AMU-DARIA LIQUIDS POTENTIAL INDICATED

Robert B. O'Connor Jr.
WaveTech Geophysical Inc.
Denver

Stephen Sonnenberg
Consulting Geologist
Denver

The Amu-Daria basin is a large, primarily gas producing basin located in the Soviet republics of Turkmen and Uzbek and in Afghanistan (Fig. 1).

It covers an area roughly three fourths that of Texas and has estimated gas reserves on a scale of several hundred trillion cubic feet. Of these, somewhat less than 50% have been produced, proven, or indicated as probable by drilling,

The basin is of current interest because roughly 66,000 sq km of exploration and producing rights in its lightly explored southern part will be put up for foreign competitive bidding by the Turkmen government later this year.

The purpose of this article is to present growing evidence that significant new oil and condensate reserves, in addition to large new gas reserves, are likely to be found in the subsalt carbonate reef complexes of Upper Jurassic age in the deeper part of the basin.

In fact, there is a very real possibility that a good portion of these reserves may be found in fields of giant or even supergiant size.

As an example, one recent discovery in the eastern part of the basin, Kokdumalak, found reserves of 5 tcf of gas, 700 million bbl of oil, and 720 million bbl of condensate from an Upper Jurassic reef structure having an areal extent of only 26 sq km.

EXPLORATION HISTORY

Exploration for oil and gas began in the Amu-Daria basin in 1929, but it was not until 1953 that the first field was discovered at Setalantepe in Uzbek, SSR.

Major basin potential was indicated by the discovery, in 1957, of Gazli field (17 tcf) and confirmed with the subsequent discoveries of Shatlyk (34 tcf), Shurtan (19 tcf), Dauletabad-Donmez (60 tcf), and a host of other fields, many of which have reserves in the multi-tcf range.

Most of the production has been from terrigenous reservoirs of Lower Cretaceous age or from Callovian and Oxfordian carbonate reservoirs of Upper Jurassic age.

Recent discoveries of much-larger-than-expected oil and condensate occurrences in the Callovian-Oxfordian in the deeper parts of the basin have led to significantly greater interest in them and to an important revision of the earlier view that Amu-Daria is strictly a gas producing basin.

GEOLOGICAL FRAMEWORK

The Amu-Daria basin is classified as a supra-rift syneclise. 1 It was formed on the ancient passive margin of the southern Turonian platform.

Structural development of interest to oil and gas exploration is that following the close of the Paleozoic era. The main structural features of the basin include the Karakum uplift to the northwest, the Kopetdag foredeep to the southwest, the Bukhara and Charjou steps to the northeast, the Murgab depression to the southeast, and the Badkhyz-Karabil step to the south (Fig. 2).

The Charjou step, the Murgab depression, and the Badkhyz-Karabil step are the focus of attention here.

A stratigraphic column of the Amu-Daria basin shows that basement consists of Paleozoic and older rocks that were deformed and slightly metamorphosed during the Hercynian orogeny (Fig. 3).

Directly overlying these are Permo-Triassic redbeds and molasse-type continental sediments that filled the basin during its early development stage. Next are shallow water marine clastics and carbonates of Lower to Middle Jurassic age.

Following them are Upper Jurassic sediments that consist first of clastic rocks of the Lower Callovian that grade into limestones of the Upper Callovian and Oxfordian and then into anhydrites and salts of the Kimmeridgian and Tithonian.

The Callovian-Oxfordian limestones contain a broad variety of reef complexes on the shelf and shelf margin parts of the basin, with the organic buildups often attaining a height of several hundred meters. The overlying evaporitic sediments range in thickness from zero to more than 1,200 m in the center of the Murgab depression and form an impervious barrier to hydrocarbons from the Jurassic rocks.

Post-Jurassic rocks in the basin consist mostly of terrigenous clastics, although there is occasional interspersion of limestones in the Cretaceous and younger rocks.

The general structure and stratigraphy of the Amu-Daria basin is summarized by the two cross-sections located as dashed lines in Fig. 2 and illustrated in Figs. 4 and 5.

The shallower section is controlled by wells and seismic coverage, and the deeper section by gravity, magnetics, and refraction seismic data.

Of particular interest are the extensional faulting and rift grabens shown in the deeper measures.

