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Global Geology
2020 Vol.23 Issue.1
Published 2020-02-25
Articles
Articles
1
Geochronology, geochemistry and geological significance ofEarly Cretaceous volcanic rocks from Niangniangshan Formation,Ningwu Basin, middle and lower reaches of the Yangtze River
PING Zhilan, WANG Li, ZHANG Yajing, XIN Wei, LI Liang, QIAN Lingyu
Ningwu Basin is one of the Mesozoic continental volcanic basins in the middle and lower reaches of the Yangtze River. The volcanic rocks of the Longwangshan, dawangshan, Gushan and Niangniangshan formations, as well as the homologous subvolcanic rocks or small intrusions, are developed from old to new in the Ningwu Basin. Zircon U-Pb dating results show the latialite phonolite of Niangniangshan Formation was erupted at 128±1 Ma (i.e., Early Cretaceous). The latialite phonolite contains moderate SiO
2
contents (57.28%-60.96%) with high Na
2
O + K
2
O contents, belonging to shoshonite series. The samples have high REE contents, and display right-inclined REE distribution pattern. They are characterized by enrichment in some large ion lithophile elements (e.g., LILEs, Rb, K), and depletion in some high field strength elements (e.g., HFSEs, Nb, Ta, Ti). All volcanic samples have relatively depleted Nd isotopic compositions (I
Sr =
0.707 197—-0.707 878; ε
Nd
(t)=-0.5—-0.9), indicating no genetic relationship with the lower crust of Yangtze plate, but a drift trend towards the EMII. The geochemical data suggest that the Early Cretaceous latialite phonolite was derived from the partial melting of an enriched lithospheric mantle metasomatized by subduction-related fluids in an arc-related setting. Based on the temporal and spatial distribution and geochemical variation characteristics of the regional volcanic rocks, it is suggested that the tectonic system within the study area changed from a subduction-related compression to an extensional environment in the early Early Cretaceous, which was caused by the ridge subduction of the Paleo-Pacific Ocean.
2020 Vol. 23 (1): 1-15 [
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234
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16
Extraction method of component waves in full waveform acoustic data and its application
ZHOU Haoyi, MO Xiuwen
In conventional slowness-time coherence (STC) method, slowness and time need to be searched at the same time, which limits the precision and lowers the efficiency. The dichotomy method combined with slowness-time coherence algorithm aims to enhance the efficiency of data processing and to improve the precision. The algorithm changes the searching pattern of conventional slowness-time coherence method to acquire the slowness of component waves in array acoustic logging data. Based on energy ratio of short time window versus long time window and slowness-time coherence method, the algorithm first acquires the arrivals of the component waves using energy ratio of short time window versus long time window method. It then uses the calculated results as the arrivals in conventional slowness-time coherence method, so the slowness-time two-dimensional searching process is simplified to slowness searching process. Based on dichotomy method, the searching pattern is further optimized in replace of the ergodic searching pattern in conventional slowness-time coherence method, which means that as the iteration proceeds, the current searching interval is reduced to half of the former, so the number of searching times is decreased. The dichotomy method combined with slowness-time coherence algorithm is applied to well L in the data processing. Compared with conventional slowness-time coherence method, for compressional wave, the searching efficiency of the algorithm is 4.53 times better, while for Stoneley wave, the searching efficiency is 1.85 times better. Compared with conventional logging data, the average absolute error of the results of the dichotomy method combined with slowness-time coherence algorithm is 1.14 μs/ft smaller than that of the conventional method, while the average relative error is 1.2 percent lower. The dichotomy method combined with slowness-time coherence algorithm shows good results in its application, which can enhance the processing efficiency remarkably while getting reliable results at the same time.
