The study presents the results of U-Pb dating of zircons and whole-rock geochemical analyses of a syenogranite located in the western Niubiziliang area, China with the aim of determining its formation time, petrogenesis and the regional tectonic setting. Zircons within the syenogranite are euhedral-subhedral and display rhythmic growth zoning, indicating a magmatic origin. Zircon U-Pb data obtained by LA-ICP-MS indicate the syenogranite formed in the Late Permian (260.7±1.5 Ma). The w(SiO2) of syenogranites is 70.82%-73.59%, w(Al2O3) is 13.49%-14.82%, and w(Na2O+K2O) is 7.85%-8.52%, and yield K2O/Na2O ratios of 1.06-1.26. Therefore, the syenogranites belong to the high-K calc-alkaline and metaluminous (A/CNK<1, A/NK>1) series which display I-type granites similarly. The syenogranites also show the geochemical characteristics of volcanic arc rocks, being enriched in large-ion lithophile elements (LILEs; K, Rb) and light rare-earth elements (LREEs; La, Ce, Sm, Nd), but depleted in high field strength elements (HFSEs; Ta, Nb, P, Ti). On the whole, trace element ratios are close to the mean of the Earth's crust, indicating a lower-crust magma source. The low Sr (161-214)×10-6 and Yb (1.08-1.80)×10-6 concentrations indicate that plagioclase and hornblende are residual mineral phases in the source. The regional geology and whole-rock geochemistry suggest that the formation of the syenogranites was related to subduction of the Zongwulong Ocean crust, and the north margin of Qaidam Block during the Late Permian was in an active continental margin tectonic setting.
The southeast depression of Qinnan Sag is a potential oil-gas exploration region in the Bohai Sea area. With the analysis of large quantity of rock thin sections, scanning electron microscope and the physical property data of reservoir, the authors studied the petrological characteristics and the evolution of pore and fluid of sandstone in the deeper strata in 29-2 structure in Qinhuangdao area. The results show that the evolutionary tendency of Paleogene sandstone reservoir porous fluid in research area is changed from alkaline porous fluid to acidic porous fluid, and back to alkaline porous fluid. There are three stages of reservoir porous evolution in Qinhuangdao area, namely sharp decrease in porosity due to mechanical compaction, increase in porosity because of corrosion and dissolution, and remarkable reduction owing to carbonate cementation.
Gravity data observed in geophysical surveys are the sum of potential field produced by all underground inhomogeneous geological sources with different scales, different patterns and different depths. The aim of gravity exploration is mainly to extract the meaningful local anomaly from the regional gravity anomaly which is helpful to detect the deep structure and delineate the favorable area for mineral prospecting. In this paper, several anomaly separation methods are theoretically discussed, including trend analysis method, finite element method, interpolating cut method and analytic continuation method. And, these methods are applied to achieve the delineation of potash-mine deposit in Vientiane. Through experiments, analysis and comparison, the separating result of upward continuation is shown to be satisfactory, for that can separate the ore body anomaly from the background field more effectively. Further, with the combination of the geological information and gravity anomaly, it infers four rich potash zones in the study area of Vientiane which is significant for future geological work.
Multiple prediction and subtraction techniques based on wavefield extrapolation are effective for suppressing multiple related to water layers. In the conventional wavefield extrapolation method, the multiples of the seismic data are predicted from the known total wave field by the Green function convoluted with each point of the bottom. However, only the energy near the stationary phase point has an effect on the summation result when the convolutional gathers are added. The research proposed a stationary phase point extraction method based on high-resolution radon transform. In the radon domain, the energy near the stationary phase point is directly added along the convolutional gathers curve, which is a valid solution to the problem of the unstable phase of the events of multiple. The Curvelet matching subtraction technique is used to remove the multiple, which improved the accuracy of the multiple predicted by the wavefield extrapolation and the artifacts appearing around the events of multiple are well eliminated. The validity and feasibility of the proposed method are verified by the theoretical and practical data example.
"bright spot" on reflection seismic profile has been regarded as one of the important indications of oil and gas, and it has been widely used in the oil and gas exploration. Moreover, with the development of new technologies, "bright spot" technique has been gradually applied to the exploration of coal, underground water and other fields, and also in the research of deep structure of the earth. Based on the reviewing of published literatures, the authors described the structural features and generation of "bright spot" on seismic profile for energy resources exploration and crustal-scale deep seismic reflection profiles, and further summarized the applications of the "bright spot" technique in energy resource exploration and deep structure research.
New Vp and Vs models of the aftershock gap between the 2013 Lushan and 2008 Wenchuan earthquakes were obtained using 7190 events recorded by 102 stations in the southeastern margin of the Tibetan Plateau. The new velocity models and event locations were inverted by the regional-scale version of double-difference tomography algorithm. The new velocity structure shows that the aftershock gap is an obviously low-velocity zone in the upper crust, whereas the aftershock regions of the Lushan and Wenchuan earthquakes show high-velocity anomalies. The low-velocity anomaly of the aftershock gap in the upper crust may indicate that the materials of the gap are weak and ductile, which is not prone to accumulate stress and cause big earthquakes. As a result, it is less likely that the big earthquakes will occur in the gap in the future.
Model performance assessment is a key procedure for mineral potential mapping, but the corresponding research achievements are seldom reported in literature. Cumulative gain and lift charts are well known in the data mining community specialized in marketing and sales applications and widely used in customer churn prediction for model performance assessment. In this paper, they are introduced into the field of mineral potential mapping for model performance assessment. These two charts can be viewed as a graphic representation of the advantage of using a predictive model to choose mineral targets. A cumulative gain curve can represent how much a predictive model is superior to a random guess in mineral target prediction. A lift chart can express how much more likely the mineral targets predicted by a model are deposit-bearing ones than those by a random selection. As an illustration, the cumulative gain and lift charts are applied to measure the performance of weights of evidence, logistic regression, restricted Boltzmann machine, and multilayer perceptron in mineral potential mapping in the Altay district in northern Xinjiang in China. The results show that the cumulative gain and lift charts can visually reveal that the first three models perform well while the last one performs poorly. Thus, the cumulative gain and lift charts can serve as a graphic tool for model performance assessment in mineral potential mapping.