WESTERN TURKMENISTAN - 1 FUTURE OIL AND GAS POTENTIAL IN SOUTHERN CASPIAN BASIN

Robert B. O'Conner Jr., Richard A. Castle, David R. Nelson
Wavetech Geophysical Inc.
Denver

Turkmenistan is the most southerly C.I.S. republic and ties on the southeastern border of the Caspian Sea (Fig. 1). Its population is small, about 3.5 million people, and except along its rivers most of its land area is extremely arid. Iran and Afghanistan form its southern border and Uzbekistan and Kazakhstan its northern border.

On Jan. 23, 1993 an important bidding round was held for producing and shut-in oil and gas fields in the western part of the country. Nine international companies registered for the round, and winning bids were submitted on three of four blocks (Fig. 2). A bid on Block 1, the only block not to be awarded, was rejected as being insufficient.

The purpose of this article and another planned for later this year is to present background information on the huge oil and gas potential of western Turkmenistan and to put the recent bidding round into perspective.

The current official estimate of remaining reserves on the blocks just tendered is 2.7 billion bbl of oil equivalent, roughly half of which is oil. The authors believe this to be a very conservative estimate as we shall attempt to demonstrate.

GEOLOGICAL FRAMEWORK

The South Caspian basin is one of the oldest producing basins in the world. Baku has been known as a major oil center since the 19th century.

Although oil and gas deposits have been recognized in the Turkmenistan part of the basin for nearly as long, serious exploration in this region did not begin until after World War II. Even until quite recently, however, the oil and gas potential had not been considered the equal of that in the Baku region. As a result, interest in the Turkmenistan part of the basin has, until recently, been significantly less.

BASIN GEOMETRY

The South Caspian basin covers about 200,000 sq km. The depocenter of the basin, where the sedimentary section thickness reaches 25 km, is approximately coincident with the southern part of the Caspian Sea.

Fig. 3 shows the bathymetry of the southern Caspian Sea, and Fig. 4 shows the present day structure of the consolidated basement, which in the central part is believed to be oceanic in composition.

Comparison of these two figures indicates dramatically the large delta complex emanating from the eastern margin of the basin and building westward into the sea. Most of the rocks comprising this lobe are known to be post-Mesozoic in age, and their massive volume suggests the potential for major oil and gas generation.

PLIOCENE STRATIGRAPHY

Geologically it is useful to divide the eastern Caspian basin into seven distinct zones (Fig. 5).

Of these, the most important in western Turkmenistan are the Apsheron-PreBalkhan ridge, which contains most of the oldest producing fields, and the Okarem-Gograndag step, which includes the newer southern coastal producing fields of Turkmenistan, and the Kizyl-Kum trough, which is coincident with the central deep of the eastern part of the basin.

The rocks comprising the main oil and gas objectives on the Turkmenistan side of the basin are Middle Pliocene in age and are described in the stratigraphic column of Fig. 6. Alternating shales, fine-grained silts, and siltstones dominate, and they are inferred to have been deposited in fluvial, deltaic, slope, and turbidity environments. Except in the Kizyl-Kum trough, comparatively little sand or sandstone is present in the section.

Within the Middle Pliocene, rocks of the lower part of the section are of the most future interest and are referred to as the Lower Red Color (LRC) series. Depositional thicknesses of these and the Upper Red Color series are very large, attaining values in excess of 4 km offshore, from which extremely high depositional rates can be inferred.

ORIGIN OF THE STRUCTURES

Sub-basinal scale, post-Paleogene structure in the eastern basin is primarily the result of gravitationally driven shale diapirism, byproducts of which are the numerous intrusive and extrusive "mud volcanoes."

Many of these are active today and display characteristics not unlike igneous volcanoes. The source shales for the diapirism may range in age from Jurassic to late Paleogene. Typically, the structures have the form of elongated ridges (Fig. 7) and are most likely the result of a transcurrent fault system that has been active in the basin since at least early Mesozoic time.

Strike-slip movements of these faults produce linear zones of stress intensification and fracturing in the sedimentary section. The increased stresses, in turn, induce shale flowage into the fractures, and the result is the formation of structural ridges and mud volcanoes. Variations in the rate of structural growth of these structures is probably related to periods of transcurrent fault movements.

