Facies Architecture of the Fluvial-Aeolian Buchan Formation (Upper Devonian) and Its Implications on Field Exploration: A Case Study from Ardmore Field, Central North Sea, UK

The Upper Devonian Buchan Formation in the Central North Sea is a typical terrestrial deposit and predominantly comprises fine to medium-grained sandstones with occasional conglomerates and mudstones. The Buchan Formation has been previously described as being made up mostly of braided fluvial sandstones; however, this study confirms the presence and significance of aeolian sandstones within this fluvial-dominated sequence. Facies architecture is investigated through analogue outcrop study, well log curves and numerical facies modelling, and the results show contrasting differences between fluvial and aeolian facies. The fluvial facies is composed of multiple superimposed and sand-dominated fining-upward cycles in the vertical direction, and laterally an individual cycle has a large width/thickness ratio but is smaller than the field scale. However, the high channel deposition proportion (CDP, average value = 72%) in fluvial-dominated intervals means that it is likely all the sand bodies are interconnected. Aeolian facies comprise superimposed dune and interdune depositions and can be laterally correlated over considerable distances (over 1 km). Although the aeolian sandstones are volumetrically minor (approx. 30%) within the whole Buchan Formation, they have very high porosity and permeability (14.1% 28%, 27 5290 mD) and therefore are excellent potential reservoirs. The fluvial sandstones are significantly cemented by quartz overgrowth and dolomite and by comparison with the aeolian sandstones are poor reservoirs. Aeolian sandstones can be differentiated from fluvial sandstones using several features: pin-stripe lamentation, good sorting, high visible porosity, friable nature and lack of muddy or conglomeratic contents; these characteristics allow aeolian sandstones can be tentatively recognized by low gamma ray values, high sonic transit time and low density How to cite this paper: Tang, L.X., Jones, S. and Gluyas, J. (2017) Facies Architecture of the Fluvial-Aeolian Buchan Formation (Upper Devonian) and Its Implications on Field Exploration: A Case Study from Ardmore Field, Central North Sea, UK. International Journal of Geosciences, 8, 902924. https://doi.org/10.4236/ijg.2017.87052 Received: June 30, 2017 Accepted: July 25, 2017 Published: July 28, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
The hydrocarbon reservoirs in the Ardmore Field (previous name "Argyll", now part of "Alma/Galia project") comprise Upper Devonian Buchan Formation, Permian Rotliegend sandstone and Permian Zechstein carbonates [1] [2] [3]. The Permian Rotliegend and Zechstein groups are important reservoirs in the North Sea and have been researched by numerous studies [4]- [9]. For the Devonian strata, the reservoir properties of Buchan Formation are highly variable, the depositional facies were poorly understood and just briefly described as fluvial deposition under a semi-arid to arid setting [2] [10], there were no reports focused on detailed facies analysis, how the facies architecture of Buchan Formation displays in the subsurface and the relationship between different facies and highly variable reservoir properties.
In this study, we have re-examined the cores and hence re-interpreted depositional facies of Buchan Formation in the Ardmore field. The results show that the Buchan Formation is composed by mixed fluvial-aeolian sandstones, and the aeolian sandstone intervals have the best reservoir quality among the Buchan formation. The facies architecture and porosity/permeability relationships of fluvial and aeolian intervals show distinct differences which are responsible for the highly variable reservoir properties. Therefore, the main aims of this paper are: (1) to characterize and interpret the main facies associations; (2) to simulate the facies architecture based on analogue outcrop study and the well log data; (3) to propose a new facies model for the fluvial-aeolian depositions of the Buchan Formation; and (4) to set several tentative criteria for recognizing aeolian sandstones in uncored wells. This is the first-time study on the fluvial-aeolian facies architecture of the offshore Devonian reservoirs in the UK North Sea. The results provide insights into facies architecture of Buchan sandstones, improve the knowledge of poorly understood Devonian in the North Sea and are useful for forecasting the quality of Upper Devonian sandstone reservoirs in the Central North Sea and other areas ahead of drilling. During this study, lithofacies description and facies identification were carried out on cores from six wells in the Ardmore field; the study on the analogue out-

Tectonic Setting of the Ardmore Field
The Ardmore Field is located on the Argyll Ridge, a large SW-NE trending Pa-

