BURIED-EUXENIC-BASIN MODEL SETS TARIM BASIN POTENTIAL

Kenneth J. Hsu
Tarim Associates for Oil & Mineral Exploration AG
Zurich

The Tarim basin is the largest of the three large sedimentary basins of Northwest China. The North and Southwest depressions of Tarim are underlain by thick sediments and very thin crust. The maximum sediment thickness is more than 15 km.

The basin had originated by early Paleozoic rifting behind island arcs. Similar back-arc basins existed in Tianshan and in the Kunlun Mountains, but those were eliminated through lithospheric subduction, or back-arc collapsing, during the mid-Paleozoic arc-arc collisions. The basinal sediments of Tarim were, however, largely spared of strong orogenic deformation.

The Tarim basins in mid-Paleozoic were surrounded on all sides by mountains, like the Black Sea of the Tethyan System, or the Western Siberian basin east of the Ural Mountains. Those relic back-arc basins evolved into a foreland basin during late Paleozoic. Tarim then became interior lowland and a site of terrestrial sedimentation, and the thickness of the Mesozoic and early Cenozoic formations is about the same over the whole region.

Tectonic deformation of continental Asia was rejuvenated, however, after the collision of India and Tibet in Eocene. Pull-apart basins formed by strike-slip faulting developed on the southern flank of Tianshan; the basins subsided while the mountains were uplifted. The Neogene Southwest depression owed its origin also to wrench faulting. The Cenozoic deposits of those basins were subsequently deformed by transpression.

Of the several Oil fields of Tarim, the three major fields were discovered during the last decade, on the north flank of the North depression and on the Central Tarim Uplift (Fig. 1). The major targets of Tarim, according to the buried-euxenic-basin model, should be upper Paleozoic and lower Mesozoic reservoirs trapping oil and gas condensates from lower Paleozoic source beds.

BASIN OVERVIEW

The Tarim basin has been investigated by geophysics and by drilling during the last 45 years. Some 24 teams from the Chinese Ministries of Petroleum and of Geology and Mineral Resources completed some 5,000 km of seismic profiling before late 1984, an additional 8,000 km before the end of 1986, and a final 5,800 km before 1987.

An area of some 220,000 sq km, or about two fifths of the basin, has been adequately explored. Additional geophysical surveys have been carried out during the last few years. The terrain difficulty and the relative lack of knowledge have rendered the exploration of Tarim a most costly and risky venture.

Motivated perhaps by this consideration, China National Petroleum Corp. decided in early 1993 to expand its cooperation with foreign firms to explore for oil and gas in Chinese onshore basins north of the Yangtze River (Fig. 2). The southeastern part of the Tarim basin was targeted as the first of 12 areas to be opened for the "risk exploration."

The difficulty and lack of knowledge may also be a reason for the optimism that the area could be one of the most profitable of the world. The very high success ratio of wildcatting in the Tarim basin during the last decade has verified the so-called Zipf's Law: the larger the poorly explored area, the more likely is the chance of finding a bonanza.

Of all the Tarim areas, the southeastern Tarim remains the least explored. The author has been told by officials at CNPC that not a single well had been drilled in the area of the five lease blocks for sale. The area certainly has room for one or more giant discoveries. Nevertheless, the high cost of exploration and the uncertainty about future pipeline capacity have been discouraging.

Tarim bidding closed in October 1993. Only three of the five blocks were ]eased.

Additional blocks in North China were opened for bidding in March 1994, and more Tarim blocks are scheduled to be offered.

HISTORICAL PERSPECTIVE

Yumen in Gansu Province was the first oil field discovered in China. Its source beds and reservoirs are Cenozoic terrestrial deposits.

Earlier exploration in the Chinese Northwest focused on lacustrine basins.

Until the 1950s, successes were modest, and exploration moved to the Chinese Northeast. The move was rewarded by the discovery of Daqing and other oil fields of North China.

