A STUDY OF THE HOLZENER ASPHALTKALK, NORTHERN GERMANY - OBSERVATIONS REGARDING THE DISTRIBUTION, COMPOSITION AND ORIGIN OF ORGANIC-MATTER IN AN EXHUMED PETROLEUM RESERVOIR

The Holzener Asphalkalk deposit is located on the southwestern flank of the Hils Syncline, northern Germany. It consists of a series of Kimmeridgian-Portlandian bitumen-impregnated limestones (cumulative thickness of nearly 200 m) overlain unconformably by Neocomian claystones and underlain by older Jurassic and Triassic sediments. The most bitumen-rich limestone layers have been extensively mined with the result that there is three-dimensional access to what is probably an exhumed series of stacked petroleum reservoirs. Geochemical, petrological and petrophysical techniques were used to gain some insight into the distribution and the source of the organic matter in two of the 13 layers of the deposit. Most disseminated organic matter was found to be soluble in organic solvents and rich in non-hydrocarbons. Its distribution appears to be controlled by ooid versus carbonate matrix abundances, the occurrence of fractures and degree of impregnation. Small (meso- to micro-) pores have high bitumen saturations whereas large pores (meso- to marco-) have much lower saturations. Mature petroleum is thought to have once filled all pores, but alteration has probably removed as much as 85% of it, mainly from the large pores. Regional geochemical considerations, pyrolysis and stable carbon isotope data indicate that the original petroleum was most probably derived from the underlying Posidonia Shale (Toarcian) source rock. Asphaltene pyrolysis data indicate that an algal lacustrine co-source might have also contributed to the deposit.