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Biotic and Sedimentological Attributes of the Organic-Rich Shales – Is it a Sufficient Tool for Sweet Spot Identification? An Example From the Polish Ordovician-Silurian Unconventional Petroleum System


As the response to the great interest in unconventional hydrocarbon resources in Poland during last decade, the Polish Geological Survey (Polish Geological Institute-NRI) works on the continuous program of recognition, investigation and evaluation of the potential prospective hydrocarbon zones in the lower Paleozoic shales of the East European Craton (EEC).

The result of the first stage of the program are maps of prospective zones and regional correlations between boreholes.

Zones are stratigraphically and spatially much more limited than the range of shale formations, previously considered as fully prospective, because only a part of their intervals fulfills the basic criteria such as: TOC content, thickness, thermal maturity of the organic matter etc.

Based on a various research methods and analyses performed in the Polish Geological Survey we present the current state of recognition of hydrocarbon prospectivity of the lower Paleozoic shales from the EEC.

Currently, one of our research goals is detailed description of sweet spots within the designated prospective zones. Since we are now looking for limited areas rather than widespread reservoirs we evaluated and distinguished several biotic and sedimentological indicators for determining the sweet spot attributes such as paleoenvironment conditions favouring organic matter preservation or brittleness of mudstones. We emphasize the great role of such subtle indicators like bioturbation index changes, taphonomy of graptolites, types of lamination, abundance and type of pyrite concretions. On the base of them we define oxic/anoxic bottom conditions, rate of sedimentation etc. what is crucial for organic matter preservation. Changes in lithology, especially carbonate intercalations, changes in amount of clays and silica may indicate the subtle changes of the geomechanical properties of shale.

As a result of our study we distinguished the most prospective facies and their distribution in space and time. We believe that both fundamental biotic and sedimentological attributes have crucial role in the identification of the areas of interests.

Such paleontological and sedimentological analyses are the first step to plan wisely the sampling for further laboratory analyses. Moreover, the detailed profiling brings us an opportunity to upscale the laboratory results and extrapolate some geochemical and petrophysical parameters to larger intervals.