The oil that escaped from Prudhoe Bay

Prudhoe Bay is a giant among world oil fields. But it could have been bigger.

Geologists think that at the end of the Cretaceous period the field contained perhaps 25 percent more oil than it did when discovered in the 1960s, Ken Bird, a geologist who is a leading expert on North Slope geology, told Petroleum News. The field reservoir, in a massive trap structure where the main reservoir rock, the Ivishak, abuts a major discontinuity in the rock sequence known as the lower Cretaceous unconformity, was fully formed and charged with oil by the end of the Cretaceous. However, an ancient land or shallow marine surface that would have been approximately level early in the subsequent Tertiary period is seen from seismic and well data to be sloping gently to the east in the subsurface. That indicates that the entire Prudhoe Bay structure became tilted to the east, with the tilting taking place progressively over an extended period of time during the Tertiary, Bird said.

As the tilting progressed, oil spilled from the field reservoir into rock strata to the west of the field, leaving the entire southeastern part of the oil reservoir devoid of oil. Oil staining, residual heavy oil and tarry oil in the lower sections of the reservoir are all that now remains of the billions of barrels of oil that must once have existed in that part of the field. And the contact between the Prudhoe Bay oil pool and underlying water slopes to the east, also a result of field tilting.

Although some of the spilled oil likely drained into what is known as the west end of the field, it is likely that substantial amounts of the drained oil flowed upwards into the younger and shallower sandstone reservoirs of the West Sak/Schrader Bluff and Ugnu formations, where the oil was degraded by bacteria to form viscous and heavy oil pools, Bird said. And, so, thanks to viscous oil production from West Sak/Schrader Bluff, some of that “lost oil” is actually being recovered, and BP’s efforts to develop heavy oil from the Ugnu may also bring some of that resource to market.

—Alan Bailey