Conceptual model of the Otake-Hatchobaru geothermal system (MANABE,EJIMA,1984)

Introduction
Geothermal wells are drilled at expenses of high costs and tremendous human and technological efforts. Geothermal wells not only are the conduit for geothermal fluids to reach the surface, but they are the only means to get a picture of the real conditions in the underground.

Like a letter from the underground, cores and cuttings (Figs.1 and 2) recovered during the drilling of a geothermal well (Figs.3 and 4) bring to surface direct information from which these underground conditions can be deducted.

The careful examination and analysis of cores and cuttings provides the data necessary to describe the geological stratigraphy around the vicinities of the drilled zones. The integration of the stratigraphy of several wells and information derived from other geoscientific studies allows to define the geological structure, location of heat source, cap rock and even to map the distribution of temperatures within the geothermal reservoir and the characteristics of underground geothermal water. All of these are indispensable aspects to construct a conceptualized model of a geothermal area.

Knowledge on the geothermal system is gradually accrued as development proceeds. Comparison of information recovered from successful wells and that from wells failing to fulfill expectations is of extreme importance to disclose the bound of areas for exploitation and to assess the dimensions of the geothermal resource.

In order to comprehensively and most efficiently utilize the information engraved in the cores and cuttings, West JEC, through many years of endeavor has developed techniques to measure and to analyze this information. This technology has been successfully applied to many geothermal fields in Japan and in other countries of the world.

The technology developed by West JEC to analyze cores and cuttings information is based upon the application of the 8 different techniques. The objective of these studies is to construct a geothermal model which best explains the results of surface exploration and that derived from wells (Fig.5). The technology developed by West JEC to extract information from cores and cuttings is base upon the following eight procedures.


1. Observation of cores and cuttings and Microscope observation of thin sections
2. X-ray diffraction analysis
3. Measurement of fluid inclusions and analysis
4. Core fracture analysis
5. Measurement of zircon crystals and analysis
6. Analysis of the chemical composition of alteration mineral
7. Rock dating and analysis
8. Measurement of physical properties of rocks
Investigation of geothermal structure through core & cuttings











Click to image
Fig.3 Core samples
Fig.4 Cuttings sample

 

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