Depositional Environment And Development Controlling Factors Of Early To Middle Cambrian Marine Carbonates In Keping Area,Tarim Basin

The Cambrian marine carbonate strata are the key layers of the deep oil and gas exploration in Tarim Basin.This thesis aimed at the Lower to Middle Cambrian carbonate strata in Xiaoerbulake profile of Keping Area in northwest Tarim Basin.The target formations were analyzed through microscopic observation,major elements,trace elements,rare earth elements,total organic carbon content(TOC)and carbon and oxygen isotope data and the depositional environment and sea level change during the formation of target carbonate rocks were discussed in this thesis.Moreover,the formation correlation and negative drift of carbon isotopes in the global context were examined.Results show that the carbonates from the Lower Yurtus,Xiaoerbulake and Shayilike Formation in the Cambrian enrich redox sensitive elements such as V,Ni and Cu which deposited under anoxic environment.However,the geochemical characteristics of the Yurtus Formation and Wusonggeer Formation indicate that the top of the formations is an oxidization-oxidation environment.That is probably because the redox environment is controlled by the sea level.The Al/Ti parameters of middle and upper Yurtus Formation and Shayilike Formation were higher than those of the Xiaoerbulake and Wusonggeer Formation.The distribution curves of rare earth elements in the study layers are relative flat,characterized by low light rare earth elements and more heavy rare earth elements,indicating an open Marine sedimentary environment.A positive?Eu anomaly can be found in Yurtus and Shayilike Formation,which suggests hydrothermal activity.Two significant negative drift of?¹³C has been observed in this study“REOCE”event at the boundary of the Lower-Middle Cambrian and"CARE event"at the Lower part of Xiaoerbulake Formation.Trace elements and carbon isotopic data suggest that the negative?¹³C drift may be caused by the upwelling of anoxic water in the bottom with?¹³C deficit,probably related to the global sea level rising.The?¹³C negative drift in the study area can be compared with the"REOCE"and“CARE”in other areas of the world,which can be used as an evidence for stratigraphic correlation.The TOC and trace element data indicate that source rocks in the Yurtus and Shayilike Formation can be considered as two sets of potential source rocks.