Investigate rock formation of the Ilan Plain, Taiwan by using petrography and mineral assemblages from cuttings

Abstract

There has been a long-standing debate over the rock formation beneath the Ilan Plain. Is the rock formation associated with the Hsuehshan Range belt which lies in the north of the plain or with the Backbone Range belt which is situated in the south? Two deep wells, HTL01 (2200m) and HTL02 (2800m), were drilled by the project of Taiwan National Energy Program-Phase II to evaluate the geothermal potential of the Ilan Plain. This study is attempt to apply petrography and mineral assemblages of cuttings taken from these two wells to delineate the rock formation beneath the Ilan Plain.The slate with a well-developed cleavage exits in 450m-1250m of HTL01, and meta-sandstone appears incrementally from 1350m to 2050m, and then diminishing with well cleavage developing in 2050m-2200m. The entire cuttings of HTL01 accommodate quartz, kaolinite and illite, but chlorite found only from 1150m to 1250m. The slate with a well-developed cleavage exits in 500m-1300m of HTL02, and meta-sandstone gradually emerge from 1300m to 2800m, but few slate and meta-sandstone found between 2050m and 2200m. The entire cuttings of HTL02 contain quartz, kaolinite and illite with quartz and illite appeared only between 1900m and 2200m. It suggests that the rock formation of HTL01 and HTL02 rather belongs to the Hsuehshan Range belt than the Backbone Range belt.