Questions over how oil formed and then became trapped in Alaska’s Cook Inlet basin are of crucial importance to people considering sinking million of dollars into the drilling of wildcat wells in a region that has for long played second fiddle to the North Slope oil province. Are there major oil fields yet to be discovered in the relatively underexplored Cook Inlet region? And in which rocks is the oil likely to be found?
Two sequences
The region contains two major rock sequences from the Mesozoic era and the Tertiary period, both with maximum thicknesses well in excess of 20,000 feet. The younger Tertiary strata lie above the older Mesozoic rocks.
Following early 20th century exploration of the Alaska Peninsula and the Iniskin Peninsula on the west side of Cook Inlet, where there is evidence for an active petroleum system in the Mesozoic, geol gists originally saw the Mesozoic as holding the best potential for oil discoveries in the Cook Inlet basin. However, when an exploration well targeting the Mesozoic under the northern Kenai Peninsula discovered the Swanson River field in Tertiary sandstones in 1957, attention immediately shifted to exploration of the Tertiary, with the subsequent discovery of a series of good-sized oil and gas fields.
Geologists established that the oil in the Tertiary reservoir sandstones had originated from source rocks in the middle Jurassic Tuxedni group of the Mesozoic sequence, and not from the Tertiary itself. But models of oil generation suggested that more oil should have been formed than had actually been found. That has led to speculation over whether the unaccounted oil has simply escaped to the surface, or whether it still exists in the subsurface, perhaps in as-yet undiscovered oil fields in the Tertiary, or perhaps in Mesozoic reservoir rocks that have never been tested by an exploration well.
Timing
From the outset geologists have puzzled over the question of the timing of oil migration from the Jurassic source rocks into the Tertiary reservoirs. Large north-northeast trending folds in Tertiary strata, in combination with similar trending geologic faults, trap the oil in the established oil fields. But geologists have determined that these structures have formed in relatively recent geologic time and are likely still forming. How could oil from much older rock formations have moved into these new geologic structures?
For a number of years the prevailing view of oil migration into the Cook Inlet oil fields has involved a two-stage process in which the oil first flowed from the Jurassic source into reservoir rocks in the older part of the Tertiary rock sequence. Subsequent upheavals of the rock strata would then have tipped the oil into the younger structures where the oil now resides.
Recent work by the U.S. Geological Survey as part of a new Cook Inlet oil and gas assessment that USGS is preparing, has proposed an alternative and simpler theory for oil migration into the known Cook Inlet oil fields. Using rock material recovered from several Cook Inlet exploration wells, USGS scientists have estimated the rate at which heat has flowed from deep within the Earth’s crust, upwards through the rocks of the Cook Inlet basin. The scientists have used a burial history model to reconstruct the timing with which Cook Inlet rocks have been buried or uplifted during the geologic past, Paul Lillis, a member of the USGS team conducting the Cook Inlet assessment, told the annual meeting of the Pacific Section, American Association of Petroleum Geologists, on May 11.
Recent oil generation
The USGS analysis has indicated that heat flow through the basin is relatively low and that along the southern and eastern margins of the basin the Tuxedni source rock is unlikely to have reached the temperatures necessary for peak oil generation. However, in the deep, more central part of the basin the Tuxedni would have been heated sufficiently for oil generation 15 million years ago, relatively late in the Tertiary period, with the rocks nearing or reaching peak oil generation at the present time. With oil forming in the Tuxedni fairly recently, the oil could have flowed directly into the Tertiary oil traps, rather than having to migrate in two stages, as previously thought, Lillis said.
But what of oil in the Mesozoic, the target of exploration before the discovery of the Swanson River field?
Geologists have for long speculated about the possibility of some oil from the Tuxedni becoming trapped in Mesozoic reservoir rocks, with Cretaceous-age sandstones appearing to have the highest reservoir potential. However, geologist have also for long worried about degradation of Mesozoic reservoir rock quality — volcanoes adjacent the basin during the Mesozoic caused ash and other volcanic material to periodically spew into the sediments in the basin. This volcanic material would have subsequently decomposed, potentially forming pore-clogging minerals. And exploration wells drilled in the lower Cook Inlet have indeed found these pore-clogging minerals in Cretaceous sandstones.
However, Robert Blodgett, an expert on Cook Inlet and Alaska Peninsula Mesozoic rock stratigraphy, feels optimistic about the reservoir quality of at least some Mesozoic rocks. Widespread clogging of rock pores by secondary minerals does not happen in all Mesozoic rock formations, with the lower Jurassic Herendeen formation, for example, being relatively clean, Blodgett said. There are a lot of good rocks down there, he said.
Jurassic oil sands
Blodgett pointed out that well Soldotna Creek Unit 22-32 in the Swanson River field had actually penetrated oil sands of Jurassic age and that a well in the Trading Bay unit, on the west side of the Cook Inlet, has produced oil from the Jurassic.
Blodgett is also something of a champion for the Kamishak formation, a potentially prolific oil source rock, Triassic in age, equivalent to the Shublik formation, the premier oil source for the North Slope oil fields. The Kamishak exists in considerable thickness along the northeastern portion of the Alaska Peninsula but, unfortunately, no Cook Inlet wells have ever drilled deep enough in the Cook Inlet basin to determine whether the Kamishak lies below the Jurassic rocks of the basin — geologists have speculated whether any oil from the Kamishak has contributed to the Cook Inlet oil fields.
The USGS scientists, in their work on their Cook Inlet oil and gas assessment, have come to the conclusion that Kamishak oil is very unlikely to exist in the Cook Inlet basin. The burial model that the scientists have developed indicates that burial of the Kamishak in the basin would have pushed the formation through the temperature window for oil generation before the late Cretaceous, thus causing oil to form before the existence of the reservoir rocks in which the oil might have become trapped, Lillis told the AAPG conference. In addition, USGS says that analyses of the composition of oils from Cook Inlet oil fields demonstrate origination in the Tuxedni, with no evidence for the existence of oil originating from the Kamishak.
Model questioned
However, Blodgett questions whether the distinctions between Tuxedni and Kamishak oils are as clear cut as USGS thinks.
And he also questions the USGS burial model, saying that on the west side of the Cook Inlet basin several wells have penetrated lower Jurassic strata at depths shallow enough to likely place the Kamishak in the oil generation window, if the Kamishak lies below the Jurassic in that region. On the other hand, with the Mesozoic being buried much deeper on the Kenai side of the basin, the USGS pessimism over the Kamishak is justified there, he said.
The bottom line on all of this seems to be that, with relatively few wells drilled outside or below known oil fields and with a general lack of high-resolution seismic data, huge unknowns and uncertainties remain regarding Cook Inlet petroleum geology. Perhaps the time has come for someone to take a chance and drill into some of the untested oil plays. The state has offered a $25 million tax credit to the first company to drill into the Cook Inlet Mesozoic from an offshore jack-up rig. And perhaps an undiscovered oil field lurks in an unexplored Tertiary prospect.
With top-notch oil source rocks in the region and an abundance of rocks that can hold and trap hydrocarbons, the stakes are high.
