Oil and Gas Basins
Tectonics of the Anadyr Basin
Anadyr basin in the Bering Sea
 Oil and Gas Reserves Marketable
ms  Chukotka Prof  Konstantinovich 
Some Reserve Estimates
 Low-Porosity Triassic Reservoirs of Barents Sea
Seeking a Positive Commitment
 exploration history
exploration in difficult Far East basins
Pet Pot Shallow-areas of Russian Arctic
Cretaceous continental volcanics from central Chukotka:
Anadyr basin in the Bering Sea

Gregory F. Ulmishek, U.S. Geological Survey, Box 25046, DFC, MS 940, Denver, CO 80225
Leslie B. Magoon U.S. Geological Survey, 3475 Deer Creek Road, Palo Alto, CA 94304

A The petroleum system is becoming accepted as a unifying concept in which various research focused at finding new hydrocarbon reserves can be conducted more efficiently.  Proposed definitions of the petroleum system vary. 
The system is understood as a group of discovered and/or undiscovered genetically related hydrocarbon accumulations that emanated from a contiguous body of source rocks and that occupy a specific rock volume.  
The petroleum system map, which shows source rocks and the genetically related accumulations and cross sections which show the stratigraphic position of the accumulations, clearly demonstrate the explorationist's interpretation of the origin and migration route of hydrocarbons. 

Three petroleum systems are compared to graphically demonstrate how the concept is used in exploration, research and resource appraisal.  The systems, from most to least explored:
Northern China in the Jizhong subbasin
Northern Alaska in the Colville basin
Anadyr basin in the Bering Sea
Although the amount of information varies for each basin, the petroleum system can be meaningfully portrayed. The exploration geologist can use the petroleum system map and cross sections to develop plays, find undiscovered commercial quantities of hydrocarbons within the system. The system map and cross sections can be used as an analog for another area that contains a seldom-explored system. The research geologist can investigate and model how a system works, either in total or part, which helps in locating new plays and in decreasing exploration risk.  The appraisal geologist can evaluate the petroleum system map from a historical view to study discovery rate process or compare similar systems that have different levels of exploration to determine the ultimate yield of the little explored system. 

Fig, 6. Isopach map of the rocks of the Anadyr basin showing the supposed offshore petroleum system and cross-section on through the onshore part of the basin. (The map is after Deter et al., 1986; the cross-section is after Marlow et al, 1983, with changes according to Krylov et al, 1988).

The offshore part of the Anadyr basin in the Bering Sea is an undrilled frontier, which contains a speculative petroleum system.  A grid of seismic lines covers the area (Marlow et al., 1983), and interpretation of the seismic data is based on drilling in the onshore part of the Anadyr basin and in the Navarin basin to the south (Fig. 5).  

Drilling in the onshore part of the Anadyr basin revealed a sedimentary section consisting of Upper Cretaceous through Pliocene rocks overlying the Jurassic-Lower Cretaceous basement and separated into sequences by pre-Eocene, pre-Miocene, and pre-middle Miocene unconformities (Voronkov, 1987).

Several, non-commercial oil and gas fields and a number of hydrocarbon shows, which demonstrate a petroleum system, were found in the deep part of the basin on the south.  

Although geochemical data are limited, the presumed source rock occurs in the Eocene-Oligocene and possibly, in the lower Miocene sequences (Ivanov et al., 1988).  In the same stratigraphic intervals, source rocks have been identified in the Navarin basin (Steffy et al., 1985).  Therefore, a potential source rock can be expected in the upper Paleogene through lower Miocene rocks in the offshore part of the Anadyr basin.
Rocks of this stratigraphic interval are present in the southern depression of the offshore Anadyr basin where they occur at depths of up to 4 km (Fig. 6)  (interpretation of seismic data Marlow et al., 1983; Krylov et al., 1988).

The rocks pinch out toward the southern and northern slopes of the depression and on the Tuman uplift on the western flank.

