The past 2 years have seen more than 30 new entrant companies from North America and the United Kingdom focusing on remaining exploration and appraisal potential in the UK Continental Shelf (UKCS). This new activity has been brought about by a flexible license system; an increasingly entrepreneurial culture resulting from the diaspora of talent from the takeovers of Amoco, ARCO, Lasmo and Enterprise; and relatively easy access to capital on the London AIM market. Most companies hope to make their mark by exploration. Observing the companies that are succeeding in the North Sea oil sector, we can see that speed, innovation and close partnerships with leading specialist technology companies have accelerated this success.
In particular, a key factor in the past two years has been the application of rock physics. This fast-growing geoscience discipline has made it possible to extract extra meaning from seismic data in terms of direct hydrocarbon indicators, indications of reservoir quality, reservoir changes over time and, frequently, "hidden" stratigraphic trapping potential.
Remaining oil potential
Blessed by an outstandingly rich oil-prone source rock, the Kimmeridge Clay, numerous excellent reservoirs from the Triassic through to the Eocene, excellent "plumbing" between mature source rock and reservoirs, and a variety of trap types, the North Sea has enjoyed a prolific 45-year history and is set to face the next 30 years with confidence.
Figure 1 shows the remaining prize, consisting of more than 60 BBoe yet to be found or produced. Many of these barrels will be found within stratigraphic traps by the use of new technology in geosciences and engineering.
If we consider a leading example of the new entrants that are enjoying success in the North Sea oil sector, we can see more clearly what it will take to deliver the oil potential suggested by this graph.
A Canadian success story
Oilexco has been especially active in the Outer Moray Firth Basin, in blocks 15/25b and c and 15/24b.
Initially, Oilexco obtained and reprocessed a 3-D seismic survey that was shot by a major oil company in 1993 and worked with GX Technology and Ikon Science to apply "state of the art" prestack depth migration and rock physics-driven inversion techniques to the data. Application of these seismic techniques (Figure 2) highlighted an extensive Paleocene-age deepwater sand channel with a large "oil effect" anomaly (Figure 3) within 15/25b, suggesting hydrocarbons were present within the Paleocene channel sands that had little, if any, structural closure and thus suggested stratigraphic trapping conditions.
This meandering upper Paleocene sand was originally encountered and tested by a major oil and gas company in 1990. The 15/25b-3 well penetrated 22 ft (6.7 m) of porous, permeable, oil-stained sand that tested at a stable rate of 2,690 b/d of 39? API oil. The initial Oilexco 15/25b-6 well location was selected to test the stratigraphic potential of this sand fairway, about
490 ft (150 m) west of the 1990 well.
The 15/26b-6 well encountered a thicker sandstone section than the one found in the 15/25b-3 well, and oil-stained sand lenses in the base of the core in the 15/25b-6 well were situated lower than an interpreted oil/water contact in the original 15/25b-3 well. A drill stem test of the new Brenda well yielded oil at an average rate of 2,980 b/d. No formation water was produced from this test, although the base of the perforations was more than 40 ft (12 m) lower than the previously interpreted oil/water contact, thus confirming Oilexco's interpretation of Brenda as a stratigraphic trap. Inversion technology provided sharp images of the stratigraphy and reservoir properties on which to base well planning and development.
The 15/25b-8 well, based on inversion of seismically derived hydrocarbon indicators, successfully tested oil. The appraisal well 15/25b-9 intersected a Paleocene sandstone oil accumulation in the western area of 15/25b. The oil/water contact encountered by this well, located .6 mile (1 km) southwest of the 15/25b-8 well, is at a different elevation from the oil/water contact to the east. The difference in oil/water contacts was confirmed by a subsequent sidetrack well located south of the 15/25b-9 bottomhole location.
Oilexco started the second phase of its Brenda drilling program with the 15/25b-10 well, following up with "clusters" of wells at 15/25b-11 and 15/25b-12, the latter concluding the appraisal of the stratigraphically trapped, subtle Brenda field.
Oilexco is currently awarding drilling and engineering contracts for its subsea development program, and first oil production is expected from Brenda in third-quarter 2006.
Elsewhere in the Moray Firth, Oilexco has completed the 15/24b-8 well, the first of two slimhole penetrations to evaluate an oil-related seismic anomaly adjacent to the MacCulloch field in block 15/24b. Time-lapse seismic data indicated that oil had not been swept from this Palaeocene sand reservoir by production from an offset MacCulloch horizontal well.
The 15/24b-8 well encountered 43 net ft (13 net m) of pay in the Palaeocene sand reservoir with an oil/water contact at the same elevation as the initial oil/water contact in the MacCulloch field. Wireline testing recovered light crude from the reservoir and measured formation pressures that indicated separation of these sands from those within MacCulloch. Well data thus support the 4-D interpretation that the sands in the anomaly have not been drained by offset production wells, proving a satellite discovery.
As these examples show, Oilexco has made effective use of its access to the rock physics, seismic modeling and inversion services provided by Ikon. In the next 2 years, the company will drill more than 10 wells targeting new opportunities that rock physics-driven exploration provides, in reservoirs from the Eocene to the Jurassic.
Where next?
The key theme illustrated in the example above is the application of new technology - rock physics - to a mature basin and the way this technology is available to new entrants via the vibrant and confident UKCS services sector.
Given that the North Sea is well-known for its test-bed status, it is certain that the central role played by rock physics in the North Sea will be mirrored in other basins in Africa, Middle East, Asia and beyond.

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