AAPG ACE 2018

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Heterogeneity in the Lower Paleozoic Shales From the Peri-Baltic Basin (Central Europe) – How Lithology and Sedimentary Features Affect Geochemical and Mechanical Properties of Rocks

Abstract

Lower Paleozoic shales from Poland were deposited along the south-western (in the present position) continental slope of the East European Craton. Dark mudstones display change in geotectonical regime from passive margin to a distal part of the Caledonian foredeep. The so-called “black shales” prompted oil companies to explore its unconventional hydrocarbon potential. It becomes clear that lithological and mechanical properties in highly anisotropic shale formations change significantly in vertical and horizontal direction. Therefore only a part of the shale formations can be considered as prospective. The main goal of current research is to indicate the most prospective zones including the evaluation of the TOC content, thermal maturity and petrophysical parameters.

We show detailed sedimentological analyses applied to several drill cores from Pomerania region (northern Poland). Four lithostratigraphic units: Sasino, Prabuty (both Ordovician), Paslek and Pelplin (Silurian) formations were divided into homogenous and heterogeneous intervals of different range. Sedimentary units defined in that way were compared to the laboratory data on the one side, as well as to the Consistent Lithological Units (CLUs) on the other side (the CLUs units were distinguished using several geophysical logs like gamma-ray, bulk density, porosity, Young’s modulus, and mineralogy). As an effect of integration, the complete aspect of heterogeneity within the investigated shales has been established. Subsequently, we defined the sedimentological and lithological features that control geochemical and/or petrophysical properties of the rocks.

As the result, we indicate that some of the rock parameters can be interpreted only during careful sedimentological study, which in our case is more sensitive than well logs. For example, (1) the relationship between TOC content and the bioturbation index, and (2) the relationship between carbonates origin and the brittleness index calculation have been detected.

Identification of the sedimentary units and their comparison to the CLUs is the first step to create the hierarchic CLUs composite, becoming proper upscaling tool. Moreover, it enables to get the samples from precisely defined intervals. Therefore, the “surprising” results during the laboratory tests and false averaging of parameters in heterogeneous shales can be avoided.