Reconnaissance hydrocarbon geology of the Anadyrsky, Khatyrsky, and Penzhinskaya Guba Cenozoic

(10–35%),plagioclase (15–40%),and quartz (up to 10%)fragments and grains of sedimentary rocks.The sandstone matrix consists of zeolites,carbonates,chlo-rite,kaolinite,leucoxene,clay minerals,sphene,and epi-dote.The presence of readily altered rocks and minerals in the clastic part provided a substantial rock fraction,which was easily altered during the regional prehnite–pumpellyite metamorphism in Sobolevskaya sandstone.This has resulted in locally decreased porosity and permeability of this formation.The open porosity of the Sobolevskaya sandstone (Table 1)is between 2.4%and 19.1%,but in the southern part of the basin where this formation produces oil,open primary porosity never exceeds 12%.Geostatic load,granular size,and sorting of the sandstone show no correlation to permeability or porosity,suggesting that the development of the sec-ondary minerals in the matrix is a significant factor in these petrophysical parameters of the rock.The per-meability of the sandstones calculated from natural gas and oil production from the Sobolevskaya sandstone is188Geologic NoteTable boratory and In-Hole Determinations of Permeability of the Sobolevskaya Formation Sandstones of the Telekaisky StructureWell number Depth (m)Permeability parallel to bedding (md)Permeability perpendicular to bedding (md)Permeability (in-hole,md)Gas (mcf/day)Oil yields (bbl/day)Water yields (bbl/day)11732–1747**88.935491**11711–1727**59.225085**41882.9–1900.20.1–1.910.1–1.8013.62**121271689.7–1731.1<0.01–0.01<0.01–0.01 1.485533**71858.9–1938.70.1–1.760.1–1.6628.23**100091863.3–1875.50.1–4.110.1–1.4961.88*895*91890–1875.60.1–1.840.1–2.43728.41*534**No core was taken.Figure 4.Achinsky oil fields of the Anadyrsky basin:(A)structural map and (B)cross section.510 kmABOil wells: map and cross-section view Reflectors (cross sections only)Equal-depth lines to the top ofSobolevskaya Formation (depth in kilometers)Cross-section reference location.between 1.5and 89md (Table 1),greatly exceeding those values determined by direct analysis of core sam-ples.In one case,within the Telekaisky structure,cal-culated permeability reaches 728md,while maximum permeability measured from core samples was only 2.4md.The Sobolevskaya sandstone yields light (API grav-ity from 39.4to 43.4j ),very low-sulfur (0.01–0.02%)oil.The carbon preference index decreases with depth from 1.47to 1.06(Ivanov,1985).Interstitial fluids analyses (Gurko et al.,1986)re-veal that the light fractions of the oils have high contents of aromatic hydrocarbons and that their concentration increases with depth.Gaseous hydrocarbons from sev-eral levels above the main oil-producing Sobolev-skaya sandstone reservoir contain high amounts of low-boiling-temperature aromatics up to C 8.Heavy paraffins in the oils reach 15vol.%in the lower part of Achinsky structure and 27%in the lower part of Telekaisky structure.The pristane/phytane ratio in the Anadyrsky basin oils is 12.67.Oil-producing structures were formed during Cenozoic tectonic activation,due to the thrusting of Al’katvaamsky and Algano-Mainitsky terranes over the Anadyrsky sedimentary basin and the collision of the Ukelayat,Emravaamsky,and Ekonaisky terranes.The faults in the upper Oligocene–lower Miocene strata served as vertical channels,along which hydrocarbon migrated from source rocks beneath already existing or syntectonic traps.Most of the steep faults do not extend above the clay strata of the Telekaisky Formation in the Anadyrsky basin.No hydrocarbons have been found associated to a monocline located immediately to the south of the Telekaisky oil field.In this region,drilling has revealed oxidized,static oil,found at the depth of 885m in fault-related fractures.Bitumens,marked by red-brown lu-minescence,were observed at depth between 2200and 2300m,but no interstitial fluid flow was observed during formation pump tests.Geophysical data,well logging,and borehole studies suggest the absence of reservoir formations in the Miocene section to a depth of 2300m.This stratigraphic level is within the inter-val that is productive in the Achinsky and Telekaisky oil fields.Khatyrsky BasinThe Khatyrsky sedimentary basin is located in the northeastern part of the Ekonaisky terrane of the Kor-yak superterrane,adjacent to the Alkatvaamsky ter-rane (Figure 1).The basement of the basin contains Carboniferous–Lower Jurassic pillow basalts and jas-pers,Carboniferous–Permian limestone blocks,Meso-zoic ophiolite complexes,and Upper Jurassic–Lower Cretaceous tuffaceous sandstone,mudstone,tuff,and pillow basalt.Upper Maastrichtian to Miocene