Petroliferous Upper Jurassic correlated across W. Siberia, N. Alaska, Arctic Islands

Michael B. Mickey
Micropaleo Consultants Inc.
Encinitas, Calif.

Boris Nikitenko, Boris Shurygin
United Institute of Geology, Geophysics, and Mineralogy
Siberian Branch of Russian Academy of Science
Novosibirsk, Russia

Jurassic deposits are very widespread in the Arctic basin ( Fig. 1 [39,131 bytes]). The Jurassic is almost entirely represented by terrigenous sediments over this vast territory.

It is during the Jurassic when very large sized structural features formed on the West Siberian Plate, the sedimentary cover of which is saturated with oil and gas. These strata are widespread in the Sverdrup basin of Canada and in northern Alaska.

In northern Russia the areal extent of sedimentary basins increased persistently during the Jurassic. Judging from the distribution of marine strata, the main direction of transgressions was easterly in the Early Jurassic, northerly in the Middle Jurassic, and northerly and northwesterly in the Late Jurassic.¹

Northern Russian regions

Based on structural features and the thickness of the Jurassic sections, three large regions have been distinguished in northern Russia. In the western region (Barents Sea Platform-West Siberian Plate), the Jurassic shows a relatively distinct two-member structure: Mainly freshwater (subcontinental) sandy-argillaceous deposits occur at the base (Lower-Middle Jurassic), giving way to mainly marine, essentially argillaceous Upper Jurassic sediments. The whole sequence is frequently characterized by hiatuses, which can attain one-two substages in size. Thickness of the Jurassic ranges from 400-600 m to 1,000-2,000 m.

We should note that this sequence of Jurassic deposits is also traced into the Kara Sea shelf and Laptev Sea shelf where they are as yet unstudied, but seem to be oil-bearing as well, by analogy with the Barents Sea shelf.

The sedimentary composition of sections, conditions of development, and occurrence of Jurassic strata in Western, Middle and Eastern Siberia have much in common. First, the cyclicity of sedimentary composition is regular and is very similar over the entire vast area of the Siberian Platform, and probably, caused by eustatic controls (Fig. 2 [39,078 bytes]).^{2 3} This is also related to the terrigenous sedimentogenesis and the rather widespread relationship of biota in the Jurassic basins. This enables the construction of a single stratigraphic scale of regional horizons for Siberia based on a combination of parallel zonal scales utilizing different groups of macro- and microfauna, applicable over the entire territory of the northern Asian part of Russia and in adjacent areas.

The main working biostratons are represented by stratigraphic layers containing a specific group of fossils and zones. The biostratons of different fossil groups are sometimes defined in very different ways (phylozones, teilzones, epiboles, assemblage zones, etc.). The scales (Fig. 3 [38,070 bytes]) consist of polytaxon zones: based on bivalves (Bivalvia: B-zones); foraminifers (Fora- minifera: F-zones); ostracods (Ostracoda: O-zones); palynozones, etc.^{4 5 6}

The whole variety of zones is considered as an operational combination of scales of biological event nature used directly for biostratigraphic subdivision, for identifying datum points in correlation of e-logs, in cyclo-, and seismostratigraphic analysis, in intra- and interregional correlation of the Jurassic both in natural outcrops and in well bores.

Some of the more distinctive biostratons are well recognized in the Jurassic sections of western Europe, Canada, Alaska, etc. Isochroneity (naturally, within the range of method accuracy) of datum levels traced on the basis of comparative analysis of ben- thic assemblages, sometimes circumboreally,⁷ was controlled by superposition on the independent correlation framework of the ammonite zonal scale.

Simultaneously, zones of narrow and wide ranges are distinguished with different characteristic assemblages for different facies. This allows correlation of different-facies intervals in transitional sections with index deposits in cases where there is a deficiency of data on one of the parallel parts of the scale and so on. Comparison of one succession of biostratons with another succession (but not correlation of a single biostraton with another single biostraton) is mandatory in correlations. In remote interregional correlations, the zonal scale based on benthos can be considered as "bioevent" scales, in which datum intervals are characterized by the unique sequence of results of a combination of biological events of a different nature (phylogenetic, chorological and ecosystem). It is the fixed sequence of events of different (independent of each other) nature that seems to show the highest probability of isochroneity in identification in different regions.

The boundaries of zones defined on different faunal groups do not coincide, and this is natural since the evolution rates, migrational and adaptational abilities, etc., of groups are not similar. A combination of all the scales provides a very detailed sequence of changes in assemblages of different groups and these combinations define the co-intervals (as we named them). This sequence of co-intervals serves as a good tool for reliable correlation at the intrazonal level.^{5 6}

Upper Jurassic targets

The Upper Jurassic is one of the main targets for oil and gas production in the central and southern regions of West Siberia. Its bio- and lithostratigraphic structure ⁸ was studied in detail ( Fig. 4 [26,446 bytes]), and sequence stratigraphic models were developed. ^{2 3 9} Several oil bearing sand reservoirs have been distinguished at different stratigraphic levels within the Upper Jurassic. The stratigraphic volume of reservoirs and clay seals is relatively precisely determined using the system of parallel zonal scales. The placing of straton boundaries on regional charts with regard to the scale of general (worldwide) stratigraphic subdivisions was made using mainly the framework of datum levels from the combination of parallel zonal scales (at moments of levelling of Arctic biota) and horizons marking worldwide (especially circumboreal) changes of biota. ⁷ The position of horizons in the general stratigraphic sequence associated with macrocyclicity and sequence stratigraphy were used.

