Abstract

The Cook Inlet Basin is a northeast-trending collisional forearc basin that extends from Shelikof Strait northeastward to the east end of the Matanuska Valley. The basin is divided into three segments including, from northeast to southwest, the Matanuska Valley segment, Cook Inlet segment, and Cook-Shelikof segment. The Matanuska Valley segment represents the collapsed onshore part of the basin. The Cook Inlet segment is a significant hydrocarbon province, with more than 1.3 billion barrels of oil and about 8 trillion cubic feet (TCF) of gas produced since 1958. No commercial oil or gas production has been established in either the Cook-Shelikof or Matanuska segments of the basin.

The basin is located in the arc-trench gap between the Alaska-Aleutian Range batholith to the northwest, representing the plutonic roots of a Mesozoic-early Cenozoic magmatic arc and the surface edifice of the modern arc, and the modern Aleutian trench to the southeast. An enormous emergent accretionary prism separates the forearc basin from the modern trench and is represented by the Kenai-Chugach Mountains. These tectonic elements are the products of subduction and associated magmatic and accretion processes that have operated for at least 200 million years. Several crustal-scale faults have modified the basin margins over its history. These include the Bruin Bay fault system, the Border Ranges fault system, the Lake Clark-Castle Mountain fault, and the Capps Glacier fault. The Bruin Bay and Border Ranges fault systems separate the basin from the arc and accretionary prism on the west and east sides, respectively, along most of its length. Eocene and younger nonmarine rocks overstep the Bruin Bay fault system along the northwest side of the Cook Inlet segment. Here the Capps Glacier fault is in a basin-bounding position, separating arc intrusive rocks from forearc basin strata. Lower Miocene strata overstep the Border Ranges fault system along the Cook Inlet segment’s east side.

The forearc basin accommodated a cumulative thickness in excess of 35,000 ft (10,670 m) of Jurassic and Cretaceous dominantly marine strata and up to 25,000 ft (7620 m) of Cenozoic nonmarine strata. The Middle Jurassic and younger Mesozoic succession comprises several unconformity-bounded packages. Compositional data from sandstones and conglomerates in these rocks record the progressive uplift and exhumation of the arc edifice and deeper plutonic roots of the Mesozoic arc. Middle Jurassic rocks of the Tuxedni Group consist exclusively of first-cycle lithic grains derived from the volcanic arc edifice (Talkeetna Formation) that were deposited in a spectrum of deep-water through shallow marine settings. Upper Jurassic rocks of the Naknek Formation consist of first-cycle arkosic sandstones and conglomerates with abundant dioritic and granodioritic clasts derived from the older Jurassic plutonic roots of the arc that were deposited in marginal-marine to deep-water settings. Upper Cretaceous strata (Kaguyak Formation and Matanuska Formation) vary compositionally and record an increase in volcanic detritus over the underlying Naknek. They were also deposited in a range of deep water through shallow water and, locally, nonmarine settings. Organic-rich mudstone and silty shales in the lower part of the Tuxedni Group are thought to have sourced most of the oil discovered in Cook Inlet segment fields.

The Mesozoic-Cenozoic contact is a pronounced angular unconformity of regional extent that likely records subduction of an oceanic spreading center during the Paleocene. The Tertiary succession in the Cook Inlet segment is complex and includes the Eocene to early Oligocene(?) West Foreland Formation, late Eocene(?) to late Oligocene Hemlock Conglomerate, Oligocene-early Miocene Tyonek Formation, the Miocene Beluga Formation, and the late Miocene to early Pliocene Sterling Formation. The overlapping ages of these formations demonstrate the time-transgressive nature of the Tertiary stratigraphy. The generally accepted model for Tertiary depositional systems in the basin includes high-gradient alluvial fans along the west and east basin margins that fed sediment to lower gradient axial-fluvial depositional systems. Relatively high accommodation during the Miocene led to deposition of thick coal seams in the Tyonek Formation and progressively thinner seams in the Beluga and Sterling Formations. Microbial gas from these coals sourced most of the gas in the Cook Inlet segment.

Sandstones of Cook Inlet Basin can be divided into two informal groups based on reservoir quality. The Middle Jurassic Tuxedni Group through Lower Cretaceous (Herendeen equivalent) sandstones comprise the “zone of diagenetic control” where reservoir quality is largely controlled by the compaction and cementation history of the rocks. These sandstones tend to have an unstable mineralogy and have been buried to substantial depths, resulting in significant modifications to depositional texture and mineralogy. The Upper Cretaceous Kaguyak (and correlative unnamed Upper Cretaceous strata) through late Miocene to Pliocene Sterling Formations comprise the “zone of depositional control” where reservoir quality is largely controlled by textural parameters related to depositional environment. Coarser-grained sandstones deposited in higher-energy environments within this zone tend to have better reservoir characteristics than finer-grained lithologies deposited in lower-energy environments. Rocks comprising the zone of depositional control have seen less burial and consist of mixed mineralogies. Eocene to early Oligocene(?) West Foreland sandstones are extremely volcanigenic and have undergone more diagenesis than older, more quartzose Upper Cretaceous strata. Oligocene Hemlock and Oligocene to early Miocene Tyonek sandstones have similar compositions dominated by monocrystalline quartz with lesser amounts of polycrystalline quartz and chert. Plagioclase, K-feldspar, and a variety of lithic fragments, including volcanic, plutonic, and metamorphic rock fragments, round out the composition of these formations. Miocene Beluga and late Miocene to Pliocene Sterling sandstones have mineralogies conducive to diagenetic alteration, but their young age (< 10 Ma) and shallow burial (< 10,000 ft) (3050 m) have resulted in only minor diagenetic modifications.

Tertiary nonmarine strata throughout the Cook Inlet segment have been deformed into a series of north–northeast-trending, discontinuous folds arranged in an en echelon pattern. Most of these fold structures formed by right lateral transpressional deformation on oblique-slip faults. Many of these faults extend into underlying Mesozoic marine rocks. These structures are attributed to the ongoing collision between the Yakutat terrane and inboard terranes in Alaska. This collision is resulting in the progressive collapse of the forearc basin from northeast to southwest (analogous to a closing zipper). All producing oil and gas fields in the Cook Inlet segment are associated with fold structures having four-way closure. Gas in most fields resulted from desorption of microbial methane as thick coal-bearing successions were uplifted along fold structures. Unconformities within the Mesozoic succession suggest multiple phases of deformation during Mesozoic time.

The Cook Inlet Basin is underexplored for petroleum. A recent USGS assessment of undiscovered technically recoverable oil and gas resources in the onshore areas and state waters of the Cook Inlet region estimated mean undiscovered volumes of about 600 million barrels of oil and about 19 TCF of gas within three total petroleum systems (TPS). All of the forecasted oil resources and about 72% of the gas (about 13.7 TCF) are expected to be found in conventional accumulations in Tertiary and Mesozoic sandstone reservoirs within the Cook Inlet Composite TPS. Of the remaining undiscovered gas, about 25% (4.7 TCF) is expected in continuous accumulations in coalbeds at depths shallower than 1800 m (5906 ft) in the Cook Inlet Coalbed Gas TPS, and about 3% (0.6 TCF) is anticipated in continuous accumulations in low-permeability (and possibly overpressured) Mesozoic sandstone reservoirs at depths greater than 6000 m (19,700 ft) in the Tuxedni-Naknek Continuous Gas TPS.