Gas Potential of the Lower Part of the Tyumen Deposits of the Pudinsky Arch
Abstract Generated abstract
This paper reports on the gas-bearing potential of the basal Tyumen deposits within the Verkhne-Kombarskoye uplift of the Pudinsky Arch, where earlier exploration had focused mainly on upper Tyumen reservoirs. Using seismic structural interpretation and testing of well No. 290, the study documents a gas-condensate flow from bed M near the cover and basement contact, with normal reservoir pressure and a high condensate-gas factor. The paper describes the structure of the uplift, reservoir test results, and the composition of the gas and condensate, noting hydrocarbon-rich gas dominated by methane and light condensate with low aromatic content. The discovery is interpreted as important for exploration, suggesting that the full Jurassic section of the Pudinsky Arch, down to the Paleozoic basement, warrants further evaluation.
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GAS-BEARING CAPACITY OF THE LOWER PART OF THE TYUMEN DEPOSITS OF THE PUDINSKY ARCH
Yu. V. Naidanov
(Presented by Professor A. V. Aksarin)
Within the Pudinsky Arch, a first-order structure, until 1971 only three gas-condensate fields were known: Luginetskoye, Ostaninskoye, and Kazanskoye. In 1971 one more was discovered—Verkhne-Kombarskoye.
The Verkhne-Kombarskoye local uplift is located in the southeastern part of the Pudinsky Arch. According to the principal reflecting seismic horizon IIa (the lower part of the Bazhenov Formation), the structure is an anticlinal fold of oval form, contoured by the 2300 m isohypse, and has a northwestern strike; its dimensions along the contouring isohypse are 14.5 × 5.5 km, and its amplitude is 45 m. The southwestern limb of the structure is gentle (40–50′), while the northeastern limb is steeper (1.5–2°). According to seismic exploration data, no tectonic disturbances are observed. In the overlying seismic markers III (the lower part of the Pokur Formation) and IVa (the base of the Slavgorod Formation), the Verkhne-Kombarskaya structure flattens considerably and, evidently, is not of interest as a trap.
The gas-and-oil-bearing capacity of the upper part of the Tyumen deposits (beds Yu-II–IV) is known within the Pudinsky Arch in the Luginetskaya and Kazanskaya structures, located northwest of Verkhne-Kombarskaya. Here, at the Verkhne-Kombarskaya area, the very lowest part of the Tyumen deposits, situated in the contact zone of the rocks of the cover and the basement (bed M), has been tested. For individual areas of the region, sporadic development of this basal bed is characteristic, when it possesses fairly good reservoir properties and is productive (Festivalnaya, Chebachya areas, etc.).
In well No. 290, drilled in the center of the Verkhne-Kombarskaya structure, during testing of bed M in the intervals 2794–2807 m and 2826–2837 m, a gas flow was obtained with a rate of about 40 thousand m³/day through a 9.65 mm choke, with a high condensate content; the average value of the condensate-gas factor was 370 cm³/m³. The reservoir pressure in the pool is 280 ata, which corresponds to hydrostatic pressure; that is, we have a gas-condensate accumulation with normal reservoir pressure. It should be noted that in the fields of the southeastern part of the Pudinsky Arch (Ostaninskoye, Verkhne-Kombarskoye, Kazanskoye), a high condensate content is observed in the gas, considerably greater than at Severo-Vasyuganskoye and Myldzhinskoye, which are located farther northwest. Moreover, the increase in the condensate-gas factor also occurs in the southeastern direction, from 204 cm³/m³ (average value) at the Luginetskoye field to 770 cm³/m³ at Kazanskoye.
According to preliminary data, the condensate is a colorless liquid with a specific gravity of 0.71–0.75 g/cm³. The initial boiling point is 30–70°C; up to 200°C, 70–85% boils off, and it boils off completely at 250–300°C. The kinematic viscosity at 20°C is 0.77–1.2 cSt, and at 50°C, 0.65–0.85 cSt. In terms of group hydrocarbon composition, methane components predominate in the condensate, their content being 65–80%; naphthenic components are significantly less abundant, 15–25%, and aromatic components are very minor, 3–6%.
According to the results of the analysis carried out, the free gas of the Verkhne-Kombarskoye field has a hydrocarbon composition; its relative specific gravity is 0.700 g/cm³. The principal component of the gas is methane, the content of which is 82.8–84%. The sum of heavy hydrocarbons (ethane–hexane) is 10.76–10.9%; nitrogen in the gas composition amounts to 2.38–3.48%. Carbon dioxide is present in small quantities, 2.64–2.67%; hydrogen sulfide is absent. The maximum amount of helium in the free gas is 0.02%, that is, the concentration is clearly noncommercial. Other gas-condensate fields of the Pudinsky Arch (Luginetskoye, Kazanskoye, Ostaninskoye) have a similar gas composition.
Obtaining a gas flow from the lower part of the Tyumen Formation at the Verkhne-Kombarskoye field is of great exploration significance; at present, practically the entire section of Jurassic deposits, from the Bazhenov Formation to the Paleozoic basement, merits attention. This, in turn, increases the prospects for gas-bearing capacity of the Pudinsky Arch as a whole.