The Ardmore Field is located on the Argyll Ridge, a large SW-NE trending Palaeozoic age tilted fault block on the south-western flank of the Central Graben in Block 30/24, UK North Sea, about 350 km south-east from Aberdeen. The field is a horst feature with the crest in the north and fault closure to the north- east. It measures 2.5 km wide and 6 km long (Figure 1(A)). A combination of dip and faulting defines the limits of the field on the north-west and south-east flanks, while dip closure defines the southern limits of the field. The major fault trends are in two main directions, WNW-ESE cut by NW-SE faults (Figure 1(B)). Top seal of the field is provided by Triassic shale to the far west, Jurassic shale in the mid-part of the field and impermeable Chalk at the north-eastern crest [10] . The trap relies heavily on the major NW-SE trending graben edge faults to the northeast and southwest of the field while dip closure occurs to the northwest and west.

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 upward- coarsening 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)).

The fluvial facies is volumetrically major (approx. 70%) in the whole Buchan

Formation of the Ardmore Field and composed by multiple fining-upward cycles (see details in facies architecture part). In this study, two types of fluvial deposit are identified: fluvial channel facies and flood plain facies.

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.

This NW-SE trend outcrop profile is located in the Dwarwick Pier (58˚37'N 03˚22'W, see Figure 1(A). for location) and displays typical fluvial sandstones of the Upper Devonian formation. Due to the North Sea Basin was under similar tectonic, depositional and climate settings in the late Devonian age [20] [21] , therefore, this outcrop is chosen as the analogue example for the geometry of fluvial sand bodies.

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 NE [22] [23] . The absence of fine-grained sediments, such as rip-up muddy clasts, might indicate that this locality was proximal to the sediment source area; and the rapid channel migration in lateral direction and consequent sediment reworking prevented the preservation of muddy sediments.

The outcrops provide a good opportunity to measure the geometry of the channel sand bodies. On the selected profile, nine fluvial channel units have been identified and named as fc1 to fc9 upwards (Figure 5(A)). The fluvial channel unit generally shows a sheet-like sand body, the thickness of an individual unit is from 0.45 m and up to 0.75 m, and each unit is composed by thin laminates around 5 - 8 cm. A single fluvial sand body is sometimes truncated by adjacent fluvial sand bodies (Figure 5(A)). The lateral extension of a single fluvial sand body can be tracked somewhere tens to nearly one hundred meters away from the selected profile, thus the width-thickness ratio is approximately 80:1 to 120:1 (Table 2).

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 basin- scale 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 expected to have occurred across the basin-scale area at about the same time, and presented a general progradation-retreat-progradation cycle of the alluvial fan- based braided system with aeolian deposits occurred mainly between two main progradation periods.

Therefore this study proposed a depositional pattern for the Ardmore Field area in the Late Devonian age (Figure 8). The lithofacies database is restricted to the Ardmore Field and the areas beyond that are conjectured based on previous studies on related areas.

The braided fluvial fan system was possibly sourced from high-relief uplands in Scotland Midland Valley, and flowed south-eastwards to the Central North Sea area [11] [31] . The Ardmore Field is believed locating near the margin of the braided fluvial fan system (Figure 8), as the presence of aeolian and associated desert lake sediments became established during more arid periods. Therefore, it could be generally divided into two patterns: the fluvial dominated and aeolian

dominated patterns. The bird-eye view slices of facies modelling results demonstrate more details of fluvial and aeolian dune geometry:

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

interdune facies are laterally extensive which suggests the stable development of aeolian deposition, either as the consequences of major channel switching and/ or the regionally significant hiatus of fluvial activity.

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/cm³), 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/cm³). These featured well

log responses, combined with uniform dipmeter patterns (not shown in the Figure), make it possible to tentatively identify aeolian facies in uncored wells (Figure 11(B)).

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. 30%) but show the best reservoir quality among the whole Buchan formation: in fluvial sandstones, the extensive quartz and dolomite cementation reduces permeability by 1 - 2 orders of magnitude; however, quartz overgrowth is absent in aeolian sandstones. Given the same tectonic setting and burial process, these contrasting differences have two implications: 1) the relationship of porosity and permeability is different in fluvial and aeolian sandstones; and 2) the fluvial and aeolian sandstones have undergone different diagenesis processes. These make the sandwiched aeolian sandstones can still keep good porosity/permeability even the surrounding fluvial sandstones were significantly cemented.

In the Ardmore field, aeolian sandstones are thin, laterally extensive, poorly cemented and permeable, and interbedded with thick, cemented and low-per- meability 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.