Onshore, the top of the oil window occurs at depths varying from approximately 1.8 to 2.6 km (Ivanov et al, 1988).  

Hydrocarbon pools and shows onshore arc dominantly concentrated in siliciclastic reservoir rocks of early and middle Miocene age.  Middle Miocene shales to 330-m thick (Yeliseev Formation) form a regional seal. These rocks are present over all the depression, and overlap inner and marginal basin highs and extend into the offshore basin.  

These data indicate that a petroleum system is probably present in the offshore Anadyr basin beyond the Tuman uplift (Fig, 6).  The exploration concept is to extend the onshore petroleum system offshore using structural data.  The offshore system is separated from the onshore system by a structural barrier, the Tuman uplift.  The age of the Tuman uplift is important to determining whether there are one or two petroleum systems.  If the uplift is young, and formed after the source rock became mature, then one system occurs. Whereas, if the uplift was formed before the source rock became mature, then there are two independent petroleum systems.  
An analogy with uplifts north of the onshore Anadyr basin suggests that the Tuman uplift is an old structure formed before deposition of the middle Miocene shale; therefore, the map of the Anadyr basin indicates two petroleum systems (Fig. 6). 
The areal extent of the offshore system (shown on map) is based on probable presence of mature source rocks: approximately along isopach 2.5 km and includes adjacent structural slopes.  The probable stratigraphic extent of the petroleum system is from Eocene through middle Miocene strata. It may also include the top of the Upper Cretaceous section underlying the source rocks, although onshore, this section is devoid of good reservoir rocks.  Exploration based on this play concept may further reduce the spatial extent of the offshore system when more data on reservoir rocks, maturity of source rocks, and quality of the regional seal become available.

These examples demonstrate that the proven extent of the petroleum system is shown by a line that circumscribes a mature source rock and genetically related oil fields and significant shows. This map is easily made in c provinces where most of the fields have been found and a substantial amount of geological and geochemical data are available.  However, even in these provinces, lateral extensions of the petroleum system can be expected based on mapping of the geometry of the rock body that contains the oil and gas fields within the system.  This extension of the petroleum system can be the basis on which a play is predicted.

In moderately explored areas, such as in the North Slope of Alaska, mapping of the rocks that constitute the petroleum system is the best way to understand the extent of the system.  Here, discovered fields are clustered in a relatively small area so more geologic information may be needed to understand the entire extent of the system.
In unexplored frontier areas, mainly structural and limited stratigraphic data, coupled with comparative analysis of similar provinces, can be used to map the speculative petroleum system and to evaluate its probable stratigraphic extent.

The map of the system in this latter case presents the maximum possible extent of the system, and commonly, this extent decreases as more data on distribution of mature source rocks, reservoir rocks, and seals become available.  At all stages of exploration, a comparison between the proven lateral and stratigraphic extent of the system and its maximum possible extent indicates potential areas for further exploration efforts.  Using the petroleum system concept, plays can be developed to find commercial quantities of petroleum in unexplored extensions of the system.

Tectonics of the Anadyr Basin

A. K. Dertev, O.I. Cuprunenko (vnigri) {from Russian}

Page 85
Anadyrsk basin is well expressed in  relief encompassing the territory of Nizhneanadyr depression, and water area of Anadyr limb and same embayment. From the south, it is bounded by Koryak bowed system, from west the Rarytkinskim anticlinorium, from north - Chukotsk bow system and Zolotogorskim anticlinorium, from east and south (on water area) - Nunivakskim and Gangutskim upheavals (fig. 1).

The majority of explorers, following geological features of the basin, esteems it as a Cenezoic pattern overlapped on difficultly constructed Mesozoic bow formations, being the foundation to sedimentary and igneous-sedimentary Cenozoic depositions [1, 4).

