Geological and Geophysical Characterization of Horizon A-I (Vartovskaya Formation) of the Sovetskoye Oil Field

F. Ya. Borkun, Yu. L. Brylkin, E. I. Leontiev, V. I. Romenko

(Presented by Professor D. S. Mikov)

The Sovetskoye multilayer oil field, discovered in 1962, is located in the central part of the Sosninsky swell of the Nizhnevartovsk arch, within the latitudinal course of the Ob River, and is associated with the Sosninsky, Sovetsky, Medvedevsky, and Northwestern local uplifts.

Horizon A-I (Vartovskaya Formation) stratigraphically occurs in the roof of the Vartovskaya Formation of Hauterivian–Barremian age. Its top is identified in well sections by a fairly distinct change from the sandy–silty deposits of the horizon to the argillaceous-clayey sequence of the Koshai member, as recorded by core materials and production-geophysical investigations. On the logs of probes measuring apparent resistivity (AR), this is reflected as a transition from AR values equal to 3.0–4.0 ohm·m opposite the roofing deposits of the horizon to 2.1–2.4 ohm·m corresponding to the deposits of the Koshai clay member. On the spontaneous-polarization (SP) log, the rocks of the horizon top are marked by a transition from negative SP values to zero values (“clay line”), which are characteristic of the practically impermeable deposits of the Koshai clays. On the natural-radioactivity (GR) and neutron-gamma activity (NGK) logs, the horizon deposits are not always clearly distinguished from the enclosing rocks, which is explained by the high clay content of the deposits of horizon A-1, the gradual lithological transition from the sandy-clayey deposits of the horizon to the purely clayey-pelitic deposits of the Koshai member, and the insufficiently high resolving power of the equipment used.

The bottom of the horizon is identified and correlated insufficiently clearly both from core data and from production-geophysical investigations. This is explained by the fact that the horizon is underlain by lithologically nonpersistent deposits consisting of an irregular interbedding of siltstones, clays, and sandstones, in connection with which complete lithological replacement of impermeable underlying deposits by permeable ones is possible, as well as the appearance of a new collector bed A-1 “a” (wells Nos. 2, 22, 24, 59). In such cases, tracing the bottom of the horizon is possible only as a conditional boundary (Fig. 1).

At the Sovetskoye field, horizon A-1 (Vartovskaya Formation) has been penetrated by 92 wells and tested in 83 wells, as a result of which its rather complex geological structure has been clarified.

Lithologically, the horizon is represented by an irregular alternation of sandstones, siltstones, and, more rarely, argillites, with the first predominating in quantity.

Sandstones are gray, light greenish-gray, less commonly dark gray with a greenish tint, vari-grained, less commonly coarse-grained. It should be noted that medium- to fine-grained sandstones are for the most part confined to the basal part of the horizon, whereas fine- to very fine-grained sandstones predominate in the top of the horizon; in this connection, the sandstones at the base of the horizon consist mainly of grains 0.1–0.25 mm in size (50–60%), with the content of aleurolitic grains varying within 15%, while the sandstones at the top of the horizon consist 55–75% of the fine-grained fraction, 0.25–0.01 mm in size. Frequently the fine- to very fine-grained sandstones may be classed as coarse siltstones. The admixture of aleurolitic material in the sandstones of the horizon does not exceed 5%. The sorting of the material in the listed varieties of sandstones is good to moderate. The average median grain size of sandstone over the area does not show sharp jumps and generally varies within 0.12 mm; however, in individual wells in the southern and southeastern parts of the Sovetskoye field (wells Nos. 38, 39, 24, 43, 69) a regular tendency is observed toward an increase in the percentage content of the sandy fraction 0.25–0.1 mm in size, which indicates the direction of supply of clastic material from the east and southeast.

The structure of the sandstones is aleuro-psammitic, psammitic; the texture is more often laminated owing to the oriented arrangement of clayey areas, chlorite, micas, and plant detritus; in addition, clayey material is present both in a dispersed state and in the form of lenses, nests, and thin discontinuous interbeds. Clastic material constitutes 85–90% of the rock; in fine-grained sandstones its content decreases to 55–70%. In mineralogical composition the clastic material is represented by quartz (40–50%), feldspars (35–40%), rock fragments (15–20%), micas, and chlorite (3–5%). In addition to the main components composing the clastic material of the sandstones, glauconite (authigenic) and, in small amounts, accessory minerals represented by garnet, zircon, and apatite are encountered.

