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Looking for subtle oil traps in NPR-A DOI scientists say 3-D seismic is essential to finding potential oil reservoirs in some of the younger rocks in the reserve Alan Bailey Petroleum News
A new U.S. Geological Survey assessment of oil and gas resources in the National Petroleum Reserve-Alaska, released in October 2010, significantly downsized the agency’s estimates of undiscovered oil in the reserve. However, much of the USGS new pessimism over potential NPR-A oil resources revolves mainly around a revised evaluation of the petroleum system in the Beaufortian sequence, one of three major oil-bearing rock sequences in northern Alaska.
And while the agency revised downwards its oil estimates for the Brookian, the youngest and shallowest of the rock sequences, the agency also commented that the greatest potential for finding new NPR-A oil exists in what are termed “stratigraphic traps” in that Brookian sequence.
A stratigraphic trap is a configuration of rock strata in which a relatively porous oil reservoir rock is juxtaposed against an impervious oil seal rock as a consequence of the sequence in which the various rock strata were formed, rather than as a result of the way in which the strata have been deformed or faulted.
Brookian traps During the meeting of the Pacific Section, American Association of Petroleum Geologists, in Anchorage, Alaska, in May scientists from the U.S. Department of the Interior said that it is possible to find huge Brookian stratigraphic traps in northeast NPR-A, in locations ideally positioned to capture oil from prolific oil source rocks at the base of the Brookian. However, explorers need to obtain high-resolution 3-D seismic data to adequately image the potential Brookian oil reservoirs, the scientists said.
During the lower Cretaceous period, subsidence of a huge basin under what is now NPR-A resulted in a massive inflow of sediment in a west to east direction, to form a series of relatively fine-grained sand bodies interspersed with relatively impervious sediments in the Brookian sequence. The rock sequence includes what geologists term “turbidites,” in which submarine flows of detritus have resulted in channels and lobes of sand embedded in an ancient ocean floor.
A variety of sophisticated seismic analysis techniques using high resolution seismic data can help locate and delineate these sand bodies.
Fine-grained The Brookian sands tend to be relatively fine-grained, resulting in less than ideal oil reservoir qualities and probably requiring fracture stimulation for viable oil production, the scientists said. And the presence of small rock fragments in the sands may lead to some degradation of reservoir quality with increasing burial depth, they said. However, the huge size of the sand bodies may compensate for the reservoir quality challenges, leading to potential oil industry drilling targets — some sand bodies extend over areas in excess of 30,000 acres.
The most promising situation for finding commercial oil pools is in the more shallowly buried sands, at depths of less than 7,000 feet, where reservoir quality is likely to be best and where trap locations relative to oil source rocks are optimum, the scientists said.
“These untested prospects potentially rival or exceed the size of the Alpine field discovery 15 years ago,” they said.
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