Geothermal Gradient Drilling And Measurements Ascension Island, South Atlantic Ocean

This Technical Report On The Phase Ii Geothermal Exploration Of Ascension Island Documents The Data Collected During Thermal Gradient Drilling And The Subsequent Thermal And Fluid Chemical Investigations. It Also Documents The Completion Of The Phase Ii Exploration Strategy Which Was Proposed At The End Of Our Phase I - Preliminary Examination Of Ascension Island. The Thermal Gradient Drilling Resulted In Seven Holes Which Range From 206 To 1750 Ft. (63-533 M) Deep, With A Cumulative Footage Of 6563 Ft. (2000 M). The Drilling Procedure And The Problems Encountered During The Drilling Have Been Explained In Detail To Provide Information Valuable For Any Subsequent Drilling Program On The Island. In Addition, The Subsurface Geology Encountered In The Holes Has Been Documented And, Where Possible, Correlated With Other Holes Or The Geology Mapped On The Surface Of The Island. Temperatures Measured In The Holes Reach A Maximum Of 130 Degrees F (54.4 Degrees C) At 1285 Ft. (391.7 M) In Hole Gh-6. When The Temperatures Of All Holes Are Plotted Against Elevation, The Holes Can Be Classed Into Three Distinct Groups, Those Which Have No Thermal Manifestations, Those With Definite Geothermal Affinities, And One Hole Which Is Intermediate Between The Other Two. From Consideration Of This Information, It Is Clear That The Highest Geothermal Potential On The Island Is In The Donkey Flat Area Extending Beneath Middleton Ridge, And In The Cricket Valley Area. Because Of The Greater Drilling Depths And The Remote Nature Of The Cricket Valley Area, It Is Recommended That Future Exploration Concentrate In The Area Around Middleton Ridge. Hole Ldtgh Was A Large Diameter Hole Which Was Drilled To Test For The Presence Of A Fresh Water Aquifer Beneath The Island. The Water Encountered Was Brackish. This Is Probably Due To The High Lateral Permeability Of The Island Which Effectively Disperses Any Precipitation To The Sea. Fluid Samples Collected From This Well Have Been Chemically Analyzed. Their Chemistry Indicates That They Are Derived From Seawater Reacting With Rocks At Elevated Temperatures. Temperatures Calculated From The Chemical Analyses Indicate That The Fluids Have Seen Temperatures At Least As High As 232 Degrees F (111 Degrees C) And May Have Experienced Temperatures Between 302 Degrees And 392 Degrees F (150 Degrees And 200 Degrees C). We Believe That These Fluids Have Been Transported Laterally From The Higher Temperature Portions Of A Geothermal System And May Not Be The Highest Temperatures Which Could Be Discovered By A Deeper Drilling Program. In Summary, Our Phase Ii Effort Has Found Evidence Of A High-Temperature Geothermal System Beneath Ascension Island. Located In The Vicinity Of Middleton Ridge. The Most Prospective Area Is We Recommend Additional Exploration In This Area Which Should Consist Of A Short Electrical Resistivity Survey To Allow The Siting Of A Deep Thermal Gradient Test Hole. The Hole Should Be Drilled To Depths Of 3000 To 5000 Ft. (914 To 1524 M), And Should Be Designed To Intersect And Test Fluids Which Are Capable Of Generating Electrical Power. Egi Reference Number Gl04131

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Field Value
Citation Date 1984-07-12T00:00:00

Harvest Information

Original ID 9F84302DF44E4AF1AE8E078A880F1277
Index Date 2014-06-17T12:13:59
Original Format ISO-USGIN
Original Version 1.2


Name Bruce S. Sibbett, Dennis L. Nielson, And Michael C. Adams

Geographic Extent

North Bound 78
South Bound -54.45
East Bound 28.65
West Bound -80.6