Geological Characteristics in Cook Inlet Area, Alaska

Home , Chickaloon Formation, Chinitna Formation, Cook Inlet Basin, Talkeetna Formation

SOCIE?I’YOF PETROLEUMENGINEERSOF AIME 6200 North CentralExpressway *R SPE 1588 Dallas,Texas 752C6

THIS IS A PREPRINT--- SUBJECTTO CORRECTION

Geological Characteristics in Cook Inlet Area, Alaska Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

ThomasE. Kelly,Jr. MemberAIYE, Mickl T. Halbouty,Houston,Tex.

@ Copyright 19G6 Americsn Institute of Mining, Metallurgical and Petroleum Engineers, Inc.

This paper was preparedfor the 41st AnnualFall Meetingof the Societyof PetroleumEngineers of AIME, to be held in Dallas,l?ex.,Oct. 2-5, 1966. permissionto copy is restrictedto an abstract of not more than 300 words. Illustrationsmay not be copied. The abstractshouldcontainconspicu- ous acknowledgmentof whereand by whom the paper is presented. Publicationelsewhereafter publica- tion in the JOURNALOF l?i’TROI.WJMTECHNOLOGYor the SOCIETYOF PETROLEUMENGINEERSJOURNALis usually grantedupon requestto the Editorof the appropriatejournalprovideciagreementto give propercredit is made.

Discussionof this paper is invited. Three copiesof any discussionshouldbe sent to the Societyof PetroleumEngineersoffice. Such discussionmay be presentedat the abovemeetingand, with the paper,may be consideredfor publicationin one of the two WE magazines. v,

The Cook Inlet basin is a narrow, Although the general characteristics elongate trough of Mesozoic and Ter- of the basin are fairly well known, tiary sediments located north of new information, as it is made avail- latitude 59° in south-central Alaska able will cause many revisions of the (Fig. 1). The basin covers approxi- stratigraphic and structural fabric mately 11,000 square miles of th~ before a complete geological picture northerripart of the Matanuska geo- is possible. This report revises syncline and is bounded by the Kenai and refines certain concepts of the and Chugach Mountains”on the east, basin which were discussed in an the Talkeetna Mountains and the earlier work (Kelly, 1963). In cer- Copper River on the northeast, the tain aspects it may be considered q Susitna basin on the north, the progress report that will require Chigmit Mountains and the Alaska periodic updating”as new information Range on the west , and the Shelikof is made available. Strait on the south. In less than ten years the basin PREVIOUS WORK has achieved su.@ a degree of promi- nence as a pet~’oleumprovince that A list of references includes many it ranks with’bff-shore areas in published reports wri~ten chiefly by California and the Gulf Coast as the members of the U. S. Geological Sur- most promising future source of large vey who investigated the mineral re- domestic reserves. sources of the Cook Inlet basin. The Oil and gas exploration in the first indications of petroleum in basin has rapidly increased in the the basin and the early attempts to past few years and is currently at find production were documented by its highest level in history. Martin (1905). Ma&her (1925), fur- ther described the petroleum poten- References and illustrations ac end tial of the southwestern part of the of paper. basin in the Kamishak Bay area. Miller, Payne and Gyrc (1957), pre- TRIASSIC ROCKS sen~ed the first comprehensive in- vestigation dealing specifically with Paleozoic and early Mesozoic’sedi- potential petroleum provinces of mentation occurred in a linear de- Alaska which included the Cook Inlet pression that occupied most of south- Mesozoic and Tertiary basin. Ayres ern Alaska. Volcanic islands and (1959), discussed the regional Tee- archipelagos, emergent throughout tonic framework of the basin as re- the belt, served as loca. .)urcesof lates to preorogenic and postorogenic sediments and provided areas for Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 units of the circumpacific geanti- reef limestone deposition. Bedded clinal and geosynclinal evolution cherts comprising a large part of (Kay, 1951). Parkinson (1962), pre- the Triassic System were probably sented the first field paper on the precipitated from volcanic silica basin’s first commercial oil field, and deposited contemporaneously with the Swanson River field. Other work- tuffaceous silts, shales and carbon- ers have recently contributed to the ates. knowledge of the complex stratigraphy Areal Extent and LithologY - Tri- and s~ructure of the basin (Kelly, assic rocks have been identified on 1961, 1963; Hill, 1963). Information the southeastern rim of the basin on the basin architecture was pro- near the town of Seldovia and on the vided by an aeromagnetic study of the western coast of Cook Inlet in the region by Grantz, Zietz, and Andrea- vicinity of Kamishak and Bruin Bays. son (1963). The excellent palyno- The Triassic rocks in the Kachemak logical studies of Wolfe, Hopkins and Bay region include limestone, tuff, Leopold (1965), provided much needed and banded chert, underlain by el- stratigraphic information on the lipsodial lava, slate, and graywacke nonmarine Tertiary rocks of the basin. which may be pre-Triassic in age. The Halbouty Alaska Oil Company STRATIGRAPHY Fritz Creek {/1well located a few miles eastof Homer on the north shore Rocks of Triassic co Recent age of Kachemak Bay penetrated several crop out in the Cook Inlet basin - hundred feet of white platey chert those older than Tertiary, are ex- which is considered to be equivalent posed in scattered localities on the in age to the surface exposures near ~rimeter of the basin, A general- Seldo”~ia. The total thickness of ized correlation chart of the basin the Triassic System in the Cook Inlet is shown in (Fig. 2). Mississippian basin probably exceeds 2000 fee~. and Permian sediments are exposed in a belt trending northwestward through JURASSIC ROCKS the southern Copper River basin and the slate-graywacke sequences com- The Jurassic rocks of southern prioin~ the Kenai-Chugach Ranges, al- Alaska, including the Cook Inlet though generally considered to be of basin, represent the most complete Upper Cretaceous ages may contain sequence of this age on the North Paleozoic rocks, (Martin, Johnson and American continent. The theater for Grant, 1915; Smith, 1939; and Ayres, Early Jurassic sedimentation was

