Abstract The Alma Field (formerly Argyll and then Ardmore) is located within Blocks 30/24 and 30/25 on the western margin of the Central Graben. Hamilton drilled the first discovery well 30/24-1 in 1969 and the field, named ‘Argyll’, became the first UK offshore oilfield when production commenced in 1975. Oil was produced from the Devonian Buchan Formation, Permian Rotliegend and Zechstein groups, and Jurassic Fulmar Formation from 1976 until 1992, when the field was abandoned for economic reasons. In 2002, Tuscan Energy and Acorn Oil & Gas redeveloped the field and renamed it as ‘Ardmore’. A further 5 MMbbl were produced until 2005, when the field was again abandoned due to commercial considerations. In 2011, EnQuest was awarded the licence to redevelop the field and renamed it as ‘Alma’. The field came on stream in October 2015 and has produced oil at an average c. 6000 bopd since start-up. Total ultimate recovery was expected to be about 100 MMbbl. As of end 2005, the field had produced 72.6 MMbbl as Argyll and 5 MMbbl as Ardmore. A further 4.3 MMbbl has been produced from the Alma Field to September 2017 (which includes about 0.5 MMbbl from a long-reach well drilled into the Duncan/Galia Field immediately west of Alma). In January 2020 EnQuest announced that the Alma Field would cease production early. The total production from the three phases of field development will be about 85 MMbbl of oil.

Abstract Carbonate strata were widely deposited in the Alberta Basin during the Frasnian. These are well exposed in the Alberta Rocky Mountains and regionally extensive in the adjacent subsurface. This study places many of its classic outcrops from the Cascade (Burnt Timber) Channel to the South Jasper Basin into a single sequence stratigraphic framework for the first time. This framework is correlated from outcrop to subsurface using sequence stratigraphic and biostratigraphic data. Improved confidence in the stratigraphic interpretation is based on new measured sections tied to photographic panoramas, combined with detailed mapping of lithofacies and stratal patterns of continuously exposed platform to basin transitions in outcrop. These data are correlated with new and revised core and well-log interpretations from the Alberta subsurface. Ten third-order composite sequences and their constituent high-frequency (fourth-order) sequences span the uppermost Givetian through Frasnian strata of the Alberta Basin. They reflect stratigraphic architecture typical of a (second-order) depositional sequence: transgression followed by regression, or basin opening and filling. The eight youngest composite sequences are defined from the Cline Channel and Jasper Basin areas using stratal and facies stacking patterns and regional correlation of sequence boundaries and maximum flooding surfaces, integrated with conodont biostratigraphy. Most sequence boundaries observed are subaerial exposure surfaces, seen in outcrop or inferred from onlap of tidal-flat or reef margin deposits onto foreslope facies. The basin was filled asymmetrically by mixed carbonate–clay successions that form dominant east to west prograding strata. Two main types of sediment comprise the basin fill: extrabasinal clay and intrabasinal carbonate. Composite sequences (CSs) and high-frequency sequences (HFSs) can be confidently correlated from outcrop to subsurface. A combination of well-log and outcrop cross sections, integrated with biostratigraphy, support these correlations. These regional (time) surfaces allow better understanding of basin evolution and architecture. The influence of the second-order sequence dominates the accommodation setting and is expressed in the architecture of composite and high-frequency sequences. For example, the tripartite character (lowstand–transgressive–highstand) of CSs in the lower and middle part of the sequence is followed by the appearance of a distinct falling stage component in the upper part of the Frasnian. An increased frequency of truncation surfaces and offlapping strata is consistent with diminishing accommodation. With progressive basin infill and shallowing paleobathymetry, foreslope declivity decreased from a minimum of 10° to less than 1.5° as the depositional system became more ramp-like. This is accompanied by a change of lowstand geometry from wedge to tabular shaped. Deposition of coarser terrigenous clastics was also limited in most of the basin to the lower part of the second-order sequence, except at CS and HFS. Restricted marine circulation onto the carbonate platforms and basin filling in the upper part of the Frasnian coincided with extensive siliciclastic silt deposition in the study area, particularly in the Jasper Basin, where an influx of terrigenous silt formed mixed carbonate–siliciclastic deposits. Silt was deposited during third- and fourth-order lowstands, bypassed into the basin, and reworked during intermittent inundation of the carbonate platforms. Beyond the basic transgressive–regressive architecture of the second-order (Givetian–) Frasnian sequence, we document detailed observations such as (1) controls affecting the onset, cessation, and extent of euxinic shale deposition in the mid-Frasnian and its relation to the second-order maximum flooding surface; (2) the relative speed and distribution of illitic basin fill within the second-order highstand; (3) the effect of basin fill and off-platform sediment transport on regional and local carbonate platform architecture, such as the configuration of in situ carbonate lowstands, initiation of reefs along favorable fairways, and overall margin stacking patterns; and (4) the magnitude of relative sea-level falls associated with the development of sequence boundaries. A comparison to previously established Frasnian sequence stratigraphic schemes within the basin is extended to other basins in Europe and Australia.

Abstract The southeast and northwest margins of the Frasnian (Upper Devonian) Cline Channel are preserved in excellent and continuous outcrop exposures at both Cripple Creek and Wapiabi Gap in the Alberta Rocky Mountains. Accretionary and interfingering platform margins allow detailed definition and correlation, from platform to basin, of significant sequence stratigraphic surfaces. Eight Frasnian third-order composite sequences are defined using stratal and lithofacies stacking patterns, regional correlation of sequence boundaries, and maximum flooding surfaces, constrained by conodont biostratigraphy. They form part of an upper Givetian–Frasnian second-order transgressive–regressive depositional sequence. Most sequence boundaries observed show subaerial exposure. Others are inferred from stratal architecture, e.g., onlap of tidal-flat or reef margin deposits onto foreslope lithofacies. The Cline Channel was filled asymmetrically from southeast to northwest along the described/studied transect. Progradation of platform margins is on a substrate of platform-derived fine-grained carbonates and extrabasinal clays that form argillaceous carbonates and calcareous shales. Stacking patterns of the composite sequences vary across the Cline Channel. On the southeast side, the second-order Givetian–Frasnian cycle is characterized by initial retrogradation followed by aggradation to retrogradation in the upper mid-Frasnian, and finally, progradation in the upper Frasnian. On the northwest side, the overall stacking pattern is aggradational. With progressive basin filling, platform edges evolved from rimmed boundstone and/or grainstone to mainly grainstone. Foreslope declivity decreased from a minimum of 10° (WD3) to less than 1.5° (WI1) reflecting more ramp-like foreslopes. Coincident with this change, lowstand geometry evolved from wedge shaped to tabular. Where slope gradients were high, lowstands are wedges, less extensive and abutting antecedent highstands. With development of ramp-like geometries, lowstands became tabular and were detached from their antecedent shelf edges with even minor falls of relative sea level. Gentle slope gradients and larger areas for shallow-water carbonate production facilitated extensive lowstand development. Assignment of strata into systems tracts of ramp-like systems is facilitated by subregional correlation. Decreasing accommodation within the second-order highstand is indicated by reduction in composite sequences (CSs) thickness and replacement of open marine with platform-interior strata as the basin shallowed and filled. Composite sequences became more asymmetric, developing thin, offlapping falling stage systems tracts in the late Frasnian, accompanied by a higher frequency of lowstands. Continuous outcrop exposures permitted the amount of relative sea-level fall to be estimated for the bounding surfaces of several CSs. Relative sea-level falls ranged from 9 to ~40 m.