Abstract

Petrographic studies of Miocene carbonates from a wide range of localities in Indonesia have shown that they have been subjected to a number of diagenetic processes including dissolution, cementation, neomorphism, silicification, dolomitisation, fracturing and stylolization. Whilst most of these processes are common to all the depositional facies types (e.g. reefal, near reef, shallow shelf and outer shelf open marine facies), the reasons for the development of reservoir rocks depend on which of the above mentioned processes have been dominant. The original particle types of the sediments play an important role in this respect. Coral reefs in which a large volume of the original sedimentary particles consist of interconnected corallites commonly form good reservoir rocks in view of the fact that the aragonite skeletons of the corals frequently suffer dissolution resulting in good mouldic porosities. Elsewhere, however, the phase of dissolution is followed by cementation with a resulting loss of porosity. The effects of neomorphism also cause porosity reduction especially with respect to fine scale porosity within the micrite matrix. Dolomitisation has a varied distribution and in many cases is associated with clay minerals. Petrographic examination indicates, however, that porosities are only rarely developed within the dolomite mosaics. The other diagenetic processes including stylolitization and silicification are much more localised features.