Figure
1.
Deformed structural style of the Paleogene Kuqa depression, Tarim basin(Lin et al., 2002).
| Citation: | De-tian YAN, Hua WANG, Jia-hao WANG, Qing-chen WANG. Sedimentary Characteristics and Reservoir Prediction of Paleogene in the East Part of Kuqa Foreland Basin. Journal of Earth Science, 2006, 17(2): 138-145, 157.
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Most of the Mesozoic and Cenozoic large-scale hydrocarbon-bearing basins in western China were formed in a similar foreland setting. Hydrocarbon exploration of the Kuqa foreland basin requires research into the sedimentary characteristics and filling evolution of the depositional sequences and their response to the basin process. Based on an analysis of outcrops, well logs and high resolution seismic data, the sedimentary system types and distribution characteristics of the Paleogene in the east part of Kuqa foreland basin were systematically studied. The results show that: (1) Three types of sedimentary systems are developed in the area: an oxidative salty wide shallow lacustrine system, a fan delta system and an evaporitic bordersea system. (2) The configuration and evolution of the depositional systems of the Paleogene in the Kuqa foreland basin were predominantly determined by foreland tectonism. Vertically, the Paleogene sedimentary sequence can be divided into three parts: the lower, middle and upper depositional system tracts. The lower and upper tracts commonly consist of progradational or aggradational sequences, while the middle part is usually comprised of a set of aggradational to transgressive third-order sequences. Laterally, the sedimentary systems in the east part of the Kuqa foreland basin spread from east to west as a whole, and the sedimentary facies obviously vary from south to north. The sand bodies of the delta front facies are excellent gas reservoirs, characterized by rather thick, extensive and continuous distribution, high porosity and permeability, and just a few barrier beds.
Being one of the largest gas fields ever discov ered in the continental exploration of China, the Kela-2 gas field in the Tarim basin has become a foundation for the reliable production of gas re sources for the"WesttoEast Gas Transmission'' project.The Kuqa depression is one of the most eco nomically important oil and gas prospecting districts in the Tarim basin, with a giant gas field found in its northern central area.Many previous studies have shown that the Paleogene Kuqa depression is a fore land basin resulting from the flexural subsidence due to the southward thrusting of the Tianshan Mountains.The establishment and distribution of oil and gas in a foreland basin are controlled by structure and the spacetime distribution of the sedimentary system(Lin et al., 2002; Ding et al., 1997).Because of the com'plex structure, varied sedimentary facies and least exploration, there are many different understandings and even arguments about basin fill ing dynamics and the depositional pattern of the depression.
In order to predict the gas reservoir, research was carried out on the spatial arrangement character istics of the sedimentary systems and the evolvement of the sedimentary setting, on the basis of previous research.Combining outcrop, well and seismic data (Van Wagoner and Mitehum, 1990), a lithofacies paleographic map is produced and the relation be tween reservoir characteristics and sedimentary sys terns is considered.Some credible outcomes are pres ented for researching oil and gas enrichment and ex ploration in the Kuqa depression.
The Paleogene Kuqa depression, located in the northwestern margin of the Tarim basin, is an elongate NE-trending foreland basin.It is about 70 km wide and 410 km long, covering an area of some 30 000 km2.It is bounded by the thrusting southern Tianshan Mountains to the north, and extends southward to the Tabei and the Tazhong uplifts. Tectonically, the Kuqa depression can be divided into several structural belts from north to south.namely the southern Tianshan thrust faulted belt, the northern marginal thrust faulted belt.the central deformed belt and the southern slope and the TabeiTazhong forebulges(Lin et al., 2002; Li et al., 2000)(Fig. 1).The deformation of the Kuqa de pression mainly occurred during the Tertiary and had a significant influence on the Tertiary basin fills.The Paleogene system in the depression is about 1 000 m thick and unconformably overlies the Lower Cretaceous.The Paleogene system includes the Kumugeliemu and Suweiyi formations, and the Ku mugeliemu Formation can be divided tWO stages of bottom sandstone and sandmudstone.
In the east part of Kuqa foreland basin, there are an oxidative, salty, wide shallow 1acustrine system, a fan delta system and an evaporitic bordersea system of the Paleogene.
The oxidative.salty, wide shallow 1acustrine system developed at the sand-mudstone stage in the Kumugeliemu Formation, and it includes shore and shallow lacustrine sedimentary facies.