SUBSAIT JURASSIC POTENTIAL

Although there is potential in post-Jurassic rocks for new gas discoveries in the Amu-Daria basin, the part of the basin of greatest interest here is the Jurassic section that underlies the Kimmeridgian-Tithonian salts.

Fig. 6 shows an outline of the Kimmeridgian-Tithonian salts, a map of the distribution of the principal fields in the basin, outlines of the Tedzhen, Yashlar, and Badkhyz sectors for reference, and an outline of the Charjou Analog area will be discussed below.

Exploration of the subsalt Jurassic section has been intense in Uzbekistan and moderate to light in Turkmenia.

Further to the south in the Yashlar sector, for example, there has only been one penetration of the entire Callovian-Oxfordian carbonate section and only one other well that might have penetrated deeper than the Kimmeridgian Tithonian.

In an attempt to predict basinal configuration and relative water depths during Oxfordian time, a smoothed isopach of the overlying Kimmeridgian-Tithonian interval was used (Fig. 7). 2

If it can be assumed that this isopach reasonably reflects the underlying Oxfordian basinal structure, the immediate inference is that most of the area in the Yashlar sector was in a depositional environment similar to that in the Charjou Analog area.

In other words, water depths and reef-building conditions in the two areas likely were quite similar.

In order to predict liquid hydrocarbon generation in the general area of the Murgab depression, detailed studies of Jurassic source rock distribution were used.

Fig. 8 shows the inferred Upper Jurassic source rock distribution, as well as contours based on well information of the condensate richness distribution, expressed in grams per cubic meter of gas.

From these data, it appears that liquids generation in the Yashlar can be expected to be at least as good as in the Charjou Analog area and that the increase of depth is less important to the gas-liquids ratio than the source rock composition itself.

Using the fields shown in Fig. 9, the reserves listed in Table 1, and an average richness factor of 250 g/cu m, the authors estimate condensate reserves in the Analog area to be on the order of 2 billion bbl.

The authors also estimate that there are roughly 1 billion bbl of oil in these fields, with Kokdumalak accounting for three fourths of this.

Given that Yashlar has an area roughly four times that of the Charjou Analog area, the authors estimate gas reserves for it to be on the order of more than 120 tcf of gas, condensate reserves on the order of 7 billion bbl, and oil reserves on the order of 34 billion bbl.

The Yashlar sector, as pointed out earlier, is virtually unexplored. However, despite this paucity of hard data, there are indications of significant liquids generation in the Yashlar sector that strongly support the inferences on condensate richness shown in Fig. 8.

These indications consist of shows from the Kimmeridgian section just above the Oxfordian in wells on the Yashlar and South Iolotan structures (Table 2).

The one well that may have penetrated the Oxfordian is the 1 Yashlar, but unfortunately it blew out and couldn't be logged. The estimates of production rates shown in the table are based on height of the blowout plume and the oil spillage around the well.

To gain an appreciation of the nature of some of the Charjou fields and the relationship between them and seismic anomalies in Yashlar, a cross-section through the Kokdumalak field is shown that illustrates the reef buildup (Fig. 10). Other fields in the area appear to have similar characteristics.

To relate this known reef to the seismic evidence in Yashlar, the authors show a segment of a seismic profile across part of the greater Yashlar structure (Fig. 11).

Because of the nature of the alternating salt and anhydrite layers in the Kimmeridgian it is not likely that the anticlinal feature on the profile is due to salt diapirism; in fact, the main evaporate sequence actually thins across the crest of the anomaly.

Detailed analysis of this and a number of other profiles in its vicinity 2 suggest the presence of reef build-up and porosity development over a large part of the structure.

In addition to this example, numerous profiles in other parts of the sector show similar anomalous structure and dips below the evaporate section, suggestive of widespread reefing.

Finally, there is one more aspect of the Yashlar sector that merits attention and, in fact, is also suggested by the profile (Fig. 11).

Associated with the deeper part of the anticlinal structure is an indication of deep-seated faulting that may have penetrated as high into the section as the Kimmeridgian salt-anhydrite sequence. Such faulting would be expected as a result of the general extension and rifting of the Amu-Daria basin throughout much of its history, and it is quite likely that the obvious arching is due to this.