1959)● similar to the Late Triassic, al- The maximum thickness of post-Paleo- though a hiatus of undetermined size zoic sediments deposited in.the deep- is reflected by a period of nonde- est part of the basin is estimated to position at the close of the Triassic. be 40,000 feet. Exploratory drilling During the Middle Jurassic, epeiro- has penetrated an aggregate of 30,000 geny transformed the paleozoic-early feet of sediments ranging in age from Mesozoic sedimentary trough into ge- ‘UpperTriassic to Recent. anticlinal and geosynclinal belts. The Tertiary rocks are discussed in Diastrophism renewed valcanic acti- greater detail in this paper because vity, and batholiths and lesser in- of their economic significance. trusive were emplaced on the north and west sides of the basin. The Naknek-Chinitna contact has been de- Cook Inlet basin became a dynamic scribed as conformable by most writ- structural basin and occupied approxi- ers; however, Barnes and Payne (1956] mately its present position. report the Naknek was deposited on Areal Extent and Lithology - An the eroded surface of older Jurassic unnamed section of Lower Jurassic formations, rocks is exposed on the southwestern Upper Jurassic marine clastic coast of the Kenai Peninsula in the rocks consisting of argillite and A thick se- green-gray siltstone, have been pene- vicinity of Seldovia. Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 quence of Lower Jurassic volcanic trated in wells drilled on the Kenai rocks known as the Talkeetna Form- Peninsula, off-shore Cook Inlet, and ation is exposed in the upper Mata- Susitna Flats. The rocks appear to nuska Valley and adjacent mountains be greatly altered in comparison to and has been penetrated in a few the surface sections exposed in the boreholes on the east side of the southwes~ern shore of the basin and basin. Middle and Upper Jurassic no direct surface-subsurface corre- sediments comprising the Tuxedni, lation has ye~ been published. Chinitna, and Naknek formations are widely distributed along the south- CRETACEOUS ROCKS western shore of Cook Inlet and in the upper Matanuska Valley. Early Cretaceous time w~ascharac- The Lower Jurassic rocks of the terized by shallowing of the seas Seldovia area consist predominately and local emergence reflected by of tuff and volcanic agglomerates. areas of erosion or nondeposition The unit is not fossiliferous but is along linear structural belts. lithologically similar, and perhaps Areal Extent and Lithology - Upper partly equivalent in age, to the Cretaceous rocks are exposed in Talkeetna Formation. The Talkeetna the Matanuska Valley along the Mal- formation is composed of volcanic keetna Mountain front, in the Chugach detritus containing fossil plants Mountains north of Turnagain Arm, and and marine invertebrates. The base in the extr,smesouthwestern part of of the formation has not been ob- the basin on the Alaska Peninsula served but the total thickness is es- south of Kamishak Bay. These rocks timated to be several thousand feet. have been encountered in several The Middle Jurassic Tuxedni Form- wells drilled on the eastern side of ation consists of sandstone, shale, the basin on the Kenai Peninsula, in conglomerate, and arkose. The thick- the vicinity of Anchorage, and in the ness of the formation is variable on Matanuska Valley. the outcrop and reaches a maximum of Rocks of Lower Cretaceous age have 7000-8000 feet on the Iniskin Penin- not been recognized in the Cook Inlet sula, according to Moffit (1927). Lasin. At the western end of the Overlying the Tuxedni and grading Copper River basin near the extreme upward with no recognizable struc- northeastern part of the Cook Inlet tural break, is the Upper Jurassic basin, a limestone unit preferred to Chinitna Formation composed of sev- as the Nelchina limestone, may be of eral thousand feet of red and dark Lower Cretaceous age although the argillaceous marine shales. meager fossil evidence for this age The uppermost formation of the is by no means incontrovertible Jurassic System is the Naknek Form- (Smith, 1939), ation consisting of a basal conglo- The Cretaceous rocks in the Mata- merate overlain by shales, arkose, nuska Valley are assigned to the and sandstone. The thickness of the Matanuska Formation, a shale, silt- formation varies greatly over the stone, and sandstone sequence of Al- area, ranging from approximately bian to Maestrichtian age (Upper Cre- 2000 feet in the upper Matanuska taceous) according to Grantz and Valley to more than 5000 feet in the Wolfe (1961). southwestern part of the basin. The The Matanuska Formation unconhxmbly overlies the Naknek Formation of TERTIARY ROCKS “LateJurassic age and attains a maximum thickness of more than 4000 feet. A gradual change occurred in the Along Ehe Talkeetna Mountains on the depositional environment of the basin north side of the Castle Mountain between the Cretaceous and Tertiary fault a well indurated arkose and con- Periods, The Mesozoic embayment that glomerate is considered to be a non- collected marine sediments and oc- marine equivalent to the lower part casional nonmarine wedges abutting

