EXPLORATORY, DEVELOPMENT PROBLEMS IN PALEOGENE CLAYSTONE OIL FIELDS, CENTRAL PRE-CAUCASIAN BASIN

Iosif A. Kliger
Grynberg Resources Inc.
Cranford, N.J.

The common methods of seismic research and well logging are lacking when used for prospecting for traps and developing claystone reservoirs.

Some important details cannot be established because of the limit of vertical resolution of the seismic survey.

The traditional method of unlimited horizontal correlation of well-logging curves must be changed by correlating within certain clinoform complexes only. Neither well log data nor the drilling in method (when a negative result is established) indicates the presence or absence of oil saturation.

A study of the following aspects that are common for all the known fields within the claystone reservoirs of the central Pre-Caucasian basin may be helpful in resolving the problems.

The traps are connected with a lower part of a thick (about 1.5 km) mass of Paleogene-Lower Neogene claystone deposits. The productive intervals are located within the Eocene-Oligocene claystone-carbonate section. The carbonate material disappears in the upper part of this section.
The alternation of solid claystones with marls, lamellar and platy argillites whose matrix couldn't contain mobile fluids, is established. Perhaps the space between the plate-like particles and the fractures creates the effective porosity.
The traps are adjacent to the tectonic mobile zones, where the structural bowings were formed under tectonic control and recent and neotectonic compressional loads created intergranular stresses and fractures.
Seismic lines clearly show the clinoform structure of the productive sequences. The presence of specific local zones of convergence of different clinoforms or of filling up of a paleodepression from different directions, and so on, is necessary.

Within the productive horizons, the oil is received from the more homogeneous intervals where the minimums of interbedding frequency are observed.

These aspects are indicators for predicting traps and for development of the fields. These considerations point out the necessity for modification of the seismic surveys (using shear waves, for example), well logging, and drilling-in methods (maximized bottomhole pressure, horizontal drilling, etc.).

INTRODUCTION

The presence of oil within claystones demonstrates a new type of reservoir.

In Russia, these reservoirs occur in Volga-Ural (pre-Devonian), West Siberian (Upper Jurassic), and Pre-Caucasian (Paleogene) basins. They are also found in a few basins in Canada, the U.S., and South America.

In the central Pre-Caucasian basin, some oil fields such as Zjuravskoe, Praskoveyskoe, and Ozec-Suatskoe have been exploited (Fig. 1, Nos. 48, 45, and 39).

Outside these fields are many oil seeps in abandoned wells in which Upper Paleogene deposits were exposed. The seeps appear a few years after drilling has been completed.

This is conclusive evidence of the existence of other fields in the described region, but the methodology of their prospecting is not well developed enough.

The seismic and well logging signs of the presence (or absence) of oil in certain points and depths even inside the existing fields are so far unclear.

The matter often is that the absence of oil influx from the bottom of a well does not exclude the possibility of resources since it could be only evidence of technological failure to strip the producing horizon.

Meanwhile, the well's production from the claystone reservoir rocks is 10-170 tons/day in the described basin. And there are wells that bring 90 tons/day of oil during the past 30 years.

The productivity rate is high enough, the formation depth relatively shallow at 2,070-2,570 m, and the productive formations are thick enough at about 100 m to establish the economic efficiency of the fields even though each third well has to be abandoned.

STRATIGRAPHY

Two structural stages are apparent, the lower one being the basement and the upper one being the sedimentary cover.

The stratigraphic column of the Pre-Caucasian oil basin includes Permian, Triassic, Jurassic, Cretaceous, Paleogenic, and Neogenic deposits. There are both fields and oil shows within any of these stratigraphic divisions.

Only Upper Jurassic and Upper Cretaceous subdivisions' sequences are limestone and limestone-dolomitic. The carbonates of the Upper Cretaceous subdivision extend in the extensive space from middle Asia to the Carpathian Mountains. But the huge united ancient basin began to break up at Paleogene time.

At the beginning of Paleogene time, the depth of the ancient sea became much greater in the central Pre-Caucasian basin, and the carbonate depositional environment was changed by deep sea sedimentation. The submergence of this platform was most intensive, perhaps, at the end of Oligocene time. The turbidites flowed to the basin, which can be clearly seen on regional seismic lines. The slope clinoforms of the Lower Neogene cover the productive Oligocene deposit complex.