The mineralogical composition of the cement is mainly chloritic, kaolinitic, kaolinite–hydromicaceous, and rarely carbonate. The type of cementation is film, pore, and contact. In sandstone interbeds with carbonate cement, the type of cementation is basal and pore.

The quantitative distribution of the cement is mainly uneven. In places it amounts to 5–8%, for the most part 10–15%, and locally, as interbeds and nests, up to 20–35%. In carbonate interbeds of sandstone the amount of cement reaches 35%.

The pore size in the sandstones predominantly ranges from 0.03 to 0.06 mm; both dead-end and communicating pores are encountered.

Siltstones play a significant role in the total volume of the rocks composing the horizon and are confined mainly to its upper part. The siltstones are light gray, dark gray, with a greenish tint, fine- to coarse-grained, often sandy and sand-bearing. In granulometric composition, grains 0.05–0.1 mm in size predominate (40–70%); the content of fine-aleuritic grains (less than 0.05 mm in size) is 10–20%, less commonly 25%. Most siltstones contain an admixture of sandy material from 20 to 40%, with the size of the sand grains most often equal to 0.12 mm.

Under the microscope the structure of the siltstones is aleuritic, more often psammo-aleuritic. The texture is more often irregularly laminated and nest-like, but a homogeneous texture also occurs.

The petrographic-mineralogical characteristics, the composition of the clastic material, the type of cement, and the character of its distribution are analogous to the above-described fine- to very fine-grained sandstones. Siltstones, like

Fig. 1. Geological–geophysical section of horizon A-1 (Vartovskaya Formation) in well No. 59 of the Sovetskoye oil field: 1—clays, 2—siltstones, 3—clayey sandstones, 4—tight sandstones and siltstones.

Sandstones in horizon A-1 (Vartovskaya Formation) may be regarded as possible oil reservoirs.

Clayey material is present everywhere in the horizon, both in a dispersed state and as accumulations in the form of pockets, lenses, and interbeds. The deposits of the horizon as a whole are characterized by a gradual increase in clayey material from the base toward the top, from a clayey admixture in sandstones and siltstones to their complete replacement by clays at the top of the horizon. Where clay accumulations are significant, the clays are dark gray, greenish gray, lumpy, and sometimes argillite-like. In the granulometric composition of the clays, particles smaller than 0.01 mm predominate (53–90%); particles 0.1–0.01 mm in size occur in subordinate amounts, and particles 0.1–0.25 mm in size are very rare (0.5–5%).

In mineralogical composition, kaolinite–hydromica and hydromica types predominate; chlorite–kaolinite types are less common, and isolated clay samples containing montmorillonite have been encountered. The values of the porosity coefficient of the horizon reservoirs coincide both according to core data and according to production-geophysical data, and vary from 14 to 32%, with the most frequently occurring values equal to 23%.

The values of the permeability coefficient, determined from core material, vary over a wide range from 0.5 to 1500 millidarcies.

The lithological inconsistency of the horizon, its polymictic composition, and the increase in clayey material at the top of the horizon are also reflected in the materials of production-geophysical investigations.

According to diagrams of the natural polarization of rocks (SP), the heterogeneity of the horizon in lithological terms is qualitatively recorded; in particular, the basal, less clayey part of the horizon corresponds to high negative SP values within the range of −55–100 mV, whereas the upper, more clayey part of the horizon corresponds to lower negative SP values from −20–30 mV to zero values opposite the Koshai clays (Fig. 1).

According to diagrams of gradient and potential microprobes, the permeable beds of the horizon are clearly recorded by a positive increase in the potential-probe readings in comparison with the gradient-probe readings (8.5 ohm·m in the first case and 5.0 ohm·m in the second case), which is explained by the presence of a clay crust opposite these beds. Impermeable clayey interbeds are registered by low, coincident readings of the gradient and potential microprobes within 2.5 ohm·m.

Dense lithological interbeds occurring in the horizon are marked by high readings of the gradient and potential microprobes, sometimes reaching values of up to 20 ohm·m. The increase in clay content of the horizon in its upper part is reflected in the microprobe diagrams by a decrease in the absolute magnitude of the positive increase in potential-probe readings over the gradient-probe readings, which indicates a decrease in the permeability of reservoirs at the top of the horizon.

The results of caliper logging are strictly correlated with the data of microprobing and measurements of the natural polarization of the horizon deposits. Opposite the permeable beds of the horizon, the diameter of the borehole is close to the nominal diameter corresponding to the size of the bit used in drilling; while opposite clayey interbeds and the Koshai clay member, an increase in borehole diameter is observed in comparison with the nominal one. The top of the horizon is distinguished by a characteristic narrowing of the borehole diameter to the nominal value, which indicates a lithological transition from the Koshai clays to the sandy-silty deposits of horizon A-1.