It is now recognized that the key difference between the Cenozoic basins of the Northwest and those of the Northeast and North China lies in the genesis of source beds: The Mesozoic/Cenozoic basins of northeastern and northern China hosted lakes as sites of anoxic sedimentation. Northwest China has, however, been relatively and since Cretaceous.

There were lacustrine deposits in earlier Mesozoic, but they were shallow lakes not favorable for sedimentation in oxygen deficient environments. When exploration was renewed in the Northwest, the explorers were no longer blinded by the possibilities in the Cenozoic and Mesozoic.

The search for the Paleozoic was first rewarded by the discovery of the rich reserves beneath the shallow pools of the Karamai field of the Junggar basin; correctness of the strategy has been verified by exploration activities in Tarim.

The first field discovered in Tarim was the small Yiqikelike field, producing from Jurassic sandstone and discovered in 1958 (Fig. 1). Upon their return to the Northwest, the Chinese in 1977 discovered the Kekeya field of the Southwest depression. Although gas condensates were found in Miocene sandstone reservoirs, geochemical characteristics of the oils indicated derivation from marine Paleozoic sources.

The search for Paleozoic oil in Tarim was rewarded by discovery in 1984 of oil in lower Paleozoic strata of the Yakela field. Oil then was found in Paleozoic reservoirs in Lunnan and Tazhong fields. These discoveries emphasized the importance of Paleozoic targets in the Tarim basin.

CNPC's concentrated efforts during the last decade have resulted not only from the 1984 discovery but also from the conviction that the Tarim basin has undergone a geologic evolution extremely favorable to the accumulation of huge amounts of hydrocarbon.

Tarim had been considered a cratonic basin in the continental interior, underlain by Paleozoic carbonate-platform strata and by Mesozoic and Cenozoic terrestrial deposits. Such a interpretations led to doubt that Tarim could ever become a major petroleum producing basin: There seemed to be no adequate source beds.

Later optimism by CNPC is based, at least in part, upon acceptance of the buried-euxenic-basin model developed by Tarim Associates (Fig. 3).

The model postulates the deposition of lower Paleozoic black shales in the Tarim's North and Southwest depressions. They belong to a sedimentary facies Distinctly different from the platform carbonates encountered in wells drilled into the Tarim uplifts and those exposed at outcrops in the Kalpin area to the northwest of the basin.

The essence of the buried-euxenic-basin model is to predict that thick source beds do exist in the central depressions of Tarim at depths that cannot be reached by drilling. Furthermore, the model presents a history of hydrocarbon migration in late Paleozoic and early Mesozoic time, before the source beds had been buried by the some 5 km thick Mesozoic and Cenozoic sediments.

The buried-euxenic model has been formulated on the basis of the author's investigations of Chinese geology during the last 15 years, and of Xinjiang geology during the last 7 years. The hypothesis has been substantiated by the conclusions published by various other investigators.3

The model postulated an early Paleozoic paleogeography of Xinjiang similar to the Indonesian Archipelago of Southeast Asia today (Fig. 4). The granites of the Kunlun Mountains were the root of an outer volcanic arc, comparable in size and significance to the Sunda-Banda Arc of Indonesia.

Numerous back-arc or relic back-arc basins were present north of the Kunlun Outer Arc. Several of those were completely eliminated during the Paleozoic by the consumption of the ocean lithosphere under those basins; they are represented by the melanges of Altai, West Junggar, Tianshan, and Kunlun Mountains. Qaidam, Junggar, and the Tarim depressions are three relic back-arc basins, which are now areas underlain by ocean lithosphere and buried under younger shallow marine and terrestrial sediments.

PALEOZOIC OIL, NORTHWEST CHINA

The discovery of oil in Paleozoic in the Karamai field has inspired the scientists of the Tarim Associates to formulate the buried-euxenic-basin model. The questions asked after the discovery of the Karamai deep pools were:

Where did the oil come from? Was it Paleozoic oil, or did the oil come from Mesozoic and/or Cenozoic lacustrine source beds?
If it is Paleozoic oil, what is the origin of the source bed or source beds? What is the a-e of the source beds?
Under what paleoceanographic or paleoclimatic conditions did the source beds originate?
What kind of sedimentary basin was the Jun-gar basin when the source beds were laid down in the basin? How did such a sedimentary basin originate?
Are there similar petroliferous basins in Northwest China or elsewhere in the world?