Per the last years, on this considered region, a lot of works were published [3, 5 - 8], generalizing materials on geological features and oil-and-gas-bearing capacity. The large bulk of geophysical works and deep drilling were executed by specialists of the Geological Survey and private concerns of USA within the limits of Navarin and Anadyr basins in Bering Sea [9, 10], have allowed on more authentic ground to compare a geological feature of the marine part of the Anadyr basin to a part located on land. The truth, in work [9]. The American explorers at interpretation of geophysical materials on water area for confrontation used obsolete (out of date) works of KMPV held on land in from the 1960 years. However for the  water area of Bering Sea the results of geophysical works introduce significant improvement. Besides the deductions of M. Marleu etc. [9] are confirmed by the newest results of works in basin of Navarin [10].

In difference from the mentioned large generalizings [3, 5 - 8], authors have delivered before themselves narrower problem - on the newest materials drilling and seismic of works to update features of a constitution and geologic progres of Anadyr basin on land 

Figure 6 From Olmishek
Isopach map ot Retiary rocks of the Anadyr basin
Supposed ofshore petroleum system
Cross-sedtion through onshore part of basin
Map after Dertec el al., 1986
Cross-section is after Marlow el al., with changes according to Krylov et al., 1988

Fig. 1. Structural - tectonic schema of Anadyr basin
1 - Boundary of Anadyr basin; 
2 - Boundary of patterns second belt (1 - Kanchalano - Onemensky bending flexure, 2 - upheaval of Dionisija, 
3 - Nizhneanadyr bending flexure,
4 - Tymninsko - Tumanskoe upheaval,
5 - Anadyr bending flexure) is east; z - isoheight of a base surface Cenozoic depositions, km (on land - on the data seismic and boring, on water area - on the data marine seismic (9, 10); 4 - outlines of the floor space, figured in a fig. 2 and 3

The land on a ground of constructing has enough of detailed maps of isopachs of the basic strata of a sedimentary jacket and to compare them to major stages of evolution in the sub marine basin.
Top page 86 

The foundation for rather friable Mesozoic Cenozoic of depositions basin are difficulty dislocated Mesozoic and, probably, Paleozoic formations observed on patterns, framing it. A heterogeneous nature of the foundation constitution is supposed. In the south Anadyrsk basin both on land, and within the limits of water area the foundation are the bow formations of Jurassic-chalk rocks of Koryak bowed system immersed under rather friable Cenozoic deposition. On water area, dredged trench samples on upheavals framing the basin from the south, the formations Jurassic and chalk age are recovered.

The most ancient formations referred to fundamental rocks, serve tufaceous sandstones with tufa hardpan and tufite, and also overlying stratas andesite-basalts, bottom chalk, open by wells on East - lake region. On materials of seismic survey of MOGT these depositions are allocated on East-lake and West-Lake Upheavals, but are not found in bending flexures (Majnitsk etc.). Apparently, these horizons occur more deeply -- maximum depth of described by MOGT for the given area (5 - 6 kms). On a survey profile of MTZ held through Majnitsk a bending flexure in its mean part, are guessed friable Cenozoic and early chalk of deposition thickness up to 10 kms. Maximum depth of a roof of the foundation on water area of 9 kms [9].

Top page 86 column 2

In north of Anadyr basin, both on land and on water area, the foundation is the igneous deposition chalk. They performed cartology from aeromagnetic to filming. On water area on northern deposit basin edge, debris of igneous formations, chalk and Neogene age are recovered.

In east part of the basin on water-area is probable Paleocene age of fundamental rocks. These assumptions are grounded on variation of nature seismic of record on profiles intersecting upheaval of Nunivak, and also on results from dredged trench samples, held by the American geologists [9, 10].

The stratigraphic schema Cenozoic repeatedly varied. In the given article, we adhere to alternative proposed by D. I. Agapitovym etc. in 1982 (table). 

Thus, the total thickness of a sedimentary stratum changes from 5 up to 13 kms. However, actual thickness of a jacket in many respects depends on an extent of fluid wash of this or that suite. By results of seismic of studies and geologic data all stratum of a sedimentary jacket is subdivided into three, geologic of a geophysical complex divided by surfaces of an unconformity.

