Mechanical Study on the Exploitation of Groundwater Resources in Crystalline Rocks – Examples of Hoping and Kinmen areas, Taiwan

Abstract:

Intact crystalline rocks of low porosity possess lower water storage. Conversely, fractured crystalline rocks contain higher groundwater resources. Therefore, knowledge of distribution and characteristics of fractures is essential to the exploitation of groundwater resources in crystalline rocks. This research makes crystalline rocks in Hoping and Kinmen areas of Taiwan as examples to integrates previous studies of distribution and attitude of fractures and in-situ stress from surface survey and underground study for estimating the tendencies of slip and dilation of fractures in terms of geomechanics, understanding the characteristics of potential fluid conduits, and benefiting the exploitation and development of groundwater resources.

The formations in downstream area of Hoping River contain late Paleozoic to Mesoic meta-granites and marbles, and few alluvium strata. Kinmen island closed to SE Chain is located in Pingtan-Dongshan Metamorphic Belt of Late Yanshan orogeny. The formations contain Mesozoic granite, gneiss, various dikes, and some alluviums.

Previous studies had conducted experiments of anelastic strain recovery on retrieved cores in Hoping. The results show that the maximum principal stress is vertical and the horizontal minimum stress is in NE-SW orientation, indicating a normal faulting stress regime with NE-SW extension. Most fractures are in E-W and N-S orientations. Results of hydraulic fracturing experiments in Kinmen display the maximum and intermediate stress is in NW-SE orientation and vertical, respectively, suggesting strike-slip faulting regime with NE-SW extension. Most fractures are in E-W and NE-SW orientations and some are in other orientations. Because of various attitudes and distributions of fractures, origin of fluid conduits is not easy to investigate and predict. Based on in-situ stress data, strikes of predicted fluid conduits in Hoping area is N-S and NW-SE while in Kinmen area is in N-S. Analysis of well logging data and tracer experiments will provide further insights into evaluating the existence as well as the distribution and attitude of fluid conduits. The outcomes will establish the basis for exploitation and utilization of groundwater resources in Hoping and Kinmen areas, Taiwan.