Research On The Sedimentary Microfacies Characteristics And Evolution Of O₁m₅¹⁰-O₁m₅⁶ In Mid-Eastern Parts Of Ordos Basin

In the study area,a set of dolomite formation was developed in the Ordovician Majiagou formation under the salt-gypsum,the ground be buried at more than 3000 m,and the thickness of up to 500 m,there are widely distributed,although its rock porosity is usually low,but it has good permeability and good reservoir physical property,at present,there has been a gathering of natural gas for commercial extraction.In the research of carbonate sedimentary microfacies,the research of sedimentary facies in the Middle East of Ordos basin is mainly based on the sedimentary facies pattern of international carbonate rocks,failure to establish a perfect phase signs for the status of the research area,the subtle characterization of sedimentary microfacies is low,the study of sedimentary microfacies on the plane is relatively weak,and The sedimentary microfacies pattern is not yet constructed.The five sections of the Majiagou Formation are the main gas-bearing system in the eastern part of the Ordos basin,this study guides the theories of carbonate petrology,sequence stratigraphy,sedimentology and natural gas geology,a series of geological studies have been carried out in the under the salt-gypsum area of the Middle East with the comprehensive characteristics of the regional geological background,field profile geological survey and the key wells.In order to understand the sedimentary microfacies type,planar spread pattern and the sedimentary model of various subsections in the study area,this study has collected date and seismic date from 30 key well sites in the research area,and two field profiles are measured and described in the east of the study area.This study is based on core logging data and material information,the MaWu5 subsection of mud crystalline limestone,which is relatively in thickness,is the top proofreading level,and the light gray thickness stable thick layer of limestone in O1m4 as the botoom,to divide and contrast the wells in the study area.The layers of the MaWu may be subdivided into O1m56-O1m510 five layers,and two comparative profiles were established in the research area.The thickness of the O1m56 in the study area varies greatly,from west to east,the thickness of the strata gradually increased,but the O1m510-O1m57 are generally more stable.The study found that the color of sedimentary rocks in the study area was mainly of light grey,less of dark grey an gray.The rock types mainly include carbonate(dolomite and limestone),evaporate(gypsum rock and salt rock)and other transitional lithology.The lithology of the O1m510 in the study area was composed mainly of dolomite,gypsum dolomite and bioclastic dolomite,the lithology of the O1m59 was composed mainly of dolomite and limy dolomite,the lithology of the O1m58 was composed mainly of dolomite and gypsum dolomite,the lithology of the O1m57 was composed mainly of dolomite,limy dolomite and limestone,the lithology of the O1m56 was composed mainly of dolomite,salt rock,dolomite gypsum,gypsum dolomite and limy dolomite.Through the actual measurement of the field profile,the observation of the key well,microscopy and analysis of laboratory data,and referring to the standard classification of carbonate microfacies and rock combination,which is proposed by Wilson.Identify themain development of two types of sedimentary facies,four types of sub-facies and twelve types of microfacies in the study area.Which is mainly composed of tidal flat and lagoon facies.The tidal plat deposition system includes 3 microfacies,such as supralittoral zone,intertidal zone and subtidal zone.The lagoon facise inculded salty lagoon subfacies,microfacies are mainly mudly dolomitic flat,dolomitic flat,gypsum dolomitic flat,dolomitic gypsum flat,calcareous dolomitic flat,grain beach flat,mudly calcareous flat,calcareous flat and gypsum salt depression.Through the study of sedimentary microfacies evolution shows that the study area O1m56,O1m58,O1m510 sub-section from the basin edge to the basin center development dolomitic flat-containing gypsum dolomitic flat-gypsum dolomitic flat-gypsum salt depression.And the lithology is formed by the transition from dolomite to salt rock,the content of the gypsum gradually increased.the O1m57,O1m59 from edge to center of basin are developed in dolomitic flat – containing lime dolomitic flat-calcareous dolomitic flat-calcareous flat,Local development grain beach facies and biology bank facies.And the lithology is changed from dolomite to limestone,and the grey matter increased gradually.The results show that,O1m5 period as the one of the largest depositional periods of deposition thickness,in general,it exhibits the sedimentary characteristics of an oscillation regressive,and the sediments are shallow,further development of short-term transgression and regression stratification,where O1m56,O1m58,O1m510 were regression stage stratification,and the O1m57,O1m59 were the transgression of stratification.Therefore,the oscillation regressive determin the longitudinal changes of sedimentary facies,at the same time,because of the barrier of the ancent uplift in the basin,also caused the differences in sedimentary microfacies between different layers with a half cycle of internal deposition.