|
DGGS publishes Wainwright well data Rock core includes sandstones with regional reservoir potential and multiple coal seams in a complex river delta sequence ALAN BAILEY Petroleum News
Scientists from Alaska’s Division of Geological and Geophysical Services have published the results of a detailed analysis of rock core from the Wainwright No. 1 well, a coalbed methane test well drilled by the U.S. Geological Survey in conjunction with the Bureau of Land Management in 2007. The purpose of the drilling program, which included the drilling of two delineation wells in 2009, was to evaluate the possibility of establishing a supply of natural gas for the Chukchi Sea coastal village of Wainwright.
To date the project has not resulted in coalbed methane development at Wainwright. But the USGS made available to the DGGS the rock core obtained from the No. 1 well. Hence the recently published DGGS report.
While the rock samples shed light on the coal resources in the rock sequence, the use of seismic data to make tentative correlations with rocks encountered by some other wells in the region suggest that thick sandstones with oil and gas reservoir potential extend over long distances, the report says.
Nanushuk formation The report says that the Wainwright No. 1 well was drilled into the upper part of the Nanushuk formation, with rock core collected continuously from depths ranging from 75 to 1,605 feet. The Nanushuk, lower to upper Cretaceous in age, is an important oil reservoir rock in the Beaufort Sea coastal region of the North Slope, associated for example with a new major oil find that Armstrong Oil & Gas and Repsol E & P USA are pursuing in the Colville Delta region. The Nanushuk strata at Wainwright, while many miles from the nearest area of oil exploration, shed light on the depositional environment of this important regional rock unit and on the reservoir potential of sand bodies within the formation.
“The Wainwright core provides an excellent view of coal bearing rocks and potential oil and gas reservoirs in the Nanushuk formation to the west of producing fields on the North Slope, and suggests additional exploration potential in these rocks in the central North Slope,” said DGGS Director Steve Masterman.
River delta complexes The report says that, while the lower part of the Nanushuk was predominantly deposited in a marine environment, the upper part of the formation typically exhibits characteristics of land-based deposition, in a river delta situation rather like the modern day Mississippi delta. Overall, the Nanushuk on the North Slope appears to have been deposited from three large river delta complexes, the report says. The ancient rivers in the west of the region would have originated from a large drainage basin to the west of present-day Arctic Alaska while, farther east, the rivers would have flowed north from the emerging Brooks Range.
The coal seams that the Wainwright drilling targeted would have formed from lush vegetation on land. However, the bottom 1,300 feet of the well reached strata with indications of a marine influence, while above a depth of 265 feet the nature of the sediments suggests deposition in an alluvial plain, the report says.
Reservoir potential Thick channel deposits, observed at depths between 1,060 feet and 1,157 feet, and between 619 feet and 667 feet, provide attractive analogs for oil and gas reservoirs to the west of the central North Slope the report says. These sand channel complexes, encased in mudstone, appear to have a regional extent and represent attractive reservoir targets. It is possible to infer that similar stacked reservoirs could be present in the Nanushuk in the subsurface of the central North Slope, an inference that is consistent with the findings from rock core obtained from exploration wells in the central and eastern National Petroleum Reserve-Alaska, the report says.
The reservoir quality of sandstones penetrated by the Wainwright No. 1 well ranges from very poor to excellent, depending on the grain size of the sand and the extent to which the rock fabric is cemented by carbonate minerals. The report suggests that the maximum burial depth of the rocks has a critical influence on the reservoir quality - while at the present day some of the rocks in the region may lie at relatively shallow depths, the rocks would have previously been more deeply buried, with rocks higher in the stratigraphic sequence then being eroded off at some time in the past. In general, that erosion appears to have increased from north to south across the North Slope, an important factor in evaluating the Nanushuk’s reservoir potential at different locations.
At Wainwright geologists have estimated that some 2,700 feet of overburden, above the existing Nanushuk strata, have been eroded, suggesting a shallow to moderate maximum burial depth, ranging from 4,381 to 5,701 feet. By comparison, elsewhere at distances about 100 miles south of the Beaufort Sea coastal area, the rocks may have once been buried to depths approaching 10,000 feet the report says. In general, the report says, there is a degradation of reservoir quality in the Nanushuk north to south in the region.
|
