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The Upper Jurassic – Lower Cretaceous Oil Shales of the Central Mid-Polish Trough

Abstract

Abstract

The Permian-Mesozoic Mid-Polish Trough (MPT) was a main depocenter of the Polish Basin, filled with a few thousand meters thick pile of sediments. Characteristic feature of the MPT is presence of very thick Upper Permian salts in the lower part of the section. This gave rise to the development of a complex system of salt structures, significantly affecting development of the subsequent Mesozoic basin. During latest Jurassic to earliest Cretaceous in the central part of the basin organic rich shale formation developed. The formation was a subject of current studies, including geochemical, thermal maturity and mineralogical composition analysis, wire log evaluation, as well as 1-D modelling of burial/thermal and hydrocarbon generation history, backed-up by seismic data interpretation.

Thickness of organic rich shale is relatively high, equal to 30-60 m. Average TOC contents is in a range of 2.5-3.5 %. The shale contains mixture of kerogen type II and III, with proportions of both varying across vertical section. Content of key mineralogical components varies in a following range: quartz 1-50 % (aver. 16 %), carbonates 3-97 % (aver. 43 %), and clays 0-60 % (aver. 37 %). Brittleness factor varies significantly laterally and vertically from 0.35-1.0 (aver. 0.65). Thermal maturity varies from 0.78 % to 0.95 % VRo (aver. 0.85 % VRo), although the data are from wells which were drilled mostly above salt structures. Modelling of the synthetic wells located in syncline developed between Klodawa and Ponetow-Wojszyce salt structures allowed to determine shale maturity as 1.2 % VRo at maximum, suggesting that shale reservoir contains mainly oil and/or condensate. Amount of generated hydrocarbons ranges from 75-170 mg HC/g TOC (oil) in less mature zone to 420-430 mg HC/g TOC (oil) and 75-100 mg HC/g TOC (gas equivalent) within the syncline. The analyzed Upper Jurassic – Lower Cretaceous shale formation have potential for shale oil exploration, and could be also considered as good quality conventional source rock.

This work was completed within the NCBiR BlueGas – JuraShale project