Abstract Alaska’s first commercial oil and gas province, the Cook Inlet Basin, of south-central Alaska, was identified by the presence of numerous oil and gas seeps in the southern part of the basin and the Alaska Peninsula. These were first recognized by Russian explorers in the 1850s. Early exploratory drilling was centered in the vicinity of these seeps, which emanate from rocks of Jurassic and Cretaceous ages. Eighteen wells were drilled in this southern area prior to 1950. After 1950, exploration shifted northward, and the fourth well drilled in the northern portion of the basin discovered the Swanson River field in 1957. This generated an exploration boom, and by 1970, seven of the inlet’s ten oil fields and 17 of the 34 gas fields had been discovered. These discoveries represent more than 98% of both the oil and gas produced to date. After the Prudhoe Bay discovery, in early 1968, activity in the Cook Inlet area declined sharply, and over the next 30 years, only three small oil fields and nine small gas fields were discovered. As of year-end 2011, the cumulative production from these discoveries was less than 20 MMBO and 65 BCFG. A declining reserve base and concerns regarding the ability to meet local gas demands led to an increase in gas exploration in the late 1990s and early 2000s. From 2000 to year-end 2011, eight additional gas fields were discovered, with the most important discoveries occurring in 2011. Cumulative production from the five pre-2011 discoveries totals approximately 25 BCFG. The three fields discovered in 2011 have yet to be developed; however, the cumulative, preliminary reserve estimates from these three discoveries are thought to be in excess of 1.0 TCFG. The total cumulative production in the Cook Inlet Basin, from 1958 through 2011, is 1.3 BBO from eight fields and just under 7.5 TCFG from 28 producing fields. Given the relatively short, punctuated exploration history of this prolific hydrocarbon basin, renewed exploration activity using modern technology will very likely produce significantly positive results. Recent evaluations by the United States Department of Energy/National Energy Technology Laboratory and the United States Geological Survey place the mean for conventional undiscovered technically recoverable resources in the range of 13 to 17 TCFG and approximately 600 MMBO.

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.

Abstract Seismic data have been collected since the 1950s in Cook Inlet Basin and have been instrumental in the discovery and production of oil and gas in the basin. This paper provides an overview of how seismic data have been and could be utilized for further exploration. It covers general aspects of data availability, seismic acquisition, processing, and interpretation analysis, with special focus on the extreme or unusual aspects of the Cook Inlet Basin environment, both logistical and geologic, that impact those processes.

Abstract This volume is designed to showcase the geo-technical elements of oil and gas fields of the Cook Inlet Basin of southcentral Alaska. It contains 10 chapters written by 16 authors and co-authors who have extensive experience in the basin. All of the papers have been peer-reviewed. The first three chapters provide an introduction to exploration, stratigraphy, petroleum systems, seismic acquisition, and reservoirs of the basin. Following these are seven chapters that describe individual fields in detail. This volume is intended to serve as a key reference to the petroleum geology of the Cook Inlet Basin for a wide audience including oil and gas explorers, technical professionals, students and those seeking more information about the origin and habitat of oil and gas in the area.

Abstract Oil exploration commenced onshore adjacent to lower Cook Inlet on the Iniskin Peninsula in 1900, shifted with considerable success to upper Cook Inlet from 1957 through 1965, then returned to lower Cook Inlet in 1977 with the COST well and Federal OCS sale. Lower Cook Inlet COST 1 well, drilled to a total depth of 3,776 m, penetrated the tops of Upper Cretaceous, Lower Cretaceous, and Upper Jurassic strata at 832, 1,541, and 2,112 m, respectively. Basinwide unconformities are present in this well at the tops of the Upper Cretaceous, Lower Cretaceous, and Upper Jurassic rocks. Sandstone of potential reservoir quality occurs in the Cretaceous and lower Tertiary rocks. All siltstones and shales analyzed are low (0 to 0.5 wt. %) in oil-prone organic matter, and only coals are high in humic organic matter. At total depth, vitrinite readings reached a maximum average reflectance of 0.65. Several indications of hydrocarbons were present. Oil analyses suggest that oils from the major fields of the Cook Inlet region, most of which produce from the Tertiary Hemlock Conglomerate, have a common source. More detailed work on stable carbon isotope ratios and the distribution of gasoline-range and heavy (C 12 +) hydrocarbons confirm the genetic relation among the major fields. In addition, oils from Jurassic rocks under the Iniskin Peninsula and from the Hemlock Conglomerate at the southwestern tip of the Kenai lowland are members of the same or a very similar oil family. The middle Jurassic strata of the Iniskin Peninsula are moderately rich in organic carbon (0.5 to 1.5 wt. %) and yield shows of oil and gas in wells and in surface seeps. Extractable hydrocarbons from this strata are similar in chemical and isotopic composition to the Cook Inlet oils. Organic matter in Cretaceous and Tertiary rocks is thermally immature in all wells analyzed. Oil reservoirs in the major producing fields are of Tertiary age and unconformably overlie Jurassic rocks; the pre-Tertiary unconformity may be significant in exploration for new oil reserves. The uncomformable relation between reservoir rocks and likely Middle Jurassic source rocks also implies a delay in the generation and expulsion of oil from Jurassic until late Tertiary time when localized basin subsidence and thick sedimentary fill brought older, deeper rocks to the temperature required for petroleum generation. Reservoir porosities, crude oil properties, the type of oil field traps and the tectonic framework of the oil fields in the west flank of the basin provide evidence used to reconstruct an oil migration route. The route is inferred to commence deep in the truncated Middle Jurassic rocks and pass through the porous West Foreland Formation in the McArthur River field area to a stratigraphic trap in the Oligocene Hemlock Conglomerate and the Oligocene part of the Tyonek Formation at the end of the Miocene time. Pliocene deformation shut off this route and created localized structural traps, into which the oil moved by tertiary migration to form the Middle Ground Shoal, McArthur River and Trading Bay oil fields. Oil generation continued into the Pliocene, but this higher API gravity oil migrated along a different route to the Granite Point field.

