The Structural Characteristics Research And Structural Zones Evaluation Of Ta Dong Area In Tarim Basin

The Tadong area which covers 9×10~4km~2 lies in the northeast part of the Tarim basin. Ths areai is still in a developing stage after 40 years'exploration. There were only 6 exploratory wells before 2001, and no breakthrough. However, Yingnan2 well got the industrial oil and gas flow in Jurassic in 2001, and Longkou1 well had a good sign of petroleum in Jurassic and Silurian in 2002,Yingdong2 well saw a good sign of petroleum in 2003. The three wells'breakthough in Jurassic,Silurian,andCambrian show an excellent exploration prospect in Mesozoic and Paleozoic stratum of the Tadong area . This paper is a subproject of the "Ten-Five"project "Large-scale gas field exploration and exploitation key technology research of Tarim Basin"in 2002-2003. Through the combination of petroleum geology, structural geology , seismic prospecting theory and comprehensive evaluation.This paper find out strata distribution characteristics,nature of basin, structure evolution,fault characteristics and define structural zones . It points out :dissect two zones, such as Yingnan, mandong.explore four zones,such as gucheng,mannan,Milan,Yingdong.prepare one zone ,such as luozhong. next available objects are Yingnan201, Mandong3,Gucheng2,Mannan1,Yingdong1,Milan3,Luozhong1. These results can be directly used in exploration of the Tadong area. 1. strata distribution characteristics Tadong area is a sediment zone on the metamorphosis basement of Pre--Cambrian, It means that it is a tremendous syncline on the basement and has the characteristics of composite basin, there is no stratigraphic section on the ground. The stratum from up to down were encounted by the wells as follows: Quaternary,Tertiary,Cretaceous,Jurassic,Devonian,Silurian,Ordovician and Cambrian. Cretaceous and Jurassic of Mesozoic are remained partly. They are sets of clastic rock and coal of continental facies. The upper Cretaceous,upper Jurassic and Triassic are lack. The thickest remains of Mesozoic is 3000 metres. Yingjisu-Luobupo sag is a Mesozoic-Cenozoic sag which developed on the denudation of lower Paleozoic. Permian,Carboniferous and Devonian are lack. 2. geology structure characteristics Tadong area which went though many tectogenesis evolved today's structure landform. Tadong area's structure evolution are united with Tarim Basin. Tadong area's structural evolution can be devided into four stages. (1)Presinian basement generation stage Tarim basin's basement is an old continental crust basement,it formed in Tarim tectogenesis before 800Ma. Continental crust basement are made of Archeozoic-lower proterozoic metamorphics and middle-upper proterozoic leucocratic metamorphics.Tadong2 well,Luobei1 well and Yingdong2 well were drilled Presinian basement. (2)Prejurassic residual palaeohigh generation stage After many tectogenesis,which include sinian continental margin rift basin,Cambrian-lower Ordovician passive continental margin basin, Middle Ordovician-Devonian continental margin compression sag basin and Carboniferous-Triassic craton sag basin. Because of compress from south-north stress field,Tadong area uplifted and denudated, generated deposit basement covered by Jurassic. (3) Jurassic-Cretaceous continental sag basin stage Lower Jurassic, because Tethys basin expand, Tarim plate and it's peripheral area were in structure extended phase after orogenic movement.It generated continental sag basin on Prejurassic residual palaeohigh and deposited sets of coal of petroclastic rock. (4)Cenozoic craton sag basin stage In Himalayan type orogeny,strong Himalayan type orogeny made Asia continental hinterland in large structural compression state.Plate remove,lots of structural zones revive,compress and strike-slip structural deformation were abundant, compress and strike-slip structure deformation developed between basin margin and mountain .Deformaton were very slight in the middle of basin. Because of regional tectonic stress,The Tadong area growed three key fault systems,there are northeast reverse fault systems,northwest reverse fault systems,near meridional strike-slip fault systems.Northeast reverse faults'generation are relatively early,they began to form in Caledonian-Hercynian orogeny time . Northwest reverse faults'generation are very late,they began to form in Indosinian-Yanshanian orogeny time. Tadong have two boundary faults which have very important meaning,that is Kongquehe Fault and Cherchen Fault.There are seven key Faults which control structural zones ,namely Yingnan2 Fault,Longkou Fault,Yingdong Fault,Alagan Fault,Weimake Fault,Yingbei Fault,Yingnan 1 strike-slip Fault. There are four key tectonic styles in the Tadong area:strike-slip structure,back thrusting structure,drape structure,fault-fold,flower structure and so on.Because this region went through many times and different natural tectonic orogeny,Trap type are parti-colored and complex in Tadong area.There are anticline,fault-anticline,Structural nose,buried hill,stratigraphic onlap trap. 3. basic peotroleum geology condition Drill information prove: There are six groups of reservoir-seal assemblage from upper to lower in Tadong area.There are four groups of reservoir-seal assemblage in Mesozoic,that is greywacke and red mudstone in Jurassic, the upper coal and sandstone segment below it in Jurassic, the lower coal and sandstone segment below it in Jurassic, thick fine sandstone segment and silty sandstone