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Since the discovery of oil in Paleogene sandstones in the Arbroath Oil Field in 1969, the North Sea has become established as one of the world's major oil-producing regions [Downey et al 2001, pp 170]. The discovery of the Buzzard oil field in the Moray Firth Basin in 2001 is the largest single discovery of oil in this region for two decades, and may yield as much as 800-1100 x 106 barrels of oil in reserves [Gray 2010, pp2 and Dore and Robbins 2005, pp 241]. The Buzzard Field is a stratigraphic trap occurring within the deep-water turbidite sandstones of the Upper Jurassic. The combination of these thick porous sandstones beds with a surrounding organic rich source rock of Kimmeridge Clay mudstones makes the Buzzard field an ideal area for oil exploration. The field itself is sealed by the Kimmeridge Clay Shale Formation and the trapping mechanism itself involves a complex stratigraphic pinchout of reservoir sands within these shales [Dore and Robbins 2005, pp 170]. The nature of these traps suggests that a significant amount of stratigraphic trapping potential remains within the deep-water Kimmeridigan Sandstone members [Gluyas and Hichens 2003, pp 44].
The United Kingdom Continental Shelf (UKCS) area is home to a large part of the North Sea Oil Province - one of the large oil-producing regions of the world. The reason for this large volume in oil reserves is that the geology of the North Sea region is dominated by sedimentary basins - large areas of thick sedimentary rock formations where the vast majority of oil and gas are found [Hyne 2001, pp 107]. The North Sea Province contains a multitude of working petroleum systems, home to vast accumulations of commercial hydrocarbons The main sedimentary basins within the UKCS can be broadly divided into a number of separate provinces, based on both the petroleum geology and location (see figure 1) [Gray 2010, pp 5]. Over the past half century more than 4,100 exploration and appraisal wells have been drilled on the UKCS and, as of January 2010, these wells resulted in nearly 400 producing fields, 50 fields which have ceased productions, and another 51 'significant discoveries' [Gray 2010, pp 2]. One of the most significant finds of the past 10 years has been the Buzzard field, which has estimated in-place reserves of 800-1100 x 106 barrels of oil [Dore and Robbins 2005, pp 241]. The Buzzard Oil Field is located in the southern part of the Moray Firth Basin of the North Sea, 50km northeast of the Scottish mainland, 20km west of the Ettrick field and on the eastern flank of the Grampian Spur, in waters approximately 300ft deep (see figure 1) [Dore and Robbins 2005, pp 245].
Figure 1: Distribution of oil and gas provinces and petroleum source rock on the UK continental Shelf [Gray 2010, pp 4]. Inset - Location of the Buzzard Oil Field with surrounding principal oil discoveries in the Moray Firth Basin [Allen et.al 2006, pp 188].
The geological history of the North Sea Oil Province has been dominated episodes of crustal extension and accelerated basin subsidence occurring mainly in the Late Jurassic, with some local extensions continuing to the early Cretaceous [Downey et al 2001, pp 170]. During the Mesozoic breakup of the super continent Pangaea, North America separated from Europe and formed what we now know as the North Sea. The North Sea is considered to be a 'failed-rift' basin, where rifting sequences started to develop as a result of continental break-up. The reason for the term 'failed-rift' is that the rifting did not go as far as to form a new ocean. Instead the rift became aborted, and continued to subside as a broad saucer as the region cooled down [Stoneley 1995, pp 104]. The Late-Jurassic and Early Cretaceous crustal extensions caused extensive rifting in this manner, and the thermal decay of the North Sea Dome saw the creation of large rift valleys or grabens. These tensional features represent a down-throw block between two normal faults of parallel strike, but converging dips [Press and Siever 1998, pp 634]. In the North Sea this process produced the Viking Graben, Central Graben and Moray Firth rift systems [Gluyas and Hichens 2003, pp 41]. The extensions also produced highly segmented fault systems (predominantly E-W and SSE-NNE) that affected the degree of sediment input into the Buzzard Basin and the stratigraphy of the region. Significant amounts of sands were introduced and deposited at the base of a slope setting, fed by a river or braided delta in an eastwards direction. These sediments were able to bypass the easterly dipping slope between the shelf and deep water and were deposited at the base of the slope [www.geoexpro.com pp 38]. Faulting significantly altered the sedimentary patterns of the basin and is reflected in the stratigraphic column of the Buzzard Basin which shows these deposits sandstones encased in Kimmeridge Clay.
In looking to appraise the region in terms of potential oil reserves, five key identifiers need to be assessed. These are:
The Source Rock - an organic rich deposit which reached optimal temperatures for oil development and was free of intensive erosion and oxidation processes.
Migration Pathway - to enable the oil to migrate to an area where reserves can 'pool' making its extraction possible and commercially viable.
Reservoir Rock - a deposit which allows for effective migration and extraction.
Seal - an impermeable deposit which acts as a barrier to further migration.
Trap - a structural or stratigraphic circumstance occurring in the reservoir rock which causes the pooling of oil reserves.
Each component needs to maximise the potential for reserves, and if predicted to work can be introduced as a basin Play Concept.
The source rocks of the Buzzard field are the organic-rich mudstones of the Kimmeridge Clay formation (rich in hydrocarbons throughout the region) [Gray 2010, pp 6]. These Kimmeridge Clays, deposited by deep marine gravitational flows, are an oil prone source rock which becomes more mature moving east through the South Moray Basin towards the Ettrick Field [Allen et al 2006, pp 254]. The Post-rift subsidence of the Upper Jurassic allowed for the Kimmeridge mudstones to mature with respect to hydrocarbon generation, along the rift axis.
