Sedimentary Facies And Character Of Palaeokarst Cust Reservoir In Majiagou Formation In Ordovician, Tabamiao Area, Ordos

The situation of petroleum resources is austere in our country,as the develop of technic and economy, marine stratum is explored necessarily,which is regarded as the second chance.It is essential to study the carbonate rock of Lower Paleozoic Eratherm in Ordos basin,because of the example of jingbian gasfield. The evolvement history of Ordos basin indicates that one of the best reservoir is the palaeokarst weathering cust which is formed during Caledonian movement.It is important to study the palaeokarst weathering cust systemically for obtaining the character hydrocarbon reservoir,which usually include sedimentary environment and karst mode and karstic palaeogeomorphology as well as diagenesis.The reservoir of gas pool in Lower Paleozoic Eratherm is the palaeokarst weathering cust in Ordovician,which is controlled by sedimentary facies and palaeogeomorphology ,as well as diagenesis.Based on the analysis of regional geologic information and core & slice,as well as well logging,and theory of Sedimentology & Karstology & Geomorphology and Sequence stratigraphy,using impression method and Sequence stratigraphy method,the auther semiquantitatively reconstruct the types of karstic palaeogeomorphology according to thickness of overlying normal stratum.At the same time,the auther also analyse character of sedimentary facies and type of diagenesis,then forecast the best area. Synthetically study achievements have been made as follows:1)The tidal-flat facies exits in Majiagou formation ,which is divided into 6 microfacies such as supratidal dolostone and intertidal dolostone and subtidal limestone, and forming one sedimentary rock association which is made up of limestone and dolostone and muddy dolostone as well as gypum dolostone.The the auther founds depositional model of droughty climate.2)Karst section is divided into vertical vadose zone and level runoff zone according to the phreatic surface.the former ,whose dual laterolog curve hoist slowly,is on top of the phreatic surface.while the latter is below it and its dual laterolog curve hoist then fall,as"U"or"弓".3)The 5 techniques,such as impression method & Sequence stratigraphy method & sedimentary method & geophysics method and residual thichness method,have advantage and disadvantage,which is used to reconstruct palaeogeomorphology. Integrated use is good to actual modality. The auther semiquantitatively reconstruct the types of karstic palaeogeomorphology using impression method and Sequence stratigraphy method.4)Karstic palaeogeomorphology intabamiao area is divided into 3 types of geomorphology that are karstic mesa & karstic ramp and karstic basin ,which are further divided into 5 types such as mesa & sloping field & wash and lowland & karsti hill. Karstic mesa distributes in the North of area and karstic basin maily in the southeast, then karstic ramp is between them.5)Factors, affect palaeokarst,are climate & geomorphology & hydrodynamic force and sea level as well as sedimentary facies & tictonism and nature of rock that include materiel & structure and physical mechanic property.6)The palaeokarst reservoir in tabamiao area,whose reservoir space is made up of intercrystal porosity &dissolved pore and dissolved fracture &slit,is controlled by the type of karstic palaeogeomorphology and sendimentary facies,accompanied with diagenesis, and its heterogeneity is obvious.7)Dolomitization and corrosion,as well as degypsification and recrystallization are favorable to reservoirs of this area;the other way round ,compaction-pressolution and Dedolomitization are bad.Pyritization hardly make a difference. 8)There are two best area based on the analyses of Sedimentary facies and Diagenesis and Karstic palaeogeomorphology,which are D43-D12 and D13-D49 well field.