Characteristics And Forming Environments Of Tuffs In Yanchang Formation In Ordos Basin

There is remarkable radioactive abnormal phenomenon in the source rocks of Yanchang formation in Ordos. It has been found that the anomaly mainly caused by radioactive uranium element, combined with partial research achievements of the National Important Basic Research Program. The tuffs and the rich uranium hydrocarbon source rocks interbed output, with the unique characteristics of low SP, high Ga, high AC and high Resistivity value in well logging curves. People pay much attention, but deeply research few on tuffs, just regarded which as stratum division and contrast marker beds, and without thorough petrology and geochemistry research work, there are not science data and conclusions to understand the tuffs' genesis yet. And all past research work had not discussed the relationship between uranium element migration and tuffs. So exploring the formational mechanism of rich uranium hydrocarbon source rocks, explaining the relationships between tuff interlayer and uranium abnormality of Yanchang formation, it has important theory and practice significance, for discussing tuffs' genetic types and the role which plays in hydrocarbon source rocks mature process.Based on the analysis of predecessor research thoroughly, with the help of synthesis using many modern analysis and test technologies, including thin slice identification, scanning electron microscope and energy spectrum analysis, X-ray diffraction analysis, elements geochemistry analysis, Lead and Oxygen isotopes analysis and so on, the petrology and geochemistry characteristics of tuff interlayer has been particular researched and discussed its attribute, the uranium element migration characteristic in the tuff has been analyzed. Through several outfield work and careful rock cores observation, combining well log data, then investigated the position of well and layer about tuffs, and the relationship between the tuffs and hydrocarbon source rocks, the distribution rule and formation environment, as well as some scientific questions had been discussed.It is divided the traditional tuffs into three types which in Yanchang formation: ash tuffs, bentonites and hot water sedimentary rocks, the formational mechanisms of tuffs and bentonites containing fall down and water-carrying. Proved the phenomena of uranium element loss and transport in tuffs in modern time, and the tuffs maybe were the source rocks of uranium element in the rich uranium hydrocarbon source rocks. At the same time, the single well and the associated wells sections of tuffs interlayer and the Chang 7₃ bottom tuffs' plane had been done. The synthesis contrast had shown that the distribution of tuffs interlayer, the hydrocarbon source rocks and the radioactive abnormal was basic consistent in Yanchang formation. So it has the inevitable relationship. The tuffs' formational environments mainly are turbidity fan and deep lacustrine facies. The altered tuffs in turbidity fan maybe influence reservoir indexes, and tuffs in deep lacustrine facies maybe were the favorable factor on the transition from oxidation to reduction environment. Meanwhile, tuffs was distinguished maybe were from the volcano arc or within plate and calc-alkaline lavas through geochemistry data.Synthesis analysis believed that the tuff interlayer was not only the stratum division contrast horizon marker bed, but also was the important belt of organic-inorganic interaction, it had the important petroleum geology significance for source rocks layer and the reservoir, at the same time, it maybe was the important factor on improving the crude oil quality in Ordos basin.