Three significant source rocks are present beneath much of the Alaska NorthSlope (Fig. 1), the Triassic Shublik Formation, the lower part of theJurassic-Lower Cretaceous Kingak Shale, and the " Brookian shale" that includesthe Cretaceous pebble shale unit and Cretaceous-Lower Tertiary Hue Shale. Although these source rocks are known to have generated oil and gas thatmigrated into conventional accumulations, including the super-giant Prudhoe Bayfield, the first attempt to produce hydrocarbons directly from the three sourcerocks was initiated in 2012.
The Shublik Formation contains a mixture of Type I and IIS kerogen, and oilin conventional accumulations sourced from the Shublik is of relatively lowgravity (23-39° API) and high sulfur (more than 1.5 percent). In contrast, theKingak and Brookian source rocks contain a mixture of Type II and III kerogen, and oil in conventional accumulations sourced from those rocks is of relativelyhigh gravity (35-42° API) and low sulfur (less than 0.3 percent). These threesource rocks occur at depths that range from less than 3,000 feet along theBarrow Arch to more than 20,000 feet in the Brooks Range foothills. Over thatrange of depth, thermal maturity of the source rocks grades from the onset ofoil generation along the Barrow Arch, through the oil window, and well into thedry gas window in the south (Fig. 1).
Shale-oil and shale-gas assessment units (AUs) - areas where organic richfacies are inferred to be in the oil or gas window, respectively - weredelineated for each source rock (Figs. 2, 3, 4) based on empirical thermalmaturity data and regional modeling (Houseknecht et al., 2012a). Both Shublikand Brookian source rocks include rock types that are brittle and in whichnatural fractures are common. Brittle lithologies include limestone, phosphaticlimestone, sandstone, siltstone, and chert in the Shublik and very-fine-grainedsandstone, siltstone, concretionary carbonate, and silicified tuff in theBrookian. In contrast, the Kingak source rock is mostly clay shale that deformsplastically, and brittle lithologies generally are absent. These petroleumsystem elements (organic matter content, thermal maturity, and brittlelithology) were among the factors considered in estimating the probability thatoil and gas can be technically recovered from the source rocks, with results of95 percent probability for the Shublik, 90 percent for the Brookian, and 40percent for the Kingak (Houseknecht et al., 2012b).
Maps of petroleum system elements were used to evaluate spatial variabilityin source rock character. A map of mostly transgressive facies in the ShublikFormation (Fig. 2) delineates areas that may contain highest organic content, based on published relations between transgressive facies and total organiccarbon (TOC) in the formation (Hulm, 1999; Robison and Dawson, 2001; Peters etal., 2006; Kelly et al., 2007). The Shublik is absent owing to non-depositionat Pt. Barrow and transgressive facies in the formation thicken basinward in aradial pattern, reaching maximum values greater than 200 ft in northeastern andwestern NPRA (Fig. 2). East of NPRA, the Shublik thins depositionally towardsPrudhoe Bay, and is truncated completely farther east beneath the LowerCretaceous unconformity (Fig. 2). In northcentral NPRA, Shublik transgressivefacies are not only relatively thin (less than 100 ft) but also containgenerally low organic carbon content (Fig. 2) and low values of interpretedoriginal hydrogen index (Peters et al., 2006). Both the TOC content (Fig. 2)and interpreted original hydrogen index (HI) increase abruptly in the vicinityof Teshekpuk Lake in northeastern NPRA, and both parameters are relatively higheastward to the Shublik truncation edge beneath the eastern North Slope(Peterset al., 2006). We infer that the best oil potential in the Shublik occurswithin the shale-oil AU (defined by thermal maturity) from Teshekpuk Lakeeastward (Fig. 2). We infer good gas potential in the Shublik in the shale-gasAU across much of the North Slope (Fig. 2).
The Kingak Shale is divided into three map areas (Fig. 3) on the basis ofseismic and well-log character (Houseknecht and Bird, 2004). A broad area innorth-central NPRA contains a series of progradational shelf sequences, withinwhich Kingak source-rock facies are mostly limited to thin transgressivedeposits characterized by low values of TOC and HI. To the east and southeast, shelf deposits are absent and the lower Kingak comprises basinal condensedshale that has higher values of TOC and HI (Houseknecht and Bird, 2004; Peterset al., 2006). The Kingak is poorly known beneath the southwestern North Slopebecause of an absence of well penetrations (Fig. 3). We infer that the best oiland gas potential in the Kingak occurs within the shale-oil and shale-gas AUs, respectively, and basinward from the shelf sequences in north-central NPRA(Fig. 3). However, the paucity of brittle facies in the lower Kingak Shale maylimit its reservoir quality everywhere.
A map of the Brookian sequence showing thickness of net high gamma-ray (HGR)log response (cumulative thickness of gamma-ray response greater than 150 API)in the oil window (based on thermal maturity) delineates thermally mature areasthat may contain higher organic content (e.g., Schmoker, 1981). The map of netHGR displays complex patterns that reflect regional accommodation, localerosion beneath sequence-bounding unconformities, and thermal maturitypatterns. In general, net HGR thickens to the east (Fig. 4); this regionaltrend is consistent with patterns of TOC and interpreted original HI (Peters etal., 2006). Brittle facies closely associated with HGR intervals in theBrookian are more common east of NPRA. We therefore infer that the best oilpotential in the Brookian shale occurs in areas that contain more than 100 ftof HGR in the oil window, east of NPRA and west of ANWR (Fig. 4). The best gaspotential likely occurs within the shale-gas AU, in areas adjacent to the TransAlaska Pipeline System where the largest thickness of HGR is observed (Fig. 4).
The USGS in 2012 completed the first-ever assessment of technicallyrecoverable shale-oil and shale-gas resources in northern Alaska. Aggregateestimates for all three source rocks range from 0 to 2 billion barrels of oiland 0 to 80 trillion cubic feet of gas (TCFG), with the ranges representing a95- to 5-percent probability of occurrence (Houseknecht et al., 2012b). Estimates for each source rock system include 0 to 928 million barrels of oil(MMBO) and 0 to 72 TCFG for the Shublik, 0 to 955 MMBO and 0 to 4 TCFG for theBrookian, and 0 to 117 MMBO for the Kingak (gas was not quantitatively assessedfor the Kingak). In all cases, the zero value at the 95-percent probabilityreflects the application of play-level risk. The Shublik is estimated tocontain the greatest oil and gas resource potential per unit area, with valuesthat rank among the top few source-rock systems in the United States.