On the Search for New Oil Fields in the Jurassic Deposits of the Lar-Yegan Oil-Bearing Region

ON THE SEARCH FOR NEW OIL FIELDS IN THE JURASSIC DEPOSITS OF THE LAR-EGAN OIL-BEARING REGION

E. E. Danenberg, N. I. Podstrigan

(Presented by Professor A. V. Aksarin)

The Lar-Egan oil-bearing region is located in the central part of the West Siberian Lowland and, in tectonic terms, is associated with the positive structure of the same name of the second order—the Lar-Egan swell, which complicates the eastern part of the Nizhne-Vartovsk arch. According to geophysical data, the swell is an elongated structure of meridional extent, measuring 100·30 km. The eastern flank of the swell plunges into the Koltogor trough, which separates it from the Aleksandrov mega-swell. In the west, the swell is separated from the central part of the Nizhne-Vartovsk arch by a saddle with insignificant amplitude of subsidence. The swell is complicated by a whole series of structures of the third order, grouped into the Sosninskaya, Alenkinskaya, Matyushkinskaya, and other groups of local uplifts.

Wells drilled within the Lar-Egan swell have penetrated a sedimentary rock complex of Meso-Cenozoic age, 2600–2800 m thick, lying transgressively on the dislocated and eroded surface of the pre-Jurassic basement.

The sedimentary sequence composing the section of the swell is characterized by a broad range of oil-bearing capacity. All identified oil shows and commercial accumulations of oil are confined to sandy horizons of Jurassic, Valanginian, and Hauterivian-Barremian age. No oil shows have been found in deposits of the Aptian stage or in stages of younger age.

It should be noted that, in terms of the degree of oil saturation, the above-mentioned horizons are far from equivalent. Along with large oil pools in the Lower Cretaceous deposits, confined only to the Sosninskaya group of local uplifts, oil shows and oil pools in Jurassic formations are widely distributed throughout the entire swell.

The Lower and Middle Jurassic deposits occur at the base of the sedimentary complex and are combined into the Tyumen Formation, composed of an alternation of sandstones, siltstones, and argillites with a thickness of 15–230 m. The sandstones of the formation are characterized by very low reservoir properties, and only two reservoir horizons are distinguished in its section, of which horizon “M” occurs at the base of the formation, and horizon Ю₂ at its top. The overlying deposits of the Vasyugan Formation are of Upper Jurassic age and, on the basis of lithological characteristics, are subdivided into two members: the lower, predominantly argillaceous one, and the upper, represented by sandstones with subordinate interbeds of argillites, distinguished as horizon Ю₁ (according to the nomenclature adopted at the meeting).

in Pravdinsk, 1965). The thickness of the suite is up to 70 m. Higher in the section lie bituminous clays of the Maryanovo Suite, 25–100 m thick, corresponding to the Upper Jurassic and Lower Valanginian.

In the Jurassic deposits, oil-saturated horizons M, Yu₂, and Yu₁ have been identified. The degree to which they have been studied is not the same; accordingly, the possibilities for forecasting further exploration also differ.

Commercial oil productivity of the Medvedev area of the Sovetskoe field is associated with horizon “M”.

During its testing in well No. 6, from the interval 2722–2705 m, an oil fountain with a flow rate of 88.2 m³/day was obtained through an 8 mm choke. The specific gravity of the oil is 0.844; paraffin content is 19.53%; sulfur is practically absent. In the direction of the crest of the uplift, the horizon is replaced by low-permeability rocks. When it was tested in crest well No. 7, at a dynamic level of 1300 m, an inflow of formation water with a rate of 1.93 m³/day was obtained, with the presence of a strongly paraffinic oil film.

At the present stage of study of the field, it may be assumed that the identified pool of horizon “M” belongs to the bedded type, lithologically bounded by impermeable rock varieties in the direction of the crest of the structure. In plan, the pool may possibly be developed only on the northeastern flank of the Medvedev uplift.

Horizon “M,” penetrated by single wells in the Sovetskaya, Komsomolskaya, and Malorechenskaya areas, is replaced by dense rock varieties, and during its testing no indications of oil saturation were observed.

The insufficient study of the distribution of zones of improved reservoirs in horizon “M,” even within a single local structure of the Lar-Egan swell, does not at present allow prediction of possible sites within it where oil pools may occur. It can only be assumed that, where horizon “M” is developed on the flanks of local uplifts facing the Koltogor trough, commercial accumulations of oil in pools of the above-named type may be discovered in it.

Oil shows from horizon Yu₂ have been recorded only in the Medvedev area of the Sovetskoe field. When it was tested in crest well No. 7, from the interval 2528–2521 m, at a dynamic level of 1300 m, an oil inflow with a rate of 6.4 m³/day was obtained. The specific gravity of the oil is 0.829; paraffin content is 20.2% and sulfur 0.12%. Within the area, the horizon is well developed in the crest of the uplift and decreases in thickness almost by half in the northern near-crest part and on the northeastern flank.

