In St. Matthew-Hall basin we distinguish two petroleum plays on
the criteria of tectonic setting, reservoir stratigraphy, dominant
trap type, and access to thermogenic gas.
Play 1 (UASM0100). Rift Sequence Play: The Rift sequence
play (play 1) is inferred, on the basis of analogy to Norton basin
(geology of latter described by Turner and others, 1986), to consist
of Paleocene to lower Oligocene fluvio-deltaic sandstones deposited
in fan-deltas along the margins of fault-bounded pull-apart grabens
during active wrench faulting in early phases of basin subsidence.
Prospects are mostly fault traps, but also include anticlines,
faulted anticlines, and sub-unconformity traps. Unmapped stratigraphic
traps are anticipated in deep, graben-floor fan systems, but porosity
at those depths (>10,000 feet) is expected to be quite low,
as observed in Norton basin. However, the deeper traps lie within
the oil window and are best positioned to capture thermogenic
gas. Potential traps in play 1 range in depth from 4,000 to 13,000
feet. Postulated source rocks are interbedded within the play
sequence and are speculated, on the basis of analogy to Norton
Basin, to include marine to non-marine shales and coal seams of
Eocene and Paleocene age. Early Oligocene coals and shales are
speculated to occur in the upper part of the sequence. These
latter rocks are probably thermally immature, but may provide
feedstock for microbial generation of biogenic gas.
Play 2 (UASM0200). Sag Sequence Play: The Sag sequence
play (play 2) consists of inferred late Oligocene shallow shelf
sandstones to submarine fan turbidite and basin plain deposits
above a prominent (seismic data) unconformity, speculated from
analogy to Norton basin to be mid-Oligocene in age. Possible
trap types are mostly gentle anticlines, but also include faulted
anticlines and fault traps. Additional unmapped traps may occur
in stratigraphically isolated shelf sandstones in the upper part
of the sequence. Sag sequence traps range in depth from 1,400
to 5,000 feet. Thermogenic gas from thermally mature rocks deep
within grabens may charge traps near the grabens. Potential traps
at shallow depths or at great distances (some up to 100 miles)
from the deep pull-apart grabens are likely to contain only biogenic
gas.
______________________________________________________ OIL AND GAS ENDOWMENTS OF ST. MATTHEW-HALL BASIN PLAYS
* Unique Assessment Identifier, code unique to play.
____________________________________________
REFERENCES CITED
Turner, R.F., Martin, G.C., Risely, D.E., Steffy, D.A., Flett,
T.O., and Lynch, M.B., 1986, Geologic Report for the Norton Basin
Planning Area, Bering Sea, Alaska: Turner, R.F., ed., U.S. Minerals
Management Service, OCS Report, MMS 86-0033, 179 p. |