of the Matanuska Formation based on cratonic source areas was semien- Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 paleobotanical evidence (Grantz and closed or silled as the Chugach and Wolfe, 1961). The arkose, named the Kenai Ranges (Seldovia geanticline Arkose Ridge Formation by Barnes and of Miller, et al, 1957), began to Payne (1956), has been a strati - emerge. In general the Tertiary was graphic enigma since its rocks were characterized by fresh-to-brackish first described by Paige and Knopf water environments, with an abundant (1907). It was thought by different supply of sediments laid down in a writers to be older than, equivalent rapidly subsiding interrnontanebasin. to, and younger than rocks assigned Subsidence and deposition were nearly to the Matanuska Formation. in balance. Although the Arkose Ridge and Mata- Considerable igneous activity oc- nuska Formations are not economically curr6d in the northern part of the important to oil and gas exploration, basin during the Tertiary as evi- the facies relationship of marine or dented by extrusive in areas ad- marginal marine rocks grading into jacent to the Alaska Range and Tal- nonmarine coarse elastics adjacent to keetna Mountains and by sills or highlands probably is repeated in the dikes in Cretaceous and Tertiary Tertiary rocks and may be helpful in sediments penetrated in well bores determining the source rock-reser- inthe.Matanuska Valley. voir rock relationships. Areal Extent and Lithology - In the The Cretaceous rocks which have Matanuska Valley the Tertiary se- been penetrated by a number of wells quence includes three formational on the eastern side of the basin are units, the Chickaloon Formation, Wish- included in the Matanuska Formation. bone Formabion and Tsadaka Formation. Abundant fossil evidence is lacking; The Chickaloon Formation, named by however, occurrence of Inoceramus Martin and Katz (1912), consists of prisms in diCch samples or cores is nonmarine clastic sediments at least used to identify Cretaceous rocks. 5000 feet thick that is exposed in Very little is known about the age many places in the Matanuska Valley. and relationships of the rocks form- The formation contains many beds of ing the Kenai and Chugach Mountains. bituminous coal and is randomly in- The strata consist of intensely de- truded by igneous s~ocks, sills, and formed metamorphics-predominantly dilceswhich are more numerous in ~he slate and graywacke type clastics- eastern part of the Matanuska Valley. containing numerous quartz veinlets. The Chickaloon Formation appears to Excluding volcanics and ultra-basic rest unconformably upon the Matanuska intrusive the Kenai and Chugach Formation at some localities (Waring, Mountains have been mapped as undif- 1936; Grantz and Wolfe, 1961); how- ferentiated Upper Cretaceous. The ever, at others it may be conformable dating originated from examination with the MaCanuska Formation (Barnes, of small.Inoceramus casts collected 1962). The Chickaloon Formation is by Johnson (Martin, et al, 1915), conformably overlain by the Wishbone from rocks north of Turnagain Arm. Formation, a sequence of coarse, Many geologists believe the Kenai clastic, nonmarine sedimentary rocks and Chugach Mountains probably coIR- 2000 to 3000 feet thick. prise rocks of late Paleozoic and The Tsadaka Formation consists of qarly-through-late Mesozoic age. sandstone and conglomerate over 1000 SPE-1588 THOMAS E. KELLY

feet thick at some localities in the separated by thin gray shales. This Matanuska Valley. The Tsadaka Form- zone contains very minor amounts of ation rests with angular unconform- lignite as compared to underlying ity on the Wishbone or Chickaloon lithologic zones. Toward the base Formation and is weakly Iithified in of the zone, the sands become in- comparison to the older Tertiary creasingly conglomeratic. A persis- rocks. Cent conglomerate interval approxi- Outside of the Matanuska Valley the mately 600 to 800 feet thick is