2020 Vol. 23 (1): 16-23 [
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191
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595KB] (
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24
Computation of waves scattered by inhomogeneousvelocity gradient using generalized Born approximation
LIU Bo, SUN Jianguo, MENG Xiangyu
The generalized Born approximation is an approximation method that represents the scattering term by the error between the exact Green's function and the approximate Green's function, mainly for the gradient scattering problem. However, so far, the research on the generalized Born approximation has only stayed in theory, and its implementation techniques are rarely reported. In order to fill this gap, the basic theory of generalized Born approximation is reviewed, and the implementation method of generalized Born approximation is discussed in this paper. In particular, the problem of calculating boundary effect elimination is discussed in detail. Finally, through model trial calculation, the calculation of gradient scattering, by comparing Born approximation and finite difference method, shows that using the generalized Born approximation to calculate gradient scattering achieves higher computational accuracy.
2020 Vol. 23 (1): 24-31 [
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188
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32
Monte Carlo simulation for pulsed neutron-neutron loggingin fractured and cavernous carbonate formation
LIU Lipan, MO Xiuwen, ZHANG Youshuo, ZHANG Weifeng
The influence of carbonate formation on Pulsed Neutron logging (such as Pulsed Neutron-Neutron logging) is quite different from that of sandstone due to the complexity of reservoir architecture and the development of fracture in carbonate reservoir. To study the factors affecting Pulsed Neutron-Neutron (PNN) logging in carbonate formation, the responses on fracture or cave are simulated by Monte Carlo method, getting the relationships among the macroscopic capture cross section (Σ), the count ratio of the thermal neutron at far spacing and near spacing detectors (RATIO), the fracture porosity, oil-bearing and shale content of fracture. The results show that PNN logging can be used to detect caves, and there exist linear relationships among the macroscopic capture cross section (Σ), the count ratio (RATIO) and the above factors. The research findings in this paper provides theoretical basis for the interpretation and data correction of the PNN logging in carbonate reservoirs.
2020 Vol. 23 (1): 32-37 [
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225
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38
Lacustrine high resolution sequence records from UpperCretaceous Nenjiang Formation in Songliao Basin
GAO Youfeng, JIAO Yuhui, WANG Pujun, CHENG Rihui, WANG Guodong
The Nenjiang Formation was continuously cored in the SK-1 borehole (China Cretaceous Continental Scientific Drilling Project-SongKe1well). Over 95.7% recovery rate, the obtained continuous core is 827.6 m in length, covering the geological age ranges from late Santonian to middle Campanian. This is the most complete and continuous sedimentary deposits in the Songliao basin till now. Here we systematically presented the lithological results of the observation of the cores in 2-5 cm thickness scale at the drilling spot, and the analysis on lithofacies and sedimentary sequence, cyclic stratigraphy. 13 types of sedimentary rocks deposited mainly in lacustrine, delta and fluvial environments were found in the Nenjiang Fromation, and they were subdivided into 19 microfacies including dolomite, marl, argillaceous limestone, turbidite, mudstone of still water, volcanic ash, channel-lag deposit, point bar, natural levee, crevasse splay, crevasse channel, floodplain, flood lake, distributary mouth bar, subaqueous distributary channels, subaqueous natural levee, subaqueous inter-distributary bay, distal bar and adjacent shore sand bar. The whole sedimentary column is composed of 630 sixth-order depositional cycles in the meter-scale, which can be further organized in 148 fifth-order depositional cycles, 38 fourth-order depositional cycles and 5 third-order depositional cycles. Meticulous depiction (centimeter level) of the whole Formation provides an approach to study its sedimentary process with high precision. The centimeter-scale description of the sequent core presented some previously unidentified dolomite and volcanic ash layers. The cyclostratigraphy of the Nenjiang Formation was controlled by Milankovitch cycled, which is helpful to discover the climatic changes of the late Santonian to the middle Campanian in Songliao Basin. In addition, the detailed description of the sequent section is significant for the upcoming oil and gas exploration in the Songliao Basin if regarding the new type of oil/gas systems.
2020 Vol. 23 (1): 38-70 [
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252
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15498KB] (
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