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Providing coverage of Alaska and northern Canada's oil and gas industry
June 2014

Vol. 19, No. 22 Week of June 01, 2014

An underexplored North Slope play

Sands deposited in deep marine environments in the Brookian sequence can be challenging to explore but offer intriguing potential

Alan Bailey

Petroleum News

Although often viewed as a mature oil province, Alaska’s North Slope has tended to see exploration and development focused on some specific hydrocarbon plays in its central and northern part. But what opportunities remain in some of the more sparsely tested exploration possibilities in the region?

On May 15, during the 2014 Alaska Geological Society Technical Conference, Marwan Wartes, a geologist with Alaska’s Division of Geological and Geophysical Services, or DGGS, talked about one of these possibilities, the deepwater sands of the Brookian sequence, the sequence of strata that constitutes the youngest of the four major rock sequences associated with oil and gas in Arctic Alaska. Wartes characterized the geologic basin in which the Brookian sequence is deposited as one of the most promising onshore exploration regions remaining in North America. It is a challenging and expensive place in which to work, but there is a lot of exploration opportunity, he said.

Scientists from DGGS and from Alaska’s Division of Oil and Gas have for several years been working with geologists from the U.S. Geological Survey and academia, researching the Brookian rocks, mapping surface outcrops around the southern and eastern sides of the North Slope and tying surface observations into what is known about the subsurface geology from seismic and well data. The idea is to provide public-domain insights into the geology, to assist and encourage oil and gas exploration.

Major basin

Onshore Arctic Alaska the Brookian rocks, ranging in ages from early Cretaceous through into the Tertiary, were deposited in a massive geologic basin that extends west-to-east between the northern front of the Brooks Range and the Beaufort Sea coast. Vast volumes of sediment, including sand and mud, were deposited into this basin over many millions of years in what would have been an ancient sea.

It is generally understood that the basin filled progressively from southwest to northeast, with the older rocks of the Brookian being found to the west, near the Chukchi Sea coast, and the younger rocks to the east and northeast. As the basin filled, sediment spilled downwards from the west into the marine basin, spreading down the side of the basin and out onto the deepwater basin floor. Sands deposited into that more deepwater environment appear to be of particular exploration interest, Wartes said. These deepwater sands have formed reservoir rocks in several North Slope oil fields, he said.

Difficult geology

But interpreting the complex Brookian geology is difficult, given the ways in which the ancient geography led to the formation of rocks of different types in different parts of the basin at equivalent times, and of rocks of similar types at different times. The ages of the rocks can be challenging to pin down; the subsurface rock strata can be difficult to image from seismic data; and the Brookian reservoirs can be heterogeneous, having rock of different types in juxtaposition with each other.

But that difficult seismic data can yield spectacular images. In one case, a cross-section of the subsurface depicts massive “clinoforms,” strata that are characteristic of sedimentation on the sloping side of a marine basin. These particular clinoforms are among the largest known on Earth, Wartes said.

Four intervals of interest

Wartes said that the DGGS-led team thinks that there are four Brookian rock intervals that are especially prospective for oil and gas: sands within what is referred to as the Cenomanian, the earliest stage of the late Cretaceous; Cretaceous sands from the Turonian, the stage that follows the Cenomanian; sands swept into the deepwater during an interval of coastal erosion in the mid-Campanian, a later Cretaceous stage; and a similar episode of sand deposition in the Paleocene, in the early Tertiary, following the end of the Cretaceous.

In the case of the Cenomanian-age rocks, the sands that flowed into the depths of the ancient marine basin are now found as thick, sometimes coarse-grained sandstone benches, with hydrocarbon reservoir potential, overlain by shale that could both seal a reservoir and act as a hydrocarbon source.

Sands of Turonian age, equivalent to the reservoir rocks of the Tarn and Meltwater fields in the central North Slope, can form individual sand bodies, encased in shales. The strata inter-tongue with the Hue shale, one of the major oil source rocks of Arctic Alaska. The Turonian rocks are characterized by a distinctive oily smell when cracked open, Wartes said.

Eroded sand

The mid-Campanian is marked by a major geologic unconformity, a depositional break, with evidence for the erosion of deep valleys along what would have been the margin of the marine basin. This erosion in some parts of the basin appears to have resulted in sand deposition within the deeper basin, Wartes said.

The Paleocene is also marked by a major unconformity, with evidence of the uplift of the Brooks Range and the central and western North Slope, Wartes said. While uplift resulted in the erosion and removal of some of the older rocks of the basin, the resulting detritus would have flowed onto the floor of the marine basin to the east and north, giving rise to basin-floor sands equivalent to the sands that form the reservoir of the Badami field, near the Beaufort Sea coast to the east of the central North Slope.

Emphasizing again the petroleum potential of these various sand-prone Brookian rock units, Wartes said that the work conducted by the DGGS-led team has provided a stratigraphic framework with which to make testable predictions of where to find the sands in the subsurface, justifying more attention to this underexplored play.





Clarifying the history of the Brookian

Geologists from the U.S. Geological Survey have been assessing the origins and ages of rocks within the Brookian sequence, the youngest of the four rock sequence associated with oil and gas in Arctic Alaska, Richard Lease, a member of the USGS team, told the 2014 Alaska Geological Society Technical Conference on May 15. The geologists have been using the results of tests on grains of the mineral zircon found in the Brookian rocks. Zircon has particular value in determining the ages and origins of rocks: Being especially stable, the mineral can persist through multiple cycles of erosion and sediment deposition; and, containing small quantities of radioactive uranium, the mineral can be used for radiometric dating.

This combination of mineral stability and radiometric dating potential can provide insights into both the ages of rocks and the sources of the sediments from which the rock formed.

West-to-east fill

The USGS scientists have confirmed that during the lower Cretaceous, sediments that formed Brookian rocks filled an ancient basin to the north of the Brooks Range from west to east. Radiometric ages from zircon grains indicated that this period of basin filling lasted from 116 million years ago to 106 million years ago and filled the basin eastward at a rate of about 50 kilometers per million years.

Interestingly, other research using zircon samples indicated that the sediments filling the basin at around that time originated from an ancient mountain belt, known as the Chukotka orogenic belt, in what is now eastern Siberia.

As deposition of the Brookian sediments continued through the upper Cretaceous, the source of the sediments appears to have shifted to the emerging Brooks Range in northern Alaska, suggesting a change in the flow of sediments from an easterly direction to a more northerly direction, the scientists have found. This apparent change in direction appears consistent with a period of erosion of the Brooks Range that started some 95 million years ago.

Other analysis of zircon samples is providing insights into the rates at which the Brooks Range has been emerging as a mountain range. It appears that the range emerged quite rapidly in the early Cretaceous, before going into a period of quiescence later in the Cretaceous and then rejuvenating to a certain extent more recently.

—Alan Bailey


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