Tertiary System in the Cook Inlet found at depths from 3500 to 5000 Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 basin is represented by the Kenai feet. Formation. The Kenai Formation is Underlying the massive sand zone is mantled by thick glacial deposits and an interval (Zone 11) approximately alluvium over most of the lowland of 2500 feet thick composed predomin- the basin but crops out in impressive ately of silty sand and shale inter- beach bluffs along the south and west bedded with occasional clean sands shores of Cook Inlet and north shore and numerous coal or lignite beds. of Kachemak Bay. Other exposures The coal seams range from a few occur in incised s~ream valleys on inches to over 30 feet thick and many the western side of the basin and on are correlative in the subsurface the south Kenai Peninsula.. for several miles. The Kenai Formation consists of in- Middle Kenai Unit - Underlying the terbedded loose to moderately indur- Upper Kenai Unit is an interval 3500- ated conglomerate, sandstone, and 5000 feet thick (Zone III) composed shale with numerous coal and lignite , of alternating siltstone and shale beds especially in the middle part. with minor thin sand stringers and In the deepest part of the basin many coal beds. Individual coal (along the coast of the Kenai Pen- seams are normally less persistent insula northwest of the town of Kenai) than the Zone II coals but locally the Kenai Formation is at least they are extremely helpful for cor- 18,000 feet thick. relation and to determine fault dis- Because the Kenai Formation is a placement because they often occur heterogeneousdeposit of nonmarine in clusters which yield distinct sediments which were depwited under electric log patterns. rapidly changing local depositional Lower Kenai Unit - Underlying environments, correlation from sur- Zone III is the principal oil hori- face exposures into the subsurface is zon in the basin known as the Hem- difficult and must be done in a lock zone. The Hemlock zone varies generalized way by comparing thick from a few hundred feet to over 10QO sedimentary zones of similar litho- feet and is composed of poorly to logic character. In the subsurface moderately sorted sandstone and con- the Kenai Formation can be subdivided glomerate, with interbedded carbon- into an upper, middle, and lower unit. aceous siltstone, shale, and coal. These divisions are arbitrarily de- The Hemlock zone is noticably per- fined and are not time-strati.graphic sistent in the Lower Kenai Formation intervals. On the Kenai Peninsula and may be equivalent at least in in the vicinity of Swanson River and part, to the arkosic sandstone and Kenai fields, at leas~ five distinct conglomerate of the Tsadaka Formation lithologic zones are recognized with- described by Barnes and Payne (1956), in the three Kenai units (Fig. 3). from exposures in the Matanuska Upper Kenai Unit - The Upper Kenai Valley. Unit at Swanson River and Kenai conl- The lowermost interval of the Kenai prises two lithologic zones. The Formation (Zone V) consists of silt- uppermost zone (Zone I) is 4500 to stone, shale and minor coal lentils. 5000 feet thick at Swanson River and This unit varies from a few hundred consists of clean, massive, fine to feet at Swanson.River field to over medium grained, unconsolidated sand 2500 feet in Cook Inlet. It overlies Cretaceous graywacke, Upper Jurassic occurrence of lignite and subbitu- marine siltstone and shale, Lower minous coal in the Clamgulchian Jurassic volcanics or intrusive, or strata at the type section (25 miles Triassic chert, depending on the south of the town of Kenai.);as de- local area. scribed by Wolfe , et al, compares Although the areal extent of in- with the Upper Kenai Unit at Swanson dividual sand or shale beds in the River and Kenai fields. Kenai Formation cannot be reliably North of the town of Kenai, the traced for more than a few miles, Halasko Bishop Creek Unit #11-11 well stratigraphic correlation wi~hin the penetrated over 9000 feet of uncon- Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 thick Tertiary sequence has been solidated sands composed of quartz greatly facilitated by recent paleo- and abundant mafic particles which botanical studies. The excellent probably were eroded from slate, work of Wolfe, Hopkins, and Leopold graywacke sequences in the Kenai (1965), and earlier studies by Hill Mountains. (1963), indicate that basinwide cor- The mafic grains are much less correlation within the Kenai Formation mon to the Upper Kenai Unit at Swan- can be accomplished with reasonable son River and Kenai fields. Quite accuracy. Onc the sedimentational possibly, the Upper Kenai Unit pene- history reflec7 ed by the strati- trated at Bishop Creek is younger graphic sequence is fitted to the than the Upper Kenai Unit at Swanson structural framework of the basin, River and Kenai, and younger than most of the problems on occurrence strata assigned to the Clamgulchian of~hydrocarbons can be explained. stage. The paucity of organic (coal Surface and subsurface exposures and lignite) material in the Bishop of the Kenai Formation contain abun- Creek well section also suggests a dant pollen and fossil leaf floras stratigraphically younger age for of varied character but virtually no these sediments. micro or megafossil animals. Fossil The basal part of the Upper Kenai leaf floras are more reliable than Unit (Zone II) and part of the thick pollen grains in age determination siltstone, shale and coal section and have been used to subdivide the (Zone III) possibly reflects the Kenai Formation into three time- Homerian provincial stage. stratigraphic units defined as the The Hemlock zone is assigned to the Seldovian (Oligocene ? and Miocene), lower part of the Seldovian provin- Homerian (Miocene and Pliocene?)and cial stage and, as mentioned pre- Clamgulchian (Miocene ? and pliocene) viously, may be equivalent, in part, provincial stages (Wolfe, Hopkins, to the Tsadaka Formation of the Mata- and Leopold, 1965). nuska Valley. It is not possible at this time to Based on Lithologic association and draw a direct correlation between the paleogeologic interpretation this provincial stages defined by Wolfe, writer ’suggested tha~the Hemlock zone ——et al, and the lithologic zones de- might be equivalent to some part of scribed above; however, the litho- the Chickaloon Formation (Kelly, logic character of outcrop samples 1963). There is not sufficient evi- containing different amounts of ex- dence to establish, unequivocally, otic taxa can be used, in a very the exact age of the Hemlock zone generalized manner, as a basis for for as pointed out by Wolfe, et al, comparing exposed rocks with rocks (1965), “The lowest part of the Kenai observed in cores and ditch samples. Formation penetrated in petroleum Zone I of the Upper Kenai Unit in the exploration wells, however, may POS- vicinity of Kenai and Swanson River sibly consist of beds that are not fields is believed to be no older present in any surface exposures than the Clamgulchian (Pliocene) pro- from which we have floras and that vincial stage. The composition, cie- may be appreciably older than the gree of lithification, and spatial Seldovian stage”. SPE-1588 THOMAS E“.KELLY 71

Based on the recent stratigraphic several of the interior subsurface revision the most serious objections fold belts reflect control by Jur- to a Chickaloon-Hemlock zone equi- assic structures and trends. valency are: (1) The Chickaloon ex- The most prominent positive struc- hibits considerably more deformation tural features of the basin are and lithification than the Hemlock geanticlinal belts which border the zone. (2) The Chickaloon grades up- basin on the north, east, and wesb, ward into the Wishbone conglomerate These belts are reflected in the

which is separated from overlying rugged !;opographyof the Alaska Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 Tertiary rocks by a marked angular Range, Talkeetna Mountains, and unconformity. Rocks immediately Kenai-Chugach Range. overlying the Hemlock appear to be The Talkeetna geanticline was unconformable. However, at least two itially uplifted during Middle Jur- intraformational unconformities with- as~ic. Uplift of the Seldovia ge- in the Upper and Middle Kenai Units anticline (Kenai-Chugach Range) may can be.inferred from the analysis of have occurred during the late-Cre- dipmeter levels and by plots of in- taceous, however, the present form terval transit time from acoustical is the result of Cenozoic deformathw logs versus depth. Tertiary rocks dip moderately into As more information from explora- the regional synclinal axis of the tory drilling is collected and inter- basin, and reversal of regional dip preted the assignment of the Hemlock is very abrupt - the profile of the zone to a part of a time-strati- basin is nearly V-shaped between graphic unit will be made with de- Swanson River and Middle Ground fendable conclusion. Shoal (Fig. 4). This writer believes that the basal Major fault zones are present on Kenai Formation (Zone V) which rests the north (Castle Mountain zone) and unconformably on Mesozoic rocks in southwest (Bruin Bay zone) edge of the producing oil fields is older the basin (Fig, 5). High angle than rocks assigned to the Seldovian faulting probably borders the west- provincial stage. Admittedly, there ern side of the Kenai-Chugach Range is no palynologic evidence available (Chugach fault, Hill, 1963); however, for this conclusion; however, the the bedrock is intensely broken writer asc~ibes to a younger than folded and covered in most places by Jurassic source for the Hemlock zone glacial outwash such that accurate oil accumulations and suspects the determination of the fault trace at source rocks are marine equivalents the surface is next to impossible. of the Lower Kenai Formation. Since The Castle Mountain fault zone Cretaceous rocks appear unlikely differs from the Bruin Bay and Chu- source rocks and oil shows in the gach fault zones in that it cuts Chickaloon Eormation (Paleocene ?) across the trend of early Mesozoic probably originated in the same erogenic belts. Ayers (1959), at- source rocks as oil in the Hemlock, tributes this change in structural an Early Tertiary source is indi- alignment to an adjustment of re- caCed, and Zone V of the Lower Kenai gional stresses to conform to the ‘ Unit may be a continental facies of sialic continental edge formed by these source rocks. continental accretion north of the Aleutian oceanic trough. The Bruin STRUCTURE AND TECTONIC HISTORY Bay and Chugach fault parallel the dominant structural trend of the The Cook Inlet Tertiary basin is Mesozoic and Cenozoic erogenic belts. principally a result of late-Creta- Both vertical and strike-slip ceous-through Tertiary deformation (rift) displacements are indicated with movements continuing to the on the faults. Displacement on the present; however, the main structural Castle Mountain fault since Early features outlining the basin and Tertiary time may be as much as 8 GEOLOGICAL CHARACTERISTICS OF THE COOK INLET AREA, ALASKA SPE-1588

