On the geochemistry of gases and noble gas isotopes (including ²²²Rn) in deep crustal fluids: the 4000 m KTB‐pilot hole fluid production test 2002–03

Abstract

Abstract The concentrations of H 2 , O 2 , CO 2 , and concentrations and isotopic composition of the noble gases (including 222 Rn), N 2 , CH 4 , and higher hydrocarbons dissolved in 4000 m deep‐seated fluids from a 12‐month fluid production test in the KTB pilot hole were analyzed. This determination of the gas geochemistry during the test in combination with the knowledge of the hydraulic data provides relevant information about the fluid hydraulics of the deep system. All gas concentrations and isotopic signatures, except for 222 Rn, showed constancy during the course of the test. This, in combination with large fluid flow rates at a moderate water table drawdown, imply an almost infinite fluid reservoir in 4000 m depth. From the change in 222 Rn‐activity as a function of pump rate, the contribution of smaller and wider pores to the overall fluid flow in an aquifer can be deduced. This 222 Rn‐activity monitoring proved therefore to be a valuable instrument for the qualitative observation of the scavenging of pore and fracture surfaces, a hydraulic feature invisible to standard hydraulic testing tools. The observance of this scavenging effect is due to (i) the continuous on‐line geochemical monitoring, (ii) the durability of the test, (iii) a change in pump rate during the course of the test, and (iv) due to the short half‐life of 222 Rn. The fluids have a 5.9% mantle He component, and a δ 21 Ne excess of 14%, and a noble gas model age of about (5.5–6.2) ± 2.0 Myr. The mean N 2 /Ar‐ratio of 516 and δ 15 N‐data of about +1.5‰ indicates sedimentary or metamorphic origin of N 2 . The hydrocarbons, amounting to 33 vol.% in the gas phase, are derived from thermal decomposition of marine organic matter of low maturity. But a key question, the identification of the potential source region of the fluids and the migration pathway, is still unidentified.