The Paleogene sediments consist of abyssal, deep sea, and slope facies that were fashioned mainly by slope clinoforms and their fondaform margins. The accumulation of clinoforms is limited in geological space and time; and defining the biostratigraphical boundaries within these sequences has not been attempted. So it has to be noted that the same names of producing formations at different fields are used more like traditional terms.

Producing formations of Eocene (Praskoveyskoe and Ozec-Suatskoe fields) are called Kumskaya (k) and Beloglinskaya (bl); and Oligocene producing formations (Zjuravskoe field) are called Chadum (chd) and Batalpashihinskaya (btp). These terms are used in the illustrations with this article.

TECTONICS

Famous oil fields within Paleogene claystones are discovered in different tectonical zones.

These zones were down-warping during Paleogene time at different subsidence velocities; but everywhere the deepwater sedimentary environment was kept.

Paleogene clinoforms were built under the control of tectonic submersion, but the resulting accumulative relief doesn't reflect previous morphostructures. The structural divergences took place at Neogene time, and even recent structural history changed some geomorphological trends and features.

However, the paleomorphology of this basin in the Paleogene and Lower Neogene sequences cannot be studied accurately because any paleoreconstructions attempted in these sequences, fashioned by clinoforms, will leave the results in doubt.

The absence of any relation between oil field location and recent structural highs has been established for each tectonic zone. It is so far unclear how geomorphological (or paleomorphological) factors affect the producing background.

Structural indicators show a monoclinal trend, complicated by narrow trenches and specific structural (cumulative) ranges (Fig. 2). The wells encounter oil at the monoclinal slopes and even at the deepest points of the adjacent trenches.

But there has been water influx received both in Well 92, which uncovered the producing horizon (chd and btp) at the same depth interval with Well 91, and in Well 29, which uncovered the horizon far higher (Fig. 3).

The list of comparisons could be continued with dozens of pairs of wells, which met the productive interval at the same depth but showed oil influx or water influx or were abandoned being dry. This occurs at various depth levels.

These and a lot of exploitation data from the fields show clearly the absence of a united hydrodynamic system within the described claystone reservoirs. The different hydrostatic pressures in neighboring wells prove that feature, also.

The lens pools' positions at different depth levels and the absence of a united hydrodynamic system within the Eocene and Paleocene claystone reservoirs cannot be explained by vertical (block) movements. Neither the well log data correlation nor the seismic data show tectonic folds of a significant overlap or throw amplitude. Nevertheless, the core analysis shows the reservoir scale fractures.

The central Pre-Caucasian basin evolution and some direct evidence (tectonic fractures; abnormal pressures and temperatures; air photographs and space images, etc.) indicate lateral compression loads within the sedimentary mantle. The local anomalies of intergranular stresses could be the oil traps placed where the lithology factors are favorable, also.

LITHOLOGY

As stated above, Upper Eocene (k and bl) and partly Oligocene (chd) hard, dark gray, and black claystones and argillites, interbedded with marls and even marlstones are produced in the eastern part of the described region, where Ozec-Suatskoe and Praskoveyskoe fields are located.

The Oligocene (chd and btp) productive formation contains relatively fewer carbonates in the western part of this region where Zjuravskoe field is located. Argillaceous claystones of Batallpashinskaya (btp) formation contain carbonates neither in the western nor eastern parts of this region. But almost everywhere an overlying marker marl bed 2-3 m thick covers the Batallpashinskaya (btp) formation.

In accord with core analysis, the argillaceous rocks containing different clay minerals (montmorillonite, greater than 65%; hydromicas less than 20%; kaolinite less than 10%, etc.) and admixes (chlorite and kaolinite) are included within the Eocene-Oligocene upright geologic section.

The random distribution of the organic substance doesn't control the oil influxes. Flows are received from both intervals of reduced and increased content of scattered bitumen and from the clean "normal" claystones where the gamma ray logging curves show their minimum.