Page 86-87 
The bottom complex engages terrigenous deposition upper chalk (upper Cenozoic - Danish), open by wells in southern and central parts of Anadyr basin (West-lake, East-lake and the tenacious area, well on Kose Geka on her western (well no SKV 8 on the Profiled area) and on southeast (SKV ?-6 in an estuary near Nygchekveem) Abramlenii and banks on igneous Nizhnemelovyh depositions with an unconformable dip. With seismic survey of MOGT and ?OB, these depositions are allocated only on East - Lake Upheaval in the median part of the basin (reflection horizon V), to western and southern beds. The bottom complex is poorly understood. These depositions, apparently, are advanced in the central, southern and southwest parts of a hollow and are absent in north. Wells penetrate more than 1000 meters of these deposits. On a survey profile of MTZ through a mean part of Majnitsk bending flexure, the presence of upper chalk deposition by thickening more than 3 kms is supposed.

The mean complex engages igneous formations Tanjurerskoy suite and terrigenous estuary - Chirynajsk and Majnitsk suites and abundant on all territory, edging away to the set of rules of large upheavals. Thickness of a complex and its structure are changeable. The igneous depositions of Tanjurerskoy suite are advanced only in north of Anadyr basin and to the south, edge away. Their maximum thickness is marked off in Kanchalano-Onemensk bending flexure and, on the data of MOV reflection seismic, compounds more than three kms. Skv # 19 drilled in a mean part of a bending flexure exposed 2041meters of these deposits. The terrigenous formations estuary- Chirynajsk and Majnitsk suites are advanced in the central and southern parts of Anadyr basin and edge away to north and east. The thickness of more than 2 kms are observed on Echinsk upheaval in CKB #18 (fig. 2); their magnification on a southern deposit edge of Anadyr basin (Majnitsk bending flexure, Chirynajsk upheaval) is supposed. Seismic survey of MOV and MOGT is good are traced these depositions, in their top of oil horizon is allocated bearing reflective horizon III. 

End page 87 top page 88

By seismic studies on water, all stratum sediment deposits area executing Anadyr basin is confidently subdivided into two complexes easily enough. Bottom, introducing rather monotonic, Akustvcheski "a transparent" stratum, and upper, described by significant lamination [9]. Comparing seismic with a cut-away view from a deep well on Kose Geka, it's established that the bottom complex marine deposition is introduced to terrigenous and igneous depositions of Paleogene and late chalk age (quaternary). Selected by seismic survey on water area the horizon {BETA} (9, 10) is well compared to reflection horizons III and IV on land, for example with a base surface mineral of depositions.  Thus, selected is on land and mean complexes are compared to lower complex on water area. Its thickness the same as and on land, extremely variable and alternating from the first hundreds meters on upheavals up to 5 kms in zones of maximum buckling, being augmented for, account of more ancient depositions. 

The upper complex occurrences with granite wash on deposits -- middle and bottom.  It engages Miocene, Pliocene and quaternary deposit and is abundant everywhere in the basin. For depositions Neogene are characteristic multiple enough fluid-filled, acoustically rigid, reflecting boundaries. On the data of seismic survey both on land, and water area the upper complex beds on two, three-bed complexes. Horizon (on marine area [9, 10]) is compared on land to a reflection horizon I, dated to a base surface of upper Miocene (Ozerninsk suite), and is a boundary surface of top and bottom (upper and lower) bed complexes. 
On land, three bed complexes of upper Miocene-Pliocene Echinsk - is allocated 

Page 88 continued

By seismic marine studies, area all stratums of sediments (deposits) performing in the Anadyr basin, is subdivided confidently enough into two complexes. At the bottom, introducing rather monotonic, Akustvcheski "a transparent" stratum, and upper, described by significant lamination [9]. On a ground of confrontation of seismic with a cut-away open with a deep well on Kose of Geka, is installed that the bottom complex on water area is introduced terrigenous and igneous depositions of Paleogene and late chalk. Selected by seismic survey on water area the horizon ? (9, 10) is well compared to reflection horizons III and IV on land, for example with a base surface mineral of depositions.  Thus, is selected on land and mean complexes are compared to one lower marine complex its thickness the same as and on land, extremely changeable and various from the first hundreds of meters on upheavals up to 5 kms in zones of maximum buckling, being augmented for, accounted by more ancient depositions.

