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Global Geology
2020 Vol.23 Issue.3
Published 2020-09-25
Articles
Articles
135
Controlling factors of sphalerite and galena deposition in Baiyinnuo'er skarn deposit, Inner Mongolia, China
MA Wanli, YANG He, WANG Keyong
The temperature, pH,
f
O
2
and Cl
-
activity have been considered to be significant physicochemical parameters which controlled sulphide mineralization. Especially, the change of pH is considered to play one of the key roles in skarn deposit because of the involvement of marble. But few scholars have evaluated the impact of each parameter on mineralization. In this study, the authors have constructed thermodynamic model, calculated the important aqueous species activities in Baiyinnuo'er deposit, and evaluated the influence of these parameters on the solubility of Pb and Zn. The results show that the log
f
O
2
decreased from -33.834 to -39.256, pH decreased from 4.0 to 3.7 and the Cl
-
activity increased from 0.25 to 0.30 in the main mineralization stage. The effect of
f
O
2
was readily ruled out because the reaction of Pb and Zn precipitation did not involve changes in the valence of the element. The decreased pH was impossible to be a mineralization factor, which was confirmed by metal solubility evaluation. The Cl
-
activity was also considered to be a negative factor. In fact, the dominant factor of mineralization was temperature, not the combination of various parameters as previously thought. Even in the skarn stage, the main factor to make the metal solubility >10% was also the high temperature. A significant increase in pH only occurred in the post-ore stage in response to the production of large amount of calcite.
2020 Vol. 23 (3): 135-148 [
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97
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149
Method for extracting angle-domain common image gathers in Kirchhoff beam migration
ZHANG Zhihou, YAO Yu, SUN Hui, YANG Junjie, LI Meng
Kirchhoff beam migration is a simplified Gaussian beam migration, which omits the dynamic information and can calculate multi-arrival traveltime, so it is a high-precision and fast seismic imaging method. In the imaging process, extracting common image gathers can be used for velocity analysis, improving the accuracy of modeling and imaging quality. Compared with the conventional common image gathers extracting methods, the angle-domain common image gathers extracting method can avoid the artifacts caused by multi-arrival seismic waves. The authors present a new method of extracting common image gathers in angle-domain from Kirchhoff beam migration and verify the method by numerical calculations.
2020 Vol. 23 (3): 149-154 [
Abstract
] (
107
) [
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910KB] (
278
)
155
Structure analysis of shale and prediction of shear wave velocity based on petrophysical model and neural network
ZHU Hai, XU Cong, LI Peng, LIU Cai
Accurate shear wave velocity is very important for seismic inversion. However, few researches in the shear wave velocity in organic shale have been carried out so far. In order to analyze the structure of organic shale and predict the shear wave velocity, the authors propose two methods based on petrophysical model and BP neural network respectively, to calculate shear wave velocity. For the method based on petrophysics model, the authors discuss the pore structure and the space taken by kerogen to construct a petrophysical model of the shale, and establish the quantitative relationship between the P-wave and S-wave velocities of shale and physical parameters such as pore aspect ratio, porosity and density. The best estimation of pore aspect ratio can be obtained by minimizing the error between the predictions and the actual measurements of the P-wave velocity. The optimal porosity aspect ratio and the shear wave velocity are predicted. For the BP neural network method that applying BP neural network to the shear wave prediction, the relationship between the physical properties of the shale and the elastic parameters is obtained by training the BP neural network, and the P-wave and S-wave velocities are predicted from the reservoir parameters based on the trained relationship. The above two methods were tested by using actual logging data of the shale reservoirs in the Jiaoshiba area of Sichuan Province. The predicted shear wave velocities of the two methods match well with the actual shear wave velocities, indicating that these two methods are effective in predicting shear wave velocity.
2020 Vol. 23 (3): 155-165 [
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88
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166
Spatial pattern analysis of forest landscape in Changbai Mountain
ZHANG Kaixin, CHEN Shengbo, WU Qiong
Land use and land cover change is one of the important topics in the global change study with the development of satellite-based remote sensing technologies. This study employed 3D landscape pattern metrics to analyze the dynamics of land cover types around Heaven Lake in Changbai Mountain area from 2010 to 2015. The results indicated that forest land occupied the most proportion and increased from 61.9% in 2010 to 66% in 2015 due to government forest protection policy. The mean elevation of vegetation, including agriculture, fo-rest, grass and shrub was much lower compared with other land cover types (water, wetland, imperious land and bare land) in both 2010 and 2015. The elevation of agriculture land increased from 1 700 m in 2010 to 1 890 m in 2015 a decreased land proportion. Considering the adjacent relationship between the local agriculture land and forest land, the transformation from agriculture land to forest land was obvious in the lower elevation area. The study showed that the selected 3D landscape pattern metrics had the ability and efficiency to describe the spatiotemporal variations of mountainous landscape, and significant correlation existed between land cover and local terrain.
