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A new understanding of the Cook Inlet Analysis of seismic data by state geoscientists reveals complexity of the west side of oil and gas basin in Southcentral Alaska Alan Bailey Petroleum News
Access to seismic data and some continuing analysis of that data by geoscientists from Alaska’s Divisions of Oil and Gas and Geological and Geophysical Surveys is leading to a new understanding of the structure of the west side of Alaska’s Cook Inlet basin, DGGS geologist Robert Gillis told the American Association of Petroleum Geologists Pacific Region annual conference on May 23. An understanding of the structure of the basin is of great importance in figuring out potential targets for developing oil and gas in the prolific petroleum province of the basin. Forearc basin The basin consists of what geologists refer to as a “forearc basin,” a regional area of subsidence that forms between an oceanic trench and a volcanic arc, in a region where one plate of the Earth’s crust is subsiding beneath another plate. In the case of the Cook Inlet basin, that subsidence began in the early Jurassic and has continued to the present day, albeit with a hiatus in the middle Cretaceous. The result is a massive thickness of tens of thousands of feet of Mesozoic and Cenozoic strata in the basin.
The Jurassic strata within the older Mesozoic section include rocks of the Tuxedni group, the main source of oil in the Cook Inlet oil fields. Sands in the younger and shallower Cenozoic section form the reservoirs for the producing oil and gas fields. Some of the natural gas in the fields has come from the Tuxedni in association with the oil, but much of the gas has come from Cenozoic coals, resulting from microbial action in the coals.
Forces exerted on the basin by northward plate movement to the south have created stresses within the basin, including shear stresses that have created a series of north-northeast trending fold and fault structures in the Cenozoic strata. These structures have become the prime targets for oil and gas exploration and development - an examination of a map of the Cook Inlet show the fields and well locations lined up along the structures.
Bounded by faults The basin is bounded by major regional faults: the Border Ranges fault on the southeastern side, on the Kenai Peninsula, and the Castle Mountain and Bruin Bay faults on the northwestern side: the Bruin Bay fault runs northeast from the Alaska Peninsula, while the Castle Mountain fault cuts across the northern edge of the basin, separating the Cook Inlet basin from the Susitna basin to the north.
Gillis explained that current understanding of the structure of the basin derives primarily from U.S. Geological Survey mapping carried out in the 1960s and 1970s. And although the oil industry has subsequently obtained much information about the basin, little of this information is publicly available.
Access to seismic However, since 2006 DOG and DGGS have been conducting some new research into the geology of the basin. Although that research has tended to focus on the basin’s stratigraphy, attention has also moved to the basin’s structure. In particular, recent access to some 2-D seismic data for the west side of the basin has enabled a new evaluation of the structure of that part of the basin, Gillis explained.
The seismic data prove particularly valuable on that west side, because much of the bedrock is obscured by recent sediments, thus making surface geologic mapping very challenging.
The conventional view of the structure of the west side of the basin is that the Bruin Bay fault runs onshore quite close to the west coast of the inlet but bends northward in the area of Trading Bay to splay into or be cut off by the Castle Mountain fault at a point somewhere to the northwest of the Beluga River gas field. Thus, the entire northwestern margin of the basin would be characterized, in effect, as a single, continuous system of major faults.
But the state’s analysis of the newly available seismic data has revealed that traditional interpretation to be incorrect, with the actual structure of the basin being much more complicated than previously thought.
Structure revealed Gillis said that an analysis of the seismic has revealed that, in fact, the Bruin Bay fault does not bend to the north at Beluga but instead fizzles out along its continuing more northeasterly trend. The area through which the Bruin Bay fault was assumed to track more northward to meet the Castle Mountain fault is in fact characterized by a swarm of discontinuous steeply dipping faults that trend in a north-northeasterly direction. In the Cenozoic, the Bruin Bay fault has less regional importance than previously thought, Gillis commented
Curiously, seismic imaging of the Bruin Bay fault shows the fault cutting through Cenozoic strata, with the West Foreland formation, one of the older formations in the Cenozoic, being thousands of feet thicker on one side of the fault than on the other. That appears to indicate a relationship between deposition of the West Foreland and movement on the fault at the time of deposition. The younger and shallower Hemlock formation, on the other hand, has the same thickness on either side of the fault, Gillis said.
Apparently field mapping done in conjunction with the state’s Cook Inlet research near Capps Glacier, on the northwest basin margin, has also revealed a relationship between the deposition of Cenozoic sediments and structural movement in the area of the basin margin.
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