As a whole, the upper sedimentary complex is more sustained on thickness, than the base does, and reaches 4 kms on water area and 5 kms on land. Thickness of the beds in the complexes is changeable. Bottom of the bed complex (Sobol'kovskaya, Gagarinskaya and Avtatkul'skaya suites), absent in north of Anadyrsk basin, and on a large part of other territory it thickens 200 - 300 m. Is more sustained on thickness, which underlay deep, and reaches 4 kms on water area and 5 kms on land. Thickness bed complexes changeable. Bottom bed complexes (Sobol'kovskaya, Gagarinskaya and Avtatkul'skaya of suite) is absent in north of Anadyrsk basin, and on a large part of other territory its thickness 200 - 300 m. It is augmented more than 2,5 kms in the south Majnitsk of a bending flexure and up to 1 km in Central. Mean bed complex (Eliseevsk and Ozerninsk suite) is observed everywhere (300 - 500 m's) Thickness is moderated on a southeast deposit edge. Under Anadyr bending flexure also are augmented up to 750 - 832 meters in its middle part (East & Ozerninsk upheaval) and in the south Majnitsk of a bending flexure (fig. 3). Upper bed complexes (Aaleksandrovsk stratum and Echinsk suite) are advanced everywhere (500 - 600 m's). The maximum thickness's are observed in Kanchalaio-Onemensk bending flexure and in the south Majnitsk of a bending flexure. In east part of Majnitsk of a bending flexure and further to the south these depositions, apparently, are absent.

The modern basin outlines are traced up to Pliocene - quaternary transient shelves. 
On depositions that are more ancient, its sizes are considerably abbreviated down to separation to more small-sized independently developing basins. The pattern is anisometric (single-ended) - southern deposit edge steep, northern flat. In the plan basin has an oval form, extended in width direction, western part which one is located on land, and east - on water area of Anadyr embayment. Tymninsko - Tumanskoe the crosscut (transverse) upheaval partitions basin on Nizhneanadyrsk bending flexure located in its western part, and Anadyrsk is east. 

Patterns more small-sized have north western and northeast direction of strata. The folds focused, as a rule, in northeast direction, for east - predominantly in a northwest direction are characteristic for western part of the basin. First report structure surrounding also are located on continuation of large patterns of a frame (configuration), embedded floor Mesozoic Cenozoic rather friable formations.
Fig. 3. The schema of isopachs weighted volumetrically- and top of Nemiotsenovyh (Ozerninsk both Eliseevsk suite and top of Nemiotsenovyh, Pliocene-quaternary of depositions (Echinsk suite and Aleksakdrov stratum - b) fig. 2 

Top page 89 text

The upheaval of Dionisiya is a continuation of Zolotogorsk anticlinorium. Chirynajsk and Ozerninsk upheaval are located on submerged Tamvatnejsk anticlinorium. Podgornensky and Majnitsk bending flexures are on submergence accordingly Velikorechensk and Kerkerveemsk sinclinoria frame, observed on Jurassic-chalk depositions of the basin. The patterns of northwest orientation are not patterns of surrounding deposits; apparently, it is a younger formation.

The following patterns of the second order are allocated: Kanchalaio - Onemensk bending flexure, upheaval. Dionisiya, Nizhneanadyrsk a bending flexure, Tymninsko - Tumanskoe upheaval, is east Anadyrsk bending flexure.

