Dolomitisation of the Early to Middle Cambrian Carbonate Rocks of Georgina Basin, Northern Territory, Austrlaia
Degree of Master of Science, 1992
The University of Adelaide
The sequence consists of shallow water peritidal and deeper water subtidal (reefal and lagoonal) lithologies and has been entirely dolomitized during a complex diagenetic history. Diagenetic events include multiple stages of dolomitization, evaporite formation, silicification, mineralization, hydrocarbon migration, compaction and secondary porosity development and occlusion. Three dolomite types are recognized: synsedimentary, replacement and cement. Synsedimentary dolomite is now a minor component and was derived from syndepositional brines in a sabkha environment. Finely crystalline anhydrite was precipitated syndepositionally with this dolomite. Replacement dolomite is volumetrically the most widespread and varies in crystal size from fine to coarse, forming xenotopic to hypidiotopic crystal mosaics. Under cathodoluminescence, it is non-zoned and has a mottled appearance with brightly to dully luminescent patches. Dolomite cement is also volumetrically important and forms coarsely crystalline idiotopic to hypidiotopic crystal mosaics. Some crystals undergo undulose extinction and have curved planes. Under cathodoluminescence, the dolomite cement is zoned, with an early non-luminescent zone, followed by a brightly luminescent zone which is followed by a dully luminescent zone. This transition is typical of a burial sequence. Coarsely crystalline anhydrite cement frequently postdates the dolomite cement.
Geochemically, the synsedimentary dolomites and the dolomite cements are distinctive. The supratidal "primary" dolomites have ?18O(PDB) values that range between -8.61 and -5.35o/oo, whereas the dolomite cements' ?18O(PDB) values are significantly more negative, ranging from -14.34 to -10.80 o/oo. Replacement dolomites have a wide range of ?18O(PDB) from -10.90 to -3.30o/oo reflecting differing solution chemistries at the time of replacement. The ?13C values of all three dolomite types are quite similar.
Both the petrographic and stable isotope characteristics imply three dolomitization episodes: a sabkha stage, which affected the peritidal lithologies, a brine-reflux stage, which affected the subtidal lithologies and a later burial stage, during which the entire sequence was pervasively dolomitized by evaporite-related, dolomite-saturated diagenetic fluids. Overall porosity in the sequence is low with higher porosities developed as secondary intercrystalline and vuggy intervals in zones no more than a few centimetres thick. The original voids within the matrix are either completely or partially infilled by dolomite and / or anhydrite cements. The reservoir potential of the studied interval is poor. To obtain subsurface porosity of reservoir grade and extent will require identification of areas with either partial preservation of intercrystalline / mouldic porosity around the fringes of the dolomitization zone or preservation of burial-stage porosity along conduits that were permeable at the time of oil emplacement, such as fault-associated conduits or along sharp lithological boundaries (both depositional and diagenetic).