Luginetskoye Gas-Condensate and Oil Field in Tomsk Oblast

Yu. V. Naidanov

(Presented by Professor A. V. Aksarin)

The Luginetskoye field is located in the Kargasok District of Tomsk Oblast and, in terms of approved reserves, is second after the Myldzhinskoye field in the Pudino gas-and-oil-bearing region. The Luginetskaya local structure lies within the limits of the uplift of the same name, a dome-shaped uplift identified by seismic exploration in the northwestern part of the Pudino megaswell. On the structural map for seismic reflecting horizon II-a (the base of the Bazhenov Formation of the Upper Jurassic), this structure is a brachyanticlinal fold, almost completely contoured by the seismic isohypse of −2320 m. Only in the extreme southeast does this isohypse fail to close over a small interval (Fig. 1). The strike of the structure is northwestern; its dimensions are 25·34 km, and its amplitude is 160 m (for comparison, we note: the amplitude of the North Vasyugan structure is 150 m, and that of the Myldzhinskaya structure is 120 m). The structural plan of the Luginetskoye local uplift becomes considerably simpler at higher reference horizons.

The field was discovered in 1967 by well No. 152, drilled in the crest part of the structure. According to the results of testing at the Luginetskoye field, two pools have been established—gas and oil—which are hydrodynamically connected with one another. The Jurassic deposits are commercially gas-and-oil-bearing; within them the productive beds Yu-1, Yu-2, and Yu-3 are distinguished. Clayey-silty interbeds are not always present between the indicated beds, and therefore their separation presents certain difficulties. In picking the boundaries of these productive beds from logging diagrams, well No. 151 was taken as the basis, since their differentiation is most clearly expressed there.

The absence of monolithic character and the hydrodynamic connection between the beds made it possible to index this entire series of beds as productive bed Yu\(_{1–3}\).

The top of the bed is identified fairly clearly on standard electric-log diagrams by the sharp drop of the KS curve at the transition from the high-resistivity bituminous argillites of the Bazhenov Formation to the low-resistivity silty-sandy deposits of the upper part of the Vasyugan Formation. In addition, the black bituminous argillites of the Bazhenov Formation are readily traced in core.

The base of the bed is distinguished less confidently, and in some wells even with a certain degree of convention, since it passes through a monotonous-

Fig. 1. Structural map of the Luginetskoye field. Scale 1:200,000: 1—seismic contour lines of reflecting horizon Pa (base of the Bazhenov Formation); 2—contour lines based on drilling data; 3—oil–water contact; 4—gas–oil contact; 5—drilled wells

…in a sequence of interbedded sandstones, siltstones, and argillites.

This convention mainly concerns those wells where the lower part of the bed is no longer productive (water-bearing). Therefore, such identification of the base does not affect the accuracy of structural constructions and determinations of the elevation positions of the gas–oil structures, or determinations of the elevation positions of the gas–oil contact (GOC) and oil–water contact (OWC), since we are dealing with a reservoir pool of the massive type. Establishing the boundaries separating reservoir fluids (GOC, OWC) presents certain difficulties. In most cases these boundaries pass through an interbedded and electrically heterogeneous interval, in which the applied BKZ method is not very effective.

Therefore, in such a section the most applicable method is determining the OWC (GOC) from the results of testing small intervals of the reservoir in the casing string; electric-log data are also taken into account.

At the Luginetskoye field, determination of the OWC was made mainly from the results of well testing, while the picking of the GOC was carried out from temporary NGK measurements and testing.

The absolute elevation of the gas–oil contact is −2225 m, and of the oil–water contact −2244 m. The working gas rates through a 12.5 mm choke were…

59–351 thousand m³/day, oil (through an 8 mm choke) 11–107 m³/day. Formation pressure in the gas pool is 240–244 ata, in the oil part 241–245 ati; the average formation temperature is 80°C.

The condensate–gas factor (stable condensate) varies from 138 to 279 cm³/m³; its average value for the field is 204 cm³/m³. The gas-bearing interval is 87 meters, the oil-bearing interval 19 meters (at the Severo-Vasyugan and Myldzhino fields the gas-bearing interval is 74 meters).

The total thickness of productive bed Yu₁–₃ is 92–127 m, the effective thickness 24–55 meters. The number of sandy interbeds reaches 9–29.

