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Getting the big picture of the Chukchi Basin-wide seismic survey by GX Technology raises tantalizing questions for would-be Arctic offshore explorers Alan Bailey Petroleum News
Haunting weather, sea and ice conditions, coupled with a remote offshore environment, have long deterred all but a handful of major oil companies from searching for oil and gas in the Chukchi Sea. But the sedimentary basins that underlie this challenging and largely unexplored region have the potential to become a world-class petroleum province.
As part of the lead up to a U.S. Minerals Management Service Chukchi Sea lease sale in February, GX Technology conducted a basin wide survey of the Chukchi in the fall of 2006. And on Dec. 13, 2007, Menno Dinkelman, chief geologist for GXT BasinSPAN programs, and Naresh Kumar of Growth Oil & Gas and consulting geologist for GXT, described some of the results of that survey to a joint meeting of the Alaska Geological Society and the Geophysical Society of Alaska.
Although GXT entered the Chukchi much later in 2006 than it had originally planned, the company succeeded in shooting 11 seismic lines in a grid pattern that encompassed the whole of the MMS lease sale area.
“We entered, for various reasons, the Chukchi much later in the season than we anticipated and we managed to get over 3,100 kilometers in November of 2006,” Dinkelman said.
Reconnaissance program Dinkelman said that the Chukchi survey was of a reconnaissance nature and part of GXT’s BasinSPAN seismic programs.
GXT originated its BasinSPAN program in the Gulf of Mexico to meet an industry need for an understanding of the broad context and structure of a petroleum province — BasinSPAN seismic programs enable the identification of major geologic features, Dinkelman said. In fact GXT presents customers with a complete package of processed and interpreted data and in the Chukchi survey the company has combined seismic data with gravity and magnetic data in assessing the subsurface geology of the region.
“It is an image-driven, basin-scale geological program … and the objective is to really highlight the petroleum system-based architecture and framework,” Dinkelman said. “The detailed new insights that you gain from this type of dataset have been very warmly welcomed by many companies that now make these datasets an underpinning for their regional exploration efforts.”
In addition to shooting seismic lines that extend across hundreds of miles of ocean, streamers that deploy geophones at 9,000 meters (29,500 feet) from the seismic air guns enable the production of seismic cross sections that bottom out in the Earth’s mantle below the base of the Earth’s crust. In the case of the Chukchi survey the bases of the seismic sections represent depths of about 40 kilometers (120,000 feet). Modeling of the sound velocities from the seismic data indicates that the top of the mantle ranges in depth from 30 to 40 kilometers in the region.
The techniques that GXT uses provide clearer images of the deep surface than would be obtained from a conventional survey, Dinkelman said.
The Franklinian And in the Chukchi that clarity is providing some new insights into the geology of what geologists term the Franklinian, the oldest sequence of rocks underlying northern Alaska.
“What we believe this data shows in a lot more detail, a lot more clarity, is that the older part of the Franklinian has undergone an extension and a compression, and a structural inversion, whereas certain parts of the same sequence also remain relatively undeformed,” Kumar said.
The fact that the Franklinian is fairly undeformed in some areas suggests that there is some petroleum potential in these rocks, a concept previously proposed by MMS geologists.
“The Franklinian cannot be completely ruled out,” Kumar said.
The new insights into the Franklinian geology may also shed some light on the formation of the Canada basin, the section of the Arctic Ocean to the north of Alaska. GXT is working with geologists from the Canadian geological survey to try to correlate the Franklinian sequence of the Chukchi with a similar rock sequence in the Banks Island region of northern Canada. A correlation between these rock sequences would support a theory that the Canada basin opened as a result of northern Alaska rotating in a counterclockwise direction from northern Canada.
“We are hoping that with the new data that we might be able to make those correlations a little bit more confirmed,” Kumar said.
In addition to the Franklinian, the seismic sections from the GXT Chukchi survey depict the complete range of northern Alaska stratigraphy, covering what geologists term, in order of oldest to youngest, the Ellesmerian, Beaufortian and Brookian sequences. The Prudhoe Bay, Lisburne and Endicott fields in the central North Slope are reservoired in the Ellesmerian, while the Kuparuk and Alpine fields are reservoired in the Beaufortian. The Brookian sequence reservoirs fields such as Tarn, Meltwater and Badami.
“The same stratigraphy that is recognized on the North Slope can be and has been recognized (in the Chukchi) through the drilling and from seismic data that has been previously interpreted, as well as in the seismic lines that we will show today,” Kumar said.
Five wells Only five exploration wells have ever been drilled in the Chukchi Sea, all in a drilling program between 1989 and 1991. One of those wells, the Burger well, found a major gas accumulation in Beaufortian rocks that are broadly equivalent to the reservoirs of the Kuparuk field.
All of those wells targeted rocks equivalent to the reservoir for the Prudhoe Bay field, in an unsuccessful attempt to find the type of 3 billion-plus-barrel field that would be needed for viable oil field development in the Chukchi, Kumar said.
But the new seismic confirms the presence of a very thick Ellesmerian through Brookian sedimentary sequence throughout what geologists term the Hanna Trough, a major sedimentary basin in the center of the U.S. sector of the Chukchi. And north of the structural high called the Barrow Arch that is associated with most major oil fields of the North Slope, the seismic depicts almost 10 kilometers (30,000 feet) of what appear to be predominantly Brookian strata.
So, there appear to be many exploration opportunities in the Chukchi, beyond what the drilling program of the late 1980s targeted, including possibilities targeting some of the younger source rocks and reservoirs.
For example, drilling on the Lomonosov Ridge in the Arctic Ocean near the North Pole has revealed a mid-Tertiary, algae bearing horizon that could represent a regional Arctic source rock.
“If that can be buried to a reasonable depth we might have a younger system that would be operating in the Chukchi basin, and we don’t have to only look for the older systems and only for the big, early generation which is what happened on the North Slope,” Kumar said.
Gas chimneys And just to get the saliva moving for any self-respecting exploration geologist, the GXT seismic shows widespread evidence of gas chimneys in the strata under the Chukchi Sea (a gas chimney shows up as a distinctive pattern on a seismic section and represents a column of natural gas bubbling up through the rock strata from a deep source).
“We’ve also seen a fair amount of gas chimneys from the data, basically throughout the dataset,” Dinkelman said. “To us that is suggestive that … the regional petroleum system that is active is far more pervasive than what we see just based on these five wells.”
However, the GXT seismic has also brought to light some of the structural complexities of the Chukchi geology, with various cataclysmic events alternately pulling apart and compressing the rocks over geologic time.
“The Chukchi, I think, from this (seismic data) is a very complicated area, and what we can document … is that you have a series of rifting and compressional events that have taken place throughout the history from early Paleozoic until late Cretaceous,” Kumar said.
One curiosity in the structures revealed in the seismic is the apparent absence of what geologists term the Herald Thrust, the major geologic fault that represents the seaward extension of the northern front of the Brooks Range.
“We were not able to show the Herald Thrust as a continuation of the front of the Brooks Range,” Kumar said. “… We did not see that kind of feature in any of the data towards the southern end of our survey.”
Given the short time window and proximity of the permanent ice pack during the 2006 open water season GXT was able to shoot about half of the seismic it had planned. The company plans to acquire additional data, filling in the seismic grid that it has now established and extending coverage out over the edge of the continental shelf into the Canada basin of the Arctic Ocean.
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