HYDROCARBON POTENTIAL OF SANDSTONE RESERVOIRS IN THE EAST SLOVAKIAN BASIN, PART 2: ZEOLITES AND CLAY MINERALS

Zeolites and authigenic/diagenetic clay minerals in Neogene sandstones of the East Slovakian Basin are closely related to the presence of locally-abundant tuffs and tuffaceous sediments. In tuffs, plagioclase crystals, glass shards, and volcanic rock fragments alter to zeolites and clay minerals. In sandstones, zeolites are formed both from the direct alteration of volcanic rock fragments, and as authigenic pore-filling minerals; clay minerals occur as diagenetic products of both volcanics and feldspars. Zeolites present include erionite, clinoptilolite, mordenite, analcime, and minor offretite, phillipsite, and laumontite. Authigenic/diagenetic clay minerals include smectite, illite, chlorite, and kaolinite. Vertical and lateral distributions of authigenic/ diagenetic mineral assemblages tend to follow published models. A complete paragenetic model of the occurrences of these minerals within the basin is not currently possible with the present, limited database.
Diagenetic/authigenic mineral suites in sandstone hydrocarbon reservoirs present special problems for exploration and production technology. These problems include: (1) well-log distortions; (2) reservoir permeability reduction through diagenesis; and (3) reduction of permeability through interactions between diagenetic clay minerals and zeolites and incompatible fluid systems. Recognition of problematic mineral suites in this study allows for the application of accepted technological solutions, which in turn could result in the increase of recoverable reserves in addition to those added by exploration efforts.