sand-stones,shales,siltstones,and conglomerates represent the basin-fill complex (Figure 6).Basin-fill thickness increases to the south,where sedimentation started and ended significantly earlier than in the northern parts of the basin.Presently,the Uglovoya oil field is the sole economic hydrocarbon deposit found in theHarbert et al.189Figure 5.Telekaisky oil fields of theAnadyrsky basin:(A)structural map and (B)cross section.km510AOil depositsGas depositsReflectors (cross sections only)1.9Oil wells:map and cross-section view lines to the top ofCross-section reference location.basin.The producing reservoirs are found in the Mio-cene strata of the Khatyrsky basin (Figure 6).The age of basin-filling sediments and the compo-sition of the tectonic basement in the Khatyrsky basin are quite variable.Basement lithologies include several thrust sheets of Paleozoic and Mesozoic ophiolites,Upper Jurassic to Cretaceous turbidites,tuffaceous sand-stone,mudstone,and tuff.The deposition of the sedi-mentary section began during the late Maastrichtian in the southern part of the basin and during the Oligocene in the northern part of the basin.Although the Khatyrsky basin has a complex internal structure of several troughs separated by internal uplifts,it is generally assumed that sedimentation both began and ended earlier in the south-ern part of the basin.The Uglovoya oil field is a complex structure lo-cated offshore in the northwest Bering Sea.Seismic data have been interpreted to show the main basin struc-ture as a fault-bounded anticline separated into several blocks by steep reverse faults.The major oil reservoir consists of sandstones and conglomerates of the lower Miocene Imlikinsky Formation (Figure 7).Indicators of hydrocarbons and increased gas content were observed during exploration drilling,beginning at a depth of 600m.A significant oil deposit was encountered between a 1638and 1658m depth.Gas content during drilling increased from 1.5to 20%,sometimes up to 30–40%,with con-centration of heavy hydrocarbons reaching 5–7%.Below 900m depth,beds of siltstone and sandstone show evi-dence of a hydrocarbon component,such as yellow lu-minescence and what is described as a ‘‘dead’’oil smell (Ivanov,1985).In the Khatyrsky basin,the Neivytvyrsky and Anol’sky Formations,which contain thick imperme-able claystone strata,separate the reservoir rocks of the Imlikinsky Formation from the source rocks of the Ionaysky Formation.Steep faults do not generally ex-tend above the base of the Vaamochkinsky Formation in the Khatyrsky basin,enabling the preservation of hydrocarbons in trapping structures.190Geologic NoteFigure 6.Stratigraphic column of the northern and southern parts of the Khatyrskybasin.Geochemically,hydrocarbons from the Khatyrsky basin are similar to those of the Anadyrsky basin.Hy-drocarbons from the Imlikinsky Formation are light oils with API gravity of37j and low sulfur content(0.13%). The carbon preference index of1.16has been inter-preted to show moderate input of the terrestrial sources (Ivanov,1985).The hydrocarbons are depleted in low-boiling-temperature hydrocarbons of the pentane-hexane fraction and enriched in hydrocarbons of the heptane-octane fraction.The ratio of C5+C6sat-urates to C7+C8saturates is0.28.The pristane/ phytane ratio in the Uglovoya oil reaches a value of 22.55(Gurko et al.,1985).Minor gas accumulations above the main producing reservoir are enriched by heavy hydrocarbons while the liquid phase is sub-stantially degassed.The Uglovoya uplift is interpreted to be underlain by the Eocene to lower Oligocene Ionaysky Formation,a thermally mature,organic-rich formation found during geological mapping and deep drilling within and adjacent to the basin.Vitrinite reflectance and catagenetic mineralogy show that these rocks have passed through the oil-generation window and they are the most likely source of oil in the Khatyrsky basin.Penzhinskaya GubaThe Penzhinskaya Guba basin(Figure8)has the most heterogeneous basement of the three basins described here.It is underlain by Paleozoic and Mesozoic ophi-olite tectonostratigraphic terranes consisting of olis-tostromes,parts of the Okhotsk-Chukotsk plutonic-volcanic belt and Paleozoic to Tertiary sedimentary units.A distinctive feature of the Penzhinskaya Guba sedimentary basin is the presence of Eocene to Oli-gocene shallow-water or subaerial basalt flows.K-Ar dates range from41to31Ma(Ivanov,1985)for these units and suggest that volcanism began in the south-ern to southwestern region of the basin and spread to the north and northeast.The progression of initial sedimentation suggests the same direction.Sediment accumulation began earlier in the southern parts of the basin,which today have the highest thickness of the sedimentary fill as well as the most siltstones and mudstones(Figure8).Another distinctive feature of the Penzhinskaya Guba basin is increased heat flow with respect to surrounding areas.No hydrocarbons have been found in the Penzhinskaya Guba basin. Similarities in structures and source formations toHarbert et al.191km5101.4Equal-depth lines to the top ofImlikinsky Formation (depth in kilometers)Cross-section reference location.