Interplay of organic matter, rock anisotropy, and horizontal shortening in bed-parallel vein development within the lower Palaeozoic shale formations from the northern part of the Caledonian Foredeep Basin (Poland)

Abstract

Bed-parallel veins that originated in organic-rich laminated rocks are considered to be related to organic matter (OM) content. The presence of OM and lamination affects the properties of host rocks and increases the bulk rock anisotropy. Bed-parallel veins occurring in the lower Palaeozoic organic-rich mudstone formations in the northern part of the Polish Caledonian Foredeep Basin were previously interpreted as having been formed as a result of horizontal shortening. The present work extends the study area c. 65 km to the south of the previous investigations to identify factors that influenced the development of bed-parallel veins on a regional scale. Application of a combined methodology including analysis of cores, thin sections, XRD and geophysical well-logs from five wells allowed for the examination of the relationship between the distribution of bed-parallel veins and the composition of the laminated host rocks. The results show that the contrasting mineral composition of horizontal lamination at micro-scale and the mean kerogen content exceeding 2.2% facilitated bed-parallel vein formation during horizontal shortening. A crucial factor for the development of bed-parallel veins is the mutual relationship of OM, rock anisotropy and horizontal shortening. Knowledge of the host rock structure and composition, coupled with the spatial distribution of bed-parallel veins in prospective successions, is useful for the effective recovery of unconventional hydrocarbons.