Study On Sedimentary Facies For M₅⁵Sub-member Of The Ordovician Majiagou Formation In The Southern Sulige Area

The Lower Paleozoic in Sulige area is rich in oil and gas resources.In recent years,many natural gas resources have been discovered in M₅⁵Sub-member of the Ordovician Majiagou Formation,which makes it more and more important in oil and gas exploration.Although much research has been made on the M₅⁵Sub-member,it is necessary to carry out more in-depth research in view of its good prospects for oil and gas resources exploration.This paper takes the M₅⁵Sub-member of the Ordovician Majiagou Formation in southern Sulige as the research object.Take petroleum geology,sedimentology,palaeontologic stratigraphy,lithofacies and paleogeography,the sedimentary facies is systematically studied based on data analysis of measured stratigraphic sections,cores,microscopic thin sections as a guid.Combined with relevant data such as the measured stratum profile,core,microscopic slice,analysis test,etc,combined with well logging techniques and methods,the formation characteristics,petrological characteristics,and sedimentary facies characteristics were systematically studied.Through research,the types of sedimentary facies are classified,the sedimentary patterns are discussed,and favorable phases of oil and gas distribution are predicted.The research are as follows:The strata in M₅⁵Sub-member in the study area are divided and contrasted.According to the formation lithology characteristics and logging data,the division marks are established.Then the M₅⁵Sub-member in the study area is further divided into two sublayers,M₅⁵¹small layer and M₅⁵²small layer.Through comprehensive comparison and analysis of the strata in the M₅⁵Sub-member,it is concluded that the stratum distribution in the M₅⁵²subsegment is relatively stable.Strata is thicker in the north and thinner in the south.The stratum thickness of the M₅⁵¹sub-section is relatively large.The central and northern regions are thicker and the southeast stratum is thinner.The study area mainly develops carbonate rocks.By classifying them and studying the petrological characteristics,they are divided into dolomite and limestone.And further divided the limestone into granular limestone,mud crystal limestone,powder crystal limestone,transition limestone.Dolomite is divided into mud crystal dolomite,mud powder crystal dolomite,powder crystal dolomite,transitional dolomite.Comprehensive analysis of information such as phase signs in the study area,it is considered that the study area belongs to the carbonate platform facies.Two sub-facies of restricted platform and open platform are developed in this area.And the sedimentary micro-facies of cloud plateau,grey plateau,cloud grey-grey cloud plateau and inner platform beach are sequentially identified.On this basis,the distribution characteristics of single-well facies and continuous-well facies in longitudinal and transverse directions are described.It is considered that the M₅⁵Sub-member of the study area is a tidal flat-limited platform-open platform deposition system.And the tidal flat facies is almost leftover due to erosion and other effects.It is a typical land surface carbonate platform deposition model.Combining the results of the above studies,it is believed that not only the depression area exists in the Yunping facies belt in the middle-eastern part of the study area,but also a large amount of magnesium ions are deposited and preserved,which is conducive to the dolomitization and a favorable sedimentary facies belt for reservoir development.