(a)(b)Figure7.(a)Structural map and(b)representative cross section of the Uglovoya oil field,Khatyrsky basin.Refer to Figure1for the geographic location of Lake Vaamochka in the northwestern Bering Sea region.the more thoroughly studied Anadyrsky and Kha-tyrsky basins suggest that the large Penzhinskaya Guba region may be a promising region for future exploration.KINEMATIC MODELGenerally,compressional structures dominate the north-ern Kamchatka Peninsula.However,sediment accumu-lation in each of these basins began in the Paleocene with rapid accumulation occurring in the Khatyrsky basin during the Eocene.Because Euler poles,which de-scribe the relative motion of the Eurasian and North American plates and the opening of the North Atlan-tic,fall in this geographic region,small variations in the relative North America–Eurasia plate motion in the North Atlantic predict very different types of interactions and motions in the northern Kamchatka region,including periods of motion commonly asso-ciated to the development of extensional back-arc basins.Chase (1978)and Uyeda and Kanamori (1979)presented one widely accepted explanation for the ini-tiation of periods of back-arc or extensional stress in overriding subducting plates.In their model,the abso-lute,or hotspot-fixed,plate motions are compared for both the overriding and subducting plates at a partic-ular coordinate.The absolute motion vectors can be compared for a single point at the edge of the overriding plate in the region of the Khatyrsky basin.If a component of motion of the overriding plate is in the direction of the sub-duction absolute plate azimuth,then back-arc exten-sional deformation may occur.For this region of the northwestern Bering Sea,the Eurasian plate is found192Geologic NoteFigure 8.Generalized stratigraphic section of the Penzhinskaya Guba basin,northeastern Sea of Okhotsk.BasementBasalt flowsLiparite flowsBasementto be moving up to 3cm/yr in a direction parallel to and in the same direction as the subducting Pacific Plate,and back-arc extension is predicted between 48and 39Ma using models of Eurasia–North America motion presented by Pitman and Talwani (1972)and Srivastava and Tapscott (1986)as shown in Figure 9.This age agrees well with the onset of extension of basin development in the Anadyrsky and Khatyrsky basins (Figure 10).When this region is considered part of the North American plate,no period of back-arc extension is predicted.Rotation about this Euler pole may have continued during the Miocene and,possibly,later,as may be inferred from Miocene to Pliocene grabens in the northern Kamchatka Peninsula region thatHarbert et al.193Figure 9.(A)Satellite altimetry of the Kamchatka Peninsula region from Sandwell (2001,personal communication).These data have been digitally shaded resulting in enhancement of the gravity anomaly structure in the northern Sea of Okhotsk region.Interpretation of major linear structures,representing normal faults,allow the extensional axis for each to be interpreted.These interpreted directions of extension are shown by arrows.(B)Simplified model shows possible rifting geometry and formation of the Penzhinskaya Guba parison of the geological units and faults exposed along the western and eastern coasts of the Penzhinskaya Guba in the northern Sea of Okhotsk can be interpreted to record a single-or two-stage extensional history of the rift.In our model,during the first stage starting at approximately early to middle Eocene,the Euler pole of relative motion may have been located at approximately 60.8j N,162.3j E.Because of a change in the subsidence rate in the Khatyrsky basin,in our model,at 35Ma (early Oligocene),we shift the Euler pole of relative rotation between the Kamchatka Peninsula and Eurasia to 62.1j N,165.9j E.are filled with continental and shallow-water sediments and Miocene to Quaternary plateau basalts.CONCLUSIONSThe Anadyrsky,Khatyrsky,and Penzhinskaya Guba sedimentary basins in northeastern Russia have been described.The structural deformation in the Anadyr-sky basin appears to have been a response to over-thrusting of the southern flank of the basin by the Alkatvaamsky