Characterization of gases in sedimentary formations through monitoring during drilling and core leaching (Balazuc borehole, Deep Geology of France Programme)

Continuous analysis of the drilling fluids and core sampling formed a major part of the Ardèche theme of the Deep Geology of France Programme when drilling the Balazuc-1 borehole in 1991. The analyses of gas concentrations in the drilling fluids are compared to those of core leaching. Both data sets show good agreement. Although higher concentrations are obtained in the drilling fluids, the drilling to core ratio shows a relatively constant value. These higher concentrations are due to crushing of the rock during drilling and the influence of the sediments surrounding the borehole. The characterization of the gases with depth indicates 3 different zones: (1) the Jurassic carbonates, with high CH4 and CO2 concentrations; (2) the Triassic upper sandstone and middle evaporitic units, with average concentrations of He, CO2, N2 and low CH4; and (3) the Triassic lower sandstone unit and the Carboniferous silts, with high He, N2 and low CO2 and CH4 concentrations. These trends indicate that diffusion is not a major process at Balazuc and that the formations remained relatively independent from each other. Nitrogen and CO2, which are the main constituents, show a reverse correlation; the high CO2 zones are related to the carbonate formations. In the Hettangian carbonates, a zone of black shales has preserved a high hydrocarbon content due to its very low permeability whilst in the other formations, any hydrocarbons originally present have escaped. The He record in the continuous measurement mainly reflects the fractured levels and the paleoporous zones. This is correlated to major fluid transfer in the formations of the lower part of the borehole, along the Uzer fault.