Gas Geochemical Evaluation of the Petroleum Potential of the Birofeld Graben of the Middle Amur Sedimentary Basin (Russian Far East)

Yatsuk A.V. and Gresov A.I.

V.I. Il’ichev Pacific Oceanological Institute feb ran, Vladivostok, Russia

yatsuk@poi.dvo.ru

The gas geochemical investigations in the northeastern part of the Birofeld graben revealed that the soil layer contains anomalous fields and zones of hydrocarbon gases confined largely to the southeastern fault and its splays. It is established that the molecular mass and weight reflecting the composition of the C₁–C₄ hydrocarbon fractions represent reliable characteristic parameters of the genetically different hydrocarbon gases in the Birofeld graben. The use of the gas geochemical coefficients (quantitative parameters) allows for the discrimination of the main genetic groups of natural gases.

No surface natural oil and gas showings are registered in the Birofeld graben, which indicates the good high properties of the cap rocks and represents a positive factor. This inference is substantiated by the results of the snow cover and soil gas surveys: most of the defined hydrocarbon anomalies are usually below fivefold values of the background parameters (0.0010%), while amounting occasionally to 0.002–0.004% (20–40 ppm).

P roceeding from the data on the reserves of coal seams [1], the minimal predicted resources of hydrocarbon gases contained in the coal seams of the northeastern and central areas of the Birofeld graben and in their gas-saturated host rocks should be as high as 5.4 × 10⁹ and 1.1 × 10⁹ m³, respectively (or 6.5 × 10⁹ m³ in total). According to the geological- technological criteria and the economic significance, this corresponds to a small gas field (1–10 × 10⁹ m³) with prospects for its development for local needs.

Fig. Schematic map of the gas geochemical zoning of the genetically different natural gases in the northeastern Birofeld graben.

It is established that the hydrocarbon gases from the igneous rocks, condensate gas, gas condensate, and oil gas accumulations migrate through fault zones from the Cretaceous sequences, which serve as both oil–gas source and oil–gas generating formations that stimulate the accumulation of hydrocarbons and the formation of their pools in the Cenozoic sequences developed in tectonically closed fault line structures along the southeastern wall of the graben and on dome shaped uplifts. The discovery of large economic gas reservoirs in the Cenozoic sedimentary complex in the graben areas under consideration seems problematic, although the defined gas geochemical properties of the primary organic matter and natural gases in the Cenozoic and Cretaceous gas source rocks imply the development of small and, presumably, medium gas, condensate–gas, gas–condensate, and oil–gas accumulations.

The anomalous zones of genetically and compositionally different hydrocarbon gas accumulations defined by the preliminary gas geochemical investigations are promising for the planning detailed oil exploration work in the studied area of the Birofeld graben (water intake well) and in the southern part of the Dimitrovo uplift (Fig.). In addition, it should be taken into consideration that the oil and gas fields in neighboring China are localized in the zone of the main graben forming fault bordering the inner uplift.

Judging from the obtained geochemical gas data, the area under consideration is highly promising for the discovery of gas and, probably, gas–condensate accumulations. The defined gas geochemical parameters, combined with the seismic exploration materials, form a basis for revealing and outlining promising graben areas appropriate for oil and gas drilling exploration works.

References:

1. Middle Amur Sedimentary Basin: Geological Structure, Geodynamics, and Fuel-Energy Resources, Ed. by Kirillova G. L. and Khanchuk A.I. (DVO RAN, Vladivostok, 2009), 424 pp.