P2498 Blocks 20/5a & 21/1a (Buchan & J2) – JOG 100% working Interest
The licence is classified as a straight to Second Term licence with a four (4) year duration to submit for approval a Field Development Plan (FDP) for the re-development of the Buchan oil field and the development of the J2 (well 20/5a-10y) oil discovery. The Buchan Andrew oil discovery and the Capri prospect will be further evaluated for possible drilling.
Buchan oil field
The Buchan oil field was discovered by well 21/1-1, drilled by Transworld Petroleum and Texaco in 1974 into a pronounced and relatively elevated horst block at the eastern end of the Buchan Horst.
The well encountered a significant oil column, in excess of 900 ft, in fractured, Upper Old Red Sandstone that flowed 890 bopd on test. The reservoir was significantly over-pressured at 3,400 psi above hydrostatic pressure. The oil is sweet and under-saturated with an average GOR of 285 scf/stb and gravity of 33°API. Subsequent appraisal drilling tested up to 5000 bopd on test (21/01-2) and demonstrated an oil column in excess of 1,900 ft (well 21/1a-6). The field was brought into production by BP in 1981 with nine development wells and initial production peaked at 54,537 bopd.
The field exceeded all initial production expectation and 148 mmstb of oil have been produced under natural depletion with aquifer support to the cessation of production (COP) in May 2017.
The reservoir pressure has over time declined by 5,000psi but has been relatively stable at approximately 2500 psi from 1992 to the COP. Oil production continued until May 2017, when the Buchan Alpha platform was shut in for safety reasons relating to the platform infrastructure.
Independent, major studies undertaken by JOG/Rockflow (2018-19) and RSRUK/AGR TRACS (2014-16) conclude that the Buchan oil field has remaining mean case producible volumes of around 80mmboe.
JOG’s Subsurface Manager, Dr Mohit Khanna, revisits the reservoir of the Buchan Field in his first geological paper: Buchan Field Paper
J2 (well 20/5a-10Y) oil discovery
The J2 discovery was drilled by well 20/5a-10Y in 2006. The well was drilled as a deviated well to test the westerly culmination of a 3-way dip and fault closed structure mapped at Late Jurassic, Sgiath Formation level and located on a structural terrace downthrown to the north of the Buchan Horst and the Buchan field.
The initial well (20/5a-10) was abandoned due to higher than expected pore pressures being encountered within the Late Jurassic interval, whilst the first side-track well (20/5a-10Z) was abandoned due to wellbore instability/collapse within the Early Cretaceous Carrack Formation.
The second side-track (well 20/5a-10Y) encountered hydrocarbons within the objective shallow marine Sgiath Formation Sandstone that flowed at 2,850 bopd plus 1.2 mmscfd (37 API, GOR 426 scf/bbl) on test. The well also encountered c. 16 metres of excellent quality, deep marine, intra Kimmeridge Clay Formation, Burns Sandstone that flowed at 4,800 bopd plus 2.6 mmscfd (39 API, GOR 500 scf/bbl) on test.
The two reservoirs are contained within different pressure cells. JOG estimates that the J2 oil field contains 20mmboe of mean case recoverable oil volumes.
Buchan Andrew oil discovery
The Andrew Sandstone reservoir forms a secondary reservoir in the Buchan oil field. Wells 21/1-1 and 20/5a-5 have penetrated oil-bearing reservoir within the Andrew Formation, these wells are located towards the crest of the structure. The trap constitutes a low relief, four-way closure with approximately 60 ft of closure and a spill point that correlates closely with observed oil-down-to depths in the wells.
Rockflow has performed a high-level evaluation of the Andrew Sandstone reservoir oil in place, the derivation of which has provided clear insight into the primary volumetric uncertainties and their potential mitigation through further maturation of the opportunity.
The Capri prospect is characterised by the distinct onlap of the Upper Jurassic onto the north east flanks of the Buchan horst which has resulted in pinch-out with a three-way trapping geometry immediately beneath the Base Cretaceous Unconformity (“BCU”), coincident with a pronounced seismic amplitude anomaly.
The position of this prospect between the Buchan field and the hydrocarbon generative area of the North Buchan Graben, the abundant evidence of mobile oil within Upper Jurassic sands in the vicinity and the clear seismic amplitude anomaly associated with the adjacent Tweedsmuir fields all support this prospect.
The absence of sandstones of sufficient thickness to be seismically correlative in the uppermost Upper Jurassic in offset wells requires a sand in this position to be sourced locally from the emergent Buchan horst. It is possible the Capri prospect represents the updip, slope apron fan deposits of the basinal Cortina prospect recognised in Licence P2170 Blocks 20/5b & 21/1d.