Kanchalaio - Onemensk bending flexure is located in a northwest part of basin and is carved out from it by upheaval of Dionisiya. As independent pattern it existed only in Pliocene - quaternary time, in other time served northern decline of Anadyr basin. The deep well, drilled in northern part of a bending flexure, has opened Pliocene - quaternary of deposition (810 m), middle Eocene (455 m) and igneous Paleocene - Eocene formation (2041 m). On the geophysical data it is possible to guess, that the thickness of the Cenozoic shelf here do not exceed 3.5 - 4 kms, from them Neogene no more than 2 kms. 

The upheaval of Dionisiya has northeast direction of strata and is on western submergence of Zolotogorsk anticlinoria under a stratum of Neogene deposition. It separates Kanchalaio -Nemensk bending flexure from Nizhneanadyr, amplitude about 4 kms. At the arch the thickness Paleogene depositions are considerably abbreviated and edge away lower and middle Eocene. Upper Eocene and Pliocene-quaternary formation are absent in east part and reach 800 - 900 m in western, mostly embedded (see fig. 3).

End page 89

In Nizhneanadyr bending flexure, the dominant inherited patterns are dull.
Top page 90

Nizhneanadyr bending flexure is the largest pattern of Anadyr basin (on land), in its limits the upheavals of Chirynajsk, Ust'-Chirynajsk, Ozerninsk, Povorotno - Aleksandrovsk, and multiple and bending flexures are observed: Podgornensky, Velikorechensk, Central, Protochnaya, Majvitsky, Oleninshaya, and Limansk. Nizhneanadyrsk a bending flexure extends in a northeast direction on a long fulcrum on 300 kms and on short - on 25 - 110 kms. On western and east deposit edge is located multiple depositions with depths submergence of up to 5-6 kms. Its mean part is raised, section of Nizhneanadyrsk bending flexure on two parts: western and east. The depths of immersion of Cenozoic depositions for positive patterns of a mean part of a bending flexure, on the data of boring and geophysical studies, compound 1.3 - 2.5 kms. Fundamental rocks drilled by boring only on Lake Upheaval (East-Lake Pattern) on depths 1326 - 1670 m. 

On southern and western to deposit edge the bending flexure is carved out from patterns of a frame by zones of explosive dislocating with amplitudes of separate disturbances from several tens up to hundreds meters. On a southern deposit edge of a bending flexure Jurassic-chalk and Paleogene formation frame are pulled, apparently, on friable Cenozoic deposition. 

Tymninsko - Tumanskoe upheaval has a sub-meridian direction of strata and partitions in the Anadyr basin on two parts. On a long fulcrum length it more than 150 kms, on short 20 - 60 kms, amplitude on a base surface of Cenozoic depositions 2.5 - 4 kms, occurrence depth of these formations of 1-1.5 kms. Here are advanced Pliocene - Quaternary and mid to upper Eocene formations. In the set of rules of upheaval are absent upper Eocene and terrigenous Eocene to Oligocene formation, to which a cut-away open SKV convincingly testified by well K-8. The same well has opened igneous depositions Paleocene Eocene (Tanjurersk suite). In cut-away open with wells a slope of upheaval - (P-12, P-23), thickness of these depositions having augmented 
On a long fulcrum length is more than 150 kms, on short 20 - 60 kms, amplitude on a base surface of Cenozoic depositions 2.5 - 4 kms, occurrence depth of these formations of 11.5 kms. Three basic developments here Pliocene-quaternary and middle & upper Eocene formations. The same well has opened igneous overturned Paleocene-Eocene (Tanjurersk suite). 

The same wells have opened terrigenous depositions upper chalk, missing on the set of rules. 

Here Cenozoic formation occurrence on early chalk fundamental rocks is at the surface. In water area trench dredge were brought up granite early of age. Tumansk upheaval was generated in late chalk. In Paleogene and early Eocene time it monitored deposit accumulation in Anadyr basin, that is precisely visible on a pinching out of a mean complex of depositions and part upper in Nizhneanadyr bending flexure as approaching Tymninsko-Tumansk upheaval.

