Retracing Organic-Inorganic Interactions in the Alum Shale (Middle Cambrian-Lower Ordovician) in Southern Sweden: From Shale Oil to Biogenic Shale Gas?
Biogenic methane occurs in the TOC-rich Alum Shale (Middle Cambrian to Lower Ordovician) in southern Sweden in an area where the shales are thermally immature, but impregnated by migrated oil formed from local heating of the rock. The shales are buried to less than 200 m and covered by a thin Quaternary cover. To explain the present-day occurrence of biogenic methane in the Alum Shale and to retrace the pathways of biogenic methane formation, an interdisciplinary analytical approach has been applied to gain data for interpretation from organic geochemical investigations, thin section microscopy, SEM/TEM, stable isotope analyses, hydrogeochemical modeling, and incubation experiments to determine methane production rates. Oil impregnation of the lower Alum Shale section is characterized by higher amounts of extractable organic matter and higher percentages of saturated hydrocarbons. In contrast, the upper section reveals higher contents of volatile C1-5 saturated hydrocarbons (methane to pentane) by thermovaporization, and obvious calcite precipitations occur with variable intensity. Such cementations are result of hydrogeochemical processes leading to precipitation of the oil degradation product CO2. In contrast, methane was stored with only slight indications of anaerobic oxidation. A hydrogeochemical modeling approach enables the quantitative retracing of methane formation in the Alum Shale as result of organic-inorganic interactions due to oil degradation. Quantities of generated methane and carbon dioxide as well as their diagenetic fate are calculated, and are coupled to numerical calculations about dissolution and precipitation of the current mineral assemblage. The results suit the current gas composition and composition of the current mineral assemblage. A conceptual model to explain the methane occurrence is that melting water after the Pleistocene glaciation and modern meteoric water may have reduced the contents of total dissolved solids in basinal brines to greater depths. Thereby methanogenesis in shallowly buried Alum Shale was enhanced. This model can be applied to explain that the biogenic methane is result of oil degradation at the lower/upper Alum Shale boundary. However, incubation experiments show that the present-day methane and carbon dioxide production rates are highest in selected intervals of the upper Alum Shale section, and thus may contribute to a conversion of immature organic material to the biogenic methane pool.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014