Stratigraphy of the Buchan Formation
The Devonian sequence in the Ardmore Field comprises a succession of the Middle Devonian Kyle limestone and Upper Devonian Buchan sandstone. The succession dips to the south-west, and is separated from the Permian by a palaeo-topographic unconformity, in which successively younger stratigraphic units in the Devonian sub crop towards the south-west. Although the pre-Permian surface has topography it also dips to the SW, this has the effect of making the oldest part of the Buchan sandstone subcrop the unconformity in the NE of the field and thus the youngest Devonian in the SW slightly deeper (Figure 1(B)).
The Middle Devonian Kyle Group is a fairly thin unit (c. 105 m. thick, Figure   1(C)) comprising limestones and minor evaporates [11]. In the adjacent Auk Field, this succession rest unconformably on pre-Devonian basement [12]; this might be similar in Ardmore as inferred from seismic interpretation. The Middle Devonian limestone is mainly encountered in Ardmore wells 30/24-03 and 30/25a-02 ( Figure 1(B)), and forms a strong intra-Devonian seismic reflector throughout the area of the field (Figure 1(C)).
The Upper Devonian Buchan Formation comprises a thick, generally upwardcoarsening succession of shales of mixed shallow marine and sabkha environment at the base, passing upwards into mainly fluvial and aeolian sandy sediments ( Figure 2). The whole Buchan succession lacks clear seismic stratigraphic markers, a combination of log and core data has been used to divide the stratigraphic units for the Upper Devonian group: B01 is the oldest unit overlying the Middle Devonian Limestone, and B11 is the youngest unit [10]. In the absence of bio-stratigraphic data, sedimentary structures and lithofacies associations have been applied to help correlation [10]. The total thickness of the Buchan Formation is nowhere documented due to the combination of erosion below the Devonian-Permian unconformity, lateral thickness variation and incomplete well penetrations. The estimated thickness is about 500 -800 m according to the seismic profile ( Figure 1(C)).

Facies Analysis
The detailed lithofacies study forms the basis of this paper, six wells were chosen because they comprises important fluvial and aeolian intervals in the Ardmore field, and have a good data availability (drilled cores with good recovery and well log data).
The lithofacies in the Ardmore field can be roughly divided into three associations according to the dominant grain size grades: conglomerates (G), sandstones (S) and fine-grain sediments (F). The further detailed lithofacies identification has confirmed eight sub-types (Table 1), the classification of lithofacies is mainly based on the original scheme of Miall [13].

Fluvial Facies Association
The fluvial facies is volumetrically major (approx. 70%) in the whole Buchan     Interpretation: the fine-to medium-grained sandstones with various sedimentary structures record the different types of channel migration and accretion. The trough cross bedding represents the result of downstream migration of sinuous-crested dunes, which was usually formed by braided channel-fill deposits under a lower flow regime [14]. The planar cross bedding usually represents the transverse and linguoid sand bars in fluvial channels, and this could be either the result of lateral or downstream accretions [14]. The inclined horizontal laminations could be interpreted as the plane-bed flow and was probably formed by the flash floods deposits within a distal sheet-flood sand-bed river system. Interpretation: these finer-grained sediments record flood plain deposition resulting from overbank flood event [15]. The lithofacies Fl might be the temporary floodplains and inactive or abandon channels. The desiccation cracks with infilled sands indicate the occasional exposed setting under arid climate.

Flood Plain Facies (Fl and Fm)
The mudstones (Fm) were accumulated in the lowest energy zones; represent a quite standing water condition following flooding events.

Aeolian Facies Association
The aeolian facies is volumetrically minor in the whole Buchan Formation of the Ardmore Field (approx. 30%). In this study, two types of aeolian deposit are identified: the pin-stripe laminated dune and discontinuous wavy laminated interdune sandstones. Interpretation: the presence of well-rounded and sorted, upper fine-to mediumgrained, muddy or conglomeratic clast-free sandstones with pin-stripe laminates are typical features of the aeolian dune deposits [16] [17] [18]. The high-angle, relatively stable graded laminates are interpreted as sand-flow deposits, formed by avalanching of non-cohesive sands on dune slip faces [16]. The highly friable nature suggests the low cementation, which is responsible for high visible porosity.

Lithofacies Sw: Discontinuous Wavy Laminated Sandstones
Description: this lithofacies is the predominant type (approx. 80%) in aeolian association and occurs interbedded with the dune sandstones; it is dominantly composed of 1 -2 m thick, very fine-to medium-grained sandstones that are moderately to well sorted, with low-angle discontinuous wavy laminates. Several local sedimentary structures can be sighted, including the mud desiccation infilled by sands (Figure 4(B)), de-water structure (Figure 4(C)), cross-bedding ripples (Figure 4(D)) and argillaceous streaks (Figure 4(E)). Vertically, there is no obvious boundary between Sps and Sw.  Interpretation: the discontinuous wavy laminates with several local sedimentary features suggest that this lithofacies represents the interdune environment.
The interdune is usually located in low-relief areas of a dune system [19]; therefore, in the given fluvial-presented setting, the interdune area would be affected by fluvial depositions. The mud desiccation infilled by sands and de-water structures indicate dry condition, while the cross-bedding ripples and argillaceous streaks can represent a wetter condition [17]. The variation between dry and wet conditions could be associated with the fluctuations of the fluvial system.

