CHARACTERIZATION OF GASES IN SEDIMENTARY FORMATIONS THROUGH MONITORING DURING DRILLING AND CORE LEACHING (BALAZUC BOREHOLE, DEEP-GEOLOGY-OF-FRANCE-PROGRAM)

Continuous analysis of the drilling fluids and core sampling formed a major part of the Ardeche theme of the Deep Geology of France Programm e when drilling the Balazuc-1 borehole in 1991. The analyses of gas co ncentrations in the drilling fluids are compared to those of core leac hing. Both data sets show good agreement. Although higher concentratio ns are obtained in the drilling fluids, the drilling to core ratio sho ws a relatively constant value. These higher concentrations are due to crushing of the rock during drilling and-the influence of the sedimen ts surrounding the borehole. The characterization of the gases with de pth indicates 3 different zones: (1) the Jurassic carbonates, with hig h CH4 and CO2 concentrations; (2) the Triassic upper sandstone and mid dle evaporitic units, with average concentrations of He, CO2, N-2 and low CH4; and (3) the Triassic lower sandstone unit and the Carbonifero us silts, with high He, N-2 and low CO2 and CH4 concentrations. These trends indicate that diffusion is not a major process at Balazuc and t hat the formations remained relatively independent from each other. Ni trogen and CO2, which are the main constituents, show a reverse correl ation; 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 ot her 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 tr ansfer in the formations of the lower part of the borehole, along the Uzer fault. (C) 1998 Elsevier Science Ltd. All rights reserved.