The shallow lacustrine facies are made up of dust-colored, gray and celadon parget-mudstone and hoar gypsolyte, mud-gypsolyte, imbedding siltstoneand cryptite.Locally, the mudstone contains phytolite. The shore lacustrine facies are made up of maroon and brown lime-mudstone, lime-siltstone, mud- siltstone, and some silver sandstone and gypsolyte outputting along layers.Sandstone elements have high maturity and psephicity, and are well graded. The quartz composition is 85%-90%.The sandstone granularity probability curve shows two stages and the skip collectivity slope is high.The composi-tion of gypse in gypsolyte is more than 90%and gypse has a granule and filiform texture.Gypse takes on a macula distribution in gypsiferous mudstone. Fold-roll bedding, contemporaneous deformation bedding and slump structures can usually be found in the mud-gypsolyte. Undulating cross-bedding, climbing ripple lamination(Fig. 2a), minitype tabu- lar cross-stratification and horizontal stratification are developed in this environment.
The fan delta system is mainly developed at the bottom sand stage in the Kumugeliemu and Suweiyi formations.A small-scale fan delta system outputting as lens is deposited at the sand-mud stage in the Kumugeliemu Formation(Fig. 2b).This system mainly develops a fan delta plain facies and fan delta front facies in this study area(Fig. 3).
The fan delta plain facies possess alluvial fan characteristics.and mainly develop debris flow and braided channel microfacies.
Debris flow is a reDresentative characteristic of the fan delta system, differing from other delta sys- tems(Li et al., 1999; McPherson et al., 1987). Debris flow is composed of brown, gray, glaucous, fuchsia thick clumpy coarse conglomerate, medium- to--fine conglomerate imbedding sandstone and mot。' tled mudstone.The conglomerate has a variety of components: pyroid rock, quartz, silex, sandstone and metamorphosed conglomerate are common, as well as a few mud pellets.The granularity of grain is greatly disparate and the grain is sub_, angular or sub。。rounded and matrix-propping.Undermining and ero- sional structures are developed in this environment.
Braided channel deposition is made up of conglomerate, gravel sandstone and gritstone, and it de- velops great-medium cross-bedding.Braided channel distributed in the sediment of debris flow is of depo- itional lenticularity.The sandstone granularity probability curve is linear three-stage, skip collectivi- y is leading and the pitching angle is subdued.
The fan delta front facies are comprised of an underwater distributary channel and estuary dam mi- rofacies.
The underwater distributary channel is made up f mauve, gray, heavy pebble-stones, gritstone im- bedding French grey, lilac sihstone, medium-to-fine sandstone.It has the scoured base of a stream-laid deposit and usually develops tabular cross-bedding. The sandstone granularity probability curve is three- stage, and it gives priority to skip collectivity(Fig. 4a).
The estuary dam is mainly made up of sihstone nd sandstone and it has an inverted layered struc- ure.Parallel bedding and minitype cross bedding are developed.The sandstone granularity probability urve is two-stage, and it gives priority to skip col- ectivity.The skip collectivity pitching angle is high (Fig. 4b).The estuary dam is a fine sorted microfa- ies in a fan delta system.
The evaporitic bordersea system is developed at he bottom sand stage in the Kumugeliemu Forma- ion and is made up of ivory-white dolomicrite, astizoic dolomite rock, cryptite, knotty limestone and algal bryozoan boundstone.
A suite of gray blocky cryptite, knotty limestone and algal bryozoan boundstone exists at the bottom of Kumugeliemu Formation in Kezileiluer outcrop. The composition of gravel in the blocky cryptite in- cludes cryptite and the gravel diameter ranges from 0.5 cm to 5 cm, mostly lying between 1 and 2 cm. Knotty limestone presents tuberculation on the surface, about 10 cm in diameter(Fig. 2c).The center of tuberculation protrudes upwards and its cireumam-. bience is sinking.The gaps between tuberculations are filled with brown-red mud.The percentage corn- position of calcareous deposit in cryptite exceeds 95%.The main content of algal bryozoan boundstone is cryptalgalaminate, and it also contains approxi- mately 5%psammyte and 5%bioclastic rock.
Many gastropods can be found in the dolostone of Kelashuhe profile.and wells TBl and KL20 1 (Potamides sp.Potamides angustus, Sigmesalia sp.),and there are crinoids in well KL20 1 and the Awate profile.
The Paleogene system is the main reservoir for-mation in the Kuqa depression (Ding et al., 2001). The Paleogene is an integrated second-order sequence in the Kuqa depression and can be divided into three parts: the lower, middle and upper depositional sys- tem tracts(Wang et al., 2004).The lower and up- per tracts commonly consist of a progradational or aggradational sequence, while the middle tract is usually comprised of a set of aggradation to trans- gressive third-order sequences or parasequence sets (Table 1).Based on the present research and practi- cal significance for exploration, groups or stages cor- responding to different depositional system tracts are the standard units used for the mapping of sedimenta- ry systems.
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The bottom sandstone stage of the Kumugeliemu Formation corresponds to the initial development of the second-order sequences.Strong orogenesis resul- ted in thick fan delta system deposits along the fore-land margin(Fig. 5).