The Analogue Outcrop in Dunnet Head, Scotland
This NW-SE trend outcrop profile is located in the Dwarwick Pier (58˚37'N 03˚22'W, see Figure 1 The medium to course-grained sandstones with red-buff and brown-buff coloured are predominant; the main lithofacies are sandstones with trough and planar cross beddings ( Figure 5(B), Figure 5(C)), which commonly has soft sediment deformation ( Figure 5(C)); the conglomerates and fine grain sediments are rarely sighted.
The depositional environment was fluvial braided channels, probably on a low angle alluvial fan [22]. The possible sediment source area was the Scottish Northwest Highland [20], and the general transport direction was from SW to

Facies Architecture Analysis
The study on the lithofacies has revealed that the Buchan Formation in Ardmore Field is composed by sandstones mainly deposited in fluvial-braided and aeolian dune/interdune settings. By using the well logging data, geometry knowledge from analogue outcrop study and the facies modelling module in Schlumberger Petrel software, it is possible to study the facies architecture in both vertical and lateral directions. The selected profile is roughly perpendicular to the fluvial flowing (NW to SE) and palaeo wind blowing (E to W) directions [24]; and the wells are in relatively close spacing (approx. 500 m) which will be helpful on increasing accuracy of lateral correlation [25]. The main parameters for Petrel facies modelling are listed in Table 3.

Vertical Architecture
In the Ardmore field, layers B04, B10 and B11 are fluvial-dominated and composed by conglomerates, sandstones and fine grain sediments. Figure 6 Sp, indicates that braided channel sands are likely to be effective reservoir.

Lateral Architecture
The parameters for facies modelling are integrated by analogue outcrop study and depended on the actual well log correlation in Ardmore wells. In the lateral direction, the well log curves and detailed lithofacies log for three wells are not laterally correlatable, which indicate that the deposition was multiple braided channels rather than a single channel with kilometre-scale width.
The facies modelling result for Layer B10 (Figure 6(B)) shows a high gross sand ratio in the profile, the channel sand bodies are closely superimposed with adjacent ones both in vertical and lateral directions. According to the relationship between sand body connectivity and channel deposit proportion proposed by Karssenberg and Bridge [28], the high gross sand ratio (67.18% for B04, 71.5% for B10 and 77.7% for B11) has increased the possibility that the superimposed sand bodies are almost inter-connected with each other, and only some thick muddy layers (>2 m) could form local flow barriers.

Vertical Architecture
In the Ardmore field, layers B07 and B08 are aeolian-dominated units and mainly composed by upper fine to medium-grained and well sorted sandstones and minor fine-grained sediments.

Lateral Architecture
The close spacing (around 500 m) of development wells in Ardmore field permits the lateral correlation of aeolian units with reasonable confidence. By comparing the well log data of selected wells, the aeolian association comprises a number of laterally correlatable horizons over kilo-meter scale across the profile and generally shows a tabular geometry (Figure 7(A), Figure 7(B)). Dune sand bodies pinch out into finer grained interdune deposits, in this case the desert lake association, towards south-east, and the bounding surfaces of each horizon are commonly the overlain interdune deposits.

Depositional Pattern for the Ardmore Field Area in the Late Devonian Age
The aforementioned lithofacies and facies architecture studies have revealed that the Upper Devonian Buchan formation in the Ardmore field comprises a succession of thick terrestrial sandstones, mainly fluvial and subordinately aeolian depositions under a semi-arid to arid climate. The climatic changes, in this case the rainfall variations in the hinterland high relief area, were the main control of fluvial distributary system [29], and would occur uniformly across the basinscale area at the same time [30]. Therefore, the changes from fluvial to aeolian facies between B04 and B07/B08 (B05 and B06 are not clear), and the reverse transition from aeolian to fluvial between B07/B08 and B09-B11 may be ex- 1) During the fluvial-dominated period, the Ardmore field was located in a braided fluvial fan with multiple conglomeratic-sandy channels and minor overbank/flood plain depositions (Figure 9(A)). The braided fluvial sedimentation was featured as poorly confined channels with rapid and frequent lateral migrations, the cross section of a single channel, according to the analogue outcrops, shows a sheet-like sand body with high width-thickness ratio. This could be explained by the weak riverbank stability due to the lack of deeply rooted vegetation in the Devonian age [32] [33] and overall arid climate which enabled the frequent lateral migration of channel deposits.
2) During aeolian-dominated period, the dune and interdune deposits were predominated and the wind direction was suggested as blowing from east to west [24]. Clearly not all the interdune deposits were aeolian origin, but also the modification by the loading occasional fluvial deposits. There were no conglomeratic-sandy channel deposits during this period, and only some distal fluvial deposits (e.g. sheet flood, floodplain, etc.) could affect the study area ( Figure   9(B)). The fluvial-origin fine grained sediments were deposited in the topographically lower area, in this case, the interdune facies. During this period, occasional desert lakes could exist when the water table was high. The dune and