Abstract In less than ten years the Cook Inlet basm of south-central Alaska has become one of this nation's most promising sources of oil and gas. From 1957 to 1966, significant discoveries include 5 major oil fields, 1 gas-condensate field, and 10 dry-gas fields with total proved reserves in excess of 500 million bbls of oil and 3 trillion cu ft of gas. Rocks ranging in age from Late Triassic to Recent are present in the basin; however, commercial oil and gas accumulations have been found only in Tertiary rocks assigned to the Kenai Formation, a thick heterogeneous deposit composed of sandstone, siltstone, conglomerate, shale, and abundant coal beds. Lateral variation of individual beds within the Kenai Formation makes correlation difficult; however, three arbitrarily defined stratigraphic units comprising at least five distinct lithologic zones are recognizable on the Kenai Peninsula where adequate well control is available. The United States Geological Survey, as a result of recent paleobotanical studies, has subdivided the Kenai Formation into three provincial stages which correspond to time-stratigraphic units. Although subdivisions of the Kenai Formation used in this paper may be roughly correlative with the provincial stages on the basis of distribution, spatial occurrence of coal within the section, and degree of lithification, they were chosen arbitrarily. The Cook Inlet basin is an elongate, deep, moderately asymmetrical structural depression which overlies much of the Matanuska geosyncline, a Mesozoic sedimentary trough. The structure of the basin is mainly a result of Late Cretaceous-early Tertiary deformation; however, the major tectonic features are a result of middle Jurassic orogeny. The principal structural features within the basin are narrow, elongate, complexly faulted anticlinal belts which trend north-northeast parallel with the grain of the major tectonic features which flank the basin. All of the oil and gas fields are located near the crest of local structural closures on the anticlinal belts. Accumulation of oil in the Hemlock Zone, the principal oil reservoir in the lower part of the Kenai Formation, probably is related to old fold belts which were uplifted before Late Cretaceous time and remained positive until Tertiary time. Gas accumulations in the middle and upper parts of the Kenai Formation are indigenous, and were generated from coal and lignite in the section. Time of structural uplift, diagenesis of carbonaceous source rocks, and stratigraphic relations between coal and reservoir host rocks control the accumulation of gas in commercial quantities. Most of the proved gas reserves of the basin are shut in because there is no market. Planned facilities for methane liquefaction, anhydrous ammonia production, and pressure maintenance of oil reservoirs soon will greatly increase the use of natural gas and the demand may exceed the supply.

Abstract The Cook Inlet basin of south-central Alaska is an intermontane structural basin approximately 14,000 square miles in area, encompassing almost 80,000 cubic miles of sedimentary rocks ranging in age from Upper Triassic to Recent. The basin coincides with most of the northern part of the Matanuska geosyncline—an arcuate Mesozoic trough located at the northwestern end of the Pacific Cordilleran mobile belt. The Cook Inlet sedimentary trough, as contrasted with the structural basin, is defined as a Paleozoic-early Mesozoic eugeosyncline that received sediments from volcanic islands which were part of the volcanic archipelago bordering the Pacific Coast of North America. Middle Jurassic epeirogeny transformed southern Alaska into arcuate geanticlinal and geosynclinal belts with the Cook Inlet basin beginning as a half graben created by complex faulting on the east flank of the Talkeetna geanticline. The Mesozoic embayment that collected marine sediments and occasional nonmarine wedges abutting cratonic source areas was semienclosed or silled as the Kenai and Chugach Ranges began to emerge following the early stages of the Laramide orogeny. During the early Tertiary, an abundant supply of nonmarine clastic sediments and carbonaceous material was widely distributed in the subsiding intermontane basin. The structural grain of the major tectonic elements describing the basin architecture is preserved in trends of local structure. Intense folding and faulting are exhibited on the north, east, and west flanks. Several major buried anticlinal trends extend in a northeasterly direction through the interior. Mesozoic hydrocarbon accumulations associated with anticlinal traps are found on the western side of the basin. Minor quantities of oil, gas, and condensate have been produced from sandstones of the Middle Jurassic Tuxedni Formation. The oil is believed to be indigenous to Jurassic beds. Oil and gas accumulations in Tertiary beds will determine the significance of the Cook Inlet basin as an oil and gas province. Present oil production comes from the Hemlock zone, a sandstone and conglomerate unit near the base of the Tertiary Kenai Formation. Entrapment has been influenced by folding and faulting along trend of an interior basin high which lies adjacent and parallel to an early Tertiary hinge belt. The Tertiary crude oils were probably derived from Eocene marginal marine strata or from upper Cretaceous marine shales which are unconformably overlain by the Tertiary sediments. Significant quantities of gas—predominantly methane—are present in the loosely consolidated sands of the upper Kenai Formation. The two conditions necessary for gas accumulation anywhere in the basin are (1) abundance of lignite or coal beds in the section to serve as source rocks, and (2) a suitable trap. The Cook Inlet basin is in its earliest stage of exploration and development. It is anticipated that many new fields will be discovered. Regional isopach maps of the interval between the Mesozoic beds and the base of the Hemlock zone are suggested as a basic approach to delineating old basin highs that may be sound Hemlock prospects. Cook Inlet should become a major gas basin regardless of its future as an oil province.