Nizhneanadyr bending flexure bedding in the east part of the basin almost wholly is within the limits of water area. It has the sizes 200 X 225 kms, isometric the form and skewed constitution. Optimum embedded parts situated on southern deposit edge. Northern flat complicated by patterns width of a direction of strata, southern steep.

On the data of a gravity exploration [2] and marine seismic, the bending flexure is complicated by explosive disturbances by amplitudes up to several hundreds meters on a southern deposit edge and zone of explosive disturbances on the basis northern. Maximum thickness of Neogene formations up to 3 kms, Paleogene - up to 1 - 2 kms and Mesozoic-Paleozoic - up to 5 kms. 

Thus, the former submissions about existence of large Anadyr basin from late chalk bottom are not confirmed.  The modern outlines of the basin will master those for depositions of Pliocene -quaternary time. On more ancient formations of Tymninsko - Tumanskoe the crosscut (transverse) upheaval basin partitions on two bending flexures (Nizhneanadyr and eastern Nizhneanadyr), developing independently in Paleogene and early Eocene time. In their constitution the complex combination fold - block of members of the various order is observed, among which one on genesis stand apart inherited and is new formed positive and below zero structure.

Page 91

1. Azaputov D. I., Ivamov I. I. A history of tectonic progressing of Penzhinsk-Anadyr of area in late Mesozoic and Cenozoic. Geotectonics. 1969, no. 1, p. 68- 82.

2. Willow of m. L., Ermakov. In. Tectonic impact area of a northwest part of shelf of Bering sea and adherent coastlines - Geotectonic, 1976, no. 2, p. 101 - 110.

3. Geology of Bering Sea and its continental frame / under edit. B. X. Egiazarova. L.: mineral resources, 1985.

4. Dranovsky YA. no., Berson G. L. Tektonics the schema of a northwest part of Tihookeansk bow belt.- Geology and geophysics, 1969, no. 3, with. 41-47.

5. Ivaiov V. V. Sedimentary basins northeast Asia. M.:  Science, 1985.

6. New data on outlooks oil of a presence of gas of Northeast USSR. / D I. Agapntov, Ju. And. Kosygin, Ju. V. Motovilov etc. - Geology oil and gas, 1983, M 7, with. 1 - 6.

7. Sketch of tectonics of Koryak upland /under edit. Ju. M. Pushcharovskogo, S. M. Til'mana. M.: a science, 1982.

8. Tjutrun I. I. Dunichev V. M. Tectonics and oil-and-gas-bearing capacity of a northwest part of Tihookeanskogo zone. M.: mineral resources, 1985.

9 Marlow M. Cooper A. Childs U. Tectonic evolution on gulf of Anadyr and Formation ?f Anadyr and Navarin Basins. Bull. AAPG. l983, vol. 67, N 4, p. 646-665.

10. Steffy D., Turner  P., Martin A., Flett T. Evolution and Petroleum Geology of the Navarin basin, Bering Sea, Alaska. Oil and Gas. J., 1985, vol. 83, N 31, p. 116 - 119, 122 - 124.

New 40Ar/39Ar ages of Cretaceous continental volcanics from central Chukotka: implications for initiation and duration of volcanism within northern part of the Okhotsk Chukotka Volcanic Belt (northeastern Eurasia).

V. O. Ispolatov1, P. L. Tikhomirov2, M. Heizler1, and I. Yu. Cherepanova3
1Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801
2Geological Faculty of Moscow State University, Russia
3Chaun Mine Geological Enterprise, Pevek, Russia
(accepted by Journal of Geology in October 2003)

We used single-crystal (sanidine) laser fusion 40Ar/39Ar method to date five samples of volcanic rocks from the northern (Chukotka) part of the Cretaceous Okhotsk-Chukotka Volcanic Belt (OCVB, northeastern Eurasia). Ages yielded by four samples from lowermost stratigraphic units range from 87.94 to 87.08 Ma, indicating that volcanism within this portion of the OCVB started in Coniacian time, at ca. 88 Ma. Ages of lower- and uppermost units lie within 2s errors of each other, implying that the volcanic pile was erupted over a very short (perhaps on the order of 1 m.y) time period.