Horizon Yu₂ is traced throughout the entire Lar-Egan swell, is characterized by sharp lithological variability, and is overlain by a bed of argillites of the Vasyugan Suite up to 50 m thick.

It may be assumed that in the most elevated parts of the Lar-Egan swell, provided that good reservoirs are present in horizon Yu₂, small bedded, lithologically bounded oil pools may be identified.

The main interest for oil exploration in the Lar-Egan swell is horizon Yu₁ (Fig. 1). Within the swell, the horizon has been penetrated on eight structures. On two of them (Medvedev and Malorechenskaya) oil pools have been established; on two (Alenkinskaya and Matyushkinskaya) oil shows have been recorded; and on four (Komsomolskaya, Zakhoryutinskaya, Sosninskaya, and Sovetskaya) negative results were obtained. It should also be noted here that, with subsequent refinement of the structure of the Komsomol uplift, it was established that well No. 111 penetrated horizon Yu₁ under non-optimal structural conditions.

During testing of horizon Yu\(_1\) in well No. 7 of the Medvedevskaya area, from the interval 2454–2448 m, an oil fountain was obtained through an 8-mm choke with a flow rate of 47 m\(^3\)/day. The specific gravity of the oil is 0.846; its paraffin content is 2.3% and sulfur content 0.7%. Within the area the horizon is lithologically inconsistent. In the northern near-crest part a decrease in its effective thickness is observed owing to argillization of the upper part of the bed. The oil accumulation is massive, underlain by water, and its contours are to some extent limited by the lithological variability of the horizon over the area. The position of the OWC is determined at the elevation of 2410 m.

Horizon Yu\(_1\) is the productive horizon of the Malorechenskoye field. Within it, it has been opened by a number of wells in the interval 2454–2500 m. The object was tested in wells Nos. 117 and 120, as a result of which oil fountains were obtained, with a flow rate through an 8-mm choke of 115–146 m\(^3\)/day. The oil obtained has a specific gravity of 0.847 and contains 2.19% paraffin and 0.6% sulfur. The thickness of the horizon is about 40 m, with only its upper part being oil-saturated. Within the field the horizon is lithologically inconsistent, which is expressed in an increase in the number of interbeds of argillites in its upper part in the section of well No. 117 and a corresponding decrease in the thickness of the sandstones. The oil accumulation belongs to the type of massive accumulations underlain by water. The position of the OWC depends on the heterogeneous lithological structure of the middle part of the bed and is conventionally placed at the elevation of −2420 m.

On the basis of the available data it has been established that the oil accumulations of horizon Yu\(_1\) within the Lar-Egan swell are confined only to the upper part of the horizon. The incomplete filling of the traps of horizon Yu\(_1\) with oil is apparently connected with the fact that the growth of the structures continued after the formation of the oil accumulations as well, but with the practical cessation of further oil migration from the oil-generating area.

It is quite possible that the unfavorable distribution of sandy reservoirs in the upper part of the horizon in the Alenkinskaya, Sovetskaya, Sosninskaya, and Zakharyutinskaya areas restricted oil migration into their crestal parts. This assumption is confirmed by the results of testing horizon Yu\(_1\) in well No. 101 of the Alenkinskaya area, as a result of which an inflow of formation water with a film of oil was obtained. At the same time, in the Matyushkinskaya area, in crestal well No. 31, horizon Yu\(_1\), which lies hypsometrically lower than in the above-mentioned areas, according to core material and field-geophysical studies, is characterized by good reservoir properties and is oil-saturated in its upper part.

Thus it may be considered that the oil accumulations in bed Yu\(_1\) within the swell depend mainly not on its hypsometric position, but on the development in its upper part of good permeable reservoirs. Consequently, if this factor is preserved, new oil accumulations in horizon Yu\(_1\) may be discovered in the Kedrovskaya, Molodezhnaya, Verkhne-Koltogorskaya, Strezhevaya areas, and other local structures.

REFERENCES

1. Gurari F. G., Nesterov I. I., Rostovtsev N. N., Rudkevich M. Ya. Principal regularities in the distribution of gas and oil accumulations. Proceedings of the West Siberian Research Geological Petroleum Institute, issue 1, 1965.

2. Danenberg E. E. The Sosninskoye oil field. Bulletin of the West Siberian and NTGU, No. 3, 1963.

Fig. 1. Correlation scheme of productive horizon Yu-1 of the Lar-Egan swell

Visible labels in the diagram include:

• Matyushkinskaya, R-91, M2A0.5B
• Komsomolskaya, R-111, A2M0.5I
• Zakharotinskaya, R-106, A2M0.5I
• Alenkinskaya, R-101, M2A0.5B
• Malorechenskaya, R-117, M2A0.5B
• Medvedevskaya, R-7, M2A0.5B
• Sovetskaya, R-20, M2A0.5B

Stratigraphic labels shown:

• Maryanovskaya Suite
• Salym Suite
• J₃