While much oil and gas interest in North America has focused recently on new so-called unconventional oil and gas plays, especially involving the extraction of hydrocarbon resources directly from oil and gas shales, the Alaska oil industry has continued along a route of seeking and developing oil from conventional porous and permeable reservoir rocks.
But with Great Bear Petroleum forging ahead with plans to extract oil directly from source rocks on the North Slope, Alaska looks set to join the unconventional oil and gas bandwagon.
And the U.S. Geological Survey, the federal agency that has for many years conducted assessments of Alaska’s conventional onshore oil and gas resources, is now turning its attention to estimating how much unconventional oil and gas might be accessible in northern Alaska and in the Cook Inlet basin.
The agency hopes to complete its Cook Inlet assessment in the late spring or early summer, and run the numbers for its northern Alaska assessment sometime in the fall, USGS geologist Dave Houseknecht told Petroleum News Feb. 8.
Unlike a conventional oil and gas play, where hydrocarbons migrate into a porous source rock to become trapped under an impervious seal r
Trapped in the rockock, an unconventional play, sometimes referred to as a “continuous” play, involves a rock unit saturated with oil or gas over a broad area, with the fabric of the rock itself, rather than an overlying seal rock, trapping the hydrocarbons in place. The much publicized “fracking” techniques used in this type of play release the hydrocarbons by smashing open the rock fabric.
Estimating the producible volumes of hydrocarbon resources in this type of unconventional play involves assessing three factors: the extent and thickness of the hydrocarbon bearing rock unit; the mechanical and oil production properties of the rock; and the likely success rates for well production from the rock, Houseknecht explained.
Essentially, a geologist conducting the assessment will use the rock properties to estimate the sizes of cells from which individual wells might be able to drain hydrocarbons and will use the hydrocarbon production characteristics of the rock to estimate ultimate production volumes for the wells.
The geologist will then statistically combine possible ranges of cell sizes and likely production volume ranges, together with ranges in the estimated extent of the complete rock unit, to derive a range of potential, extractable hydrocarbon in the play as a whole.
USGS conducted an assessment of North Slope coalbed methane resources in 2006. And, although there are likely to be substantial unconventional North Slope resources in impermeable, “tight” sands, USGS needs access to appropriate 3-D seismic data to delineate the tight sand bodies, Houseknecht said. There are probably resources in tight sands, even within existing North Slope productions units, but individual sand bodies are probably limited in extent, he said.
Focus on source rockSo, the agency is focusing on potential oil and gas production direct from source rocks, starting with the Cretaceous Gamma Ray Zone and Hue shale, and the Triassic Shublik formation, two prominent source rock intervals that have generated much oil in the North Slope oil fields, Houseknecht said. Could the source rocks in these intervals be tapped directly for oil production, using hydraulic fracking?
Existing seismic data tied into data from existing wells give geologists a good handle on the geographic extent and thicknesses of the source rock units. But, given the total lack of any track record of unconventional oil and gas production on the North Slope, estimating the rock’s production characteristics, the parameters needed to estimate the well drainage cell sizes and well productivity, is one of the biggest challenge in conducting an unconventional resource assessment in northern Alaska, Houseknecht said.
Essentially, the geologists have to find analogous rocks from the Lower 48, rocks that have been used for unconventional production and that appear to have somewhat similar characteristics to those on the North Slope, in order to infer the required North Slope production characteristics.
Quite a bit is, however, already known about one key rock property: the distribution of thermal maturity, the measure of the extent to which the rock has been heated to generate oil or natural gas. In general, for example, the thermal maturity is relatively low on the crest of the Barrow arch, a major geologic structure along the Beaufort Sea coast, but increases to the south.
Gamma ray responseFor the Cretaceous sources USGS is using the gamma ray response, a hydrocarbon content indicator, from existing well logs to infer hydrocarbon-rich rock depths and thicknesses at different locations, Houseknecht said.
And the good news is that the thickest high-gamma-ray concentrations occur along a trend of thermal maturity appropriate for oil generation, he said.
However, a prevalence of carbonate minerals has distorted the gamma ray responses in the Shublik, causing the USGS geologists to resort to a more complex procedure to locate the likely sweet spots in the Shublik source — using data from the Phoenix well, offshore north of the Colville River Delta, USGS is correlating the thickness of the likely prime hydrocarbon-bearing zone of the Shublik across multiple wells.
And there is evidence from existing well penetrations on the North Slope that the Shublik is fractured and is brittle enough for fracture stimulation, Houseknecht said.
There’s encouragement but quite a bit of uncertainty, he said.
Cook Inlet basinIn the Cook Inlet basin, USGS has already committed to an assessment of conventional resources but is now also assessing coalbed methane resources and potential gas production from impermeable or “tight” gas sands.
Direct gas production from source rocks, in particular from rocks in the Jurassic Tuxedni group, the main oil source for the Cook Inlet oil fields, is also a possibility, although the paucity of well penetrations into this deeply buried rock unit make it impossible at present to make a quantitative assessment of this play, Houseknecht said.