Brookian structural plays in the National Petroleum Reserve, Alaska

As part of the U.S. Geological Survey assessment of undiscovered oil and gas
resources in the National Petroleum Reserve-Alaska (NPRA), two structural plays were
assessed in thrust-faulted and folded Upper Cretaceous rocks of the Brookian
megasequence. These are the Brookian Topset Structural Play and the Torok Structural
Play, located in the Brooks Range foothills and the southern part of the coastal plain,
within the Tertiary-age frontal part of the Jurassic to Tertiary Brooks Range orogenic
belt. A new regional structural interpretation, developed through regional seismic
analyses, reconnaissance field investigations, and new thermal constraints, guided the
geologic evaluation and risking of these plays. Volumetric parameters were derived from
seismic reflection data, well data and oil and gas field analogs.
The fundamental elements of the Brookian Topset Structural Play, exemplified by
the undeveloped Umiat oil field, include: (1) reservoirs in Nanushuk Group and
uppermost Torok Formation shallow-marine to nonmarine sandstones draped over
anticlines caused by structural thickening in underlying Torok mudstones; (2) seals
provided by overlying shale drapes in the Nanushuk, and locally by thrust faults; (3)
Torok, gamma-ray-zone (GRZ) or pebble shale source rocks; (4) remigration of
hydrocarbons from early formed (Late Cretaceous) stratigraphic traps disrupted by 60 Ma
thrusting, into newly formed structural traps. The 60 Ma thrusting was probably
accompanied by new generation and migration of natural gas resulting from late
structural thickening and tectonic loading. Subsurface data from the Umiat field and
other seismic reflection data within the play area indicate that the structural traps are
commonly compartmentalized by thrust faults.
The fundamental elements of the Torok Structural Play, exemplified by the
undeveloped East Kurupa gas accumulation just south of NPRA, include: (1) reservoirs
in lower Torok Formation basin-floor sandstones, and perhaps in small sandstone bodies
intercalated with middle to upper Torok mudstones; (2) structural traps provided by
folded sandstone bodies above thrust faults within a pervasively deformed passive-roof
duplex beneath the foothills and a less-strongly deformed domain to the north; (3) seals
provided by Torok mudstone, both stratigraphically above the sandstone bodies and
smeared along bounding thrust faults; (4) Torok, GRZ, pebble shale, or Kingak Shale
source rocks; (5) a migration and charging scenario similar to that of the Brookian Topset
Structural Play. According to our estimates, the Brookian Topset Structural Play contains 137
million barrels of technically recoverable oil and 10.6 trillion cubic feet (TCF) of
technically recoverable, non-associated natural gas, with both values representing the
mean estimate (expected value). The Torok Structural Play contains about 35 million
barrels of technically recoverable oil and 17.9 TCF of technically recoverable, nonassociated
natural gas.