Transgression and torridity climate caused con- densed and oxidative salty lacustrine setting that makes red rock series mingled with limestone and gessoed salty rock deposited in the study area.
Blocky conglomerate and sand-conglomerate is mainly sedimentary matter in the northern study area.The fan deRa front facies extends to well DQ5 10cated in the center of the depression.Conglomer-ate, glutenite and conglomeratic gritstone are mainly deposited in wells S54 and Yangl。The granularity of sediment in wells DNl 1。K1, T1 and LT2 becomes fine and the sediment is mainly made up of conglom- eratic gritstone, gritstone and silver sandstone.This reveals that the source area is located in the east part.The sandstone granularity probability curve of wellS54 shows channel deposits of fan delta plain facies.The fan delta front facies are commonly devel- oped in the southwest of the study area, and the de- posit is mainly silver sandstone and siltstone.
The sand-mudstone stage in the Kumugeliemu Formation corresponds to a set of aggradational to transgressive third-order sequences.0n the one hand。the north foredeep is formed from the sharp tectonic compression in this period.On the other hand, different tectonic actions led to the inconsonance of subsidence and even to uplifted erosion in the northern study area.The maximum depressed amplitude and the thickest strata are in the Kezhilein ver profile.The thickness of strata in the Yiqikelike and Tuzhiluoke profiles becomes small, and corre- sponding strata in the Tagemazha profile are absent。As a whole, the sand-shale stage in the Kumugeliemu Formation takes on a configuration pattern of thick in the northwest and thin in the southeast.
Under the control of tectonic movement, the fan delta system is rapidly shrunk in the Kezhileinver profile and is widely developed in the northeast.The sedimentary granularity of the strata in wells K1, Yangl and$54 is coarse。and the sediments are main- lY gritstone and fine sandstone.These characteristics imply that the source area is located in the east part (Fig. 6).
The depressed amplitude of depression in this stage is greater than that of other Paleogene stages and the shallow lacustrine distribution is wide.Shore lacustrine in the south depression is wider and the distribution of the coastal bar is broader.According to well log characteristics and sandstone granularity probability curves, a small-scale fan delta front facies are developed in wells S44 and S83.
In this period, tectonic movement is weakened to lag phase and basement subsidence is related to stress slack and snapping back.The depression is passively filled and the sedimentary sequences are controlled by the change of lacustrine level.The Suweiyi Forma- tion strata are thick in the north and thin in the south, with the maximum thickness located at Yiqikelike profile(268.4 m).Compared with the sand-shale stage in the Kumugeliemu Formation, the changes of subsidence center and strata distribution pattern embody the discrepancy of tectonic movement in different evolutionary phases.
A fan delta system iS broadly developed in this period and the strata are made up of conglomerate, silver sandstone.siltstone and interlayers of sandshale rock.The sediment of the fan delta front facies is propoftionally higher than other sediments in the Kezhileinver profile and the sediments are more de- veloped in the fan delta plain facies in the Yiqikelike and Tuzhiluoke profiles.These characteristics indi- cate the difference of source area.According to the well data.the sediments are mainl silver sandstone and sihstone in the southeast study area and the sedi- ments of the underwater distributary channel and mouth bar are rather developed.The depression gra- dient in the southwest study area iS very flat.Shal- low lacustrine facies are developed and the sediments are mainly interlayers of sihstone and mudstone.
The formation of the Kuqa foreland basin is a re- suh of lithosphere flexural isostatic adj ustment resul- ting from the thrust 10ad.The subsequent cessation of tectonic load and erosion of the mountain area 1ed to rebounded uplift during a period of tectonic quies- cence(Lin et al., 2002).The characteristics of the depositional architecture of Paleogene sequences represent the basin fills from thrust loading subsidence to rebounded uplift(Fig. 8).During the initial devel- opment of the tectonic sequences, strong orogenesis resulted in thick fan delta system deposits along the foreland margin.At the same time, rapid subsidence generated by foreland thrust load during the early de- velopment of a tectonic sequence caused the rapid in- crease in accommodation.1eading tO a general trans- gression.Subsequently, the weakening of foreland thrust and the decrease in relief reduced the develop- ment of a syn'。tectonic fan delta system along the ba-- sin margins, while deltaic systems prograded toward the central basin, forming the upper regressive SUC- cession of a second--order sequence as a result of a de-- crease in subsidence rate related to the tectonic quies- cence and rebounded uplift(Lin et al., 2002).
As a whole, Paleogene depositional systems re- present a territorial depositional cycle from trans- gression to regression. Correspondingly, sediment granularity is from coarse to fine and then coarse again, and the surface of the water body is from shallow to deep and then to shallow.Due tO the influence of the sedimentary environment。dark mudstone in the sediment is rare, however, gritstone and silver sandstone reservoirs are widely developed.
