Geology
The central horst of the field is approximately 400 meters thick on an east-to-west orientation. This central horst is made up of Old Red Sandstone from the Devonian and Lower Carboniferous periods (Edwards 1991). The horst is surrounded by middle to late Jurassic sands which are fluvial and aeolian interbedded with siltstones (Hill 1979). The quality of these sands though varies both vertically and laterally (fig 2). The central horst is thought to contain 370 million barrels (59,000,000 m3) oil while the surrounding Jurassic sands are expected to contain significantly less, approximately 38 million barrels (6,000,000 m3) oil (Wood Mackenzie 2002).
The most important factor of this field is the overpressure of the central horst. This overpressure has been created by what is thought to be an uplift of 2500 m, of which 1000 m occurred during the Lower Cretaceous Period (Hill 1979). The pressure data that was collected from the appraisal wells show that there is an overpressure of 3,200 lbf/in² (22 MPa). More importantly the transition zone from normal pressure to this overpressure happens over some extremely thin Lower Cretaceous layers, which in parts are only 30 m thick.
These initial findings suggested that this would be a marginal field which would be difficult to extract from. This data also suggested that extraction would be limited due to a low porosity and permeability. However, as with all fields, as more was investigated from production data, new facts about the geology were revealed. In this case, the Buchan field is fairly fractured due to its location on an old fault line. Therefore, there are numerous fissures in the reservoir which increase its porosity and permeability.
Three other major problems of the field’s geology would have to be overcome during the drilling stage (Hill 1979). Initially there were problems with the geophysical data of the field. As mentioned, the field is highly fractured due to its location. This lowered the quality of the seismic data, giving poor reflections. As a result, geophysics could not be used to accurately define the top depth of the reservoir; initial depth estimates were out by as much as 133 m.
The pore pressures of the layers over the reservoir are also very dangerous, as they vary greatly from one to the next. There is normal pressure in the Upper Cretaceous limestone layers, but between this normal pressure in the limestone and the over-pressured reservoir there is only a small layer of Lower Cretaceous shale. In parts this shale is only 30 m thick separating the above layer and the over-pressured reservoir. As a result pore pressure in the shale at the top is close to normal but rapidly approaches 3200 psi (22 MPa) as it comes in contact with the reservoir. This circumstance was to prove difficult when selecting the appropriate mud weights during drilling. Furthermore the Shale layer did not have even layer depth throughout varying from 30 to 200 m so making it difficult to extrapolate pore pressure in different sections (Hill 1979).
During the drilling of appraisal wells in the reservoir two major loss zones were discovered. Fractured zones such as those encountered around the Buchan field can lead to a complete or serious loss of the entire mud stream as it is absorbed by the formation. Both cases involved the penetration of the reservoir at a much shallower depth than originally considered due to vague geotechnical data. The first case, in well 21/1-2, resulted in considerable mud losses and a significant section of the hole had to be abandoned so the drilling could take a different path. This option was not available in the second case, in well 21/1-4, and the hole had to be abandoned eventually.
Furthermore, the geological makeup of the matrix of the reservoir did not contribute to the production of the field to the extent that was hoped. It was initially assumed that full production would peak at 72,000 bbl/d (11,400 m3/d) and have an average production rate of 48,000 bbl/d (7,600 m3/d). However, the field only reached a peak of 32,000 bbl/d (5,100 m3/d) and the field average was significantly lower than that.
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