Geological Structure and Petroleum Potential of the Luginetskoye Field

Yu. V. Naidanov

(Presented by Professor A. V. Aksarin)

The Luginetskoye gas-condensate–oil field is located in Kargasok District, Tomsk Oblast. Tectonically, it is confined to the dome-shaped uplift of the same name, which lies within the northwestern part of the Pudino arch. At the field (as of January 1, 1970), 14 wells had been drilled: 151—157, 159, 160, 162—164, 166—167 (Fig. 1). The geological structure of the field includes rocks of Paleozoic, Mesozoic, and Cenozoic age. Since the petroleum potential at the Luginetskoye field is associated only with Jurassic formations, these will be considered in greater detail.

Paleozoic deposits compose the basement and have been penetrated to a shallow depth (28—38 m) by only two wells, 151 and 160; in lithologic composition they are carbonates and basic effusives. Their exact age has not been established; presumably they are Lower Carboniferous—Upper Devonian.

At the boundary between the Paleozoic and the Jurassic, the so-called weathering crust has been encountered. In well 151 it is characterized by core, while in well 160 it is distinguished only from logging data. In composition it is an argillaceous-carbonate rock of brownish-yellow color; the thickness of the weathering crust is 6—26 m. Its approximate age is Triassic. Deposits of the Jurassic system are present in the volume of three formations: the Tyumen, Vasyugan, and Maryanovka. Continental deposits of the Tyumen Formation, of Lower–Middle Jurassic age, lie with an unconformity on the basement rocks. The overlying Vasyugan Formation of nearshore-marine deposits is dated as Upper Jurassic. Both formations consist of frequently interbedded sandy-silty and argillite rocks. The thickness of the Tyumen Formation is 123—130 m, and that of the Vasyugan Formation is 60—74 m. The Jurassic ends with marine sediments of the Maryanovka Formation, represented by argillites, bituminous at the base (the Bazhenov member). The thickness of the Maryanovka Formation is 66—99 m. The total thickness of the Jurassic deposits is 249—293 m.

Deposits of the Cretaceous system are represented by all stages and substages. Its total thickness is 1,854—2,077 m. The thickness of the Paleogene is 254—278 m. Quaternary sediments are composed of sands and clays of modern aspect; their thickness is 10—40 m.

Until now, in the course of geological-exploration work in the prospective lands of Tomsk Oblast, gas-condensate fields without an oil rim have been encountered (Severo-Vasyuganskoye, Ust-Silgin-skoye), or gas-condensate fields with an oil rim of noncommercial character (Myldzhinskoye, Kazanskoye). An exception to this is the Luginetskoye field, which is a gas-condensate–oil field;

Fig. 1. Structural map of the Luginetskoye field: 1—contours of the roof of productive horizon Yu-1-III according to drilling data; 2—gas-bearing contour; 4—number of the drilled well.

i.e., it has oil reserves of commercial significance. Commercial gas-condensate productivity is associated with the Jurassic deposits, among which bed Yu-1-III is distinguished. In essence, these are three beds which stratigraphically encompass two formations—the Vasyugan Formation (bed Yu-1) and the Tyumen Formation (bed Yu-II-III). However, the absence of a reliable clayey break between these beds does not allow them to be indexed separately; therefore the productive bed has been given the name Yu-1-III.

In the wells drilled in the field, the roof of bed Yu-1-III was penetrated within the depth interval 2,260–2,339.2 m. The upper boundary of the bed is rather clearly expressed on standard electric-log diagrams by the sharp drop of the SP curve during the transition from the high-resistivity Bazhenov member to the low-resistivity sediments of the upper part of the Vasyugan Formation. The base of the bed is distinguished less confidently, since it passes within a monotonous thickness of sandy–silty–argillaceous rocks of the Tyumen Formation. The total thickness of the productive bed is 92–126.8 m; the effective thickness, i.e., the total thickness of permeable interbeds excluding the dense partings that separate them, is 24.4–54.8 m. The number of sandy interbeds in the effective part reaches 9–29. Sandstones and the coarsest-grained varieties of siltstones serve as the commercial reservoirs for gas and oil.

