Sedimentary Characteristics Of The Carbonate Shoal From Cambrian To Ordovician In The Tarim Basin

Based on the geologic survey study and analysis of outcrop profiles, well drilling/logging/core data, hundreds of outcrop and core samples slices and geochemical data, and the geological 2-D seismic interpretation of the Tarim basin, and combined the pre-study achievement, the sedimentary characteristics of carbonate shoals of Cambrian-Ordovician in the Tarim basin has been studied.According to the Sedimentology, a carbonate shoal has been defined and a standard of dividing the shoal types and a standard of grains content of the different part of the carbonate shoals has been developed. The research work has made clear that the Tarim basin has conditions to develop carbonate shoals, and that four types carbonate shoals have been developed from Cambrian- Ordovician, which are algal shoals, intraclastic shoals, oolitic shoals and bioclastic shoalsThe geological and geophysical response characteristics of carbonate shoals from microfacies to macroscopic features have been conclude based on the thin sectionscore rocks-well drilling/logging-seismic-outcrops data. Algae grains have cohesive property to form algae lumps easily, and the algae grains in dark are cemented by sparry calcite. The algae reef(mound) are interbedded with the algae shoals, and the scale and the amount of the Algae-Reef-shoal Interbed body are descending from Cambrian to Ordovician, while the algae grains diameter are increasing;The intraclastic shoals begin to develop from the early Ordovician, and the intraclastic shoals developed like a layer cake with the thin layer and thick layer two patterns; The geophysical response characteristics of the oolitic shoals are similar to that of the intraclastic shoals. In the main part of oolitic shoals, the components percentage of oolitic grains is high, with few other type grains developing, and the diameter of grains is big. While in the fringe parts of the oolitic shoals, other type grains are increasing and the oolitic grains diameter is descending; The bioclastic shoals begin to develop from Middle Ordovician, and in the main part of bioclastic shoals, the echinoderm fragments are the main components, and the grains are cemented by sparry calcite, the echinoderm fragments diameter is big.when it come to the fringe parts of the bioclastic shoals, the grain types become more and the diameter is desecending, the grains are cemented by microcalcite. All the four types shoals are deposited in periods, and each period shoal is corresponding to one high frequency cycle period, and the shoal are developed in the middle to late part of the high frequency cycle period.Four depositional stackying patterns of the carbonate shoals have been founded from Cambrian to Ordovician, including: Reef-shoal Complex, Algae-Reef-shoal Interbed, Thick-Layer Cake Aggradation shoal, and Thin-Layer Cake Retrogradation shoal. Based on the simulation experiment that schematic representation of wave breaking and refraction pattern over a shoal, the distribution characteristics of a shoal has been crushed by wave has been summarized. Therefore, the depositional facies and the distribution characteristics of the carbonate shoals have made cleared from the upper Qiulitage Group of Cambrian to the Lianglitage Formation of the Ordovician.It has been discussed and verified that the climate, the sea level change, and the geomorphologies created by tectonic movements, the platform margin structure are the important controlling factors for the carbonate shoals development from Cambrian to Ordovician. Furthermore, the main controlling factors of four stackying patterns of the carbonate shoals have been studied. Futhermore, the exploration potential of the four stackying patterns of the carbonate shoals has been discussed. According to the above study, â… to â…£ four shoal reservoir grades have been divided and their distribution areas have been delineated on the basis of multipeiods shoals superimposed from the Xiaqiulitage Group of the Upper Cambrian to the Lianglitage Formation of the Middle Ordovician.