Our answers to those questions are as follows:

The deep Karamai oil came from marine source beds, not from Permian lacustrine oil shale as some American scientists postulated.
The source beds are mainly Paleozoic black shales, interbedded with oil reservoirs of turbidite sandstone.
The source beds of the Karamai were formed in a deep-sea anoxic environment.
The late Paleozoic Junggar basin was a relic back-arc basin like the present-day Black Sea. A back-arc basin had originated by seafloor-spreading behind an active island arc, and the relic back-arc basin was formed when the frontal magmatic arc was shifted away from the site.
Petroliferous basins similar to Junggar are the Tarim and Qaidam basins in China, and the Western Siberian, Kazakhstan, Caspian, Black Sea, South Kara, and Varents-North Kara basins in the former U.S.S.R."

A great hindrance to the understanding of Chinese geology is the traditional geosynclinal theory of mountain building. The crust of the earth, according to the theory, is divisible into two categories: stable areas and mobile belts. Mountains are mobile belts, and mountains of the Tethyan type are supposedly formed by the compression of mobile belts between two stable blocks.

The Junggar and Tarim basins are separated by the Tianshan Mountains (Fig. 5). According to the classical theory, Tianshan was the mobile belt between the two stable cratons, jun,-gar and Tarim. Tianshan was a site of geosynclinal sedimentation during the Paleozoic, while the cratons, underlain by continental crust, were overlain by thin shallow marine sedimentary cover.

Gravity studies first suggested, however, that the Tarim and Junggar are underlain by thick sedimentary sequences. This conclusion was verified by seismic and by drilling of deep boreholes.1 Seismic profiling has revealed that the maximum thickness of the sediments under Junggar is about 15 km, under Tarim more than 15 km (Fig. 6). Those basins are thus neither underlain by stable craton, nor are they intracratonic basins.

The three great basins of Northwest China have also been called "intramontane basins." The designation refers to their present geographical position, but those basins are certainly distinct in many respects from the intramontane basins of the American West.

This comparison is improper; even the Cenozoic structures are different. Whereas normal faulting predominates in the Basin and Range province, the Cenozoic faults bounding the basins of Northwest China are mainly wrench faults or upthrusts. The Paleozoic structures of Tarim are completely different from those of the Basin and Range. The Paleozoic rocks of the latter, deformed by nappe tectonics, constitute the 'basement" of the intramontane basins. The Paleozoic rocks of Junggar, Tarim, and Qaidam, as shown by seismic profiling, are mostly flat-lying; they are little deformed and are the target formations of oil exploration.

If Junggar and Tarim were neither Paleozoic intracratonic nor intramontane basins, what were they? What is the cause of basin subsidence which allowed for the accumulation of a sedimentary sequence 15 km thick or more? What is the origin of the holes in the midst of a continent?

The author was asked this question while touring the Soviet Union in the 1970s, because there seemed to be "deep holes" in the midst of a craton in the former U.S.S.R., such as the Western Siberian, Kazakhstan, Caspian, Kara, and other basins. The question still applies: What is the origin of such "deep holes" in continental interior?

One can resort to simple logic: We could enumerate all the alternative possibilities and find our answer through the elimination of the answers that are not viable working hypotheses. We know that all basins owe their origin to subsidence under stress, and there can be only three stress systems possible near the surface of the earth. This is the reason, according to the current theory of faulting, that there are only three types of structures possible in crustal deformation: horizontal extension, horizontal compression, and horizontal shearing producing normal, reverse, and wrench faulting.

