ÁKOS TÖRÖK

Budapest Technical University, Department of Engineering Geology, [email protected]

ABSTRACT: Pore Occlusion by Dolomitization, a New Insight into the Diagenetic History of an Epeiric Ramp

As most low energy ramps the Middle Triassic epeiric ramp of Southern Hungary contains very few grain-rich facies and reservoir potential rely on secondary porosity created by diagenesis (dolomitization) or fractures. The present paper describes the porosity and diagenetic evolution of the Anisian mid- and outer ramp part of the 700 m-tick sequence that show partial dolomitization. Dolomites occur in limestone as irregular mottles and as poreand fracture-filling cements.

Replacement-type scattered dolomite rhombs with inclusion-rich, very dull cores and limpid non-luminescent outer zones represent the initial phase of dolomitization. The isotopic composition of these Sr-rich, nonstoichiometric dolomites (δ¹³C + 2.3^o/_oo, δ¹⁸O -3.6^o/_oo) is similar to that of the calcitic micrite (δ¹³C +2.6^o/_oo, δ ¹⁸O - 4.0^o/_oo) indicating that no external fluids were introduced during dolomite formation, i.e. shallow burial diagenesis, a "marine" pore-fluid and a partly closed system. Fracture-related saddle dolomites are responsible for pore occlusion and their presence marks elevated formation temperatures and a maximum burial in the Cretaceous.

Multiple generations of calcite-filled fractures were formed during burial diagenesis generally having dull or no luminescence (δ¹³C +1.8^o/_oo, δ¹⁸O -6.4^o/_oo). The karst related processes include dissolution, calcite precipitation and partial replacement of dolomites by complex zoned bright yellow calcite. All dissolution processes were followed by intense calcite cement precipitation (δ¹³C = -5.0 to -7.2^o/_oo, δ¹⁸O = -7.4^o/_oo) eliminating pre-existing porosity.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado