic line B–B' through Rosebank well 205/1-1 (see C for location). (B) Seismic line B–B' flattened on the Top Balder surface. (C) Amplitude extraction on the near-Base Colsay 3 surface, showing a channel-like feature denoted by a subtle drop in amplitudes (see A, B and Fig. 17). Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

Sequence T45 volcanics: seismic character

The sequence T45 formation lavas and sediments are seismically the most challenging to pick within the offshore lava sequences owing to their chaotic laterally discontinuous seismic facies (Fig. 15a, b), often making them indistinguishable from the sequence T40 Colsay (1 and 2) Member sedimentary rocks. In comparison with the equivalent succession within the FIBG, the Malinstindur Formation often shows a chaotic, laterally discontinuous facies with extensive development of lava interbeds and weathered tops (Passey & Jolley 2009). This complicated physical character in the field may go some way to explaining the chaotic facies displayed in the seismic data, as being a result of complicated interfingering relationships of lava and sediments.

Sequence T45 volcanics (equivalent in time to the Enni Formation lavas of the FIBG) are not present in the released Rosebank wells; however, they are seismically easy to resolve across the area, being the topmost sequence of basalt. An isolated late-stage sequence T45 lava flow field appears to occur to the west of the Corona Ridge, appearing to be fed away from the area of the Anne-Marie well (6004/8a-1: Fig. 16). This suggests that this area was a focus for intrusive and accompanying extrusive flow-field activity during the sequence T45 period.

Fig. 16.
Fig. 16.

Top basalt surface highlighting an extensive upper sequence T45 lava flow field to the west of Rosebank (Enni Formation equivalent). Based on lava-flow morphology, two possible areas of eruption of this flow are indicated, to the NW of the Anne-Marie well and from an area located NW, slightly off the dataset edge. Importantly, the lava field appears to be locally sourced and, therefore, demonstrates that lava was not simply being fed away from the Faroe Islands during upper sequence T45. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey). Figure C photograph by Gisli Thorsson.

This apparently locally fed nature of at least some of the sequence T45 lavas illustrates that, although equivalent in time and volcanic facies to the Enni Formation, FIBG, the Corona Ridge eruptions were locally sourced, probably from fissures. These Corona Ridge lava fields (Figs 7 and 16) do not have a physical connection to those of the Faroe Islands, highlighting the multi-source eruption style of basalts flow fields within the Faroe–Shetland Basin. However, the timing of eruptions between the FIBG and those identified on the Corona Ridge indicates that a highly interconnected subsurface magma plumbing system must have operated across the region.

Colsay Member Intra-Basaltic Drainage System, West of Shetland

Sequence T40 (Base Colsay 3) fluvial channel system

Within the sequence T40 lava fields around the Corona Ridge, the near-Base Colsay 3 reflector (Fig. 17) forms a prominent, laterally extensive high-amplitude reflector with a series of constrained well-ties (e.g. to well 205/1-1, Rosebank: Figs 14 and 15). This reflector represents a time surface developed at a particular stage in the lava-field development. This picked reflector does not, therefore, represent one physically continuous lava flow but, rather, a series of contemporaneous, anastomosing flows erupted onto the sequence T40 landscape.

Fig. 17.
Fig. 17.

Amplitude extraction on the near-Base Colsay 3 surface over the study area. A prominent sinuous channel-like feature can be seen west of well 205/1-1 (Rosebank), denoted by a subtle fall in amplitude. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

Amplitude extraction on the Base Colsay 3 lava surface denotes a series of channel-like features represented by a subtle drop in amplitudes (Fig. 17b, c), the most prominent of these is located to the west of the Corona–North Westray Ridge, on the fringes of the Corona Basin. The channel feature appears to resemble that of a fluvial braided channel, with a SW–NE axis (Fig. 17). The imaged channels are relatively large, being between 400 and 1000 m in width, and imaged over an area of approximately 400 km2.

Two aspects support the interpretation of the incised channel systems as a fluvial drainage network. (1) When the imaged drainage system is compared against well-imaged fluvial systems imaged within the sequence T45 top-basalt surface (Fig. 18a, b), it can be seen that the drainage system is denoted in the same manner as that imaged at the base of Colsay 3 (Fig. 14c, d), in the form of a subtle drop in seismic amplitudes. (2) The occurrence of the central axis of the drainage network with a zone of maximum thickening of the Colsay sequences (see the following subsection) would suggest that the imaged channel network may represent an important Colsay Member-related drainage system.

Fig. 18.
Fig. 18.

Figure illustrating the similarity in seismic character of fluvial drainage channels within the top basalt surface (sequence T45) and within the near-Base Colsay 3 surface. Note that figures are displayed at the same scale. (A) Amplitude extraction on the top basalt surface (sequence T45), showing a SW–NE-trending fluvial channel. (B) Amplitude extraction on the near-Base Colsay 3 surface. Both channels show the same drop in amplitude, in addition note the similarity in scale of the two channel systems. (C) Corresponding seismic line X–X’ from (A) showing the nature of the amplitude drop across the fluvial channel axis on the top lava reflector (D) Corresponding seismic line Y–Y’ showing the nature of the amplitude drop across the channel axis on sequence T40 lava surface at the base of Colsay 3. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

In a attempt to better image the channel network visible in the amplitude extraction, RGB (Red, Green, Blue) spectral decomposition was carried out (Fig. 19), which splits the seismic data into a series of discrete frequency domains, and is particularly useful for delineating stratigraphical features, such as channels (Partyka et al. 1999). Owing to the attenuating effect of the basalts on frequency (Ziolkowski et al. 2003), we concentrated the spectral decomposition in low-frequency ranges. A range of frequency blends were used, until a blend involving frequencies R=18 Hz, G= 15 Hz, B= 30 Hz was discovered to suitably highlight the channel-like features clearly. The spectral decomposition appears to better delineate some of the channel features in higher detail, displaying the sinuosity of the features and also a slight widening of the drainage zone (Fig. 19).

Fig. 19.
Fig. 19.

(A) RGB frequency decomposition carried out on the near-Base Colsay 3 surface in the area of the fluvial channel network seen in Figure 17. (B) Mapped channels (black) illustrate the sinuosity of the channels and the slight widening of the channel system towards the SW. It also appears that channels have drained away from the Cambo area. The imaged drainage system appears to become constricted towards the west of well 205/1-1, this may be the result of the lava-field morphology. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

Aerial distribution of the Colsay drainage system

Delineation of individual sequences of the Colsay Member (Colsay 3, 2 and 1) is challenging seismically, as individual reflectors forming Colsay Member sandstones are seismically laterally discontinuous. Therefore, to partly circumvent this issue and gain at least some broad understanding of the drainage system and distribution of the Colsay Member, a contoured isochron thickness map was created between the upper sequence T45 volcanics lava surface and the Base Colsay 3 surface (within sequence T40) (Fig. 20a), allowing for the overall thickness variations of Colsay 1, 2 and 3 to be assessed over the area studied.

Fig. 20.
Fig. 20.

(A) Isochron thickness map showing the cumulative thickness of the Colsay Sandstone Member and lavas. Note the SW–NE zone of thickening extending north of the Cambo discovery to the west of the Rosebank. (B) Isochron thickness map with the Base Colsay 3 fluvial drainage system overlain. Note the rough correspondence of the drainage system with the SW–NE zone of thickening. This suggests that the drainage system established at the base of Colsay 3 may have been long lived throughout the deposition of subsequent Colsay 1 and 2 sediments. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

It is apparent that when the mapped channel network is superimposed onto the Colsay isochron thickness map, the channel network corresponds to a broad SW–NE axial zone of thickening of the Colsay units (and associated lava flows) that can be seen extending from north of the Cambo discovery in a NE direction to NW of the Rosebank prospect (Fig. 16b). When the present-day basin structure is added (see Moy 2009), it can be seen that both the zone of thickening and the drainage network appear to be neatly bounded within a mini-graben bounded by a SE-dipping fault within the Corona Basin and the NW-dipping basin-bounding fault of the Corona Ridge (Fig. 21).

Fig. 21.
Fig. 21.

Map showing the cumulative isochron thickness of the Colsay Sandstone Member sediments and lavas, showing the underlying basin structure (after Moy 2009). Note the correspondence of the drainage system and the thickening of Colsay sequences with the basin structure, suggesting some possible fault control. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

Importantly, as the imaged drainage system occurs in the lower units of the Colsay Member (Base Colsay 3), the apparent thickening that is seen in sequences above this may suggest that the depocentre and drainage system established at the beginning of Colsay 3 times was long lived throughout the subsequent deposition of Colsay 2 and 1 (and eruption of intermittent lava flows). Therefore the imaged drainage network may represent the first establishment of a long-lived Colsay Member intra-basaltic drainage-pathway with a SW–NE axis on the lava field (Fig. 22).

Fig. 22.
Fig. 22.

(A) Possible Colsay drainage pathway operating during sequence T40 time. (B) Possible drainage direction of the Colsay Member drainage system. Data courtesy of PGS (Faroe–Shetland Basin MegaSurvey).