The alternation of solid claystones with schistose and slatey, flaky, lamellar and platy argillites is observed everywhere. There are no oil influxes from solid claystone intervals. These rocks and marls, because of the nature of their effective porosity (Table 1), form lateral and vertical impermeable barriers within the Paleogene sequences. Their matrix cannot contain fluids, and interstitial fluids are immobile. But the rate of solid claystones and marls is only about 20% within the Eocene-Oligocene deposits.

The data in Table 1, except the last three lines, are related to the conventional surface conditions. The effective porosity should be reduced within the depth conditions and, counting upon the high rate of the residual water saturation, it can be told that the matrix of plate argillites doesn't contain mobile fluids; nevertheless, the wave transit time, special formation resistivity, and natural radioactivity characteristics are different for platy argillites, solid claystones, marls, and marlstones (Table 1).

The exfoliation planes within schistose, slatey, flaky, lamellar, and platy argillaceous beds have mainly horizontal direction, and the quantity of these planes reaches 4,000-6,000 units/m. These fissures' opening is about 1-15 mkm. In addition to exfoliation planes, vertical fractures may also be observed. The quantity of these fractures reaches only 10-15 units/m, but their opening is 3-5 mm.

The interconnected voids of exfoliation planes and fractures create available pore space for oil migration and accumulation in front of the lithological shields. Even platy argillites with a low hydrophobic rate can make such lithological shields.

The well log data distinguish between plate argillites, solid claystones, marls, and marlstones (Table 1). But there are no differences in the well log characteristics that can distinguish between wells that contain oil, those containing water, and those with neither.

According to this feature, there is no physical basis for predicting, by modern geophysical methods, the type of saturation of these formations. The presence of oil may be established after formation tests only. The drillstem tester was used during the drilling of Wells 62, 63, and 77 in Zjuravskoe field (Fig. 3), and no oil influxes were produced. However, after through-casing formation tests, those wells produced oil.

The drillstem tester was used 26 times at Praskoveyskoe field and 14 times in Ozec-Suatskoe field. The oil influx was received from seven intervals at Praskoveyskoe field and from three intervals in Ozec-Suatskoe field.

After through-casing formation tests, 13 more intervals of Praskoveyskoe and four more intervals at Ozec-Suatskoe fields showed fluid influxes.

Perhaps the low efficiency of the drillstem tests might be explained by conditions of the testing, when the maximal drawdown was chosen. But it is far more difficult to predict the pressure in-place when the productive formations are remarkable for different abnormal reservoir pressures.

The ratio between the abnormal reservoir pressure, measured by the bottomhole pressure gauge, and normal hydrostatic pressure changes from 1.28 to 1.7 at Zjuravskoe field, from 1.1 to 1.2 at Praskoveyskoe field, and from 1.06 to 1.2 at Ozec-Suatskoe field (Table 2).

As it can be seen from Fig. 2, the wells listed in Table 2 are located close enough (less than 2 km), but this abnormal ratio is different in each of them. Perhaps the abnormal pressure keeps open the exfoliation planes and fractures.

SEISMOSTRATIGRAPHY

The productive Paleogene claystone complex is fashioned by clinoforms, which are reflected on seismic lines.

Angular unconformities observed on seismic sections are produced by non-parallel strata and depositional sequences. The downlap, onlap, and toplap on seismic sections represent progradational sequences, i.e., evidence of dominant lateral filling of the basin.

The stratigraphical position of the index reflection "F" within Eocene-Oligocene sequences is different in the western and eastern areas of the central Pre-Caucasian basin. This index reflection "F" is associated with the Beloglinskaya (bl) formation in the western area (Zjuravskoe field).

Nevertheless, though the corresponding individual reflection dominates in the composite event, the modeling shows that the conditions of reflection interference also vary along the seismic lines due to laterally inconsistent sequences. And, as shown in Fig. 2, the thickness of the Beloglinskaya (bl) formation itself varies also across Zjuravskoe field.

Seismic line 12 (Fig. 4) is paralleled by the geological section (Fig. 2), and they coincide between Wells 91 and 92. Here some fondaforms pinch out, and the boundary of a zone where some clinoforms coming from different directions converge is marked (Figs. 2, 4). This zone also can be seen clearly on other seismic lines, and it is mapped (Fig. 3).