A seismic profile (Fig. 8) across the Apsheron-Prebalkhan ridge, the largest of the diapiric ridges, makes it clear that the source of the uplift is very deep, in fact at depths of 10 km or more. Reflections from Mesozoic carbonates should occur at or near the base of section, but the data are not sufficiently good to disclose their presence.

Fig. 9 is an interpretation of the earth configuration that could have given rise to such a seismic section. In the model, the source of the diapiric shales is attributed to the Lower-Middle Jurassic section, but younger shales could be involved as well.

The main reasons for inferring a Jurassic source are the presence of carbonate breccia of Jurassic age in a mud volcano near Cheleken and the fact that the seismic profile indicates very deep roots.

The great importance of the shale diapirism in the region is that a mechanism is provided for the formation of large anticlinal structures that form early and continue to grow throughout time. This means that regardless of the timing of hydrocarbon migration, closed structures will have been available to trap them. Analysis of numerous individual structures in the region confirms this growth model.

PLIOCENE SOURCE ROCKS

Source rocks for hydrocarbons found in the Pliocene are most likely indigenous to the Pliocene itself, although Miocene and Oligocene sediments may also be significant contributors in certain areas of the eastern basin.

Chemical conditions throughout the region were generally reducing during deposition, which has led to the preservation of much of the organic matter. While the density of organic materials is low due to the extremely high sedimentation rates, nevertheless the total volume of organic materials is high as a result of the massive Pliocene thickness. Shales and silts alternate throughout the section, the latter providing conduits for efficient dewatering of the shales.

This is unlike the U.S Gulf Coast, where great thicknesses of shale with very little interspersal of sands and silts were accumulated during transgressive stages and which, upon rapid burial, became geopressured. With geopressuring the shear strength of the shales was drastically reduced, and widespread listric faulting took place.

There appear to be no listric faults in the eastern South Caspian basin, which suggests the shales are able to expel their interlayer waters. The implication of such a sedimentary section is that a system is created that can be expected to be highly efficient for large scale hydrocarbon generation, expulsion, migration, and structural trapping, provided the temperatures were favorable for generation in the first place.

TEMPERATURE REGIME

Temperature gradients near the coast are relatively low, with the result that despite great depths of burial the entire Pliocene section is in the oil generating window today. Along the onshore Apsheron-Prebalkhan ridge the top of the oil generating window, 70 C., is found in the 2,200-2,500 in range and the bottom of the window, roughly 165 C., is well below 6,000 in. Lopatin diagrams, derived both from well data and seismic analysis, indicate that in the offshore areas south of the Apsheron-Prebalkhan ridge most of the Pliocene has been in the oil generating window since the end of the Apsheron stage.

HYDROCARBON GENERATION

By combining a knowledge of the total organic carbon content of the rocks with estimates of the temperature distribution over time and space, it is possible to calculate roughly the total hydrocarbon generation per square kilometer from Middle Pliocene rocks throughout the eastern Caspian region.

The results of this calculation are given in the map of Fig. 10, where the hydrocarbon generation density is mapped in millions of metric tons per square kilometer.

If we make use of information on the recoverable oil and gas reserves that have been discovered in the onshore part of the Apsheron-Prebalkhan ridge, we can calibrate this hydrocarbon density map and use it to estimate the future upside potential elsewhere within the general region. This is done in the final section of the article.

MIOCENE, OLIGOCENE

Over much of the eastern South Caspian basin, comparatively little is known about the oil and gas potential below the Pliocene because of the generally large drilling depths.

In Azerbaijan, Oligo-Miocene rocks are known to contain good source rocks, perhaps the main source of hydrocarbons in the western part of the basin. They may also be an important source in Turkmenistan as well.

In the Kizyl-Kum trough, Miocene reservoirs of reasonably good quality have been penetrated. To the south, onshore, the preserved Miocene on the structural crests is noticeably thinner and of poorer reservoir quality.

Based on seismic data, however, thicknesses appear to increase away from the structures in this region, suggesting the possibility of sub-regional stratigraphic trapping on the flanks of the larger ridge structures, and regional stratigraphic trapping on the basin flank at the Messerian Step boundary.

Oligocene and older rocks are even less well known in this region. They are probably similar to the Pliocene rocks but coarser and less well-sorted.

