The one that got away: Did Mukluk oil end up in the Kuparuk field?
BP’s 1982 Mukluk well, drilled from a gravel island in the nearshore waters of the Beaufort Sea off Alaska’s North Slope and costing in excess of $1 billion, has gained notoriety as the most expensive dry hole in oil industry history. Testing a potential oil trap where rocks of the Sadlerochit group, analogous to the reservoir rocks of the nearby giant Prudhoe Bay field, abut a regional break in the rock strata known as the lower Cretaceous unconformity, BP expected the well to encounter a massive oil pool. In the event, all that remained of any oil that might once have existed in the Mukluk structure was extensive oil staining.
And people have tended to view the Mukluk failure as resulting from oil leakage through the rocks that should have sealed the oil trap.
But recent petroleum system computer modeling by scientists from computer services company Schlumberger suggests that the Mukluk oil may have in fact drained south into the Kuparuk River field. If that is correct BP, with a 39 percent ownership interest in Kuparuk, could unknowingly have been producing oil originally from Mukluk, earlier knowledge of which might have saved a few dollars from the company’s exploration budget.
On May 11 Schlumberger’s Kenneth Peters told the Pacific Section, American Association of Petroleum Geologists, that a new 4-D computer model of the North Slope petroleum system might have prevented the Mukluk debacle, had the model been available at the time of the Mukluk drilling. Using seismic data and covering much of northwest Arctic Alaska, the model can play speeded up, visual images of how the rocks appear to have been deposited and then subsequently deformed, displaced, eroded and re-oriented as a result of upheavals in the Earth’s crust at various times.
This model indicates that about 97 million years ago there would have been a structure at the Mukluk prospect that would have accumulated oil, along the lines that BP had expected, Peters said. However, later tilting of the rock strata somewhere around 60 million years ago would have caused the accumulated oil to migrate through sands that would have formed an oil conduit along the lower Cretaceous unconformity, with the unconformity providing an escape route for the oil, rather than providing a mechanism for sealing the oil trap. The model suggests that the oil would have flowed southeast into the reservoir sands of the Kuparuk field, with some oil also migrating towards the northwest.
The danger of snapshots And the lesson to be learned from this new insight? Rather than just looking at static snapshots of current structures where oil may be trapped underground, it is important to consider how these structures have evolved over time and how this evolution relates to events and processes in the regional petroleum system, Peters said. The 4-D modeling of an oil and gas basin can reduce exploration risk, he said.
—Alan Bailey
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