GEOCHEMICAL CHARACTERIZATION AND DISTRIBUTION OF UNCONVENTIONAL HYDROCARBON PLAYS OF THE LOWER CRETACEOUS OF NW GERMANY

Within the Lower Saxony Basin (LSB) of NW Germany sediments of the Lower Cretaceous are known to contain abundant organic material at different stratigraphic levels. We present a geochemical high-resolution study of sediments of the German Wealden (Berriasian) and compare the data to Barremian and Aptian organic-rich sediments.

Geochemical investigations including XRF, as well as pyrolysis methods and molecular and carbon isotope analyzes have been performed.

Berriasian sediments have been deposited in a predominantly lacustrine environment that experienced marine ingressions. Facies changes within the Wealden are likely related to precipitation changes affecting lake level variations. The observed depositional changes are associated with variations of the organic facies. Periods of high lake levels, associated with anoxic to dysoxic water conditions, are obvious from the stable carbon isotopic composition of the organic carbon which is highly enriched in 12C suggesting that the organic matter experienced a significant contribution from the microbial precursors. Low lake levels were associated with the deposition of hydrogen-depleted organic matter, which either relate to land plant and/or highly oxidized material.

The Barremian to Lower Aptian organic-rich Dark Shale Sequence contains variable amounts of terrestrial and aquatic organic matter deposited in a marine environment. The sediments contain rather high concentrations of C28 iso-steranes, which could be related to observed higher abundances of coccolithophores.

Although, basin sediments of the Wealden are highly variable the anoxic to dysoxic facies types containing type I kerogens are excellent sources for in-situ oil and gas (at higher maturities). Favorable targets for the extraction of unconventional hydrocarbons in the NW German Basin would likely be sections of Wealden 3 and 4.

The Barremian and Aptian organic-rich shales exceeding partly 100 m of thickness contain type II kerogens, and would generate oil and gas at higher maturities in the basin.

The Lower Saxony Basin is tectonically segmented, and the segments set the regional boundary conditions for the distribution of unconventional Lower Cretaceous plays.