Brookian promise to east
Houseknecht: 1002’s oil-prone rocks could charge stratigraphic, structural traps
In its last assessment of the ANWR 1002 area in 1998 the U.S. Geological Survey estimated the federal lands held a mean of 7.7 billion barrels of recoverable oil, 75% of which was in the western section and 25% in the eastern (see map in part 1 of this article in the July 21 issue of Petroleum News).
When you add in recoverable oil from state lands offshore and Native lands in the 1002 area, the total mean (50% probability) was 10.4 billion barrels, per a 2015 report by Paul Decker, at the time lead petroleum geologist at the Alaska Department of Natural Resources’ Division of Oil and Gas.
The only well in the 1002 area, the KIC No. 1, was drilled in the eastern section, referred to as the deformed area because of the known presence of significant folds and geologic faults in the strata.
There are major structures in this part of the 1002 area. They’re not like structures in the central North Slope such as in the Prudhoe Bay area - the main thrust faulting and folding are limited to the area generally east of the Canning River, with the degree of deformation increasing to the east, USGS geologist Dave Houseknecht, an expert on North Slope petroleum geology, told Petroleum News.
Many of the older rocks that sourced and reservoired oil in the central North Slope were eroded out at some time in the geologic past. The older rocks are preserved in some sunken faulted blocks offshore under the Beaufort Sea, but it is not clear whether this phenomenon extends under the onshore region.
Houseknecht said that although there is uncertainty regarding the presence of two major source rocks, the Shublik and the Kingak, in the eastern section of the 1002 area, there is outcrop evidence for the presence of a thick source rock interval in the younger Brookian sequence, including the pebble shale unit, GRZ, and Hue Shale. And potential Brookian reservoir rocks are definitely present.
Unfortunately, in the eastern section the “Brookian is thick and very badly deformed … making it very difficult to interpret what lies below the Brookian,” Houseknecht said.
The information from the KIC No. 1 well was not available to the USGS, while over the years it has been accessible to a very limited number of people in DNR’s Division of Oil and Gas.
Stratigraphic trapsIn the western section of the 1002 area, the subsurface strata are relatively undeformed. The exploration interest there would primarily be in stratigraphic traps, traps created from the way the sediments that formed the rocks were deposited.
With known significant thicknesses of Brookian strata in the region, Houseknecht said the exploration plays would be analogous to those in which major recent oil finds have been made by Armstrong Energy, Repsol and ConocoPhillips in the Nanushuk and Torok formations to the west of the central North Slope.
He said that given the wide spacing between the vintage 2D seismic lines collected in the ANWR 1002 area, it would be possible to hide between the lines a stratigraphic trap on the scale of the big Brookian Nanushuk Willow (ConocoPhillips) or Pikka (Armstrong/Repsol/Oil Search) discoveries.
As previously reported, although only those oil companies that co-own 2D seismic data shot in the ANWR 1002 area in the mid-1980s will have subsurface data prior to a U.S. Department of Interior lease sale in December (see related lease sale news item in this issue), Houseknecht is leading an effort to get more geologic information released prior to the sale that will help bidders better understand the geologic potential of the region.
He and his team have conducted field work in and near the ANWR 1002 area during the past two summers, as well as completed a grounds-up reprocessing of 1,451-line miles of the vintage 2D seismic, the purpose of which was to glean more detail than was initially apparent. Although Interior’s Office of the Solicitor ruled against releasing this data, the reprocessed seismic gave Houseknecht’s team a better understanding of the area. While they are not permitted to show images of the reprocessed data, their public presentations use analogs from offshore and state lands west of the ANWR border, plus they can show images of 2D and 3D seismic data licensed from seismic companies.
The Brookian sequence, he said, shows much hydrocarbon potential in the ANWR 1002 area. There are structural hydrocarbon traps in the east, stratigraphic traps in the west and outcrops of oil-saturated sandstone in both areas. Summaries of this information were published by the USGS in 1999 and recent work is enhancing the details of those results.
Brookian could charge stratigraphic and structural“The presence of Triassic (Shublik formation) and Jurassic (Kingak shale) source rocks is a significant uncertainty in the 1002 area. Those units are truncated by the Lower Cretaceous unconformity across a large part of the northeastern North Slope as indicated by well penetrations and seismic data east of Prudhoe Bay and west of the Canning River, and by outcrop data in the Sadlerochit Mountains,” Houseknecht said.
“The Triassic and Jurassic source rocks may be present in the northeastern 1002 area if they are preserved in graben basins. Regardless, the presence of a thick (up to 200 meters) and rich (up to 26 weight percent total organic carbon) Brookian source-rock interval is known in the 1002 area from outcrop studies.”
In the eastern 1002 area “those Brookian source rocks were imbricated by Cenozoic thrust faults. In that area, the source rocks have immature thermal maturity values in outcrop and are modeled to be in the oil window in the subsurface. Thus, the Brookian represents a viable, oil-prone source rock that could charge both stratigraphic and structural in the 1002 area,” Houseknecht said.
Brookian’s three partsOn a regional scale, he said in a July 10 email to Petroleum News, “Brookian rocks can be divided into three parts (1) Lower Cretaceous Torok-Nanushuk, (2) Upper Cretaceous strata (includes Seabee, Tuluvak, Schrader Bluff, Prince Creek, and part of Canning formations), and Cenozoic Canning and Sagavanirktok formations.”
The regional tectonic setting of these three intervals varied significantly, Houseknecht said, “resulting in rapid and high-volume sediment accumulation during 1 and 3, and slower and lower volume sediment accumulation during 2. So, even though all three intervals were characterized by prograding shelf margins, the volume of sand deposited in each is quite variable.”
Consequently, he said, “the size and geometry of reservoirs may vary among the three and there likely will be a ‘learning curve’ involved in calibrating 3D data collection and interpretation as exploration for stratigraphic traps in the Upper Cretaceous and Cenozoic intervals occurs” across state lands and into the 1002 area.