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Iniskin exploration: what are the odds? The peninsula on the west side of Alaska’s Cook Inlet has oil potential but has thus far eluded a significant hydrocarbon discovery
By Alan Bailey Petroleum News
A clearly observable large anticline in sedimentary strata, coupled with the existence of several surface oil seeps, will excite any worthwhile oil explorer. And, with both of those oil-suggesting features known about for many years, the Iniskin Peninsula on the west side of Alaska’s Cook Inlet, almost opposite the town of Homer, has long intrigued those interested in finding new sources of oil and gas.
New proposals for state exploration licenses encompassing the peninsula indicate renewed interest in the area. The state’s Division of Oil and Gas has received two competing proposals, has decided that exploration of the peninsula would be in the state’s interests and has invited competitive bids for a license. The state has not divulged the identities of those who have made the proposals. In the summer of 2013 Hilcorp Alaska conducted a mostly onshore 2-D seismic survey in the area.
But what are the odds of finding commercial quantities of oil or gas?
Early attention The peninsula and its oil seeps attracted companies during early Alaska oil exploration. The Alaska Petroleum Co. drilled a couple of wells near Oil Bay on the southern end of the peninsula in 1902 and 1903. The first of these wells reached a total depth of 305 meters, encountering natural gas below 58 meters and a “considerable” oil flow at a depth of 213 meters, according to unofficial reports. A water influx cut off the oil flow. The second well encountered oil shows but had to be abandoned at a depth of 137 meters. The company subsequently drilled two further wells in the same general area, with one of these wells encountering some oil and gas.
Also in 1902 and 1903, the Alaska Oil Co. tried drilling a couple of wells near an oil seep north of Dry Bay, on the southeast side of the peninsula. These wells were abandoned at shallow depths without encountering oil.
The major anticline that straddles the peninsula is called the Fitz Creek anticline. Between 1936 and 1939 the Iniskin Bay Association drilled a well to a depth of 2.7 kilometers into the anticline, encountering oil and gas shows. During the 1950s other investors drilled two further wells: the Beal No. 1 and the Antonio Zappa No. 1, with both of these wells encountering oil shows and the Beal well flowing gas at 4,000 cubic feet per day.
No further wells have been drilled on the peninsula since that time.
Different geology Although the peninsula lies directly opposite gas fields on the Kenai Peninsula, on the east side of the Cook Inlet, the geology at Iniskin is quite distinct from that of the producing oil and gas fields of the Cook Inlet basin. Essentially, while the producing fields have reservoirs in a sequence of Tertiary strata, the rocks around the Iniskin Peninsula are from an older rock sequence, Mesozoic in age, that lie underneath the Tertiary rocks in the region of the developed fields. And, while the sediments that formed the Tertiary sequence were laid down on land from a system of rivers, the Mesozoic sediments are predominantly marine, laid down in an ancient sea.
But the source of oil in the Cook Inlet oil fields is known to lie within the Tuxedni group, a sequence of rocks of Jurassic age, within the Mesozoic. Rocks from the Tuxedni group are observed at the surface on the Iniskin Peninsula. And with younger Jurassic rocks, including large thicknesses of sandstones, overlying the Tuxedni on the peninsula, a known oil and gas source that would account for the observed oil seeps lies in proximity to other rocks that could perhaps host oil pools.
DGGS research A team of geoscientists led by Alaska’s Division of Geological and Geophysical Surveys has been conducting research into the geology of the Cook Inlet basin and recently reported the results of some geological mapping on the Iniskin Peninsula. Among its results the team described a 6,000-foot section of sandstone and siltstone of the upper Jurassic Chinitna and Naknek formations.
Although geologists view the Naknek as a possible oil reservoir rock in the Cook Inlet region, there is considerable regional variation in the composition of the formation, with the formation having quite low porosity and permeability in the area of the Iniskin Peninsula. But, while the Mesozoic rocks of the peninsula may not in themselves be particularly ideal as oil reservoirs, there are faults and fractures in the rocks that could provide conduits within which hydrocarbons could flow.
Oil-stained rocks One of the discoveries made by the DGGS team was an exposure of oil-stained rocks within the Tuxedni group, on the southeast side of the Fitz Creek anticline. The oil staining occurs around a fault, exposed at the surface, supporting the concept that fractures and faults control the flow of oil in the subsurface, and suggesting the possibility of unconventional oil reservoirs in the Mesozoic of the peninsula area, the team reported.
Research conducted by the U.S. Geological Survey in the 1960s suggested that known oil seeps, and oil shows encountered in wells, are controlled by rock fractures and fault zones, the team reported.
However, in another new discovery, the team found oil-stained sandstone at the surface in the Chinitna formation. The lack of fracturing in the rocks at this location suggests that at least some of the Jurassic rocks on the Iniskin Peninsula hold enough permeability to support oil migration, thus raising further questions over the petroleum system that has led to those long-known oil seeps.
With exploration licenses in the offing, new oil exploration may shed further light on this enigmatic region.
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