on it in Jurassic, which is not an normal group of reservoir-seal assemblage.There are two groups of reservoir-seal assemblage in Paleozoic,that is silty sandstone segment and fine sandstone segment below it in Silurian,which is not an normal group of reservoir-seal assemblage either. dolomite in Sinian with limestone and dolomite in Cambrian and mudstone on it in Ordovician. It is proved by exploration for years. There are two sets of hydrocarbon source rock, they are Jurassic source rock of Mesozoic, source rock of Cambrian—lower Ordovician. There are quite distinguishable in distribution and old generative feature. Generally speaking, the maturity of Jurassic source bed is low, most of these are premature to low mature, so they have less contributor to the Tadong area. However, Cambrian—lower Ordovician hydrocarbon source rock has high organic matter abundance,the type is better than Jurassic source rock and has a large distribution and it is overmature. It is the main contributor to the oil and gas reservoir discovered in the Tadongarea. 4. zone partition and evaluation The Tadong area is a typical composite congruent basin.It can be divided into two structural layers,which are paleozoic and Mesozoic-cenozoic structural layers.There are big difference between the two structural layers in tectonic framework,sedimentary formation, oil and gas reservoir conditions.So we district structural zones and choose targets according to the two structural layers. The Tadong area went through tension-convergence cycle.It generated clinoform in north, salient in south and a sag in middle,which get shallow from west to east. The Tadong area Paleozoic can be divided into three second-order structural units(one clinoform,one salient,one sag).There are Kongquehe clinoform,Tadong low salient,Manjiaer sag.we divide it into eleven structural zones,which are Korla,Qunke,Daxihai,Mannan,Gucheng, Tadong,Yingdong, Milan,Luonan, Luozhong, Mandong zones. Tadong area Mesozoic-Cenozoic can be divided into one structural nose,two clinoform and three sag,which are Kuerle structural nose, Kongquehe clinoform,Tadong clinoform, Manjiaer sag,Yingjisu sag,Luobupo sag. we divide it into nine structure zones,which are Qunke,Daxihai,Longkou,Yingnan,Yingbei,Weima-Kaiping,Yingdong, Luonan, Milan zones. After evaluation,Yingnan and Mandong zones have been found big condensate pools at present.Gucheng,Yingdong,Mannan zones'regional structural location is very good,it is not only gas migration's long-term directional area,but also fossil oil-gas pool that was not be cracked and key layers'reservoir are relative good,soever petroclastic and carbonate rock.so forenamed five zones are the first-class zones. Tadong,Longkou and Weima-Kaiping zones were all drilled wells,which got low oil-gas flow. Luonan,Luozhong and Milan zones'exploration are very low,but reservoir and generating oil-gas pool's conditions are similar toYingdong zone. so forenamed six zones are the second-class zones. Kuerle, Qunke, Daxihai, Yingbei zones'exploration are very low,we didn't find any oil-gas sign in adjacent wells,such as Kunan1,Kunan2,Qunke1 and Anan1 wells. reservoir facies tract is weak or oil-gas resource conditions is poor. so forenamed four zonesare third-class zones. 5.The direction of oil and gas exploration (1) Three direction of oil exploration a.Silurian stratigraphic onlap trap zone in south clinoform of Manjiaer sag. Those traps generated in upper Caledonides,it is advantageous to catch oil and gas of Cambrian-lower Ordovician generating rock's generating and bleeding.Upper conservancy is well,because its' buried measurement is shallow, it is not cracking.We found good oil-gas sign inTazhong32 well. Big stratigraphic onlap trap are existing in this zone, buried measurement is shallow.According to material on hand, it may have big danger of exploration.getting oil-gas at once,.petroleum exploration sence is very pretty. b. Caledonides palaeohigh region around Manjiaer sag,such as Yingdong,Gucheng,Milan zones.We explore Cambrian/Sinian carbonate rock primary oil-gas pool in this direction,it is similar to Yingdong2 well Cambrian super thick dolomite reserior.Trap formation time were consistent with generating rock's generating and bleeding,oil and gas pool generated early, upper conservancy is well,it is an another important petroleum exploration region in the Tadong area. Gucheng2 well were to explore this world at present,but it doesn't in advantageous facies tract, reservoir is dangerous. c. generating rock of Jurassic coal-bearing series in Yingjisu and Luobupo sag. Jurassic coal-bearing series generating rock's mature degree are lower,but mature region's area is 2000Km2 in deep sag, petroleum total sources are 50000-100000 thousand tons.So we should explore Jurassic authigenic coal type pool in Yingdong and Luobei furrow. (2)Two directions of condensate pool exploration a.Anticline trap in Manjiaer,such as Mandong and Mancan zones.There are deep-big faults near these traps,which can link fossil pool cracking gas sources. Silurian super thick silty sandstone's reserior is good in crack growth region,which may form unnormal gas pool,such as Mandong1well.so we should maintain prospect for condensate pool in Manjiaer sag. b.drape structure in Mesozoic,for example Yingnan2 etal.according to Yingnan2 well condensate pool's generating type,it have "exchange"under it(upper-Ordovician or Cambrian-Sinian fossil reserior).Then advantageous Mesozoic drape structure must have two...