Page 317 - Petroleum Geology

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Much of the North Slope of Alaska is a Naval Petroleum Reserve, and the
United States Navy (through the U.S. Geological Survey) spent nearly 10
years exploring it (1944-1953). They drilled on and round the Barrow Arch
looking for pre-Cretaceous subcrops (finding one small gas field); and further
south, looking for Lower Cretaceous paralic sands in closed anticlines (find-
ing a small oil field and a small gas field). This was intelligent exploration,
and the information gained was to lead to more successful drilling by others
later. In 1964, after a quiet decade, Atlantic Richfield (ARCO) - as it was
soon to become - and Humble joined the search.
They drilled first in the south, about 100 km south of Prudhoe Bay, where
steep, narrow anticlines formed long and rather sinuous trends, separated by
broad, gentle synclines (a structural style that will be discussed in Chapter
15). These anticlines were found to have sheared cores, and the Upper Creta-
ceous sands had poor porosity and permeability. The Cretaceous here is
dominantly regressive. They then turned their attention to the north, partly
because the geology seemed more attractive, and partly because the State
leasing in the north was more favourable for exploration in allowing larger
areas than the Federal leasing in the south (Morgridge and Smith, 1972; Jami-
son et al., 1980).
In the Prudhoe Bay area, the Barrow Arch brought the pre-Cretaceous se-
quence to depths much shallower than in the Colville trough to the south,
where at least 9 km of sediment accumulated. The discovery well was drilled
on the local culmination of the structure, and penetrated the gas cap of the
main reservoir. The next well, drilled 11 km to the southeast, on the flank
of the structure, had an oil column of 120 m.
Dips on the south flank were found to be very gentle, about 2". There are
normal faults in the pre-unconformity sequence in both flanks, and the den-
sity of them obscures the dip on the north flank. The stratigraphy is shown
in Fig. 13-5. The most important reservoir, the Sadlerochit (Permian-Lower
Triassic), is the consequence of a regressive episode in a generally transgres-
sive pre-unconformity sequence. This regression was southwards from a land
area to the north (where the Beaufort Sea now is). Thinning of the Sadlero-
chit onto the structure from the south indicates structural growth beginning
in the Permian (probably). Faulting was no later than early Cretaceous. The
post-unconformity sequence is regionally regressive, but Lower Cretaceous
(Barremian) mudstones seal the subcrops in the Prudhoe Bay area (Fig. 13-6).
Whereas the sediment source for the pre-unconformity sequence lay to the
north, that of the post-unconformity sequence lay to the south or south-
west, with the evolving Brooks Range.
There is general consensus that the source of the petroleum in the Prudhoe
Bay field is in the Lower Cretaceous marine mudstones on the unconformity
(Morgridge and Smith, 1972; Jones and Speers, 1976). Geochemical evidence
does not preclude a pre-Cretaceous contribution. The total organic carbon
content, which is considered to be a guide to source rock potential, was found