Analysis of the negative effects of groundwater exploitation on geological environment in Asia

This paper summarizes the negative effects on geological environment caused by groundwater exploitation and its distribution. There are seven main types of the geological environment negative effects, which are generally as follows:(1) Constant decrease of groundwater level is mainly distributed in China(East Asia), India(South Asia), Tajikistan(Central Asia) and Saudi Arabia(West Asia);(2) land subsidence occurs mainly in eastern plains of East Asia and west Siberian Plain of North Asia;(3) seawater intrusion occurs mainly in China, Japan and South Korea in East Asia, Philippines and Indonesia in Southeast Asia, the Indian coastal areas in South Asia;(4) groundwater level decline caused by groundwater exploitation in oil fields;(5) mining collapse is mainly in 50° to 70° north latitude band;(6) the total area of karst collapse in China of East Asia is as much as 197.05 km^2; and(7) ground fracture is mainly distributed in the North China Plain, Fenwei Plain and the Yangtze River Delta. Asia can be divided into 6 zones in terms of the geological environment negative effect caused by groundwater exploitation. According to analysis, with the increasing intensity of human activities, geological environment issues become more and more serious, therefore it is vital to control the human activities within the scope of 5× 10~5 people/km^2 to 9.9× 10~5 people/km^2 for the effective control of the size of the affected area by geological environment problems.

Geological characteristics of the Nankai Trough subduction zone and their tectonic significances

The Nankai Trough subduction zone is a typical subduction system characterized by subduction of multiple geological units of the Philippine Sea Plate(the Kyushu-Palau Ridge,the Shikoku Basin,the Kinan Seamount Chain,and the Izu-Bonin Arc)beneath the Eurasian Plate in the southwest of Japan.This study presents a geophysical and geochemical analysis of the Nankai Trough subduction zone in order to determine the features and subduction effects of each geological unit.The results show that the Nankai Trough is characterized by lowgravity anomalies(–20 mGal to–40 mGal)and high heat flow(60–200 mW/m2)in the middle part and low heat flow(20–80 mW/m2)in the western and eastern parts.The crust of the subducting plate is 5–20 km thick.The mantle composition of the subducting plate is progressively depleted from west to east.Subduction of aseismic ridges(e.g.,the Kyushu-Palau Ridge,the Kinan Seamount Chain,and the Zenisu Ridge)is a common process that leads to a series of subduction effects at the Nankai Trough.Firstly,aseismic ridge or seamount chain subduction may deform the overriding plate,resulting in irregular concave topography along the front edge of the accretionary wedge.Secondly,it may have served as a seismic barrier inhibiting rupture propagation in the 1944 Mw 8.1 and 1946 Mw 8.3 earthquakes.In addition,subduction of the Kyushu-Palau Ridge and hot and young Shikoku Basin lithosphere may induce slab melting,resulting in adakitic magmatism and the provision of ore-forming metals for the formation of porphyry copper and gold deposits in the overriding Japan Arc.Based on comparisons of their geophysical and geochemical characteristics,we suggest that,although the Izu-Bonin Arc has already collided with the Japan Arc,the Kyushu-Palau Ridge,which represents a remnant arc of the Izu-Bonin Arc,is still at the subduction stage characterized by a single-vergence system and a topographic boundary with the Japan Arc.

Hydrocarbon migration and accumulation along the fault intersection zone-a case study on the reef-flat systems of the No.1 slope break zone in the Tazhong area, Tarim Basin

Understanding hydrocarbon migration and accumulation mechanisms is one of the key scientif ic problems that should be solved for effective hydrocarbon exploration in the superimposed basins developed in northwest China. The northwest striking No.1 slope break zone, which is a representative of superimposed basins in the Tarim Basin, can be divided into five parts due to the intersection of the northeast strike-slip faults. Controlled by the tectonic framework, the types and properties of reservoirs and the hydrocarbon compositions can also be divided into five parts from east to west. Anomalies of all the parameters were found on the fault intersection zone and weakened up-dip along the structural ridge away from it. Thus, it can be inferred that the intersection zone is the hydrocarbon charging position. This new conclusion differs greatly from the traditional viewpoint, which believes that the hydrocarbon migrates and accumulates along the whole plane of the No.1 slope break zone. The viewpoint is further supported by the evidence from the theory of main pathway systems, obvious improvement of the reservoir quality （2-3 orders of magnitude at the intersection zone） and the formation mechanisms of the fault intersection zone. Differential hydrocarbon migration and entrapment exists in and around the strike- slip faults. This is controlled by the internal structure of faults. It is concluded that the more complicated the fault structure is, the more significant the effects will be. If there is a deformation band, it will hinder the cross fault migration due to the common feature of two to four orders of magnitude reduction in permeability. Otherwise, hydrocarbons tend to accumulate in the up-dip structure under the control of buoyancy. Further research on the internal fault structure should be emphasized.