A certain increase in sandiness is observed in the central part of the structure, as well as in its northeastern parts.

The sandy beds of the Cretaceous deposits (Kulomzino, Tara, Pokur, and Ipatovo suites) have been tested in a number of crest and flank wells; everywhere inflows of formation water were obtained, or no inflow was obtained at all.

The basal bed at the contact of the platform deposits and the Paleozoic (the so-called weathering crust, bed M) was tested in well No. 160; a small quantity of formation water was obtained.

The gas-oil pool of bed Yu₁–₃ is underlain throughout its entire outline by water. The waters are calcium chloride, with mineralization of 47–53 g/l. Static levels in several wells (156, 160, 161, 163) have absolute elevations of +97 m, +62 m, +118 m, +96 m.

The recharge area for the Jurassic waters is the southern border of the West Siberian Lowland. The regional discharge area of the groundwater is apparently located in the waters of the Kara Sea.

Taking into account the influence on the pool of the gas part (“cap”), the field regime may be considered mixed, i.e., elastic gas-drive + elastic water-drive. Special attention should be given to the prospects for gas-bearing capacity of the basal bed M of the Pudin megaswell as a whole. Recently the gas-bearing capacity of the lower part of the deposits of the platform cover here has been proved by the discovery of the Verkhne-Kombarskoye field (south-southwest of the Pudin megaswell) and, quite recently, by the establishment of a gas pool in bed M at the Ostanino field (southwestern flank of the megaswell), i.e., a zone of possible development of bed M is emerging within the southwestern slope of the megaswell. It is characteristic that the productive bed M in well 425, the discovery well of the gas pool on the Ostanino structure, has a very low resistivity for these regions (3.8 ohm·m), although in practice cases are known of obtaining significant oil gushers even with such resistivity of productive beds (Malorechenskaya — 3.6 ohm·m; Olenya — 3.5 ohm·m areas).

Interesting data could also be obtained at the Luginetskoye field, located at the northwestern end of the Pudin megaswell, but there, on the southwestern flank of the structure, there are no wells that have penetrated the Paleozoic; therefore it is impossible to judge with confidence the distribution of the reservoir and the nature of saturation of bed M.

In the future, within the Luginetskoye structure it is planned to drill a parametric well with the objective of penetrating the Paleozoic formations to a considerable depth (300–500 m) and studying their oil and gas potential. In view of the above, it would be advisable to place such a well in the southwestern part of the structure. Thus, the Paleozoic formations within the Pudin megaswell undoubtedly constitute a potential base for a further increase in gas resources (and possibly oil as well).

According to laboratory analyses, the free gas of productive bed Yu\(_{1-3}\) of the Luginetskoye field has a hydrocarbon composition. Its principal component is methane, the content of which ranges from 78 to 92%. The sum of heavy hydrocarbons (ethane–hexane) varies from 6 to 16%. Nitrogen in the gas composition amounts to 2–12%; carbon dioxide is present in small quantities, 0.1–1.6%; there is no hydrogen sulfide. No regular pattern has been identified in the variation of gas composition over the area of the field, but an increase in the sum of heavy hydrocarbons is noted in wells located near the oil-bearing contour (159, 163, 168).

The relative specific gravity of the gas with respect to air is 0.58–0.72 g/cm\(^3\), and the lower heat of combustion is 8000–9700 kcal/m\(^3\). The maximum helium concentration is 0.03%.

According to surface analyses, the oil has a specific gravity of 0.815–0.851 g/cm\(^3\).

The initial boiling temperature is 35–124° C. Light fractions boiling up to 200° C account for 18–42%; those boiling up to 300° C account for 38–65%. According to analyses, silica-gel resins in the samples are noted up to 8%, paraffin up to 4%, and sulfur up to 0.46%.

In the group composition, methane hydrocarbons predominate (27–50%); naphthenic hydrocarbons are less abundant (15–34%), and aromatic hydrocarbons still less so (15–27%).