2020 Vol. 23 (3): 166-172 [
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67
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351
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173
Determining water well sites based on electrical structure in Taobei District of Baicheng
SHEN Xintu, LIU Lijia, LI Peng
Drilling a well randomly would lead to high uncertainties and could not meet the demand of water supply from the increasing population. It is in dire need to improve success rate of well drilling. The high-density resistivity method offers us a good choice. In this study, high-density resistivity method is used for groundwater survey in five villages in the Taobei District of Baicheng. The data obtained by high-density resistivity method is inverted through Res2dinv software. It is found that the electrical structure is characterized by a horizontal layer distribution, and the resistivity shows a "high-medium low-low" feature from shallow to deep. Moreover, obvious electrical gradient zones are identified in the strata below each section, which are interpreted as tectonic weak zones, i.e., the faults. The low-resistance anomaly areas are inferred to be favorable aquifers. The results show that high-density resistivity exploration is an efficient and practical method for determining water well sites in rural areas and can provide a guide for finding water resources in the area.
2020 Vol. 23 (3): 173-179 [
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94
) [
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180
U-Pb-Hf isotopes of Neoarchean Shuimowan tonalites in southern margin of North China Craton: constraints on petrogenesis
HAO Leran, YANG Haotian, YANG Donghong
This study presents zircon LA-ICP-MS U-Pb geochronology, trace elements characteristics, whole rock geochemistry composition and zircon Hf isotopic data of Shuimowan tonalites in Dengfeng City. The results show that zircons from Shuimowan tonalites have oscillatory growth zoning, relatively high Th/U ratios (0.8-1.7) and upturned zircon rare earth elements (REE), indicating that they are magmatic origin. Shuimowan tonalites yield a weighted mean
207
Pb/
206
Pb age of 2 522 ±9 Ma, indicating they were formed in the latest Neoarchean. The tonalites are characterized by relatively low SiO
2
contents, high MgO and Na
2
O contents, high REE (∑REE = 322×10
-6
-354×10
-6
), obvious fractionation of REE, enrichment in LREE and feeble negative Eu anomaly (Eu/Eu
*
= 0.97-0.99). The samples are also enriched in large ion lithophile elements and depleted in high field strength elements, with negative Nb, Ta, P and Ti anomalies. The
ε
Hf
(
t
) values of zircon grains from Shuimowan tonalites range between 3.5 and 5.3. Combined with regional tectonic evolution, it is suggested that Shuimowan tonalites were derived from partial melting of mantle peridotite metasomatized by the slab-derived felsic melt and formed in the arc magma tectonic setting related to plate subduction. Research on the Neoarchean Shuimowan tonalites provides a constraint for the study of the Pre-Cambrian tectonic evolution in the southern margin of North China Craton.
2020 Vol. 23 (3): 180-190 [
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113
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191
Influence of wettability of shaly sandstone on rock electricity parameters
ZHAO Yongqiang, PAN Baozhi, SI Zhaowei, LIN Fawu
It has been found that the existence of chlorite and illite will lead to the rock wettability of oil affinity. Chlorite and illite are developed in shaly sandstone of Yingcheng Formation in Longfengshan area, the saturation index
n
of 7 rock samples is very high, with the highest of 14.57, whereas the cementation index
m
is low. The X-ray diffraction of clay and whole rock analysis of 7 shaly sandstone samples are carried out, and the relationship between chlorite and illite content vs.
m
and
n
is established. It is concluded that the low
m
value and high
n
value are caused by the wettability of rock. In order to verify the influence of wettability on the parameters of rock electricity, this paper discusses the method of making artificial shaly sandstone and the control of wettability. This study provides certain reference for determining the saturation model of shaly sandstone and improving the logging interpretation accuracy of shaly sandstone reservoirs.
2020 Vol. 23 (3): 191-198 [
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92
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466KB] (
260
)
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