Micropaleontologic investigations of the Upper Jurassic in northern Alaska made it possible to locate a number of datum levels from the West Siberian zonal scheme that allow a more detailed subdivision and correlation of the Upper Jurassic sections in Alaska. In Alaska, the B19 and B20 B-zones are well recognized within the Oxfordian-Kimmeridgian interval. Based on foraminifera, the subdivision is more detailed-the F19, F20, F21, F22, F24, and F26 F-zones were all recognized (Fig. 3). Judging from literature, fossil assemblages, characteristic of F24 and B20 (the latter in the Deer Bay formation) were also found in the Awingak formation of the Sverdrup basin.¹⁰

Alaskan similarities to Siberia

Using the parallel zonal scales, the Upper Jurassic sections in Alaska are well correlated with the Siberian oil and gas bearing strata (Fig. 3 and Fig. 5 [14,944 bytes]). Bed-by-bed correlation of the Alaska sections is possible on this basis. Judging from the authors' data and publications, the lithostratigraphic structure of the Upper Jurassic sections, and even the sequence of the main eustatic events are very similar in northern Alaska, Siberia, and the Sverdrup basin.

The equivalents of the West Siberian oil bearing bed U10 (base of the Georgiyev-Bazhenov cyclite) and the Oxfordian sand beds of the West Siberian oil bearing horizon U1 are recognized rather clearly in the Upper Jurassic of Alaska. Fig. 5 compares the thickness of Upper Jurassic strata between Northern Alaska and Siberia.

Thickness of the Oxfordian-Kimmeridgian sandstones of the Upper Kingak subformation vary from a few meters to 60 m. The thickest composited Oxfordian-Kimmeridgian section encountered to date is in the neighborhood of 700 m. As many as seven fourth-order depositional cycles have been recorded from the Oxfordian-Kimmeridgian section in Alaska. Oxfordian deposition represents outer shelf to upper slope sandstones, siltstones, and shales. Kimmeridgian deposition is characterized by somewhat sandier middle to outer shelf deposits.

Sandstones are inequigranular, greenish, frequently enriched in glauconite, and are well correlated with those of the Upper Sigov subformation in West Siberia. The Lower Oxfordian sandstones of the Upper Kingak subformation are correlatable with the lower part of the West Siberian oil-bearing horizon U1.

The comparative analysis of bio-, sequence-, and lithostratigraphy of these basins provides the opportunity to divide the Upper Jurassic of northern Alaska in more detail and to determine the stratigraphic volume (stratigraphic confinement) of oil-bearing reservoirs, and in perspective it may lead to bed-to-bed subdivision and correlation of oil-bearing sand sequences of the Kingak formation of Alaska.

Summary

While the exact equivalent Upper Jurassic deposition for the North Slope and western Arctic Islands lies undrilled beneath the northern Siberian continental shelf, the slightly more southern Siberian platform and Siberia basin boreal sections offer one large advantage: They represent a mixing of boreal and tethyal faunas allowing for better age control of the boreal assemblages.

This, coupled with the use of overlapping zones based on four different fossil disciplines (forams, ostracods, bivalves, and palynomorphs), produces very detailed age control. The detailed age control, in turn, allows us to compare the oil- and gas-bearing strata between the three geographic areas.

The results of this comparison may aid in evaluating the eastern National Petroleum Reserve-Alaska area for the upcoming lease sale.

References

1. Zakharov, V., Mesezhnikov, M., and Ronkina, Z., et al., The paleogeography of the north of U.S.S.R. for Jurassic, Novossibirsk, Nauka, 1983, 191 pp.
2. Pinous, O.V., Sahagian, D.L., and Nikitenko, B.L., High resolution sequence stratigraphic analysis and sea-level interpretation of the Middle and Upper Jurassic strata of the Nyurolskaya depression and vicinity (southeastern West Siberia, Russia), Marine and Petroleum Geology, 1999, in press, 17 p.
3. Shurygin, B., Pinous, O., and Nikitenko, B., Sequence stratigraphic interpretation of Callovian and Upper Jurassic (Vasiugan horizon) of the southwest of western Siberia, Russian Geology and Geophysics, Vol. 40, 1999, 25 p., in press.
4. Shurygin, B., The stratigraphic volume of zones on bivalves and their situation in a Lower and Middle Jurassic scale of north of Siberia, Russian Geology and Geophysics, No. 11, 1987, pp. 3-11.
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10. Wall, J.H., Jurassic and Cretaceous foraminiferal biostratigraphy in the Eastern Sverdrup basin, Canadian Arctic Archipelago, Can. Petrol. Geol., Vol. 31, No. 4, 1983, pp. 246-281.

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The Authors