THE GEOLOGICAL HISTORY of northern Alaska has resulted in four distinct rock sequences. From oldest to youngest, these sequences are known as the Franklinian, Ellesmerian, Beaufortian and Brookian. People also refer to the Franklinian as the pre-Mississippian sequence and the Beaufortian as the rift sequence.

The oldest rock sequence, the Franklinian, formed on a stable continental platform before middle Devonian time (about 400 million years ago).The sequence contains a wide range of rock types, some of which may have been laid down as sediments on
subsea slope deepening to the south.

The Franklinian sequence is often considered nonprospective “basement” due to its high thermal maturity and generally poor reservoir quality. However, shows of migrated oil are common in basement penetrations along the Barrow Arch; wells in the Point Thomson area have penetrated zones of dolomites with reservoir potential; and the Point Thomson gas condensate reservoir includes Franklinian carbonates. Economic production from pools in the Franklinian remains a possibility at some point in the future.

Franklinian sequence deposition ended across most of northern Alaska with a cycle of middle to late Devonian mountain building and metamorphism.

The Ellesmerian

Ellesmerian sediments, eroded from uplifted Franklinian rocks in a landmass that lay mostly to the north of the modern Beaufort Sea coast, spread southward and accumulated in the coastal and marine settings of an ancient basin known as the Arctic Alaska basin. Deposition of these sediments on a continental margin, sloping to the south, persisted into early or middle Jurassic time.

Deposited in highly varied marine-to-nonmarine settings over at least 150 million years, Ellesmerian strata constitute a diverse suite of rock formations, including prolific petroleum source rocks, excellent reservoirs and strong seal units that collectively define a self-contained, world-class petroleum system.

The strata of the Ellesmerian sequence tend to thin to the south, under the North Slope, because of the increasing distance from the source of the sediments in the north.They also tend to thin to the north of the North Slope, in the area of the ancient Ellesmerian landmass, in part because deposition was truncated against the landmass and in part because later uplift caused erosion of any sediments that had earlier been deposited.

The Beaufortian sequence

The Beaufortian sequence dates from between early to middle Jurassic and early Cretaceous and resulted from sediment deposition during major rifting or pulling apart of the earth’s crust. People have proposed several hypotheses for this rifting. However, most geologists interpret the rifting as a result of the opening up of the Canada basin of the Arctic Ocean by a counterclockwise rotational movement of the North Slope Ellesmerian landmass away from equivalent platform rocks in Arctic Canada.

The east-west trending structural high known as the Barrow arch developed along the present Beaufort Sea coast. According to the most widely accepted Beaufortian rift model the arch formed in multiple uplift phases.The northern flank of the arch slopes steeply in a system of faults toward the Canada basin of the Arctic Ocean.The southern flank slopes very gently.

Widespread surface erosion along the Barrow arch probably occurred several times but culminated during the early Cretaceous to form an unconformity of regional east-west extent. This lower Cretaceous unconformity forms an important hydrocarbon migration and accumulation element for many of the oil fields on the North Slope, including the Prudhoe Bay field.

Most of the Beaufortian sediments eroding from the rising Barrow arch likely drained off the gentle southern flank of the arch, where they later became buried deep beneath younger sediments of the Brookian sequence. Other erosion products from the Barrow arch no doubt drained into the depths of fault-dropped blocks on the north side of the arch. Beaufortian sedi- ments also accumulated in a variety of mostly shallow marine settings on the uplifted margin of the Barrow arch.These sediments formed important sandstone reservoirs in subtle low points on the arch or perched on rift-related fault blocks stepping off the arch to the north. Key examples include the Lower Cretaceous Kuparuk formation sandstones of the Kuparuk River and Point McIntyre fields and the Upper Jurassic Kingak formation sandstones of the Alpine field.

The Brookian

Also in late Jurassic and early Cretaceous time the Brooks Range started to form, sending thick sheets of thrust-faulted rock to the north.These thrust sheets loaded and depressed the earth’s crust and caused a deep depression called the Colville basin to start to sink along the northern side of the range, between the range and the Barrow arch.

Sediments eroded from the Brooks Range thrust sheets poured into the Colville basin, progressively filling the basin from southwest to northeast and forming the Brookian sequence. Brookian sediments also spread out over the Barrow arch and onto Alaska’s continental margin during Cretaceous-through-Tertiary time.

In very general terms, the older, lower Brookian sequence sediments tend to consist of shales and sandstones deposited in water hundreds or thousands of feet deep.The rocks higher in the sequence typically consist of sandstones and shales associated with coastal plains, river deltas or other shallow-water environments.

While sediments filled the Colville basin, the area of active sedimentation moved eastward. As a result, the Brookian rocks tend to become younger from west to east in the basin.

Nowadays Quaternary sediments cover the older bedrock along the North Slope. Most Quaternary deposits consist of unconsolidated sand and gravel, containing re-worked Brookian sediments along with materials from the present-day Brooks Range. Overlying these deposits are river-deposited silts and sandy silts that include variable amounts of organic matter. In addition to river deposits, windblown sands within the Quaternary sequence mark cold, dry Ice Age conditions.

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