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Ancient rocks deep under ANWR might hold oil and gas Predating Prudhoe Bay by 100 million years, rocks could boost reserves Alan Bailey Petroleum News Contributing Writer
Much of the oil and gas interest in Arctic National Wildlife Refuge centers on the rock strata that are associated with the established oil fields on the North Slope. However, geologists say there are some intriguing possibilities for finding hydrocarbon accumulations in some of the older rocks that lie beneath ANWR.
At a recent meeting of the Alaska Geological Society Jim Clough, energy section chief of the Alaska Division of Geological and Geophysical Surveys, described some massively thick, ancient limestones and dolomites that outcrop in the Sadlerochit and Shublik Mountains just south of the coastal plain. These so-called carbonate rocks range in age from about 345 million years to 780 million years, predating the Prudhoe Bay reservoir rocks by upwards of 100 million years. Despite the great age of the rock formations exposed in the mountains, there's a possibility these same rock strata form oil and gas reservoirs deep under the coastal area.
“There's a lot of interesting things going on in these ... rocks,” Clough said, “and a number of companies are getting very interested in this.” An ancient sea Clough described how close examination and mapping of the rock outcrops indicates the existence hundreds of millions of years ago of a shallow marine ramp, sloping gently towards the southeast. The stratification of the rocks provides evidence of several periods when the sea level rose and fell, sometimes exposing the relatively newly formed rocks as land with lakes and ponds, and sometimes filling the area with intertidal flats. Some of the marine deposits contain fossils of early life forms.
The oil reservoir potential of the rocks comes in part from the erosion that occurred when limestones and dolomites emerged from the sea during ancient times. Geologists working in the Sadlerochit and Shublik mountains have found within the strata widespread evidence of features typical of modern karst limestone terrain, in which water leaches holes and caves in the ground. Take these erosion features, add in rock fracturing and you finish up with the fluid-holding fissures and holes of a potential oil reservoir. In addition, the granular nature of some of the rocks lends itself to holding hydrocarbons. Do the rocks exist to the north? But, do these same rock formations exist under the land to north and in positions where they could suck in oil and gas from hydrocarbon source rocks?
With at least 2,500 meters of rock in the mountains, it's difficult to imagine that the rock formations would fizzle out in just a few miles, Clough thinks. However, a major fault runs along the north side of the Brooks Range and no one really knows how much the rocks have been shifted, moved and eroded across that fault.
There's good reason to believe that the rocks in the area have moved substantially. For example, the older rocks in the area show more affinity with rocks in Siberia than with rocks elsewhere in North America. Also, people have found some evidence that the plate of the earth's crust that contains the present North Slope has rotated counterclockwise at some time.
So, although there's a good chance that the limestones and dolomites of the Sadlerochit and Shublik Mountains extend northwards under the coastal plain, extrapolating the geology in this way becomes a speculative exercise, hindered by a lack of well information in the ANWR area.
Fuzzy seismic The fuzzy nature of the deeper seismic plots for ANWR compounds the problems of interpreting the older geology under the coastal plain — seismic surveys have tended to focus on the younger rock strata.
“They're really looking at the shallower rocks,” Clough told Petroleum News Alaska.
According to Ken Bird, project chief of the U.S. Geological Survey petroleum studies in Alaska, the seismic shows younger, potential hydrocarbon source rocks under the coastal plain sitting directly on an ancient landform. That landform may well contain the types of rock observed in the mountains to the south.
“In an ideal situation you'd have rich source rocks sitting directly on carbonate rocks,” Bird said, “and if these carbonate rocks have been exposed for some period of time you can develop all kinds of reservoir characteristics through leaching and karst formation.” Under pressure, oil and gas will migrate into cavities in the carbonate rocks, Bird said.
Bird likens the situation to some oil fields in China where oil has migrated into ancient hills buried by younger source rocks.
Oil and gas plays The U.S. Geological Survey has identified two plays in which the ancient rocks provide oil and gas reservoirs. One play involves the relatively undeformed rocks in the northwest part of ANWR, while the other play occurs to the southeast, in rocks that have been deformed close to the Brooks Range. In the undeformed play, the source rocks directly overly the potential reservoir, while in the deformed play the reservoir rocks have pushed up against the source rocks.
Scientists at the USGS have estimated a fairly high probability that these plays would contain oil and gas. However, uncertainty about the extent to which the ancient limestones and dolomites exist under the coastal plain leads to a high risk factor for hydrocarbon discovery.
It all comes down to a lack of solid information. Do the carbonate rocks thin out or disappear to the north of the Sadlerochit and Shublik Mountains?
“Until someone drills some holes into it we really won't know,” Clough said.
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