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New Lisburne play at Jacob’s Ladder Prospect believed to have Karst features, with buried caves and holes forming reservoirs, as in the Yates field in west Texas By Alan Bailey Petroleum News Staff Writer
When, in October, Alaska’s Division of Oil and Gas approved unitization of Anadarko’s Jacob’s Ladder prospect 10 miles southeast of the Prudhoe Bay, the division said that economic benefits to the state of approving the unit include the “evaluation of a previously unrecognized play type on the North Slope of Alaska.” (See story in last week’s Petroleum News.)
The division’s statement refers to a prospect in the Wahoo formation of the Lisburne Group. The Lisburne group pervades the subsurface of northern Alaska and outcrops along the northern margin of the Brooks Range. The group consists of Pennsylvanian and Mississippian limestones and dolomites.
“It has been proposed that the Lisburne (Wahoo formation) prospect may yield a range of 20-660 million barrels of oil equivalent,” the division said of Jacob’s Ladder.
Jacob’s Ladder also includes a sizable prospect in the Ivishak formation of the Sadlerochit, equivalent rocks to the reservoir of the Prudhoe Bay field.
The Lisburne field, discovered at about the same time as the Prudhoe Bay field, has a combination structural and stratigraphic trap in the Lisburne Group. And the U.S. Geological Survey has long recognized petroleum plays in the Lisburne of northern Alaska.
So what is different about the Lisburne play at Jacob’s Ladder? Karst features It seems that there is evidence pointing to the existence of what are known as Karst features in the Lisburne at Jacob’s Ladder. Karst topography, named after a region of Slovenia, typifies terrain in which limestone and other carbonate rocks occur at the Earth’s surface — water dissolves and erodes the limestone to form extensive caves, deep hollows and holes. When an ancient Karst topography becomes buried under younger rocks, the caves and holes can form excellent reservoirs for oil and gas.
In the Lisburne field, petroleum has reservoired in fractures in the rocks, rather than in Karst features. The fractures would have formed as a result of stresses imposed on the rocks at some time in their history. However, the fractures in the Lisburne were leached during surface weathering prior to deposition of younger rocks, division petroleum geologist Paul Decker told Petroleum News. The Yates field People are comparing the Lisburne prospect at Jacob’s Ladder with the geological characteristics of the Yates field of west Texas. The Yates field, discovered in 1926 and originally containing in excess of 1.5 billion barrels of oil, has its most prolific reservoir in Permian dolomites of the San Andres formation. These dolomites display extensive Karst features, including open caves and enlarged fracture systems. The caves and other Karst features form major components of the reservoir’s petroleum carrying and flowing capabilities.
Incidentally, geologists have speculated about the existence of similar prospects in northern Alaska in ancient Precambrian limestones and dolomites that outcrop in the Sadlerochit and Shublik Mountains in the Alaska National Wildlife Refuge and that are thought to extend deep under the ANWR coastal plain.
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