Prospects for the Oil and Gas Potential of the Central Sikhote-Alin Synclinorium

B. E. Kopaev

(Presented by the Department of Combustible Minerals)

The central part of the Sikhote-Alin synclinorium is composed of sedimentary deposits of the Triassic, Jurassic, and Cretaceous systems with a total thickness of up to 7–10 km. In places these rocks are cut by dikes of acidic and intermediate composition. Apparently, it was precisely this circumstance that caused the area to be regarded as unpromising with respect to oil and gas. A number of new data compel this opinion to be changed and the area to be considered promising.

In 1964, during exploration of the Nikolaevsk polymetallic deposit, well No. 70, located 10 km east of Tetyukhe, discovered a significant inflow of gas consisting mainly of methane. In 1948, during mining work in the upper reaches of the Krasnaya and Berezovaya rivers, the Tetyukhe expedition geologist K. V. Aleksandrov recorded an oil show in one of the ditches, consisting of viscous chocolate-colored bitumen. It filled cracks in Cretaceous sandstones. The bitumen, collected in a small quantity, ignited readily from a match. Laboratory determinations of the bituminological characteristics of samples taken from sandstones and siltstones showed that it differed in no way from that characteristic of the Cretaceous strata of the Suifun and Suchan-Dadyanshan synclinal zones. Here, however, it should be noted that investigations by the thematic party of the Primorye Geological Administration, conducted in this area in 1963, did not succeed in confirming this oil show.

Of interest in this respect are the geological-survey works of the geologists of the Iman expedition, who carried out mapping in the northern part of the synclinorium, covering the upper and middle course of the Bikin River. Geologists B. L. Khomenko and V. M. Skripko, in the basin of the Bachelaza River, a right tributary of the Bikin River, in 1954–1958 noted rocks strongly resembling tarred sandstones. The sandstone samples were brown in color and, according to V. M. Skripko, gave a blue glow under a luminescope. Unfortunately, a more thorough study of these samples could not be carried out, since they were all sent to the laboratory of the Vostsibnefterazvedka trust, and the results of the analyses were not reported from there.

In 1958, a report was received from a resident of the settlement of Ulunga, E. S. Mogilnikov, concerning oil films he had observed in the basin of the Ulunga River. The informant stated that from the bottom of the body of water, from time to time, [[unclear: text continues on next page]]

gas bubbles rise, after which a bluish film with bright coloration, reminiscent of oil, remains on the surface of the water. When a burning match was brought to the gas bubbles, they flared up with a bright flame. Analysis of water samples with oil films, taken by V. L. Khomenko and V. M. Skripko from the Mogilnikov water body indicated above, showed the presence in them of salts of naphthenic acids.

In the summer periods of 1963–64, the thematic oil party of the Primorsky Geological Administration undertook geological investigations here, as a result of which the following was established. Along the right bank of the Bachelaza River in its middle course, a unit of Lower Cretaceous sandstones was discovered. The sandstones show no traces of thermal metamorphism or of any secondary alterations. In appearance these are fresh, weakly fractured rocks with subordinate interbeds of dark-gray siltstones. In the deluvium, samples were found which, when split, had a brown color—evidently the same as that noted by the geologists V. M. Skripko and B. L. Khomenko. In luminescence-bituminological investigation of these samples, no increased content of bituminous substances was established. However, their amount, as in the entire batch of samples collected everywhere from the Cretaceous unit of this area, was \(2–6 \cdot 10^{-4}\%\), with an average organic-carbon content on the order of 0.1–0.2% by weight. This makes it possible to compare the Cretaceous deposits of the central Sikhote-Alin synclinorium with the Cretaceous deposits of zones that are considered promising for oil and gas in Primorye and where structural-prospecting drilling for oil is now being carried out. These include the Suifun and Suchan-Dadyanshan synclinal zones.

During the author’s reexamination of E. S. Mogilnikov’s report, oily petroleum films were observed. The oil showing is confined to a body of water supplied by a spring, bordering the deluvial apron of the left bank of the Ulunga River in its middle course. The water body has running water and, thanks to underground springs, does not freeze for a long time in winter. Its bottom is composed of sand-and-gravel mixtures. The character of the occurrence and the appearance of the petroleum films remained as before. When the ground was loosened at the indicated place, gas bubbles rose from the bottom of the water body, and together with them oily films with bright interference coloration appeared on the water surface. The films constantly changed their shape and stretched out in smooth lines. Water samples taken together with the films, when studied in the laboratory, showed the presence of bitumens and salts of naphthenic acids.

When this place was visited in 1964, it became clear that the area of the oil showing is composed of sandstones and siltstones analogous to those described and examined in the basin of the Bachelaza River. The age of the sedimentary formations, based on finds of pelecypod and echinoid fauna, was determined as Cenomanian–Turonian. According to Yu. A. Ivanov and V. G. Plakhotnik, the Cretaceous deposits, as a rule, lie transgressively on older rocks. They are weakly metamorphosed and extremely variable in facies. Investigations by the thematic party also established that the oil showing is located 1 km upstream from the monzonite massif on the Ulunga River, noted by A. I. Savchenko and E. P. Izokh in 1954. By analogy with other oil showings known in Primorye, it may be assumed that the oil showing in the Ulunga River area is connected with a tectonic fault covered by alluvial deposits. Evidently this fault passes somewhat upstream of the site of the oil showing, along the bank of the Ulunga River.

Thus, there is every reason to consider the area of the central Sikhote-Alin synclinorium promising with respect to oil and gas, and to raise the question of conducting drilling operations here.