Depositional Environment And Controlling Factors Of Cambrian-Ordovician Marine Carbonates In The Tarim Basin

This dissertation comes from the"Thirteenth Five-Year Plan"national major special sub-topic"Research on Geochemical Characteristics of Ordovician-Cambrian Effective Source Rocks and Comparison of Oil and Gas Sources in the Tarim Basin".The development and distribution of high-quality hydrocarbon source rocks in the Lower Paleozoic marine carbonate rocks have always been the key and difficult issues that restrict oil and gas exploration in the Tarim Basin.This paper,studying the formation environment of marine carbonate rocks in the Lower Paleozoic,reveals the formation environment and controlling factors of the Cambrian-ocean carbonate rock source rocks in order to provide a theoretical basis for finding hydrocar-bon sources with high organic abundance in the Tarim Basin.From the Early Cambrian to the Middle Cambrian in Tadong Area,the water environment changed from oxidation to reduction.During the deposition period of the Upper Cambrian Turshaktag Formation,the redox environment of the water was turbulent and variational.The Early Ordovician Heituao formation deposited in a relatively reduced environment,while the seawater was relatively oxygen-rich during the sedimentation period of the Middle-Late Ordo-vician.Because the Tazhong area deposited in the platform facies and the water wasshallow,the bottom water was a partial oxidation environment in the of the Ordovician ocean in Tazhong area.The nutrient element parameters of the Cambrian-Ordovician rocks reveal that the primary productivity of the Early Cambrian was relatively low,and the content of nutrients increased between the Middle-upper Cambrian and upper Ordovician.It maight be that upwelling carried a large amount of nutrients that caused aquatic organisms to grow.However,in the Middle-late Ordovician,the content of marine nutrient elements was significantly reduced,and the corre-sponding marine primary productivity also decreased.Carbon isotope indicates that the?¹³C values with global geochemical events are nega-tively drifted at the boundary of the Lower-Middle Cambrian in the core section of the Xiaoer-bulak section and the Tadong 2 well.However,this negative?¹³C drift might be caused by an upwelling of the anoxic water.The deep water was depleted of oxygen but rich in ¹²C,which led to the deposition of carbonate rocks with lower?¹³C values,causing negative?¹³C drift.During the deposition period of the Upper Cambrian-upper Ordovician in Tadong area,the water was anoxic,while the upwelling carried the nutrients on the sea floor to the coastal zone or the ocean surface,which promoted the biological bloom in the surface water.During the deposition of the Lower Cambrian,the large amount of siliceous rocks caused by the seabed hydrothermal activity had a great dilution effect on the organic matter enrichment,resulting in a lower TOC value for the lower part of the Yuertuosi siliceous rock.The paleo-productivity and reduced environment are the controlling factors for the enrichment of organic matter in the upper-middle Yuertuosi formation,so the upper-middle rock is a good source rock formation.