However, we caution that it should be noted that the Colsay isochron thickness map does not just give the relative thickness of Colsay Member sandstones but also represents the sum-thickness of any lava sequences which may overlie or interfinger with the Colsay Member interval. Therefore, an area represented as having a relatively ‘thick’ Colsay succession may also contain an undetermined thickness of lava flows owing to lava flows exploiting the same topographical low as the drainage system. Although the gross thickness of Colsay sequences is likely to be higher in these ‘thick’ areas (Fig. 23), it could equally be subdivided by a high proportion of lava flows (Fig. 23), so that the net thickness of individual interbeds may actually be relatively thin.

Fig. 23.
Fig. 23.

Figure illustrating that as topographical lows are exploited by both lava flows and fluvial systems, areas within a lava field that also contains the highest gross thickness of intra-basaltic systems are also likely to posses the highest occurrence of individual lava flow units. In this circumstance, although the gross thickness of interbeds may be high in such areas, the net thickness of each individual bed may be quite thin.

Discussion

As the Colsay Member represents the main reservoir interval in the Rosebank discovery, the identification of drainage pathways is significant. However, considerable uncertainty exists with regard to the proportion of volcaniclastic and silicilastic sediment within the drainage system away from areas of well control. The Rosebank reservoir itself has sandstones of varying porosity and permeabilities, principally related to the amount of volcaniclastic material, which includes unstable mineral phases that degrade porosity and permeability during diagenesis (Helland-Hansen 2009). However, the fluvial-dominated Rosebank sequences show evidence of marginal-marine tidal reworking of siliciclastic sediments within the reservoir succession (Duncan et al. 2009). This suggests that winnowing and sorting of volcaniclastic sediment could be vital in removing volcanic clays (Varming et al. 2012). It remains questionable whether the Corona Ridge drainage system imaged here could have undergone the same kind of reworking, as the marginal-marine conditions seen within Rosebank suggest that this area was close to the Flett shoreline at the time and, therefore, the Corona Ridge drainage system dealt with in this paper may have been located too far inland to experience any marine transgression during Flett times.

From the combination of phytogeographical and heavy mineral data (Jolley et al. 2005; Jolley & Morton 2007), it is thought that sediment dispersal within the pre-sequence T40 upper Palaeocene included elements from the west (i.e. Greenland) and east of the West of Shetland platform, with the distribution of argillaceous sediment being strongly controlled by the presence of the NW–SE transfer lineaments. This sediment transfer system breaks down with the basin flank uplift evident at the start of sequence T40 (see Morton et al. 2012).

The Colsay drainage system identified here is orientated broadly perpendicular to this NW–SE axis, lying oblique between the inferred traces of the Westray and Clair Transfer Lineament (Ellis et al. 2009) (Fig. 18b). Assessing the direction of drainage through the imaged drainage system is difficult. Based on the morphology of the drainage system alone, which appears to splay in a SW direction, it could be assumed that the drainage direction was from NE to SW. However, our data suggest that the imaged channels were draining from an area of lava-covered palaeo-highs around the Corona–Westray ridge, into the fault-bounded basin, where the lava-field morphology further acts in constricting/channelling the drainage system (Figs 20–22). These palaeo-highs are also potentially the locations of fissure systems responsible for the extrusion of much of the lava and contemporaneous volcaniclastic material through localized phreatomagmatism and fire fountaining. Without an input of siliciclastic material into the lava field from an arm of the coeval Flett Delta (Fig. 24), this drainage system was probably dominated by volcaniclastic sediments that now provide poor reservoir potential. Despite questions of reservoir potential, the identification of this drainage system opens up the possibility of other intra-basaltic plays existing within both the UK and Farorese sectors. However, the lack of remaining clearly defined large-scale four-way dip closures (compared against the clear closure structures of Cambo and Rosebank) may mean that trap definition will be a major risk in the hunt for future prospects.

Fig. 24.
Fig. 24.

Schematic palaeogeographical reconstruction of the North Atlantic during sequence T40 times (after Naylor et al. 1999; Jolley & Morton 2007; Ritchie et al. 2011). For the drainage system imaged within this paper to contain prospective reservoir, a clastic ‘clean sand’ input is needed from either the Foinaven or Flett sub-basin areas. It is currently uncertain whether this existed.