Basins produced by wrench faulting are pull-apart basins. Locally thick Neogene deposits in the Southwest and Kuqa depressions of Tarim are accumulations in Cenozoic pull-apart basins. However, the thick and widespread lower Paleozoic sedimentary sequences of the Tarim and Junggar basins are very different from the coarse clastics in long and narrow basins formed by wrench faulting. The Junggar and Tarim basins were obviously not Paleozoic pull-apart basins.

Basins formed under compression are foreland basins. The Molasse basin of Swiss Midland is the classic example. The basin was formed when its underlying basement was thrust under the advancing Alps. This type of deformation has been called A-subduction when one slab of continental crust is thrust under another slab of continental crust.

The upper Paleozoic sediments of Junggar and Tarim are foreland basin deposits. The Devonian, Carboniferous, and Permian strata of the Tarim basin are indeed shallow marine and terrestrial carbonate and clastic rocks, typical sediments of the molasse association deposited in a tectonic setting similar to that of the Swiss Molasse.

During the late Paleozoic the Tarim basement was being thrust under the advancing nappes from Tianshan, when the Tarim Molasse was deposited. The lower Paleozoic sediments encountered on the Tarim Uplifts are, however, definitely not foreland-basin sediments, nor are the turbidities or radiolarian cherts found in the melanges around the Tarim and Junggar basins (Fig. 5).'

Those lower Paleozoic sediments belong either to the carbonate-ortho-quartzite or to the flysch association; they are not molasse deposits of foreland basins. The lower Paleozoic Junggar and Tarim basins thus could not have been foreland basin deposits.

If the Junggar, Tarim, and Qaidam are neither lower Paleozoic pull-apart formed by wrench faulting nor lower Paleozoic foreland basins formed by compression, the only alternative is that they are lower Paleozoic extensional basins.

From a survey of the actualistic examples, the three types of basins of extensional origin are (1) rift valleys in continental interior, (2) spreading oceans, and (3) back-arc basins. If the Junggar, Tarim, and Qaidam are continental rift valleys, they should be underlain by continental crust. The thickness of the crust under the basins argues against such a postulate; the areas are underlain by oceanic lithosphere (Fig. 4).

The nature of the sediments also falsify the continental-rifting hypothesis. The sediments on the border of a rift basin should be terrestrial deposits, predominantly coarse-grained clastics. This is not the case for the Tarim and Junggar sediments on the peripheral uplifts. The lower Paleozoics of the Kalpin Uplift, for example, are mainly shallow marine carbonate strata (Fig. 5), and their presence rules out the hypothesis of a Paleozoic rift valley.

Another type of extensional basin is the rift-valley on mid-ocean ridges. If this were the case, the lower Paleozoic sequences on both sides of the spreading ocean should be characterized by passive margin sequence. The lower Paleozoic rocks in the Kuruktag Mountains north of the Tarim basin are, however, deep marine sediments. They were once considered passive margin deposits (Fig. 5), but are now interpreted as sediments on an active margin. This facies development contradicts the postulate that Tarim is the relic of a spreading ocean.

As Sherlock Holmes said, after all the possibilities are eliminated the remaining alternative, however improbable, must be the truth. It is through this process of logical thinking that I came to the conclusion that the Junggar and Tarim basins were lower Paleozoic extensional basins formed behind island arcs.

The sedimentary sequences under the basins are thus much thicker than the coeval sequences in the mountains. Such thick sediments can only accumulate in a basin underlain be a thin crust, 5 or 10 km thick at most. That Junggar basin is indeed underlain by ocean crust has been verified by the investigations of the Stanford group.4 The geophysical and geological arguments that the Tarim is also underlain by the ocean crust of a back-arc basin rely mainly upon privileged information presented in a Tarim report jointly distributed by CNPC and Tarim Associates. The most impressive evidence is afforded by the discovery of magnetic lineations under Tarim which is the evidence of seafloor spreading behind island-arcs (Fig. 7).

