DIAGENESIS OF SILURIAN REEFAL CARBONATES, KUDIRKA OILFIELD, LITHUANIA
N. Stentoft*, P. Lapinskas** and P. Musteikis+
*Geological Survey of Denmark and Greenland, Øster Voldgade10, DK-1350, Copenhagen K, Denmark. Author for correspondence: email nis@geus.dk
**Institute of Geology, T. Ševčenkos 13, 2600 Vilnius, Lithuania.
+Vilnius University, Čiūrlionio 21/27, 2009 Vilnius, Lithuania.
Upper Silurian carbonate reservoir rocks at the Kudirka oilfield, southern Lithuania, have a complex diagenetic history. This consists of the following stages: compaction and dewatering; early lithification; minor fracturing; ?first generation leaching; precipitation of first- and second-generation inter- and intragranular calcite cements; recrystallization of lime mud, sparry calcite cement and bioclastic grains; chemical compaction and the formation of stylolite-associated fractures; precipitation of dolomite, pyrite and silica; a second phase of leaching, with stylolite surfaces acting as conduits for aggressive fluids; and finally oil emplacement.
The Upper Silurian carbonates are interpreted to correspond to a variety of reefal deposits. In all the types of reef rock analysed, late diagenetic leaching has enhanced reservoir quality. However, no clear correlation could be found between rock texture and reservoir quality. This is mainly because the very common crinoid fragments acted as substrates for the growth of pore-occluding syntaxial calcite cement. The lack of correlation between rock texture and reservoir quality can also be attributed to subordinate localized stylolite-related dolomitization.