Study On The Characteristics Of Tufftypes In The Chang 7₃ Oil-bearing Rocks,the Upper Triassic,Ordos Basin

The Upper Triassic Yanchang Formation in the Ordos Basin is a typical large-scale inland lacustrine sediments,and the volcanic activity around the basin is relatively frequent.The Chang-7₃ of Yanchang Formation formed a stratiform tuffaceous rock interbedded with high-quality hydrocarbon source rocks with strong hydrocarbon generation and hydrocarbon expulsion capacity,and was widely distributed.In this study,through the field outcrop profile study,thin-section rock microscopy,whole-rock X-ray diffraction analysis,electron probe,scanning electron microscopy,and whole rock geochemistry experimental testing techniques,the mineral composition characteristics,rock types,and source regions of Chang-7 tuff in the Yanchang Formation of the basin were studied in detail,and then we will discuss the important role of tuff material in the generation and accumulation of oil and gas.A large number of thin-bedded,lamellar tuffs are formed in the Chang-7₃ source rock formation of the Yanchang Formation in the Ordos Basin.The orderly distribution of grain size and composition reflects the fact that the tuff is a direct drop of volcanic eruption during the same period and features multiple eruptions..The tuffaceous rock types in the study area are mainly tuff,tuffite,and tuffaceous fine sandstones,collectively known as tuffs.Because they often contain black organic matter and are confused with terrigenous debris,the identification of tuffs is very important.The tuffs were identified from the three aspects of field outcrop profile,rock mineral combination and diagenesis feature.The tuff-like detrital minerals mainly include felsic,dolomite,and pyrite,among which the feldspar is mainly alkaline feldspar such as albite and potash feldspar.The discovery of ammonitic feldspar may indicate that the formation of tuff is related to deep hydrothermal fluids.The appearance of dolomite and pyrite reflects the alkaline lake reducing environment.Through the analysis of all-rock geochemical data,it can be concluded that tuffs have two sources of neutrality and medium-acidity.Based on the comprehensive analysis,tuff has important petroleum geological significance for source rocks and reservoirs,among which the major parent hydrocarbons of Chang-7₃ may be mainly thermophilic organisms such as Colloidal Phosphorus,Algae,and Spherical algae.Its degree of development is most closely related to volcanic ash materials that provide a large amount of calories and abundant nutrients.