On the natural radioactivity (GR) and neutron-gamma activity (NGK) logs, the deposits of the horizon are not always confidently distinguished from the enclosing rocks, which is explained by the high clay content of the horizon deposits and the insufficiently high resolving power of the equipment used. The intensity of natural gamma radiation against the rocks of the horizon varies from 6 gamma (for sandy varieties of rocks) to 12 gamma (for clayey rocks), while the intensity of neutron-gamma activity varies, respectively, from 1.05 to 1.5 conventional units.

One of the principal physical parameters—specific electrical resistivity \((\rho_{\mathrm{pl}})\)—is characterized by an especially wide range of variation; its values for permeable beds vary from 2.0 to 34.0 ohm·m. For oil-saturated, strongly clayey reservoirs with relative clay content from 0.4 to 0.65, values of \(\rho_{\mathrm{pl}}\) are typical that change correspondingly from 5.5 to 2.5 ohm·m; high values of specific electrical resistivity, 6–30 ohm·m, correspond to less clayey oil-saturated reservoirs. The specific electrical resistivities of water-saturated beds \((\rho_{\mathrm{pl}}^{\mathrm{w}})\) of the horizon have fairly persistent values from 1.5 to 4.0 ohm·m; the most frequently occurring values of \(\rho_{\mathrm{pl}}^{\mathrm{w}}\) are from 2 to 3 ohm·m (51.2% of all cases) (Fig. 2).

When comparing the parameter of the ratio of the specific electrical resistivity of the invaded zone \((\rho_{\mathrm{iz}})\) to the specific electrical resistivity of the drilling mud \((\rho_{\mathrm{m}})\) for oil-saturated and water-saturated beds of the horizon, it was found that the values of this parameter coincide for both types of beds most often within the range from 4 to 6; although some tendency toward their differentiation is observed, it is so weakly expressed that it is impossible to use this parameter to determine the nature of bed saturation.

Fig. 2. Histogram of the distribution of specific electrical resistivity \(\rho_{\mathrm{p}}\) of sandy and aleuritic reservoirs of horizon A-1 (Vartovskaya Formation) of the Sovetskoye oil field: 1 — water-saturated and oil-saturated beds; 2 — oil-saturated beds according to test data; 3 — water-saturated beds according to test data.

The best possibilities in this sense are offered by the parameter of the ratio of the electrical resistivity of the flushed zone ($\rho_{\mathrm{fz}}$) to the electrical resistivity of the formation ($\rho_{\mathrm{fl}}$)—$\dfrac{\rho_{\mathrm{fz}}}{\rho_{\mathrm{fl}}}$; since 46.3% of all the cases considered had, for oil-saturated beds, values of the parameter within the range from 1 to 2, and 51% of all water-saturated beds from 2 to 4. But the ambiguity in determining the character of bed saturation by this parameter, with a limiting parameter value equal to 2, would be 46.3%.

The most probable value of the ratio of the diameter of the flushed zone ($D$) to the borehole diameter ($d$) proved to be equal to 4, and the ratio of the electrical resistivity of the formation ($\rho_{\mathrm{fl}}$) to the electrical resistivity of the drilling mud ($\rho_{\mathrm{m}}$) was within the range from 1 to 3.

For assessing the character of saturation of the horizon beds, it is possible to use their electrical resistivity, taking as the boundary of separation the value of $\rho_{\mathrm{fl}}$ equal to 4 (Fig. 2). From Fig. 2 it is evident that $\rho_{\mathrm{fl}}$ for water-bearing beds lies within the range from 1.5 to 4 ohm·m, and for oil-bearing beds—from 2 to 34 ohm·m; i.e., the values of $\rho_{\mathrm{fl}}$ from 2 to 4 ohm·m constitute the zone of ambiguity. If beds with electrical resistivity above 4.0 ohm·m are regarded as oil-saturated, we are justified in expecting to obtain oil from them with a probability equal to 1.0, but in doing so a portion of the oil-saturated beds with $\rho_{\mathrm{fl}} = (2—4)$ ohm·m, which fall within the zone of uncertainty, is lost. For the given sample this loss amounts to 18.6%.

Proceeding from the geological and geophysical characterization of horizon A-1 (Vartovskaya Formation), it is necessary to note its sharply expressed mineralogical, lithological, and physical heterogeneity, in connection with which the conclusion follows that statistical methods must be used in its study, with mandatory consideration of the degree of clayization of its reservoirs.