Oil accumulation in the Buzzard basin is located down-dip, which provides an ideal migration pathway from the oil mature source rock, to the reservoir rocks above. These faults provide a weakness in the rock formation along which oil can migrate vertically. Tectonic tilting accentuated this relief significantly and aided the vertical migration direction of this oil [Allen 2006, pp 187].
The Buzzard Field reservoir (and the reservoirs of the Southern Moray Basin in general) compromise syn-rift structureless and laminated Late Jurassic sandstone units within Kimmeridge Clay source rock [Allen et al 2006, pp 254]. The sandstone units are formed by the accumulation of sediment around the graben margins during the late Jurassic, forming thick beds of sedimentary rock 3000 to 6000 ft (900 to 1800m) thicker than sedimentary rocks in adjacent areas [Hyne 2001, pp 146]. The thick Buzzard Sandstone member comprises base of slope, submarine gravity flow sands with excellent porosity and permeability with 15% and 34% porosities and permeabilities between 200mD and 18 Darcy's characteristics [Dore and Robbins 2005, pp 241]. By far the most abundant unit (in terms of potential oil) is the Upper Unit, which contains 80% of reserves. The unit has a fairly uniform thickness across the field and is characterised by a high net-to-gross ratio and excellent porosities and permeabilities [Allen 2006 pp 187].
The top and lateral seals of the Buzzard Field are provided by Kimmeridge Clay Formation Shales. The base seal is defined by the underlying Kimmeridge Clay and pre-rift Heather Formation Shales. There are also some examples of lateral seals being provided by faults in certain places [Allen et al 2006, pp 254].
The Buzzard oil field is unusual in its development as it is dependent on a complex stratigraphic trap. The trapping mechanism itself involves a complex stratigraphic pinchout of reservoir sands within shales of the Kimmeridge Clay Formation, combined with a downthrown faulted closure against Palaeozoic rocks. [Dore and Robbins 2005, pp 241]
Discussion and Interpretation:
The Buzzard Basin play is an Upper Jurassic Turbidite sandstone reservoir sourced by oil mature contemporaneous Kimmeridge Clay mudstones. The migration pathways are determined by upward dip movement and some tectonic tilting. The trap is stratigraphic in nature, and the accumulation hosted in base of slope sandstones in a deep-marine depositional environment [Allen 2006, pp 254]. There is a very definite sequence of necessary geological events that must be followed to in order to produce a commercially viable oil field. Each of these events will be considered in relation to the five steps outline above evaluate the Buzzard Oil Field's reserve potential.
The Source Rock:
Oil is formed where there is a favourable balance between the production of organic matter and its destruction. Within the source rock depositional environment organic matter must be produced in some quantity, and buried within the sediment of a subsiding basin before being destroyed by oxidation [Press and Siever 1995, pp 579]. The depositional environments of the Kimmeridge Clay mudstones satisfied this balance by providing a set of conditions where productivity was high (coastal areas where organisms were able to thrive) and the supply of oxygen in bottom layers was not sufficient to oxidise all of this organic matter.
The Migration Pathway:
Once oil has been formed in the source rock (via slow thermal chemical reactions) it needs a mechanism by which it can migrate into nearby permeable beds. In the Buzzard Oil Field, this migration is produced by faulting (with the source rocks located down-dip). The area also benefited from tectonic tilting in the Tertiary which amplified trap volume and aided migration, and was further improved by the interbedded nature of the reservoir and source rocks [Allen et al 2006, pp 204]. As the majority of the hydrocarbon migration has been vertically (due to the tilting and fault mechanisms), the oil field was able to maintain its position within the geographical boundary of its source rocks, as the oil was not able to migrate across wide-ranging horizontal pathways [Gray 2010, pp 6].
The reservoir rocks must be both porous and permeable to allow migration through the beds. The Buzzard Sandstone member comprises base of slope, submarine gravity flow sands with excellent porosity and permeability characteristics. The Reservoir itself is subdivided into four stratigraphic units which are highly porous and permeable. These characteristics make them excellent reservoir rocks. A more porous reservoir will allow for more migration. Increased permeability is required to allow the oil to be pumped out in commercially viable quantities.
Seal and Trap:
In order for a reservoir to be tapped there needs to be an effective seal and trap mechanism which concentrate supplies to one exploitable location. The seals are provided by impermeable shales, which are not permeable or porous. These properties prevent further migration through the matrices of the rock itself. The Buzzard Basin is a stratigraphic trap known as a pinchout trap. The deltaic depositional environment of the Kimmeridge sandstone units in the Buzzard Basin are a complex sequence of depositional deformaties [Gluyas and Swarbirck 2004, pp 164].
The Buzzard Oil field is considered as a commercially viable oil field because its depositional environment produced a juxtaposition of permeable and impermeable sediments which formed stratigraphic traps for oil production. The organic rich Kimmeridge Clay mudstones were deposited in deep-water environments, free from oxidation and extreme temperature conditions. These became the ideal place for oil to form. These rich source rocks are ideally placed for oil exploration as they are surrounded by highly porous and highly permeable sandstone units, and was able to migrate through these interconnected pores, up the flanks of the basin, where it was trapped and concentrated in a pinchout trap [Hyne 2001, pp 25]. The Buzzard represents a high yielding reservoir because of the specific set of conditions that were in place during both the mudrock and sandstone deposition, and also the stratigraphic alterations that took place at the time.