The 3200 km long Okhotsk-Chukotka Volcanic Belt (OCVB) is located in the northeastern Eurasia (fig. 1). Extending for about 3200 km and comprising roughly 2 million km3 volcanic and associated plutonic rocks, the OCVB is among the largest subduction-related volcanic provinces of the planet. At present, geochronology of the OCVB is uncertain; this obstructs development of efficient stratigraphic models and severely impairs understanding of Mesozoic evolution of northeastern Eurasia and entire Circum-Pacific system. Our study provides new data on timing of initiation and duration of large-volume eruptions within the northern (Chukotka) segment of the OCVB (central Chukotka area). Primary focus is placed on oldest volcanic units that were not previously characterized by the 40Ar/39Ar method. We recognize that five new 40Ar/39Ar dates reported in this short paper are insufficient to unequivocally constrain the age of this continental-scale volcanic province. Thus, we refrain from drawing fundamental conclusions or extrapolating our results over the entire volcanic belt. Rather than that, we intend to highlight the problem of the OCVB geochronology and stimulate new discussions and studies in this area.

Regional tectonic background

The Cretaceous OCVB stretches along the southeastern margin of Mesozoic Verkhoyansk-Chukotka tectonic province and is sharply discordant to its structural trends (fig. 1). Volcanic rocks are undeformed and overlie Verkhoyansk-Chukotka Mesozoic structures with angular unconformity (e.g., Zonenshain et al., 1990; Sengцr and Natal’in 1996). Compositionally, volcanics vary from basalts to rhyolites; intermediate and felsic rocks predominate (Kotlyar et al. 1981). In regional geodynamic evolution, volcanism postdated accretion of the Okhotsk block, Kolyma-Omolon Superterrane, and Chukotka terrane to the Verkhoyansk passive margin of the Siberian craton (fig. 1). This sequence of collisions terminated earlier island arc volcanism, caused deformations and emplacement of granitoids, and led to formation of composite Verkhoyansk-Chukotka orogen (Parfenov and Natal'in 1985, 1986; Parfenov 1991, Layer et al. 2001). Subaerial, chiefly explosive volcanism of the OCVB occurred afterwards and was related to northward subduction of the Kula oceanic plate under the newly assembled tectonic collage (e.g., Parfenov and Natal'in 1986; Zonenshain 1990; Zonenshain and Kuz'min, 1992). This Andean-style volcanism ended when the Verkhoyansk–Chukotka collided with the Okhotomorsk block, and subduction zone shifted eastward (e.g., Parfenov and Natal’in 1986).

exploration history
ARCO has signed preliminary agreements with two local governments covering E&P in the Soviet Far East.
ARCO's protocols with regional councils of Magadan and the Chukotka Autonomous Area pave the way for ARCO to negotiate for exclusive rights for onshore and offshore E&D. Target areas will be identified in subsequent negotiations. The councils will help ARCO obtain required legal approvals of the Soviet and Russian Federation governments.

Chukotneftegasgeologia January 12, 1993

In a deal that could lead to a joint venture agreement, IPC and Chukotneftegasgeologia have a joint study agreement covering an area in the Chukotka region. The two will assess the technical and economic feasibility of producing small onshore oil fields in the ANADYR ANDKAHTYRKA BASINS,aimed at yielding exports for western markets.

Crude from the two basins has a high wax content, but it is low in sulfur and the fields are close to the Pacific Coast.

 IPC said depending on results of the study, it could enter a production agreement with Chukotneftegasgeologia in the next 2-5 years.