EWOLUCJA POROWATOŚCI W PIASKOWCACH KAMBRU Z POLSKIEJ CZĘŚCI MORZA BAŁTYCKIEGO

Abstract

ORIGIN OF POROSITY IN CAMBRIAN SANDSTONES FROM THE POLISH SECTOR OF THE BALTIC SEA

Abstract. Middle Cambrian sandstones are the main oil- and gas-bearing strata in the Baltic Sea region. The most prospective are the sandstones containing thin interbeds of sandstone-mudstone heteroliths. The sandstones are interpreted as tidal coastal and shelf sands with tidal channel fills. The research included standard microscopic investigations, cathodoluminescence analysis (CL), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) studies, and sedimentological analysis. Most of the sandstones are represented by quartz arenites. They are characterized by highly variable porosity ranging from nearly zero to over 20%. Porosity evolution of the sandstones was influenced by sedimentary environment, cementation, compaction, dissolution, burial depth and susceptibility to fracturing. It is estimated that the process of mechanical compaction caused loss of primary porosity by 10%. Cementation reduced it by further 25%. Chemical compaction also played a significant role. Among cementation processes, the most important was silicification which in two stages: during eo- and mesodiagenesis under progressing subsidence. Secondary porosity due to grain and cement dissolution was of lesser, only local importance. The sandstones show fracture porosity. The fractures are either empty or filled with carbonates, quartz and bitumens. Together with microstylolites, they were potential paths for hydrocarbon migrations. It has been confirmed that the Cambrian sandstones form a fracture-pore type of reservoir rocks.