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= Chukchi Sea Hydrocarbons =

File:Chukchi fields.jpg

Petroleum Fields in the Chukchi Sea

History

The hydrocarbon potential of the Arctic is well known and demonstrated by the reserves of Russia, Canada, and the United States who constitute 3 of the 5 top oil producers as well as Norway which ranks at 15. The Reserves of the United States is buoyed by the Prudhoe Bay field which bolstered reserves by 25 billion barrels, approximately 40%. The same hope exists for new discoveries in the arctic seas. Exploration began in water much later than arctic land for good reasons. The first was that oil seeps existed in places like Prudhoe Bay allowing their easy identification without sophisticated instruments. the second reason which led to the delayed exploration of offshore arctic is the physical environment. The sea ice is present for most of the year. Work can only progress three months before winter preparations must begin again. When many well plans may be as long as 60-90 days, this is a great complication. In the late 1980's and early 1990's several companies bought leases in the Chukchi Sea. To date, 7 test wells have been drilled, and none have found the massive pay that is likely in the area. The common counterexample used is that it took 9 test wells to find the largest field at Prudhoe Bay.

Introduction

The Chukchi Sea is home to vast amounts of hydrocarbon reserves. There has been concentrated interest around the northwestern coast of Alaska since the early 1990’s. Estimates of reserves vary and despite the massive investments, no company to date is producing from any wells in the Chukchi Sea. The area of chief concern is the Burger block area. Royal Dutch Shell has spent billions of dollars in this area trying to pinpoint the massive reserves of the arctic.  Many greatly anticipated the results of Shells most recent test wells. But in 2015, Shell abandoned its 7 billion dollar investment primarily due to the low cost of oil. Additional factors were that the test wells within the burger field indicated lower amounts of oil than expected. This article will cover the geologic interest of this area and include the geological setting, sediment source, depositional environment, the stratigraphy, and the basin evolution from rifting. Lastly, this article will focus on the future of the Chukchi and analyze what geologists’ newest thoughts about the migration of oil and gas in the area.

The Chukchi began to be explored in the 1990’s. Royal Dutch Shell has bought multiple leases and drilled into 4 sites. These are all named in line with a food theme and are: Klondike, Popcorn, Crackerjack, and Burger test wells. Chevron has a single test well called the Diamond. This area is difficult to explore and work due to the weather. There are only 3 months of warm weather with little enough sea ice to work. Therefore, it is not conducive to drill exploratory wells which may take more than 3 months. It took Shell 7 years to drill the Burger J test well. In 2015, Shell abandoned its most recent test well called the Burger J. It was drilled to 6800 ft into the burger dome structure and samples contained oil and gas, but it did not contain the great reservoir that is in the area. Shell geologists now think that there was a gas flush that pushed the oil further north, but with the price of oil low, we will have to wait years for another test well to be drilled.

Geological Setting

Stratigraphy and Tectonics

The Chukchi Sea is relatively shallow as the continental shelf extends to sea. The area of interest is the burger block area. It was formed through the sedimentation and is part of the Chukchi basin which is a rift basin. It is Mesozoic in age and the major sections that we are concerned about for reservoirs are bounded by 4 uncomformities. The stratigraphic sequence form oldest to youngest, starting with the oldest at the bottom is the Franklinian, Ellesmerian, Beaufortian, and Brookian. The Franklinian sequence is 400 million years old and is present throughout the Chukchi and North Slope Alaska. It is pre-Devonian and was also formed before the basin began rifting. The Ellesmerian sequence is the next oldest. The Ellesmerian dates from the late Devonian to the mid-Jurassic and is also predating the beginning of the basin rifting. The Ellesmerian is present in all of the Alaskan oil fields including the massive Prudhoe Bay field to the east. It is responsible for the massive reserves in the Prudhoe bay location as well as contributing to other Alaskan fields being exploited. The next layer of interest above the Ellesmerian is the Beaufortian. The Beaufortian was deposited from the Jurassic to the Cretaceous. It also marks the start of the basin rifting. Large amount of tectonic activity caused rifting and these large rifts caused numerous half-grabens. These large fault blocks were then filled with reservoir quality sediment washed from above. Rifting continued on a large scale and created the Canada Basin of the Arctic. Above the Beaufortian is the Brookian. The Brookian formed in the Cretaceous and Tertiary time periods. The Brooks Range being created increase the amount of sedimentation during these rifting events. The rifting occurred between the Devonian and the Jurassic. The elevation and erosion increased the sedimentation into what is now the Chukchi Sea. There is a large failed rift called the Hanna Trough that cuts from south to the north through the Chukchi Sea. The trough formed after the Ellesmerian strata was deposited and contains 38,000 ft of Ellesmerian rock. Seismic has been mapped of the area and reveled the base of the crust in an attempt to garner more information of the thermal history of this part of the basin.  

Source Rock and Reservoir Rock

The Ellesmerian is the largest reservoir currently targeted. It is responsible for the billions of barrels in Prudhoe Bay. Several hundred feet of high quality reservoir sand create potential for oil. The Moho has been mapped to ascertain the hydrocarbon window and thermal history of the region. This can be used to follow the maturation of kerogen to gas and then follow the gas migration to see where it may have flushed the oil. The source of the hydrocarbons tend to be the shale layers dispersed throughout the sequences including the Kavik, Ivishak, and Kingak shale layers.

Trap, Seal, and Migration

The amount of rifting and development of the basin led to the area having a massive amount of folds and stratigraphic wedge traps. The Burger structure is a 25 mile dome that is on the east side of the Hanna Trough where Shell hopped oil would be present. Half-grabens contribute to wedge style structural traps and anticlines are present as structural traps. There are stratigraphic pinch outs that exist but the massive structures are being pursued rather than smaller isolated picnhouts.

Future

David Houseknecht is the chief for the U.S. Geological Survey's Energy Resources Program for Alaska. He proposes that oil is still in the area, it has simply been itinerant due to gas being flushed from the mainland to the north and further offshore. The drilling sites all indicated that oil was present and Crackerjack flowed oil from the Triassic and had large pay areas. Shell is keeping quite about the highly anticipated Burger J test well in the Burger block area. According the Shell, the J well yielded disappointing results. Many suspect that a gas flush has taken place and pushed the oil north. This may explain why oil exists to the west of the Hanna Trough but not at burger which lay to the east of the trough. The theory is that gas originating in the fold and thrust belt in the Brooks Range. There is evidence that this gas flush has happened to other locations further east toward Prudhoe Bay. It is conceivable that the active tectonics would mature kerogen and allow its migration. however, acceptance of this gas flush idea could also mean that the oil was flushed from the entire area long ago. Even then, it is probable to have some remaining due to the stratigraphic wedges, but it may not be the multi billion barrel reservoir anticipated.