AN ISLAND-APC MODEL

The buried-euxenic-basin model postulates four stages of geologic evolution:

Sinian and early Paleozoic platform sedimentation on relic arcs and deep-marine sedimentation in back-arc basins in Xinjiang (Fig. 8f).
Late Paleozoic foreland-basin sedimentation in north Tarim (NTa in Fig. 8f).
Mesozoic and Paleogene continental deposition, subsidence under sedimentary load.
Neogene pull-apart basin, wrench faulting and extension.

Interpreted on the basis of such a tectonic model, the Tarim uplifts and depressions, from north to south, and their ages are:

Kuqi depression: Sinian/early Paleozoic remnant arc, late Paleozoic foreland basin (comparable to Subalpine Molasse), Permo-Triassic uplift, Mesozoic/Paleogene interior basin, Neogene depression.
Tabei uplift: Sinian/early Paleozoic remnant arc, late Paleozoic foreland basin (comparable to Subalpine Molasse), Permo-Triassic uplift, Mesozoic/Paleogene interior basin, Neogene depression.
Northeast depression: Sinian/early Paleozoic back-arc basin, late Paleozoic foreland basin, Mesozoic/Cenozoic continental interior basin.
Central uplift: Sinian/early Paleozoic remnant arc, mid-Paleozoic uplift (foreland-deformation belt), Mesozoic/Cenozoic continental interior basin.
Southwest depression: Sinian/early Paleozoic back-arc basin, late Paleozoic relic back-arc basin evolving into foreland basin, Mesozoic/Cenozoic continental interior basin, Neogene pull-apart basin on southern margin.
Tanan uplift: Sinian/early Paleozoic back-arc basin, late Paleozoic accretionary complex (uplift), Mesozoic/early Cenozoic continental interior basin, mid- or late-Cenozoic uplift (in part).
Takiliktag uplift: Sinian/early Paleozoic volcanic arc, late Paleozoic remnant arc, Mesozoic and Cenozoic Kunlun Mountain.
Southeast depression: Paleozoic back-arc basin, Mesozoic and Cenozoic continental interior basin, Neogene pull-apart basin.

The Alpine models of Subalpine Molasse, Plateau Molasse, German Molasse, and Jura Mountains are useful structural analogs to interpret the upper Paleozoic structures of the Tarim basin. The oldest (upper Devonian) Tarim molasse sediments were deposited in depressions close to the front of Tianshan. The older foreland basin deposits were folded and thrust during late Paleozoic after an arc-arc collision of the Mid-Tianshan and Kalpin/Kuruktag Remnant Arcs.

The "Sub-Tianshan" (equivalent to Subalpine) structures are very important features for petroleum geology, Even more important are the folds and thrusts in the foreland-deformation belt of the Central Tarim uplift. Those disharmonic structures can be called "buried Jura," and they were formed as compressional deformation in Tarim continued during the late Paleozoic and early Mesozoic.

Those are the major oil-bearing structures of Tarim.

PETROLEUM GEOLOGY

The Yiqukelike oil field near the Tianshan Foothills is typical of the numerous small oil fields discovered in Northwest China during the 1950s, when exploration concentrated on oil from lacustrine beds in continental basins. The Jurassic pay zone is only several hundred meter; deep, and the production is insignificant.

The Kekeya No.1 well was drilled into the Miocene of the Yecheng depression. The unexpected high pressure of gas condensates led to the blowout not only of the discovery well but also of two development wells, and the unexpected finding of hydrocarbons derived from a marine source pointed out Tarim's Paleozoic potential.

The expectation of oil from Paleozoic source beds was certified by the discovery of Yakela field in 1984. The discoveries of the Lunnan field in 1987 and Tazhong field in 1989 cut oil pay in targeted Paleozoic zones and in overlying Mesozoic reservoirs.