and Mainitsky terranes.In the Khatyrsky basin,the southern part of the basin is overthrusted by the Ekonaisky terrane.This suggests that intersuper-terrane movements continued to occur after the Kor-yak superterrane collided to the continent and led to the development of thrust faults in the Cenozoic formations.Movements along these faults were ac-companied by formation of isoclinal anticlines in the Povorotno-Telekaisky uplift group (Figure 3),which hosts all known oil and gas deposits of the Anadyrsky basin,and the Uglovaya structure,which hosts all known oil and gas deposits in the Khatyrsky basin.In our model,continued deformation of this region is related to back-arc-type rotational extension of the Kamchatka Peninsula away from northeastern Russia resulting in the formation of the Penzhinskaya Guba basin.Along the extreme northeastern part of the Sea of Okhotsk,the Penzhinskaya Guba shows an elevated heat flow with respect to the surrounding region.Extensional structures related to this proposed motion may have been observed by satellite altimetry.REFERENCES CITEDAnonymous,1985,USSR energy atlas:Washington,DC,CentralIntelligence Agency,79p.Anonymous,1997,Oil and gas resources of the West Siberian basin,Russia:Office of Oil and Gas,Energy Information Adminis-tration,U.S.Department of Energy,Washington DC,220p.194Geologic NoteFigure 10.Correlation of accumulation curves,possible initiation of back-arc-type extension,and selected tectonic events,northern Kamchatka Peninsula,Russia.Chase, C.G.,1978,Extension behind island-arcs and motions relative to hot-spots:Journal of Geophysical Research,v.83, p.5385–5387.Didenko,A.,W.Harbert,and A.Stavsky,1993,Paleomagnetism of the Khatyrka and Maynitsky superterranes,Koryak Highland, Far Eastern Russia:Tectonophysics,v.220,p.141–155. Gurko,N.N.,N.A.Galischev,and L.F.Stepina,1985,Pristane/ phytane ratio as indicator of migration of hydrocarbons in a gas phase(in Russian):Geokhimiya,v.3,p.393–399.Gurko,N.N.,V. F.Vasil’tseva,T.N.Kandakova,and V.S.Sobolev,1986,Geochemical characteristics of the Chukotka oils(in Russian):Geologiya Nefti i Gaza,v.1,p.58–61. Harbert,W.,P.Kepezhinskas,K.Krylov,V.Grigoriev,S.Sokolov, M.Aleksutin,A.Heiphetz,and yer,2000,Paleomagnet-ism and tectonics of the Kamchatka region,northeastern Russia:Implications for the development and evolution of the northwest Pacific basin:Polarforschung,v.68,p.297–308. Heiphetz,A.,W.Harbert,and L.Savostin,1994,Reconnaissance pa-leomagnetism of the Olyutorsky superterrane,northeast Russia, in D.K.Thurston and K.Fujita,eds.,1992Proceedings Inter-national Conference on Arctic Margins,Anchorage,AK Minerals Management Service OCS Study MMS94-0040,p.223–228. Ivanov,V.V.,1985,Sedimentary basins of northeastern Asia(in Russian):Moscow,Nauka,209p.Kazimirov,A.D.,K.A.Krylov,and P.I.Fedorov,1987,Tectonic evolution of the back-arc basins(using south of the Koryak Highland as an example)(in Russian),in Y.M.Pusharovsky, ed.,Essays of geology of the northwestern part of the Pacific belt:Moscow,Academy of Sciences of the USSR,p.200–225. McLean,H.,1979,Review of petroleum geology of Anadyr and Khatyrka basins,USSR:AAPG Bulletin,v.63,p.1467–1477.Nokleberg,W.J.,et al.,1998,Summary terrane,mineral deposit and metallogenic belt maps of the Russian far east,Alaska,and the Canadian Cordillera(CD-ROM media):U.S.Geological Survey Open-File Report98-136.Persits,F.M.,G.F.Ulmishek,and D.W.Steinshouer,1997,Maps showing geology,oil and gas fields and geologic provinces of the former Soviet Union(CD-ROM):U.S.Geological Survey Open File Report97-470E.Pitman,W.C.III,and M.Talwani,1972,Sea-floor spreading in the North Atlantic:Geological Society of America Bulletin,v.83, p.616–649.Shapiro,M.N.,and V.G.Ganelin,1988,Paleogeodynamic interrelations in the large blocks of the Mesozoic of the northeastern USSR(in Russian):Geotektonika,v.5,p.94–104. Sokolov,S. D.,1992,Accretionary tectonics of the Koryak-Chukchi segment of the Pacific belt(in Russian):Transactions of Geological Institute Russian Academy of Sciences,v.479, 181p.Sokolov,S.D.,and S.G.Byalobzheskii,1996,Terranes of the Koryak Highland,Northeastern Russia(in Russian):Geotec-tonics,v.6,p.486–496.Srivastava,S.P.,and C.R.Tapscott,1986,Plate kinematics of the North Atlantic,in P.R.Vogt and B.E.Tucholke,eds.,The western North Atlantic region:Geological Society of America, Decade of North American Geology,v.M,p.379–404. Uyeda,S.,and H.Kanamori,1979,Back-arc opening and the mode of subduction:Journal of Geophysical Research,v.84, p.1049–1061.Zonenshain,L.P.,M.I.Kuzmin,and L.M.Natapov(authors),B.M.Page(ed.):American Geophysical Union,GeodynamicsSeries,v.21,242p.Harbert et al.195。