Hydrocarbon potential of the Zechstein Main Dolomite (Upper Permian) in western Poland: Relation to organic matter and facies characteristics

- Verified the current views on the facies distributions in the Zechstein Main Dolomite.
- We propose new assumptions for the facies description.
- We determined geochemical characteristics of organic matter in each facies.
- The organic matter has good source rocks characteristic and good hydrocarbon potential.
- The new facies pattern influenced on understanding source rocks-oil correlation and petroleum prospecting.

The Zechstein Main Dolomite (Ca2) in the area between Grotów Platform and western part of the Gorzów Platform, in western Poland, represents the 3rd order sequence of the Polish Zechstein. The deposits are mainly the HST carbonate facies. Totally, fifteen HST microfacies associations were identified and they represent the carbonate platform (shallow subtidal-supratidal deposits), the platform slope (upper-lower slope deposits) and the basin environments. The deposition of the Ca2 was controlled by the pre-Zechstein paleomorphology and the systems of tectonic dislocations. On this background the geochemical results are presented and their source nature characterized in the context of the facial model of the Ca2. The results of geochemical analyses indicate that most of samples collected in the study area contain the epigenetic hydrocarbons. In the Ca2 carbonates the TOC values range from near zero to 5.2 wt.%, and these of hydrocarbons contents reach up to 9.6 mg/g rock. Despite such significant variability, the average values are low and do not exceed 0.5% TOC and 2.5 mg HC/g rock. All geochemical parameters indicate that the Ca2 carbonates from the study area represent poor to fair oil-prone source rocks with local good and/or excellent hydrocarbon potential. The macerals composition reveals predominance of the oil-prone Type II kerogen in all studied microfacies. In the kerogen composition organic-mineral associations and solid bitumens prevail. The organic matter from the Ca2 was deposited under anoxic conditions in the hypersaline depositional environment. The maturity indicators: vitrinite reflectance Ro, temperature Tmax and biomarker parameters indicate that the maturity of organic matter corresponds to the final phase of the “oil window” and to the early phase of the “gas window”.