The large scale development of high natural gamma sandstones has been discovered in the Chang 73 Submember of the Triassic Yanchang Formation in the Ordos Basin, China. High natural gamma sandstones consist of terrige...The large scale development of high natural gamma sandstones has been discovered in the Chang 73 Submember of the Triassic Yanchang Formation in the Ordos Basin, China. High natural gamma sandstones consist of terrigenous detrital rocks with particle sizes ranging from silt to sand. They represent turbidite deposits characterized by high gamma ray values that are more than 180 American Petroleum Institute (API) units on a natural gamma ray log profile. For a long time, very high natural gamma sandstones had been identified as high-quality source rocks, such as oil shales, from conventional well log profiles, such as natural gamma ray well logs. Therefore, predicting the distribution of high natural gamma sandstones was studied. The sedimentary, lithological, and well log characteristics, as well as the genesis of the high radioactivity of high natural gamma sandstones were analyzed in the Chang 73 Submember. Thorium (Th), uranium (U) and other radioactive elements were found, carried by deep hydrothermal activity, and probably resulted in the formation of a relatively high radioactive zone in the cross-section, where high natural gamma sandstones usually develop in large quantities. This caused many turbidite sand bodies, which should have a continuous distribution in the cross-section, to appear to have a discontinuous distribution, when using conventional well log profiles, such as natural gamma ray well logs. From the above mentioned apparent discontinuous distribution of turbidite sand bodies in the cross-section, a continuous distribution can be predicted. It is obvious that the prediction of areas of continuous turbidite sand bodies in the cross-section usually corresponds with areas where high natural gamma sandstones are developed in large quantities. Exploration and development practice demonstrated that the developed method is fast and effective in predicting high natural gamma sandstones in the Chang 73 Submember.展开更多
This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye,such as clay-sized black mudstones and clay-sized oil shales,which are rich in black organic matter(...This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye,such as clay-sized black mudstones and clay-sized oil shales,which are rich in black organic matter(including oil and asphaltene),in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks(including tuff,sedimentary tuff and tuffaceous sedimentary rock)with high hydrocarbon generation capacities.Thus,these rocks can be defined as clay-sized tuffaceous source rocks.Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member.Through the macroscopic observation of drill cores and outcrop profiles,microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis(including major,trace and rare earth elements),this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks,especially clay-sized tuffs,have the following characteristics.First,the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white,yellowish brown and purplish grey,and mixed colors occur in areas with strong bentonite lithification.Second,the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization,forming abundant flaky aluminosilicate minerals with directional arrangement.In thin sections under a polarizing microscope,the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green.Third,the rock samples plot in the igneous rock field in the TiO2-SiO2 cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams,indicating that the samples are derived from the same source.The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin,China.展开更多
基金Project(18GK28)supported by the Doctoral Scientific Research Starting Foundation for the Yulin University,ChinaProject(20106101110020)supported by the University Research Fund of Science and Technology Development Center of Ministry of Education,ChinaProject(BJ08133-3)supported by the Key Fund Project of Continental Dynamics National Key Laboratory of Northwest University,China
文摘The large scale development of high natural gamma sandstones has been discovered in the Chang 73 Submember of the Triassic Yanchang Formation in the Ordos Basin, China. High natural gamma sandstones consist of terrigenous detrital rocks with particle sizes ranging from silt to sand. They represent turbidite deposits characterized by high gamma ray values that are more than 180 American Petroleum Institute (API) units on a natural gamma ray log profile. For a long time, very high natural gamma sandstones had been identified as high-quality source rocks, such as oil shales, from conventional well log profiles, such as natural gamma ray well logs. Therefore, predicting the distribution of high natural gamma sandstones was studied. The sedimentary, lithological, and well log characteristics, as well as the genesis of the high radioactivity of high natural gamma sandstones were analyzed in the Chang 73 Submember. Thorium (Th), uranium (U) and other radioactive elements were found, carried by deep hydrothermal activity, and probably resulted in the formation of a relatively high radioactive zone in the cross-section, where high natural gamma sandstones usually develop in large quantities. This caused many turbidite sand bodies, which should have a continuous distribution in the cross-section, to appear to have a discontinuous distribution, when using conventional well log profiles, such as natural gamma ray well logs. From the above mentioned apparent discontinuous distribution of turbidite sand bodies in the cross-section, a continuous distribution can be predicted. It is obvious that the prediction of areas of continuous turbidite sand bodies in the cross-section usually corresponds with areas where high natural gamma sandstones are developed in large quantities. Exploration and development practice demonstrated that the developed method is fast and effective in predicting high natural gamma sandstones in the Chang 73 Submember.
基金Project(18GK28)supported by the Doctoral Scientific Research Staring Foundation for Yulin University,ChinaProject(20106101110020)supported by the University Research Fund of Science and Technology Development Center of Ministry of Education,ChinaProject(BJ08133-3)supported by the Key Fund Project of Continental Dynamics National Key Laboratory of Northwest University,China。
文摘This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye,such as clay-sized black mudstones and clay-sized oil shales,which are rich in black organic matter(including oil and asphaltene),in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks(including tuff,sedimentary tuff and tuffaceous sedimentary rock)with high hydrocarbon generation capacities.Thus,these rocks can be defined as clay-sized tuffaceous source rocks.Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member.Through the macroscopic observation of drill cores and outcrop profiles,microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis(including major,trace and rare earth elements),this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks,especially clay-sized tuffs,have the following characteristics.First,the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white,yellowish brown and purplish grey,and mixed colors occur in areas with strong bentonite lithification.Second,the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization,forming abundant flaky aluminosilicate minerals with directional arrangement.In thin sections under a polarizing microscope,the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green.Third,the rock samples plot in the igneous rock field in the TiO2-SiO2 cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams,indicating that the samples are derived from the same source.The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin,China.