The identification of this SW–NE-trending drainage system also raises interesting questions on a more regional context in regard to a sediment-routing direction through the lava sequences during sequence T40. Sediment routing, in general, through the sequence T40 interval is not well understood. The sediment routing from Greenland is often assumed to be in a east–west direction; however, there is no evidence within the younger FIBG interbeds of a significant siliclastic input from Greenland. However, within the Brugdan well (6104/21-1), drilled on the East Faroes High (Figs 1 and 2), some evidence does exist for T40 Greenland-sourced sediments (Morton et al. 2012). Therefore, we propose that the influence of the lava-field morphology may have been an underestimated factor, and acted to ‘divert’ sediments during sequence T40 tangentially through the lava flow fields, potentially causing a complicated array of sequence T40 depocentres and sediment-routing pathways.

The Rosebank discovery itself appears to be on the fringes of the imaged drainage network/pathway (Figs 16 and 18) and its relationship with the drainage system discussed within this paper is not currently clear. Although the two systems may be interconnected in some way, the imaged drainage system discussed in this paper appears to be spatially restricted in width to a 10 km zone, possibly as the result of constriction and control of lava-field morphology (Fig. 5). We therefore propose that the imaged system may be a separate Colsay drainage system from that which formed the Rosebank reservoir.

Conclusions

Within this paper we have integrated well data on stratigraphy and depositional environment with 3D seismic interpretations, to identify the geometry of a regional sequence T40 Colsay Member intra-basaltic drainage system with the Palaeocene lava sequences. From this we can make the following conclusions.

  • The broad stratigraphy of the Faroe Island Basalt group (FIBG) is reflected in the volcanic successions of the Corona Ridge, illustrating that the changes in lava stratigraphy and nature of eruption facies seen in the FIBG are replicated regionally across the basin.

  • The offshore lava sequences were probably erupted from multiple sources, which, although time equivalent with the FIBG and demonstrating similar volcanic facies, were not fed exclusively from the Faroe Islands.

  • A regional well analysis shows that the distribution of lava sequences varies along strike within the Faroe–Shetland Basin. sequence T40 lava flows occur extensively through the Faroe–Shetland Basin, supporting the interpretation of these flows as being part of a regional highly effusive phase. Outside of the FIBG, lower sequence T45 volcanics appear to be restricted to the Corona Ridge area. These locally sourced volcanics are closely comparable in eruptive geometry to the volcanic facies of the Malinstindur Formation FIBG. The upper sequence T45 volcanic flows appear to be spatially very restricted, only having been penetrated to date (based on release well information) in the Longan well (6005/15-1a) located to the SW of the area. A geographically restricted late-stage upper sequence T45 flow can be seen being fed away from the Anne-Marie area.

  • Few studies currently exist dealing with drainage system development on lava sequences. In order for a clastic intra-basaltic depositional system to develop, a sustained hiatus in volcanic eruption must occur. If not, then intra-basaltic products will be mainly restricted to in situ weathering, producing soil profiles. However, if a fluvial system does develop, lava-field topography appears to play a important role in controlling where a drainage system develops, with a drainage system often developing in lows of a lava field (e.g. in-between lava lobes, at the sides of lava flows, etc.). Over the medium to long term, periods of drainage system sedimentation will be punctuated by lava eruption, where a lava is likely to invade and occupy the same topographical lows occupied by the drainage systems themselves. This creates a very variable and complex sedimentary system.

  • A sequence T40 Colsay Member drainage system appears to have extended from north of the Cambo discovery in a NE direction to NW of the Rosebank prospect. The axis of this system also appears to coincide with a thickening of the Colsay Member sequences, suggesting that it was a long-lived drainage pathway.

  • Although difficult to ascertain, we propose that the drainage system is largely locally sourced, with drainage occurring off lava covered palaeo-highs around the Corona Ridge area, and draining in a SW–NE direction.

  • Without the input of a clastic sand source from outwith of the lava field, the imaged drainage system is likely to be heavily dominated by volcaniclastic sediments (i.e. those derived from fragmental volcanic material and from the direct weathering of basalt sequences). This may provide a high exploration risk of any intra-basaltic prospects in the area.

  • The morphology and geometry of the lava fields during sequence T40 probably acted to ‘divert’ sediment input into and around the margins of the lava fields. This would have led to a complicated array of sequence T40 depocentres and sediment-routing pathways

Acknowledgments

This paper forms part of the SINDRI project (C46-37-01). PGS are thanked for the donation of the Faroe Shetland MegaSurvey, without which this research would have been impossible. Spectral decomposition was carried out using Foster Findlay Associates’s (FFA) GeoTeric software. Stuart Archer, Adrian Hartley and James Wheeley are thanked for the discussion on fluvial systems. Simon Passey and Huw Llewellyn are thanked for discussions on lava sequences and Tony Doré and David Mudge for constructive reviews. N. Schofield would particularly like to thank John Underhill for discussion on stratigraphy.

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