On Oil Shows in Primorsky Krai

B. E. Kopaev

(Presented by Prof. I. V. Lebedev)

Work on oil and gas exploration in the territory of Primorsky Krai began in 1953 (work by the Institute of Geology and the Regional Geological Institute of the Academy of Sciences of the USSR, and by the Eastern Oil-and-Gas Expedition), but oil-prospecting operations have not yet been developed on the proper scale.

In the territory of Primorsky Krai, natural seepages of liquid oil to the surface have not been observed, nor have products of subaerial weathering of oils been encountered. During drilling operations (1959–1962), within the Suifun and Suchan-Dadyanshan synclinal zones, information was obtained on the oil-bearing character of the Mesozoic and Cenozoic complexes. According to Yu. V. Kuznetsov, bitumen and oil shows were repeatedly noted in the cores of drilled wells. They took the form of smears and drops of a vaseline-like consistency filling cavities and fractures in variegated sandstones. They also caused the brown coloration of calcite veins cutting the sandstones in various directions. Chemical analyses of bitumens collected in such cases made it possible to assign them to bitumens of the petroleum series.

The most characteristic oil shows observed in recent times are described below.

On 10 October 1962, in mine No. 10/16 of the Suchan coal basin, at the 517 m level, during the working of coal seam P-12 “Barsuk,” an inflow of oil was discovered. The oil was released from a tectonic fracture in a bed of coarse-grained sandstones lying in the roof of the coal seam, and also from numerous small fractures feathering off from the main one. The oil spread over the surface, partly impregnating the sandstone. The oil seepage was examined by a commission composed of representatives of the “Suchanugol” trust and the Primorsky Geological Administration. This was the first strictly documented show of liquid oil in Primorye.

An analysis of the oil, carried out in the laboratories of the Primorsky Geological Administration and VNIGRI, showed that it belongs to methane oils containing solid paraffins. At a temperature of \(+10^\circ\mathrm{C}\) the oil acquires a vaseline-like consistency. The initial boiling point is \(+70^\circ\mathrm{C}\); specific gravity is \(0.8219\ \mathrm{g/cm^3}\) at a temperature of \(+20^\circ\mathrm{C}\). Component composition of the oil: oils—98.6%, resins—1.4%; asphaltenes are absent. In its optical characteristics the oil resembles metamorphosed oils of the Minusinsk type.

The sedimentary formations composing the area consist of interbedded argillites, siltstones, and fine-, medium-, and coarse-

coarse-grained sandstones, gravelites, and conglomerates that make up the Suchan series. The thickness of the Suchan series reaches 800–1000 m. It rests on the eroded surface of Late Permian granitoids and gabbroids. At the base, the section of the Suchan series ends with a bed of basal conglomerates (60–70 m).

The geological structure of the Suchan synclinal zone in the area of mine field No. 10/16 is characterized by the presence of a number of narrow, linearly elongated synclinal folds with gently sloping troughs and steep limbs. The junctions of synclinal folds form anticlinal-type structures with steeply dipping limbs, broken by numerous dislocations that cut both the crystalline basement and the sedimentary cover.

To obtain a more complete picture of the nature of the oil show in mine field No. 10/16, borehole No. 1819 was drilled to a planned depth of 1200 m.

In siting the borehole, the assumption was made that in the area of the oil shows there is an uplift of the Upper Paleozoic basement, and that the Lower Cretaceous terrigenous strata that wrap around this uplift form an anticlinal oil trap.

Drilling of borehole No. 1819 was accompanied by gas logging and by the necessary set of core-testing operations.

At a depth of 400–600 m the borehole penetrated a zone of disjunctive disturbance, represented by crumpled coarse- and fine-grained sandstones. From the slickensides observed here it may be concluded that the fault plane has a steeply dipping (70°) character.

The disturbance zone was marked by an anomalously high gas content in the clay mud. On the gas-logging diagram, in the intervals 500–600 m, an increased content of combustible gases of up to 7% is recorded.

Analysis of gas desorbed from the clay mud showed the following composition: 1) sampling depth, sample No. 8 — 560.5 m, CO₂ — 0%; O₂ — 2.05%; H₂ — 11.11%; saturated hydrocarbons amount to — 10.71% and rare gases — 64.91%. 2. Sampling depth, sample No. 7 — 564.4 m, CO₂ — 0.86%, O₂ — 20.0%, H₂ — 5.77%; saturated hydrocarbons No. 8.40%.

Luminescent-bituminological analysis of samples taken from the core of borehole No. 1819 showed that the content of soluble bitumen A in the rock is 0.0082% by weight of the rock, while in the crushing zone (400–600 m) the amount of bitumen A increases to 0.011%.

In the present case, the oil show in the roof of seam P-12 and the gas show in borehole No. 1819 may be associated with one and the same tectonic disturbance, playing the role of a fracture-type collector.

Borehole No. 1819 did not encounter liquid accumulations of oil. Having penetrated the Suchan series through its full thickness, it entered Late Permian granitoids.

Elevated gas contents were noted in a whole series of other boreholes. Laboratory investigations of these gases indicate their affinity to petroleum gases.

The nature of the oil shows in Primorsky Krai still remains unexplained. It is important to note only the fact that all of them, without exception, are associated with disjunctive disturbances and fractures in the thickness of sedimentary formations. Whether they are associated with oil pools of deeper horizons or not remains a matter of supposition. Of particular interest is the oil show in the Suchan basin, where over most of the area of its distribution the Suchan series lies on the crystalline basement.

Further investigations will help to clarify the nature of the oil shows.