Compaction of Moderately and Deeply Buried Rotliegend Eolian Sandstones from the Fore-Sudetic Monocline, Poland

Biernacka, Julita¹, Grzegorz Lesniak² (1) Uniwersity of Poznan, Poznan, Poland (2) Oil and Gas Institute, Krakow, Poland

Eolian sandstones show the best reservoir properties within the continental deposition­al facies of the Rotliegend, SW Poland. A number of small gas fields have been discovered in the Fore-Sudetic Monocline and this area is still prospected for hydrocarbons. However, in spite of many studies concerning diagenesis, quantitative data were insufficient for map­ping diagenetic patterns and for reservoir quality assessment. The focus of our study was to quantify the effects of compaction and cementation, and to recognize a regional porosi-ty-depth trend. Samples from burial depths of 1,5 to 4,5 km and from 18 wells were ana­lyzed. Thickness of the eolian sandstones ranges from tens of meters to more than 700 m. These sandstones are fine- to medium grained sublitharenites / subarkoses and presumably had similar original porosities. Nevertheless, their present-day reservoir qualities vary con­siderably and there is no clear porosity-depth trend. Intergranular volume and cement quan­tities suggest that compaction has been the main cause for porosity reduction. If we assume a diverse effect of the Cretaceous uplift (100-1000 m), and omit 10% of samples with anom­alously high porosity, the porosity loss with depth may be approximate with an exponential function. The major porosity-occluding phases are hematite-illite rims, quartz and authigenic illite. Samples with anomalously high porosities contain smaller amounts of these minerals, although all of them are present. It is concluded, therefore, that the high porosity (average 20%) has been preserved due to either hydrocarbon emplacement or fluid overpressure. Part of the samples with retarded diagenesis was taken from gas-bearing horizons.