Sequence And Sedimentary Microfacies Analysis Of Lower-middle Ordovician,Shunnan-gucheng

The paper used outcrops,cores,lamellas,logging and seismic data,at the same time,used sequence stratigraphy and sedimentology to correlate low-order sequence divisions of the Middle-Lower Ordovician in the Shunnan area of the Tarim Basin.There are 2 two-order sequence interfaces(T₈⁰ and T₇⁴)and 3 three-order sequence interfaces(T₇⁸,T₇⁶,and T₇⁵)identified.There are2 four-order sequence interfaces identified in the Yingshan Formation based on seismic contact relationships,log response characteristics,and carbon and oxygen isotope curves.One of the four-order sequence interfaces is the internal interface of the lower part of the Yingshan Formation(T₇^6-1),another one is the internal interface of the upper part of the Yingshan Formation(T₇^5-1).Based on the sequence interface that has been identified,the Middle-Lower Ordovician strata in Shunnan area was divided into four third-order sequences:SQ1(Penglaiba Formation),SQ2(lower member of Yingshan Formation),SQ3(upper member of Yingshan Formation)and SQ4(Yijianfang Formation).SQ2?SQ3 can be subdivided into 4 four-order sequences:Ysq1(the first member of Yingshan Formation),Ysq2(the second member of Yingshan Formation),Ysq3(the third member of Yingshan Formation)and Ysq4(the forth member of Yingshan Formation),to establish a high-precision sequence stratigraphic framework for the target area of the study area.Based on this,the carbonate platform structure was finely dissected,and 2 stage reflection structures were identified within the platform margins of Penglaiba Formation and Yingshan Formation in Shunnan-Guchengxu area.The external morphologies of the platform margins of the Penglaiba Formation and Yingshan Formation are mound-like,and the interiors are intermittent-blank reflections.The reflections in each period are accretionary in the longitudinal direction,and no apparent migration occurs in the platform margin.The platform shoal are mostly mound-like or lenticular-like messy reflections,and are distributed along the edge of the platform,providing constraints for the next step in sedimentary microfacies research.Based on the sequence analysis and the anatomy of the platform structure,the sedimentary microfacies of the Middle-Lower Ordovician in the Shunan area were identified and studied.Nine types of sedimentary microfacies(MF1?MF9)can be identified,and four types of sedimentary microfacies are summarized.The combinations are:thick layer high-energy intraplatform shoal micro-phase combination(MA1),thin layer high-energy intraplatform shoal micro-phase combination(MA2),middle-low energy limited intraplatform shoal micro-phase combination(MA3),and low-energy limited tidal flat microfacies combination(MA4).There is a good correspondence between the sedimentary microfacies combination within the fourth-order sequence in the study area.Through the analysis of the two kinds of seismic attributes of the RMS amplitude and average instantaneous frequency of the sequence of the Middle Or Lower Ordovician in the south of the Shunan area,the seismic facies are characterized.Each sequence surface distribution of sedimentary microfacies has been studied with the logging data and the sedimentary microfacies characteristics.SQ1?SQ2 is a restricted platform facies,developing lagoon(MF8/MF9),intraplatform shallow(MF8),and mimicking shoal(MF3)or algal shoal(MF4).SQ3?SQ4 is an open platform facies,developing inter-shoal sea(MF6?MF8),intraplatform shallow(MF8),high-energy grain shoal(MF1/MF2)and low-energy shoal(MF3?MF5).Sedimentary microfacies in the intraplatform are mainly affected by sea level changes,micropaleomorphism,platform structure,and fourth-order sequence.Finally,a model has been established for the development of sedimentary microfacies in the Shunnan area.