Combination Patterns and Depositional Characteristics of Ordovician Carbonate Banks in the Western Tarim Basin,China

The combination patterns and depositional characteristics of the carbonate banks are investigated based on outcrop sections,thin sections,and carbon isotopes of Ordovician in the western Tarim Basin,China.Four carbonate bank combination patterns are deposited in the Ordovician, western Tarim Basin,including:Reef-Bank Complex(RBC),Algae-Reef-Bank Interbed(ARBI), Thick-Layer Cake Aggradation Bank(TLCAB),and Thin-Layer Cake Retrogradation Bank (TLCRB).All combination patterns show clear periods vertically.The RBC is mainly composed of reefs and bioclastic banks,and the dimension of the RBC depends on the scale of the reefs.Bioclastic banks deposits surround the reefs.The range of the ARBI is determined by the scale of algae-reefs, algae peloid dolomite microfacies and algal dolomite microfacies deposit alternating vertically. TLCAB and TLCRB are deposited as layer-cakes stacking in cycles and extending widely with cross bedding developed.The grains of TLCAB and TLCRB are diverse and multi-source.With the impacting of relative sea level change,biological development and geomorphology,the ARBI,TLCAB or TLCRB,RBC are successively developed from the Lower Ordovician Penglaiba Formation to the Middle Ordovician Yijianfang Formation.The depositional environment analysis of Ordovician indicates that the RBC and ARBI can form effective oil and gas reservoirs,and the TLCAB and TLCRB have the potential to form the huge scale oil and gas reservoirs and to be the crucial targets of exploration for the Ordovician carbonate banks in the future.