The gas-saturation pressure of the oil is 144 atm; the viscosity of the reservoir oil is 0.59 cSt; the volume coefficient is 1.29 m\(^3\)/m\(^3\). Stable condensate has been studied in surface samples; its specific gravity is 0.713–0.754 g/cm\(^3\), and its initial boiling point is 28–76° C. Up to 200° C, 75–92% boils off; it boils completely up to 250–300° C. In terms of its group hydrocarbon composition, it is methane–naphthenic with a small aromatic content. Taking into account the great practical importance of the oil and condensate as raw material for the Tomsk petrochemical complex, their commercial properties have been studied in the fuel technology laboratory of the Tomsk Institute under the VNIPI program. Gasoline distillates of the oil and condensate are characterized by the absence of sulfur compounds and by low octane numbers in neat form—44–47 for the oil and 48–67 for the condensate. However, all gasolines have high susceptibility to tetraethyl lead (TEL).

All gasoline fractions of the condensate from the IBP to 150° C satisfy the requirements for grade A-72 gasoline, but with the addition of TEL; the IBP–100° C fraction and all subsequent fractions meet the GOST requirements for A-66 automotive gasoline. Light distillates from the IBP to 120° C, with the addition of 0.82 g/kg TEL, comply with the GOST for A-74 motor gasoline.

Gasoline fractions of the oil from the IBP to 180° C, after the addition of ethyl fluid in an amount of 0.82 g/kg, will fully meet the GOST for A-66 automotive gasoline. All fractions with an end boiling point above 180° C have octane numbers below 66 (with TEL added) and can be used only as gasoline components.

The 120–240° C oil fraction meets the GOST for grade T-1 fuel in all parameters.

The 120–240° C and 120–280° C condensate fractions, in all parameters except acidity and initial crystallization temperature, correspond to grade T-2 fuel.

Consequently, in obtaining on-specification fuel for air-breathing jet engines (VRD), they require only alkaline treatment and dewaxing or the addition of a depressant.

The indicated distillates cannot be used to obtain T-1 and TS-2 fuels. Diesel-fuel distillates of the oil meet the GOST

for diesel summer fuel. As diesel fuels from the condensate, the residues boiling above 150°C and above 200°C were tested; both tested fractions are characterized by very high cetane numbers—above 56. The residue above 150°C, in all indicators except the pour point, corresponds to diesel fuel “L” for high-speed engines.

The residue boiling above 200°C will correspond to fuel “Z,” but only after dewaxing. It should also be noted that the condensate residue above 200°C, after alkaline treatment, meets the GOST requirements for illuminating kerosene.

Illuminating kerosene can be obtained from the oil with a yield of 35%. However, these fractions cannot be used as tractor fuel because of their low octane numbers.

The 350–450°C oil fraction is a good feedstock for catalytic cracking.

In processing the oil, fuel mazuts of grades 40, 100, and 200 can be obtained, as well as viscous road bitumens.

The results of the analysis studying the group hydrocarbon composition of the gasoline fractions of the oil and condensate indicate their exceptional value as feedstock for petrochemistry, pyrolysis processes, and catalytic reforming.

In studying the group hydrocarbon composition of fifty-degree fractions boiling above 200°C, it was established that in the specified condensate fractions the content of aromatic hydrocarbons is 12–15%, and benzene derivatives predominate in their composition. The content of aromatic hydrocarbons in the 50-degree oil fractions is 29–46%. Depending on the temperature limits of fraction selection, the content of paraffin-naphthenic hydrocarbons may be 50–71%, decreasing as the boiling temperature of the fractions increases.

By hydrocarbon composition, the oil of the Luginetskoye field belongs to the paraffin-naphthenic type.

The potential content and characteristics of distillate and residual oils, determined by the method of adsorption separation on silica gel, make it possible to consider that 8.3% distillate oils and 4% residual oils with a viscosity index (VI) of 85 can be obtained from the oil. According to the technological classification (GOST-912-66), the oil of the Luginetskoye field belongs to class I, type T₁. In terms of the content of base oils, the oil corresponds to group M₂ and subgroup I₁. In terms of paraffin content, the oil belongs to kind P₂. Consequently, this oil has the code IT₁M₂I₁P₂.

The practical value of the oil and condensate as feedstock for petrochemistry has already been noted above. In conclusion, it should be emphasized once more that, owing to the insignificant content of aromatics, the predominance of paraffinic hydrocarbons in the composition of the fractions, and the complete absence of sulfur, the condensate of this field is an exceptionally valuable feedstock for pyrolysis and may be recommended for use as feedstock for the petroleum refinery whose construction is planned near the city of Tomsk.