The source bed for Tarim oil has been a controversial issue. Neither the shallow marine deposits on the Kalpin uplift (Fig. 5) nor those on Central Tarim uplift (Tazhong Structure in Fig. 1) are good source beds. Foreign visitors to Tarim have been shown seismic profiles indicating the presence of the very thick sequence of lower Paleozoic sediments (up to 10 km or more) in Tarim depressions. The top of several formations can be traced from the uplift into the basin, so that it was commonly assumed that the basinal sediments are the same as those encountered by drill on the uplift.

Our "buried-euxenic-basin" model suggests, however, that the basinal sediments belong to a different facies.

As shown by Fig. 8, the shallow marine carbonates on the Central Tarim uplift are those of a relic island arc. The basinal sediments of a back-arc depression should be deep marine.1 3 Indeed, deep-sea Cambrian, Ordovician, and Silurian sediments have been described in the southern Kuruktags (Keluketage Mountains) on the northeastern rim of Tarim. They include black shales, radiolarian chert, and flysch sandstones and were mapped as "passive marginal formations" of Kalpin/Kuruktag (Fig. 5). We have concluded that the thick basinal sediments in the North depression of Tarim should be the same facies of deep-water sedimentation.

The total thickness of Cambrian and Ordovician black shales at the Kuruktags outcrops is considerably more than 50 m. The absence of benthic faunas in those shales indicates sedimentation under anoxic conditions. From the assumption that those sediments are the northern continuation of a coeval sequence in the North depression of Tarim, a rough order-of-magnitude estimate of possible hydrocarbon reserves in Tarim has been attempted (see table).

This was the basis of my prediction in 1986 that there might be 50 billion tons of hydrocarbons under the desert sands of Tarim. Of all the factors involved in this calculation, the most uncertain is the thickness of the source beds. The likelihood of having 50 m of black shales with 2,000 ppm hydrocarbon content in the central depressions of Tarim is discussed in the CNPC/Tarim Associates report.

The tectonic history of Tarim and a study of seismic profiles indicate a great Variety of trapping possibilities. The multiplicity of trapping factors in the various Tarim fields verifies this impression. There are anticlinal traps of the type of the Foothill fields of the Canadian Rockies, and there are conformity/structure traps of the type of the Karamai field of the Junggar basin.

The best prospects, in our opinion, are the anticlinal traps of the Central Tarim uplift, where a 230 km long anticline has been described with a closure of more than 2,000 m. Eight domes have been identified on the anticline, and the total area of those closures is 724 sq km.

Mudstone or shale above unconformity in Tarim serves as a seal to trap oil beneath the unconformity. The presence of structures such as faulting or overthrusting may be necessary to enhance the trapping through the provision of lateral closures. The Lunnan field of Tarim could also be categorized as an accumulation of this type.

Unconformity traps are imperfect. Oil presumably migrated updip from Cambro-Ordovician sources did locally leak through the pre-Carboniferous unconformity and was trapped in Triassic reservoirs by minor thrust faults and gentle warping.

Gas condensates found in the Miocene reservoirs of the Kekeya field may have been trapped in structures below a major Neogene upthrust, which is commonly a near-surface expression of wrench faulting, a structure found all along the edge of the Southwest and Southeast depressions of Tarim.

The Mesozoic and Cenozoic sequence of Tarim includes thick beds of coarse clastics; there is no lack of potential reservoir beds. Coarse clastic sediments of late Paleozoic age in the foreland basin are also reservoirs or potential reservoirs. The lower Paleozoic reservoir beds are mainly carbonates, with fracture and solution porosities; the presence of very porous zones of karst is indicated by occasional lost circulations during drilling into the carbonate pay zones below unconformities.

Hydrocarbon indications in upper Mesozoic and Cenozoic formations are on the whole insignificant. Production from the Jurassic is minor. The only significant Tertiary reservoir beds are the Miocene sandstones of the Kekeya field in the Southwest depression. The gas condensates, in our opinion, were migrated into Tertiary traps from lower Paleozoic source beds at a relatively late date, long after their burial under the Mesozoic and Tertiary sedimentary blanket.

