Study On Palaeozoic Marine Sedimentary Systems In Kongquehe Area Of Tarim Basin

With the development of oil and gas exploration, some of fertile oil and gasfields are discovered in Tarim Basin in recent ten years. Kongquehe area is morethan 50,000 sq. km.in the Northeast of Tarim Basin with less great progress for oiland gas at present there, while there is a great advance for oil and gas in Luntaiand Manjiaer depression, the adjacent area, and his resource rocks are Cambrian,Carboniferous and Jurassic. These strata developed in Kongquehe area also,which makes us to study sedimentary system and sedimentary facies ofPalaeozoic in Kongquehe areas, to learn distributions of the zones sedimentaryfacies in space, searching for the direction for oil and gas exploration in thisarea.1. APPROACH TO STUDYING SEDIMENTARY SYSTEMStratigraphic division and correlation of drilling wells and field stratigraphicsections is a basic work to set up a standard of stratigraphy in the study area,comparing with the surfaces of seismic reflections. Identification and descriptionof the sedimentary facies of each main stratigraphic unit in the outcrops, welllogging and seismic frofiles is another work based on the standard of stratigraphy.Microfacies can be identified in the outcrops and well logging, and subfacies canbe done in the seismic profiles only.On the base of particular division for facies to outcrop facies and log facies ,confirm the advantage facies of corresponding layer, cooperate with seismicfacies.On the base of above , analyzing cause of formation of facies and thedrainage syetems,confirm the sedimentary system and mke map of it.The mapreflects the facies constitution,scale,distributing scope of each sedimentary system.On the base of studying ,distribution of resource rock,reservoir rock and cap rockis analyzed.Discussing the association and configuration of them. 2.TYPES OF SEDIMENTARY SYSTEM There are marine carbonate and clastic depositional modes of Palaeozoic inKongquehe area. Marine carbonate depositional mode suit studying of depositionfor Cambrian-early Ordovician,Marine Carbonate deposition is divided 4 faciesdistricts in this studying,including: 1) Trough basin facies district: mean deepwater belt of long strip crack,occur in crack of continental margin or subside ofexpansion of back arc. 2) Transition facies district: it lies between platform faciesdistrict and trough basin, is equal to open sea shelf facies, open sea shelf faciesmargin, forelope facies of platform of Wilson`s model of carbonate facies belt; 3)Platform facies district: It lies in between transition facies district and continent,the topography is smooth, the depth of water is generally in 0-30m, the deepestwater of open-platform centre district reach oxygenation level. Shallow facies onedge of platform, open platform facies, restricted platform facies and platformevaporite facies and organic reef facies compose platform facies district Marine facies clastic deposition have different sedimentary systemassociations ,because of different of structural area and the era, topography, theresource of deposition, energy of water., Deltas facies-neritic facies and neriticfacies-submarine fan sedimentary syetems are developed in Caohu sag and Kuerlenose uplift of Kongquehe slope in Silurian, deltas facies-neritic facies and neriticfacies, bathyal facies-submarine fan sedimentary syetems are developed inKongquehe slope;Delta facies-littoral facies-neritic facies sedimentary systemsare developed in Devonian; Lagoon facies -orangic reef facies(barrier island) –neritic shelf facies , delta -neritic shelf facies and tidal flat facies -neritic shelffacies sedimentary system are developed in Carboniferous. Marine carbonate and clastic sedimentary systems are developed inKongquehe area in middle-late Ordovician. There is carbonate deposition in Weilisalient . There are platform facies district and transition facies district from northto south, platform facies district includes open-platform facies and organic reeffacies; Transition facies district includes platform foreslope facies and organic reeffacies, In Caohu sag and Kuerle nose uplift develop delta facies–littoral, neriticfacies sedimentary systems. Deltas facies-neritic facies and neritic facies, bathyalfacies-submarine fan sedimentary systems are developed in the Kongquehe slope. 3. CHARACTERISTIC OF ASSOCIATIONS OF RESOURCE ROCK,OILRESERVOIRS AND CAP ROCK TO PALAEOZOIC RESOURCE ROCK Discovering through the researches of sedimentary facies and sedimentarysystem, marine facies argillite and carbonate rock containing rich organic matterare good resource rock that are in lagoon of restricted platform and bathyal faciesin Cambrian-early Ordovician,it grows hydrocarbon potentiality enormously.Theresource rock are developed in Kongquehe area, thickness of resource rockbecome thin from Southwest to Northeastern in Caohu sag and Kuerle nose uplift,the thickness is from 1250m to 250m; Resource rock become thick gradually fromthe north to south in Kongquehe slope, the thickness is from 1500m to 500m;Atthe same time the dark mudstone of the lagoon facies is good resource rock inCarboniferous, it distribute Caohu sag only , distribution of resource rock becomethick from north to south gradually, The largest thickness is close to Manjiaer sag,the maximum is 400m. A large number of delta facies and submarine fan of marine clastic depositionand a small amount of organic reef facies are developed in middle-late Ordovician,Silurian, Devonian and Carboniferous. These facies are good facies belt for thereservoir.Favorable facies belts for the reservoir are distributed in the wholeKongquehe slope in middle-late Ordovician and Silurian, Only being distributed inCaohu sag in Devonian and Carboniferous Resource rock of Jurassic is the regional cap in Kongquehe area, the maindeposition is mudstone of deeper-lake facies and dark mudstone and coal layer ofswamp facies.Its distribution trend becomes thick gradually from south to north.Its largest thickness appears in eastern area of Kongquehe slope, the largestthickness reaches 250m .