ROLE OF SULFUR AND CARBON IN THE ELECTRICAL-CONDUCTIVITY OF THE MIDDLE CRUST

Electrical conductivity measurements were made on black shale samples from deep boreholes under laboratory conditions corresponding to more than 10 km burial depth. At these conditions, large differences in res istivity were observed for shales of different diagenetic/ metamorphic grade, although the total organic carbon (TOC) and sulfide content ar e comparable. A bituminous black shale (low diagenesis) showed reversi ble temperature dependent changes of resistivity from 10(5) Ohm m at r oom temperature to 10(4) Ohm m at 300 degrees C and 250 MPa. These are typical values for dry crustal rocks. For a very low grade metamorphi c black shale (transition from high diagenesis to beginning of metamor phism) a decrease in resistivity of at least 1 order of magnitude to a value of about 1 Ohm m was observed at the same p,T conditions. This type of black shale shows permanent changes in resistivity. The high c onductivity achieved in this sample is mainly caused by the transition of primary pyrite to pyrrhotite and the increase in connectivity betw een the conducting phases produced by the more elongated geometry of t he newly formed sulfides. Both factors result in an increase in conduc tivity at in situ conditions. These results confirm that the known par ameters of TOC content, amount of primary sulfides, and coalification stage are important for determining the conductivity of black shales. But the analytical investigations of the samples after the experiments indicate that the degree of foliation and sulfide transformation at l ow metamorphic conditions also plays an important role. The foliation and the coalification as well as the geometrical form of the newly for med sulfides are connected with the diagenetic/metamorphic overprint o f the shales. Midcrustal conditions (temperature, pressure, low oxygen fugacity) seem to promote the mobilization of sulfur and/or iron and result in increased connectivity of conducting components. Therefore s ome highly conducting zones in Earth's crust might be caused by a comb ination of organic carbon at least in a weak metamorphic stage and sul fides.