The implications can be seen from Fig. 12, which is a smoothed isopach map of the Kimmeridgian-Tithonian interval. This map shows that there was large scale structural movement during Kimmeridgian time.

The feature labeled "Murgab graben" is consistent with the transverse rifting across the basin postulated by Khain et al. (1991). 1 The feature labeled "Karabil monocline" is simply the north flank of the Karabil step.

The features labeled "Yashlar-Andchoi arch" and "Shakhmolla arch" appear to be lesser horst blocks within the same general stress regime.

The important point, though, is not the mechanical exploration of these features but the fact that they were most probably developing during the same period as the Oxfordian reef development.

To give some idea of their scale, the main part of the Shakhmolla arch is on the order of 100 km in length and 30 km in width. Based on the 300 m or more of growth during Kimmeridgian-Tithonian time, the authors would infer that the arch would probably have had substantial vertical movement during Callovian-Oxfordian time as well.

It seems certain that such growth would have influenced sedimentation and reef development significantly.

It is not possible to say exactly how porosity might have been affected without further drilling or detailed analysis of seismic data.

One can only speculate that somewhere on these structures water depths would have been optimum for laterally extensive barrier reefs and coextensive porosity development. If such extensive reef development were to occur, it is clear that the reserves estimates made above could be several times too small.

CONCLUSIONS

The authors have tried in this paper to demonstrate the great liquids potential of the subsalt Jurassic in the deeper Amu-Daria basin and to provide a plausible rationale for believing that it does exist. The key points are:

Widespread Upper Jurassic source rocks that have generated huge quantities of gas and are known to have been capable of substantial oil and condensate generation in the area of interest.
Inference of a shallow water marine basin of Oxfordian age having a southern shelf margin that underlies the Yashlar sector and that appears to be a mirror image of the well-known Charjou shelf margin on the north flank of the basin.
Substantial gas and liquid reserves in the Charjou Analog area that can be used to project reserve estimates into the Yashlar sector.
Known reef structures in the Charjou Analog area of substantial size, and seismic evidence for numerous similar reef structures in the Yashlar sector.
Seismic evidence of contemporaneously growing, large scale structures that most probably affected sedimentation and could have influenced reef development in such a way as to produce continuous barrier or fringe reef complexes with extended porosity development. The implication of this on potential reserves in the sector is very significant.
Direct evidence of substantial flows of oil and condensate from Kimmeridgian wells at Yashlar and at South Iolotan, which strongly support the inferred oil and condensate potential of the Yashlar sector.

Concentration in this article has been on demonstrating the liquids potential for the Yashlar sector. Many of the same arguments can be brought to bear on the potential of the Tedzhen sector. In addition, although the argument is somewhat different, there are implications for major oil and condensate reserves in the Badkhyz sector as well, related to the same Upper Jurassic source rocks.

ACKNOWLEDGMENT

The information presented in this paper represents the synthesis of a very large amount of work by various groups within the Ministry of Geology of the U.S.S.R. and within Turkmengeologia of Turkmen SSR.

In particular, the authors thank K.A. Kleschev, V.S. Shein, V.A. Teplitsky, and V.S. Slavkin of Vnigni and N.T. Souyounov of Turkmengeologia for their considerable assistance in assembling the information presented and in suggesting approaches to its interpretation.

The authors also thank GeoInterTech for permission to publish the results.

REFERENCES

Khain, V.E., B.A. Sokolov, K.A. Kieschev, and V.S. Shein, Tectonic and geodynamic setting of oil and gas basins of the Soviet Union: AAPG Bull., Vol. 75, No. 2, 1991, pp. 313-325.
Unpublished Vnigni studies, Moscow, 1991.
Clarke, J.W., Petroleum geology of the Amu-Dar'ya gas-oil province of Soviet Central Asia; U.S. Geological Survey Open-File Report 88-272, 1988, 59 p.
Vasil'yev, V.G. and others, in Gasovyye i gazokondensatnyye mestorozhdeniya, I.P. Zhabrev, editor: Moscow, Nedra, pp. 251299 (summary in Petroleum Geology, Vol. 21, Nos. 1 & 2).
Unpublished Vnigni communication, Moscow, 1991.