Depositional history of an Upper Carboniferous organic-rich lacustrine shale from East Greenland

A 15 m thick Upper Carboniferous lacustrine unit from East Greenland has been studied in detail by sedimentological, palynological and organic geochemical methods with the purpose of elucidating the depositional history and assessing the implications for hydrocarbon exploration.

The analytical results indicate that the lower, shaly part represents quiet deposition of clay-sized mineral grains, calcite, fish, and ostracod remains in a deep anoxic lake. The upper, sandy-shaly part was deposited in prograding fluviatile mouth bars and in protected bays where organic debris and plant material were concentrated. The lake formed rapidly, probably due to tectonic activity, and its depositional history was simple, reflected by a single coarsening-upward sequence.

The two main depositional environments are easily separated on the basis of a number of analytical parameters, which mainly reflect anaerobic versus aerobic decomposition of the organic matter. The lower hypolimnic facies has a high content of organic carbon and a high Hydrogen Index. The organic content is dominated by amorphous kerogen; trilete spores are abundant compared to monosaccate pollen. The organic matter is classified as Type I/II kerogen with a good to excellent potential for liquid hydrocarbons. In contrast, the shales of the upper epilimnic facies have a varying organic content and a low Hydrogen Index. The organic content is dominated by woody and coaly material; monosaccate pollen are frequent compared to spores. The organic matter is classified as Type III/IV with a poor hydrocarbon potential.

The thermal maturation level was determined by vitrinite reflectance, spectral fluorescence, coloration (TAI), pyrolysis, and GC data. All these data suggest that the shale is at the onset of petroleum generation. The extract composition is characterized by a high content of saturated hydrocarbons in the waxy range and a characteristic odd/even predominance, and thus is similar to petroleum generated from other lacustrine source rocks.