The geothermal gradient of the Tarim region is now abnormally low. Mesozoic and lower Tertiary rocks of the Tarim basin are not mature source beds. The Tarim source beds are the lower Paleozoic rocks. Nevertheless, oil is produced from lower Paleozoic reservoirs in the Central Tarim uplift below 6,000 m; oil must have migrated into Paleozoic traps long before the source beds were buried to the present depth.

Once migrated into traps, oil tends to be preserved as such because of the present low geothermal gradient of Tarim. Hydrocarbon leaving the source rocks at a later date of deeper burial became gas or gas condensates.

A RECOMMENDED STRATEGY

Success of the current practice for oil exploration, as developed by the industry in North America, the Middle East, and Europe, can be traced to the common history of the tectonic evolution of those regions. Jurassic and Cretaceous source beds deposited on passive margins of those continents are ubiquitous, so that the presence of the source beds, as a rule, is not the first-order problem.

Maturation is also no big problem where thick deltaic sediments have been laid down. Thus the emphasis could be placed on developing techniques to search for traps, and seismic profiling claims a major share of the exploration budget.

Asia/Pacific geology differs from Atlantic geology: Source beds formed during the global anoxic crises of Cretaceous and Jurassic periods are commonly deformed in the Asia/Pacific region, forming slates or phyllites in mountain chains. The most promising source beds are those deposited on ocean crust, which is note, trapped within continental interior. It is thus not surprising that the largest oil fields in the former U.S.S.R. are in the West Siberian, Kazakhstan, South Caspian, and other basins underlain by trapped ocean crust.'

According to the buried-euxenic-basin model, source beds should not be a problem in the Tarim area. One could thus concentrate the effort to unravel the tectonic evolution of Tarim and search with specific working hypotheses for the various types of traps represented by the fields found so far, especially the faulted anticlines on the Central Tarim uplift.

To search for Paleozoic oils in Paleozoic reservoirs trapped by Paleozoic structures requires a knowledge of the Paleozoic geologic history of Tarim - unlike the Cenozoic history of the Cenozoic basins, which is self-evident on seismic profiles.

To decipher the Paleozoic geologic history of Tarim, as I found out during the last 15 years, requires a model. One has to understand the Paleozoic geology of China in general and that of Tianshan and Kunlun Mountains in particular.

The geometry of seismic reflectors alone does not help us a great deal to understand the history of Paleozoic history of the basin. The model can be adopted from an area where the tectonic evolution is clearly understood, such as the thin-skinned deformation of the Swiss Alps and Jura Mountains or the back-arc basin collapses and arc-arc collisions in the Indonesian Archipelago. The model contributes to understanding of profiling data.

We have innovated the buried-euxenic-basin model, and the model has been successful in the interpretation of geology of Xinjiang and genesis of the hydrocarbon occurrences in Junngar and Tarim basins.

REFERENCES

Hsu, K.J., "Relic back-arc basins: Principles of recognition and possible new examples from China," in Kleinspehn, K.K., and Paola, C.(eds.), Frontiers in Sedimentary Geology: New Perspectives in Basin Analysis, Springer-Verlag, 1988, pp. 245-263.
Hsu, K.J., Li, J., Wang, Q., and Sun, S., "Melanges around the Junggar basin," proceedings of the First International Conference on Asian Marine Geology, China Ocean Press, 1990, pp. 25-38.
Aplonov, and others, "Relic back-arc basins of Eurasia and their hydrocarbon potentials," The Island Arc, Blackwell Scientific Publications Pty. Ltd., Vol. 1, 1992, pp. 71-77.
Coleman, H.G., "Continental growth in NW China," Tectonics, No. 8, pp. 621-635. Analysis. Springer-Verlag, N.Y.,245-263. Hsu, K.J., "A West Pacific model for the geologic evolution of Tianshan and Kunlun Mountains and a hypothesis for the origin of Junggar and Tarim Basins (abstract)," Prof. 12th Geodynamics Res. Inst. Symposium, November 1992, Texas A&M Univ., College Station, TX.