Origin of hydrocarbon gases accumulated in the Middle Cambrian reservoirs of the Polish part of the Baltic region

The origin of natural gases associated with oil and condensate accumulations within the Middle Cambrian sandstone reservoirs on the Polish part of the Baltic region was characterized by means of molecular analyses, stable carbon isotopes of methane, ethane and propane, and stable hydrogen isotopes of methane. Gases generated from the Upper Cambrian-Tremadocian source rock succession by hydrous pyrolysis at 330 C for 72 h was used to characterize thermogenic gas and to identify the microbial methane input in the natural gas accumulations. A insignificant component of microbial methane is only present in gases from the B3 offshore field and from two inflows in the B7-1/91 borehole. The traps within the Middle Cambrian sandstone reservoirs had already been formed and sealed between the Late Cambrian and the Early Ordovician time span when migration of microbial methane took place along the fault system. The traps were successively supplied with thermogenic gaseous hydrocarbons, condensate and oil generated from the same source organic matter of the Upper Cambrian-Tremadocian source rock succession at a successively higher maturation stage.