The gas-bearing capacity of bed Yu-1-III was established from the results of testing wells 151–157, 159, 162–164, 166. Working gas rates on a 12.5-mm choke were 59.3–351 thousand m³/day, with drawdown on the bed

30.3–79.9 ata; free flow rates reach 959.4 thousand m³/day. According to measurements with bottom-hole pressure gauges, the reservoir pressure in the gas pool ranges from 240.8 to 244.3 ata. Along with the gas, a considerable amount of condensate flowed from the wells. The condensate-gas factor (stable condensate) varies from 138.2 to 278.8 cm³/m³. Its fluctuation can evidently be explained by separation conditions, as well as by the position of the test interval relative to the gas-oil contact.

When testing the reservoir below the plane of the gas-oil interface, oil gushers were obtained with flow rates from 17 to 69.8 m³/day through an 8 mm choke, while the drawdown created on the reservoir varied from 55.4 to 171.9 atm. The reservoir pressure in the oil part of the reservoir is 241.8–245.2 atm. In connection with the presence of gas, oil, and water in the reservoir, two contacts have to be distinguished in the field: gas-oil (GOC) and oil-water (OWC). The boundary between oil and gas in the reservoir should be regarded as a transition zone rather than as a sharp boundary. Conventional electrical logging methods are unreliable for picking the gas-oil contact, except under ideal conditions when the rocks composing the gas-oil reservoir have a comparatively large thickness and are lithologically uniform. Such conditions do not exist at the Luginetskoye field. Therefore, the main material used in substantiating the gas-oil boundary was the results of well testing, with the use of RC and BKZ diagrams. In general for the field, the gas-oil contact was taken at an absolute elevation of −2,222 m. The position of the oil-water contact was also established mainly from the results of well testing. With the aid of electrical logging curves, because of the screening effect in large probes and the frequent absence of reservoirs in the zone of the assumed OWC, it is not possible to pick the oil-water contact. The standard radiolog methods applied also prove to be of little effectiveness. The reason for this is evidently the same water content in the oil and in the formation water, given the low mineralization of the latter. According to testing, the oil-water contact was accepted at an absolute elevation of −2,244 m.

Proceeding from the accepted elevations of the GOC and OWC, the gas-saturated thickness varies from 1.2 to 42.4 m; the oil-saturated thickness—from 2.8 to 12 m.

The gas pool of the Luginetskoye field is a reservoir pool, of massive type, and is underlain by an oil pool. The gas-bearing interval is 85.2 m, and the oil-bearing interval is 22 m. For comparison, it should be noted that the height of the gas pool at the Severo-Vasyuganskoye and Myldzhinskoye fields is 74 m. The oil-bearing interval of the Luginetskoye field is considerably smaller than in “purely” oil fields, for example, the Malorechenskoye (38 m) and Sredne-Nyurolskoye (33 m) fields. Thus, according to the results of testing and partly of logging, only one productive reservoir, Ю-1-III, has been identified, although many intervals were tested, located within a considerable stratigraphic range from the weathering crust at the boundary with the Paleozoic up to and including the Ipatovo Formation. Consequently, the field is single-reservoir. The composition of the reservoir gas is mainly methane (82–90%); carbon dioxide amounts to 0.6–0.7%, nitrogen + rare gases 1.2–1.6%, and helium—up to 0.02%. The condensate is colorless; its specific gravity is 0.713–0.732 g/cm³; initial boiling point 28–65°; sulfur content is insignificant (0.01–0.1%). The oil has a specific gravity of 0.815–0.849 g/cm³, initial boiling point 35–124°; paraffin content up to 2.6%, sulfur—up to 0.42%.

The bottom waters of reservoir Ю-1-III are of calcium chloride composition, with mineralization of 46.7–53.4 g/l; there are no visible areas of discharge or recharge. Therefore, the reservoir regime is considered presumably elastic water-drive.