10,000 feet (Gates, et al, 1963). the Soldotna Creek Unit. The Sol- The principal subsurface structural dotna Creek Unit 41-4 well was com- features within the basin are narrow pleted through selective perforations elongate anticlinal belts which trend from 10,234-10,565 feet for approxi- in a north to northeast direction mately 4350 BOPD and 1150 Mcf gas per similar to the structural grain of day on a 48/64-inch choke with 490 the major tectonic elements (Fig. 5). psi tubing pressure. Several of the subsurface anticline In late 1963 the field was complete-

are reflected at the surface by topo- ly developed on an 80-acre spacing Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 graphic or drainage anomalies and a pattern containing 60 producing oil few are mappable from surface dips wells, 5 gas wells, and 11 dry holes. along beach bluffs and river valleys Estimated ultimate recoverable oil which expose sections of the Kenai reserves range from 175 to 225 mil- Formation. At least four parallel, lion barrels. buried anticlinal trends are present Structure - Swanson River field is in the subsurface. All of the oil located on the crest of a doubly and gas fields found to date are plunging, elongate anticlinal fold. located near the crest of local struc- Accumulation is confined to “anarea tural closures positioned on the anti- seven miles long and less than two clinal belts. miles wide (Fig. 6). Although uplift on all of the fold Seismic profiles show that the belts has occurred during the Terti- regional fold on which Swanson River ary, several reflect pre-Tertiary de- field is located is more than 20 formation. Structures on these belts miles long and 10 miles wide between are the locale of the known oil fields. opposing flanks. Therefore, the pre- Other fold belts are believed to be sent producing area is representative much younger and experienced initial of starved anticline. Other strati- uplift during the Tertiary. Gas ac- graphic and structural oil pools will cumulations are commonly associated probably be discovered on the region- with local structures on these fold al fold. Birch Hill gas field dis- belts but to date no oil accumulations covered in 1965, is located on the have been found. north plunge of the Swanson River anticline, 3 miles north of pro- GEOLOGY OF OIL AND GAS FIELDS duction in Swanson River field. The anticline is cut by several Swanson River field, the first I transverse normal faults downthrown large economic oil accumulation found on the north (Fig. 7); the fault in southern Alaska, was discovered planes dip moderately (50° - 600). by Richfield Oil Corporation in Displacement on the faults varies August, 1957. The initial test from 50 feet to a maximum of 500 (Swanson River Unit 34-10) was drill- feet. ed in a remote area of the Kenai The structure contour map of Swan- National Moose Range after a recon- son River field (Fig. 6) is drawn on naissancereflection seismic survey the “D’tmarker horizon, a 10-30 feet provided dip and strike control which thick coal bed within Zone 11 of the indicated anticlinal closure at depth Upper Kenai Unit. The “D” marker in an area defined by a prominenb coal is the most persistent indivi- geomorphic anomaly. The well was dual sedimentary bed in the field. completed in the Hemlock zone of the The “D” marker immediately overlies Kenai Formation through perforations the “D” sand gas accumulations in from 11,150-11,215 feet, for 900 BOPD the Soldotna Creek Unit portion of of 33° API gravity oil. the field. Development of Swanson River field The flanks of the anticline steep- began slowly with no more than two en with depth and the fold is slight- wells drilling at any one time until ly asymmetrical toward the west so early 1960, when a southerly exten- that maximum structural relief on gas sion to the field was completed in producing zones lies west of the SPE-1588 THOMAS E. KELLY 9

maximum structural relief on the Her- einmultiple sand reservoirs in the lock oil horizon. The structure sec- Middle and Upper Kenai Units. tion (Fig. 7) shows a possible uncon- Casinghead gas, although not in formity at the base of the conglomer- great volume in the Hemlock reser- ate beds of Zone I (Upper Kenai Unit). voirs is important in pressure main- The sands of Zone I are gently arched tenance. and conform with the structural at- The dry gas accumulations at Swan- titude of the unconformity. Beds be- son River occur at different strati-