A favorable sedimentary facies zone can be con- firmed through contrasting the characteristics of sedi- mentary sand bodies deposited in different sedimenta- ry environments(Graham et al., 1993).The strata of the Paleogene sediment are tremendously thick, and there are a variety of Paleogene sedimentary sys- terns in the Kuqa depression.The compositional and structural maturity of the reservoir are lower, and the reservoir petrography properties are diverse in the different areas and sedimentary environments. The fan delta plain facies, especially debris flow, are developed in the northern study area.However, the sediment granularity is coarse, the sediments are poorly graded, and physical properties are bad.Most of the sediments have poor porosity and permeability (Kuang et al., 2003).
The fan delta front facies are developed in the southern study area, sediment granularity is fine, the sediment has good grading, and physical properties are good.As a whole, the physical properties of the reservoir are closely related to the sedimentary sys- terns.DHe to reconstruction by lacustrine water, the sand bodies of the delta front facies after resorted have high maturity and little matrix; moreover, the distribution of sand bodies iS more extensive in plane and the connectivity of sand body is better after re- construction.So, the delta front facies are the most favorable sedimentary zone in the study area, fol- lowed by the coastal bar and delta plain facies.
ACKNOWLEDGMENTS: This work was supported by the Knowledge In- novating Proj ect of the C: hinese Academy of Sciences (No.KZCX3-SW-147)and the National Key Basic Research Development Program (No. G1999043303).We are grateful to Doctor Zhang Liqin for valuable modification suggestions.| Ding, D.G., Liu, W.X., Cui, K.R., et al., 1997. Tectonic Analysis and Hydrocarbon Domainsin the Mesozoic and Cenozoic Foreland Basins of Tarim. Petroleum Geology and Experiment, 19(2): 97-107 (in Chinese with English Abstract) |
| Ding, X.Z., Guo, X.P., Peng, Y., et al., 2001. An Analysis of Cretaceous-Tertiary Sedimentary Facies and Reservoirs in Tarim Basin, Xinjiang. Acta Petrologicaet Mineralogica, 20(2): 146-155(in Chinese with English Abstract) |
| Graham, S.A., Hendrix, M.S., Wang, L.B., et al., 1993. Collision Successor Basin of Western China : Impact of Tectonic Inheritance on Sand Composition. 1. Geological Society of American Bulletin, 105: 323-344 doi: 10.1130/0016-7606(1993)105<0323:CSBOWC>2.3.CO;2 |
| Kuang, H.W., Gao, Z.Z., Zhai, Y.H., et al., 2003. Characteristics of Tertiary Reservoir in Kuqa Depression of Tarim Basin. Acta Petrolei Sinica, 24(1): 25-30 (in Chinese with English Abstract) |
| Li, W.F., Gao, Z.Z., Peng, D.T., et al., 1999. Comparative Study of FanDeltas, Braided River Deltas and Meandering River Deltas of Mesozoic Eart henin Kuche Depression, Tarim Basin. Acta Sedimentologica Sinica, 17(3): 430-434(in Chinese with English Abstract) |
| Li, W.F., Wang, C.S., Gao, Z.Z., et al., 2000. Sedimentary Evolution of Mesozoic Erain Kuche Depression, Tarim Basin. Acta Sedimentologica Sinica, 18(4): 534- 538 (in Chinese with English Abstract) |
| Lin, C.S., Liu, J.Y., Zhang, Y.M., et al., 2002. Depositional Architecture of the Tertiary Tectonic Sequences and Their Response to Foreland Tectonism in the Kuqa Depression, the Tarim Basin. Science in China (Series D), 45(3): 250-258 |
| McPherson, J.G., Shanm, U.G., Moioia, R.J., 1987. Fan-Deltas and Braid Deltas: Varieties of Coarse-Grained Deltas. Geol. Amer. Bull., 99: 331-340 doi: 10.1130/0016-7606(1987)99<331:FABDVO>2.0.CO;2 |
| Van Wagoner, J.C., Mitehum, R.M., 1990. Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops: Concepts for High-Resol ution Correlation of Time and Facies. AAPG, Methods in Exploration Series, 7: 1-55 |
| Wang, H., Wang, J.H., Yan, D.T., et al., 2004. Studies on the Key Technique of Exploration in Yaken Area of South Tianshan Mountain, Xinjiang. China University of Geosciences Press, Wuhan (in Chinese) |
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De-tian YAN, Hua WANG, Jia-hao WANG, Qing-chen WANG. Sedimentary Characteristics and Reservoir Prediction of Paleogene in the East Part of Kuqa Foreland Basin. Journal of Earth Science, 2006, 17(2): 138-145, 157.
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