This narrow lane where Upper Eocene and Oligocene clinoforms coming from different directions converge is characterized by relatively higher reservoir pressures (Well 72, Table 2) and permeability of the rocks. There were no oil influxes inside this zone. It could be explained by advantageous conditions for oil emigration towards the zones of lower reservoir pressure.

The thickness of both Chadum (chd) and Batallpashinskaya (btp) clinoforms enlarges in the southwest direction. As a matter of fact, the oil pool is limited to this direction by the abrupt slope of Zjuravskoe anticline (see southwest end of the seismic line in Fig. 4).

At the junction area (Vorobyovskaya) other clinoforms coming into the basin at the same time (also historically named Chadum and Batallpashinskaya) from the east-northeast are productive also around an anticlinal bowing. The western boundary of Vorobyovskaya oil field probably coincides with the boundary of the above mentioned convergent sedimentation (Fig. 3).

According to the seismic data, the edges of the upper parts of Chadum (chd) and Batallpashinskaya (btp) fondaforms may be mapped as shown in Fig. 3. This means that the oil influxes at the northern part of the area (Well 85, Fig. 3) were received from another clinoform reservoir.

It is possible to assume that Zjuravskoe field is limited by the edges of a certain clinoform complex from the west and northwest. Those clinoform complexes that converge with Zjuravskoe field from the north and west contain claystone reservoir rocks and oil.

The picture of seismic reflections is rather different in the eastern part of the described region (Praskoveyskoe field). The index reflection "F3" is associated with the Kumskaya (k) formation, which contains marls and marlstones here. The index reflection "F3" was chosen for paleoreconstruction because of its lithological and thickness homogeneity. Four types of seismic facies that correspond to certain geological features may be marked and mapped.

At the left end of seismic line 1 (Fig. 5), the upper part (t0 = 2.18-2.20s) of the index reflection "F3" shows a thin clinoform that might correspond to deposits of bottom currents because of the current bedding structure, according to the core analysis.

This type of deposits--Lower Beloglinskaya (Lbl) formation--contains reservoir rocks that are productive at certain locations. This formation doesn't extend all over Praskoveyskoe field.

The overlying beds are the mudstones (the deposits of mud streams) of Beloglinskaya (bl) formation. At the northern part of the described field, the mudstones cover the marlstones of Kumskaya (k) formation. The seismic picture of these areas (Fig. 5) is shown at the right end of seismic line 1 (t0 2.16-2.20s).

An abrupt descent (see central part of the line in Fig. 5) marks the end of the mud stream deposits, and a large accumulative depression within the Beloglinskaya (bl) and Chadum (chd) intervals can be mapped by seismic data.

The reflections of the mud stream deposits form seismofacies of the second type. The third type might correspond to the deposits that fill the above-mentioned accumulative paleodepression from different directions.

They can be observed at the right side of Line 1 on Fig. 5 (t0 = 2.10-2.18s). The thickness of the productive Chadum (chd) formation is enlarged within the depression. The specific carbonate-terrigenous accumulative bodies (bowings) within the Chadum (chd) formation contain a lot of carbonate material, and no oil influxes were received from here.

These bodies are reflected on seismic lines as shown in Fig. 5. This is the fourth type of seismofacies.

Fig. 6 presents the well log curves from the wells within the paleodepression. The curve showing the interbedding frequency corresponds to the data on the productivity of each formation. There is much evidence that the carbonate deposits containing oil are remarkable for the minimum interbedding frequency. This factor, which shows the vertical heterogeneity of formations, is very useful when indicating the productivity of terrigenous sequences, also.

The author had recommended the interbedding frequency factor for predicting the intervals in certain wells for perforation, and the use of this factor was successful.

For example, analyses show that oil influxes are received from the more homogeneic (minimum of interbedding frequency) upper part of Chadum (chd) formation at Zjuravskoe field (Fig. 7). Where the lower part of Chadum (chd) formation is more heterogeneous, no influxes were received (Wells 63, 73 in Fig. 7).

This feature has to be explained in terms of clinoform structure, while it will be proven that that factor is useful for other regions.