low the unconformity have much steep- graphic intervals in the Upper and Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 er dips. Note that fault planes ap- Middle Kenai Units. The gas is found parently do not cross the unconformity. in association with lignite, coal Reservoir and Reservoir Fluids - and other carbonaceous debris and is Oil production in Swanson River field undoubtedly indigenous to the Kenai is from intersti~ial porosity in sand- Formation. stone reservoirs of the Hemlock zone Parkinson (1962), describes the of the Lower Kenai Formation. Indivi- gas intervalsas “A” and “B”, “D” and dual sands of the Hemlock zone are 11EII , which approximately correspond variable in extent, porosity, and per- to Zone I, II, and III of this paper, meability. The upper sand is the most The accumulations in Zone I and consistently productive reservoir; how- 111 are stratigraphically controlled- ever, it is commonly erratic and brok- Zone II accumulations occur on foot- en so that the net thickness varies wall blocks. The structural cross from one well to another. Producing section (Fig. 7) illustrates the sands range in depth from about 10,200 distribution of the known gas pro- to 11,000 feet. ducing intervals and the nature of The field comprises approximately entrapment. 4500 productive acres with a verti- The dry gas accumulations at Swan- cal oil column of 300-600 feet. The son River are minor in comparison net oil sand thickness ranges from to Kenai, Beluga, and other fields 75 feet in the Swanson River Unit to that produce gas only. It is dif- over 300 feet in the Soldotna Creek ficult to explain why the gas oc- Unit. The porosity of the sandstone currences at Swanson River is not reservoirs averages above 25 percent; more prolific. Potential reservoirs perrneabilitiesrange from 1 to over are in close proximity to thick coal 3000 millidarcies and average about beds but are water bearing. The gas 100 millidarcies. Volume of oil in sands comprise-thin and lat@rallydis- place is approximately 1000 barrels continuous bodies, or, in the case of

per acre foot. thick sands of Zone I they have a ● The Hemlock zone reservoir is high- limited vertical gas column. ly undersaturated with a maximum gas- oil ratio of 400 cubic feet per barrel. KENAI GAS FIELD Oil gravities and gas-oil ratios range from 30° API gravity and 116 stan- The Kenai gas field, a huge methane dard cubic feet per stock accumulation, is the only other pro- tank barrel at the north end of the ducing field in the Cook Inlet basin ! field to 38° API gravity and 400 stan- on which there is sufficient avail- dard cubic feet per stock tank barrel able information at this writing. at the south end of the field. The Kenai field was discovered by Union separate reservoir blocks created by Oil Company of California and Mara- faulting are acted upon by modified then Oil Company in October, 1959. edgewater drives and gas expansion. The discovery well was drilled on a The gas accumulations at Swanson large doma,lseismic structure in River field are of two types: search of Hemlock ‘oil accumulation. (1) Casinghead gas produced in assoc- The Hemlock zone was non-productive iation with crude oil from the Hemlock and the well was plugged back and =One. completed in the upper portion of the (2) Dry Methane gas which occurs Kenai Formation through perforations 10 GEOLOGICAL CHARACTERISTICS OF THE COOK INLET AREA, ALASKA SPE-1588

between 4455 and 4590 feet for 31,000 HABITAT OF OIL AND GAS Mcf per day on a 7/8-inch choke, with 1634 psi flowing pressure. Cretaceous paleogeology is most use- The present estimated recoverable ful in explaining oil accumulations in reserves exceed one trillion cubic Tertiary sands of the Hemlock zone. No feet of gas. The field is only par- early Cretaceous (Aptian) or Middle tially developed and as of 1966 con- Cretaceous rocks have been recognized tains four producing gas wells in the basin, and Upper Cretaceous (1 dual), nine shut-in gas wells, and racks, although widespread in the sub- two dry holes. surface, are absent from the crests of Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 Structure - The Kenai structure Swanson River, Middle Ground Shoal (Fig. 8) expressed in the Upper Kenai and several other anticline. Although Formation is a broadly elliptical, the structural relationship of the north-south trending anticline, pro- Lower Kenai Formation (Hemlock zone) bably transected by several dom-to- to the Jurassic is known only in a few the-north faults with throw of 150- widely scattered areas in the basin, 200 feet. The anticline is probably the evidence strongly suggests that more complex than shown on the struc- structural accumulation of oil is re- tural contour map and cross section; lated to old belts which were uplift- however, the density of available well ed before Late Cretaceous and remain- control is not sufficient to map the ed positive until Tertiary Lime. fold in detail, The north-south stratigraphic cross The Kenai anticline seems to be re- section (l?ig%10) shows an interpre- Iated to structure in deeply buried tation of the unconformable Tertiary- Mesozoic rocks because the arching in Mesozoic boundary from the Matanuska the Upper Kenai strata roughly coin- Valley to the southern Kenai Penin- cides with a sharp magnetic anomaly sula. The area of erosion or nonde- in the basement (?) complex (Grantz, position in the center of the figure et al, 1960). The fold is more elon- represents the Swanson River ant,i- gate and narrow on deeper horizons and cline. Cretaceous rocks on the structure closure is believed to in- flanks of the uplift onlap and trun- crease with depth. cate Jurassic strata. Reservoir and Reservoir Fluids - The A subcrop map with the Quaternary gas sands of the Upper Kenai Formation and Tertiary rocks removed shows the at Ksnai field are clean, unconsoli- relation of oil occurrence to pre- dated, fine to medium grained quartz Tertiary structure (Fig. 11). shown sands, on the map are several postulated rift Net gas sand thickness ranges from (strike-slip) movements where the pro- about 100 to over 300 feet. Prod- ucing area (inliers) appear as drag sities calculate in excess of 30 per- folds. Such an explanation is wholly cent; however, some sands which are interpretive based on geomorphic gas bearing have low permeabilities alignments and subsurface scratigraphic and have not been produced. Inasmuch variations. The Hemlock zone appears as these reservoirs have not been to be persistent over much of the artificially stimulated, their pro- basin but the Middle and Lower Kenai ductivity is unknown. In addition to Units vary greatly in thickness and variations in porosity and perme- amount of coarse elastics present in ability, the discontinuity and len- the section. Therefore dissection of ticularity of individual sands makes the basin into rift blocks may explain it difficult to predict, prior to the rapid change in the sedimentary drilling, which beds within the zone character of the Middle and Lower Kemd are likely to be gas productive. The Units. gas sands of the Upper Kenai Unit are Rocks ranging in age from Upper Jur- shown in the structure section (Fig.9). assic to Tertiary have been mentioned Note that apparently correlative sands as probable source beds for Hemlock are not uniformly productive, regard- oil accumulations by ‘different writers. less of the structural elevation. The Cretaceous rocks thus far SPE-1588 THOMAS E. KELLY 11