DERIVATION OF POTENTIAL

Total recoverable reserves that have been discovered to date on the onshore portion of the Apsheron-Prebalkhan ridge are roughly 3 billion BOE.

If we combine this information with the hydrocarbon generation density information given in Fig. 10, we may then calibrate the latter and make predictions about the future upside potential in regions where there are little or no proved reserves.

CALIBRATION APPROACH

The first step in calibrating the upside potential is to divide the region into migration zones that have the property that hydrocarbons generated within a given zone will remain in that zone or migrate updip and out of it without entering any other zone.

In Fig. 11 we have divided the eastern basin into four migration zones, three of which are further split into onshore and offshore sectors. This partitioning is based on the structural dip gradients of the LRC regional structure.

Having defined the migration zones, we can now convert the hydrocarbon generation density figures in each sector into estimates of the recoverable oil potential. If we make the assumption that two thirds of the ultimately recoverable oil and gas in the Apsheron-Prebalkhan ridge has already been found in this sector; that is, in Sector I-onshore, we are in effect estimating the total reserves that will be recovered in the sector. By comparing this number with that resulting from integrating the hydrocarbon generation density map over Sector I onshore we obtain a conversion factor that can be used to convert the hydrocarbon generation density distribution of Fig. 10 into potentially recoverable reserves for the other sectors.

The results of this conversion are shown in the bar chart of Fig. 12, where three types of reserve numbers are given for each of four regions, namely, the estimated ultimately recoverable reserves, the actual discovered recoverable reserves, and the difference between the two, which is defined as the "upside reserves potential."

Obviously, the utility of the calculated upside reserves potential depends upon several assumptions. The "two-thirds" assumption discussed above is quite conservative, since there is a considerable amount of undeveloped potential on the trend in the form of deeper objectives, untested fault blocks and structures, and untested stratigraphic trapping possibilities.

Two other assumptions are especially important. First, given that large hydrocarbon generation and migration exist, it is assumed that there are structures of comparable scale to those on the Apsheron-Prebalkhan ridge to trap the hydrocarbons as they pass through the system, and second it is assumed that there are reservoirs of comparable characteristics to contain the hydrocarbons.

In most cases we have large, early growing structures so that the first requirement is at least partly met. For the second, while reservoirs exist throughout most of the Middle Pliocene over the region, their ultimate producibility is not accurately known at this time, especially in the LRC.

As a result the upside reserves potentials must be viewed as "maximum" estimates of what could exist in the respective sectors. In any case, without trying to be quantitative, these results tell us that we should expect much larger reserves in Sector I offshore, Sector II, and Sector III than have been found already.

SUMMARY

The Pliocene of the eastern South Caspian basin has significant potential for additional reserves beyond those already discovered.

The depositional and evolutionary conditions of the region strongly support the conclusion. Massive thicknesses of sediments were deposited under chemically reducing conditions, thus providing the necessary reservoirs, source rocks, and migration paths out of the basin.

Since the end of Upper Pliocene, part or all of this section has been in the oil generation window with the result that very large amounts of organic materials have been converted and still are being converted to mature hydrocarbons. The presence of diapiric shales in the geologic section and their continued movement throughout time have provided the structural closures to trap the migrating hydrocarbons.

The broad inference in which one can have considerable confidence is that whether or not large reserves have yet been proven on any particular structure, additional large reserves are very likely to be found on it as it is further developed in the future. This is the general expectation from such a hydrocarbon system.

In the next article we shall review some of the producing fields in detail and show from an engineering viewpoint why substantially larger reserves should be expected on most of them than have been proven to date.

ACKNOWLEDGMENTS

The authors express their appreciation to the government of Turkmenistan for making possible the studies upon which this article is based. They also express their great indebtedness to H.O. Isahnov, director general of Turkmenneft, M. Ashirmamedov, then chief geologist of Turkmenneft, and the highly qualified scientific staffs under them for their generosity in sharing so freely their vast knowledge of oil and gas occurrence in western Turkmenistan. Next they acknowledge the importance of the doctoral thesis of D.A. Babayan for the understanding of Pliocene hydrocarbon generative potential in the eastern South Caspian basin. The insights afforded by the thesis provide the basis for estimates of the upside reserves potential of the region. Finally, the authors thank GeoInterTech for permission to publish this article.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.