Significance of Identifying Aeolian Deposits
Aeolian facies is commonly featured as widespread occurrence and good reservoir properties, which usually make aeolian sandstones attractive exploration targets [34]. In the Ardmore field, the reservoir quality of volumetrically major fluvial sandstones was significantly suffered by cementation of extensive quartz overgrowths and authigenic dolomites [1], porosity ranges from 6% to 24%, but permeability never exceeded 300 mD (Figure 10(A)). Conversely, aeolian deposits possess volumetrically minor component but both dune and interdune facies show good reservoir quality (Figure 10(B)).
Therefore, it is important to identify the aeolian association as early as possible during exploration or development. Due to the presence of conglomerates and highly cemented sandstones, the well log responses of fluvial sandstones are commonly featured as medium-low gamma ray value (50 -80 API), low sonic transit times (<75 μs/ft) and medium-high density (>2.5 g/cm 3 ), and each conglomerate-sandstone group is separated by an overbank-associated muddy layer whose gamma ray value is greater than 100 API (Figure 11(A)). However, the aeolian deposits are sandy-dominated with high compositional and textual maturity, high porosity, low cementation and absence of conglomerates and muddy contents, therefore, these features can be manifested on well log curves by low gamma ray value (consistently less than 75 API), high sonic transit times (consistently greater than 80 μs/ft) and low density (<2.5 g/cm 3 ). These featured well

Implications on Devonian-Associated Reservoir Explorations
Devonian strata in the North Sea have been perceived little hydrocarbon potential for a long time [35], but several fields in both UK and Norwegian North Sea have confirmed the industrial oil productions from Devonian-associated reservoirs in recent decades [10] [36] [37] [38].
In this study, we confirm that the Buchan formation in Ardmore Field is composed not only by fluvial deposits but also the presence of aeolian components. Aeolian components possess volumetrically minor percentage (approx. In the Ardmore field, aeolian sandstones are thin, laterally extensive, poorly cemented and permeable, and interbedded with thick, cemented and low-permeability fluvial sandstones. Since the North Sea area had similar depositional setting in the Late Devonian age, therefore, careful facies identification is important for determining the presence of aeolian sandstones. This study has improved the knowledge of depositional setting in the study area and provided several tentative criteria for recognizing aeolian sandstones for the uncored wells; the results can have broad applications on future exploration in Devonian targets of the North Sea or other places with similar provenance.

Conclusions
A renewed attempt on facies identification and modelling has been made by using lithofacies, analogue outcrops, well logging data and software simulation.
This study has announced the presence of aeolian sandstones within the fluvialdominated Buchan formation in the Ardmore field, which has not been clearly identified in the previous time.
Fluvial-associated sandstones are volumetrically major facies type (approx. 70%) and typical braided origin. They are composed by superimposed finingupward cycles, and each cycle represents a deposition of channel bar or channel fills; none of channel deposits can be correlated among the inter-well distance, but the high channel deposition proportion indicate that the channel sandstones are likely to be interconnected with each other.
Aeolian-associated facies form a volumetrically minor (approx. 30%) but important reservoir in the studied field. They comprise superimposed dune and interdune sandstones and are laterally correlatable over kilometre scale. Both dune and interdune sandstones have good reservoir quality as a consequence of good sorting, roundness, absence of muddy and conglomeratic contents, and low cementation. These features allow them to be tentatively identified, especially for those uncored wells, by using the combination of well log responses: low gamma ray, high sonic transit time and low density.
Understandings on the scheme of this fluvial-aeolian facies system demonstrate the possibility of effective reservoir potential in Devonian strata, sandstones deposited in different facies would undergo different diagenesis, in this case, the sandwiched aeolian sandstones could still hold good reservoir property even the surrounding fluvial sandstones were highly cemented. This should have an important impact on reservoir identification, appraisal and discovery.