Dietrich Hugo Welte

Variations in the content and composition of organic matter in sediments underlying active upwelling regimes: a study from ODP Legs 108, 112 and 117

Abstract An overview is presented of organic geochemical studies on sediments underlying the active upwelling cells off Northwest Africa (ODP Leg 108; Site 658), off Peru (Leg 112; Sites 679, 681, 684, 686) and off Oman (Leg 117; Sites 723, 725, 728). The investigated sediments are all characterized by high organic carbon contents (0.5–9% Corg), being the sedimentary expression of an increased primary bioproductivity induced by upwelling currents of deep nutrient-rich waters. The organic matter is predominantly of marine origin with variable admixtures of a terrigenous component; the highest relative contribution is found at Site 658. The extractable lipids are composed of a wide variety of compounds, of which the most characteristic and abundant compounds were quantified. Long-chain unsaturated ketones usually dominate. Steroids, alkanediols and their corresponding keto-ols are also present in high concentrations, although strong variations in their abundances were noted. In addition, organic sulphur compounds are omnipresent and reveal strong variations in their distribution, but do not make a major contribution to the total amount of extractable lipids.

Does coal mining induce methane emissions through the lithosphere/atmosphere boundary in the Ruhr Basin, Germany?

Abstract Underground coal mining is associated with the release of substantial quantities of coal bed methane. While the quantities of methane released from mine shafts are well known, little information is available about the extent of direct firedamp emissions across the earths surface. On some farm lands, inflammable firedamp escapes have been reported to persist for more than 30 years. These incidents are restricted to hard coal mining areas in the Ruhr Basin and may indicate a causal link between underground coal panels and firedamp surface emissions. To investigate this, a study was initiated in which the emission and consumption rates for methane at the lithosphere/atmosphere boundary were measured using flux chambers. Only emission sites are presented here. As a result, methane emissions were traced not only inside coal mining regions, but for the first time outside the area of mining activities alongside natural normal faults also. The methane release at these faults was found to be correlated negatively to the atmospheric pressure. Furthermore, gas permeabilities for different rock units were calculated from hydraulic conductivities of these rocks and fluid characteristics. They allow the determination of the range of methane emissions possible through different rock units. Having compared these theoretical emission rates with the observed ones, the reason for some of the emission points might be natural, but others can be explained only by underground coal mining, disrupting the rock fabric, increasing the permeabilities and hence giving way to firedamp emissions.

Geochemistry and Petrography of Organic Matter in Northwest African Continental Margin Sediments: Quantity, Provenance, Depositional Environment and Temperature History

This report synthesises the results of organic geochemical and organic petrographic analyses of sediments fron deep sea drilling sites on the Northwest African continental nargin. The investigation of the quantity, type, and composition of the organic matter and of the depositional environment leads to the recognition of a number of organofacies. Based on the organofacies, the sediments can be classified according to their differing hydrocarbon potentials, although the organic matter in all the sediments encountered has been determined to be iirmature, the onset of thermal hydrocarbon generation having not been reached at present burial depths.