identified are the least likely source oil in horizons above the Hemlock beds. This writer believes the source lends support to the concept that the beds are of Early Tertiary age. oil is from Tertiary source rocks If the oil is from Jurassic source which have thus far eluded detection. beds, it had to remain virtually insitu The argument that oil in these hori- during diagenesis, strong lithifica- zons has migrated along fault planes tion, folding and long hiatus before from deep source rocks (Jurassic) is migrating into the Hemlock. Occ~r- not impossible; but this writer does rences of petroleum under such con- not ascribe to a tortuous migration

ditions would indeed be unique. principle and believes that faults Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 Several of the recently discovered are barriers to migration rather than fields in Cook Inlet (offshore)pro- conduits. duce from sandstone reservoirs that The best explanation for oil accumu- are stratigraphically higher in the lation in the sands which overlie the Kenai Formation thar.the Hemlock zone. Hemlock by several thousand feet is Whether or not these accumulations that the accumulation related to un- migrated from the same source rocks conformities within the Kenai Form- as the Hemlock zone accumulations can ation and is also controlled by pre- only be theorized from the information Tertiary structural trends. available. However, the discovery of

REFERENCES

Ayres, M. G., 1959, Regional Geology of the Cook Inlet area, Alaska: unpublished report.

Barnes, F. F., 1962c, Variation in rock of Tertiary coals in Cook Inlet basin, Alaska: U. S. Geol. Survey Prof. Paper 450-c, p. c14-c16.

—2 and Payne, T. G. , 1956, The Wishbone Hill district, Matanuska coal field, Alaska: U. S. Geol. Survey Bull. 1016, p. 9-51.

Gates——. G. O. and Grvc. G.. 1963, Structure and tectonic history of ‘ Alaska: Am: ~sso~. petro. Geologists, Memoir 2, p. 264-277. W>—u Grantz, Arthur, and Jones, D. L. , 1960, Stratigraphy and age of the Matanuska Formation, south-central Alaska: Art. 159 in U. S. :, Geol. Survey Prof. Paper 400-B, p. B347-B350.

—9 and Wolfe, J. A., 1961, Age of the Arkose Ridge Formation, south-central Alaska: Am. ASSOC. Petro. Geologists Bull., v. 45, no. 10, p. 1762-1765.

—1 Zietz, Isidore, and Andreasen, G. E., 1963, An aeromagn@tic reconnaissance of the Cook Inlet area, Alaska: U. S. Geol. Survey Prof. Paper 316-G, p. 117-134.”

Hill, M. L., 1963, Occurrences of petroleum in the Cook Inlet area, Alaska, World Petro. Cong. VI, Frankfurt, sec 1, Paper 39. ‘Kay, Marshall, 1951, North American geosynclines: Geol. Sot. America Memoir 43. Kelly.. . T, E., 1961, PhoCo&ealow. ..A quick, economical tool for oil hunt&s: Oil and–Gas ~bur., ~. 59; no. 47, p. 265-274. Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

—$ 1963, Geology and hydrocarbons in Cook InleC basin, Alaska: Am. Assoc. Petro. Geologists, Mere.2, p. 278-296.

Martin, G. C., 1905, The petroleum fields of the Pacific Coast of Alaska: U. S. Geol. Survey Bull. 250, p, 37-50.

—) and Katz, F. J., 1912, Geology and coal fields of the lower Matanuska Valley, Alaska: U, S, Geol. Sruvey Bull. 500, 98 p. Martin, G. C., Johnson, B. L., and Grant, U. S., 1915, Geology and mineral resources of Kenai Peninsula, Alaska: U. S. Geol. Survey Bull. 587, 243 p.

Mather, K. F., 1925, Mineral resources of the Kamishak Bay region: U. S. Geol. Survey Bull. 773-D, p. 177-178.

Miller, D. J. Payne, T. G., and Gryc, G., 1957, Geology of possible petroleum provinces in Alaska: U. S. Geol. Survey Bull. 1094,131 p.

Moffit, F. H., 1927, The Iniskin-Chinitna Peninsula and the Snug Harbor district, Alaska: U. S. Geol. Survey Bull. 789, p. 48-55.

Paige, Sidney,and Knopf, Adolph, 1907, Geologic reconnaissance in the Matanuska and Talkeetna basins, Alaska: U, S. Geol. Survey Bull. 327, p. 24.

.Parkinson,L. J., 1962, One field, one giant..,the story of Swanson River: Oil and Gas Jour., v.60, no. 13, p. 180-183.

Smith, P. S., 1939, Areal geology of Alaska: U. S. Geol. Survey Prof. Paper 192, p. 36-73.

Waring, G. A., 1936, Geology of the Anthracite Ridge coal district, Alaska: U. S. Geol. Survey Bull. 861,57 p.

Wolfe, J. A., Hopkins, b. M., and Leopold, E. B., 1965, Tertiary stratigraphy and paleobotany of the Cook Inlet region, Alaska: U. S. Geol. Survey, Open File Rept. p. 1-76.

---- Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

—— ..——-——.—..-—— L–—— I L-

L– --!- 50 MILES

Fig. 1 - Cook Inlet Basin, Principal Oil and Gas Fields GENERALIZED CORRELATION CHART OF TERTIARY ROCKS COOK INLET BASIN, ALASKA

NORTH SOUTH MATANUSKA VALLEY KENAI PENINSULA Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

GLACIAL DEPOSITS 8 ALLUVIUM

0I-=&l —.— —— PLIOCENE CLAMGIJLC~lAN STAGE l&NAl FORMA7101V HO MERIAN STAGE

MIOCENE SELOOVIAN STAGE * ; k OLIGOCENE # ,,, ,,, ,,, ,,,, ,, ; ; : ../ . /,../,.,, ,, ,.,, ,,, /,/,,,/,,,//'/'`,'``'`'/''"```'''``''''''`''`' ,,,,,,,,,,,,,,,,,.., ,,, ,,, ..:; ;fl,;/..>!’!’’’’’’’’’’”;;; :;; :::; :;~:: EOCENE , ;;;:;:;;;;;;:,,, ,,, ,,,;; ,,,fj: ///;,,,,,,,,.7///,,;;:<;j;j;/,. . ./>/./,., <;: ,<; <<;;;;,,,,, ,,, ,,. .,.,,,,,, z,,, ,,, , ./,,,,,,,,,,, ,/ /.//,/,/,7///?. , /,.,,,,,,, PALEOCEfAE CHJCKhLOON FM. ,,, ,,, ,,, ,,, ,, /,. . >...,/.,/.,,,”. , .,,,.,,.,/’ ,,, ,,,, /./.,,/.//?/.. j,,,, ,,, ,,,,,,,, ,,,,,,,.,,/. /,/////, /, /?,,,,,,,,,.,, ,,,, ,,, ,,, ,,,,.. // /..,..,,,:<;; s ,,, ,,,, ,., .//, .,, ,,, ,, e ,,, .,,<<;;;;:; jj; ;:: f:!x, ; ;;;;;,,,,,/ ///<..,,;,,;; :::/ ; hfA7ANuSKA FORMATION ;;2 ;;;:;;;;;;:;;; :; fi; ;;;;:; ;;;;;;:;;;;;; ; ;:;,,,,..,,.. : ARKOSE RIDGE FM. ;;; ;:; ;;; ;~ ; :;;<:,;:::;.’ ::,

! $ vOLCA171CS 8 INTRuSIVE ROCfS(TALKEETIYA FM.)

!?’ t! 5 LIMESTOME, CHER7’ @ VOLCANIC ROCZS [E I

Fig. 2 - Generalized Correlation Chart Cook Inlet Basin, Alaska

Fig. 3- Type Log of Kenai rormation Sedimentary Units and Lithologic Zone WEST EAST MIDDLE GROWID KE!(A1 SWAHSON RIVER SHOAL Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

. Fig.4- East-West Cross-Section, Cook Inlet Basiq

Fig. 5 - Tectonic Map, Cook Inlet Basin . ,

I I .i =w----;----~l----,--fn-`-----:----~-----!-----i--ti-\-~ 1

● Q

j Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 —,......

I 1( e

I i---- ! f I i I .~ Q# . 1 10

“1 ,0’ :-V , !“ ‘$)1, .

]

~, + ......

,. .---..”~:–--;----––––;-- ....’

f . ..-; “ i “ ; “ .; ./ ~)\ ... o I ..+. -,,’ ~ Iwv...-..._ ..._~.._._ ..------...... ----- . .---- u’ .+_ ...... - -+ . .. . . , [ f)’

Fig, 6 - Structure, Top of “D” Marker Coal Bed Swanson Rive-r_ Field, Kenai Peninsula, Alaska SOUTH STRUCTURAL CROSS SECTION NORTH SWANSON RIVER FIELD KENA[ PENINSULA, ALASKA

S,*,, e!C.,,c,”,, YOM,,, ,, Cd,,e”,, $m.*!# .1 C4!llwou 9,.*!, d C’41,1.m!o S,wo 4! Cohtwme Slwmrd .1 C4fom. Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 smvne - umr ua s- GW, (6?,, ?43+ - R* LhW .=21.- S.m,v em w,,, #/#t, w.- m“? Lcmltl,.,s $*M” muu./,m., *

6500 ——— — — ______..______— -SW{

Fig. 7- North-South Cross-Section Showing Nature of Shallow Gas Accumulation, Swanson River Field Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

Fig. 8 : Structure, Top of one Correlative Producing Sand, Kenai Field

.— SOUTH NORTH

wASILLA DEEP CREEK SWANSON RtVER

Fig.’ 10”’- North-South Stratigraphic Cross-Section, Matanuska Valley to Southern Kenai Peninsula-- Interpretation of Mesozoic-Tertiary Boundary STRUCTURAL CROSS SECTION

KENAl GAS FIELD Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021 NORTH COOK INLET BASIN, ALASKA SOUTH

Unmn o,! cd C.l,ferrm Unmn Od of C.hmrm U.m. 0,1 01 Col,fomm umo. 081 of Cahforme R,lw, lmt 33-30 K,”o# U“,! 34-31 Km., u“,f !4-6 K..o! u.!! 41-19

.5000 ______.–.- ...... — -----.— . .--s1? - --- ..31> --+~

,5500 ———– . ...––. -. . -5500

Fig, 9 - North-South Cross-Section, Shallow Kenai Gas Sand Interval, Kenai Field

. Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/66FM/All-66FM/SPE-1588-MS/2087697/spe-1588-ms.pdf by guest on 25 September 2021

I 1 1. .O...:”zzlzl !&#’ L 1 J& CRETACEOUS m JUI?A SSIC MA RiNE

JURASSIC VOLCA N/CS

X,xx L-1xxx JURASSIC lN~FFUSIVES

>>..’,,, ,,:.:,’1:...... n...... TRIASSIC

Fig. 11 - Subcrop of pre-Tertiary Rocks, Cook Inlet Basin. Known Producing Areas Interpreted as Drag Possible Explanation Folds Along Major Rift Zones. . for Presence of Jurassic Inliers