DIAGENESIS AND THE EVOLUTION OF GIPPSLAND BASIN RESERVOIRS

Abstract

Key findings of this study on the nature, distribution and influences of diagenetic modifications to the Latrobe Group, Gippsland Basin, are: The Latrobe Group sandstones do not exhibit the regular porosity/depth patterns regarded as typical for many sedimentary basins. Dolomite cements are widely though variably developed in sandstones that are characteristically medium-to very coarse-grained, matrix-poor, texturally and mineralogically mature. These cements fill pore spaces, exhibit partial framework replacive textures, and may comprise as much as 30 per cent of total rock volume. In Flounder, Snapper, Tuna, Marlin, and probably other fields dolomite cementation is the major cause of porosity reduction. Sandstone porosity, notably in the above examples, is largely secondary and principally derived from the dissolution of precursor dolomite cements. Advanced dissolution results in exceptionally high porosity and permeability, and is largely responsible for the character of Snapper, Marlin, Tuna, Flounder and probably other reservoirs. Dedolomitization appears to be closely associated with hydrocarbon emplacement. Another important form of secondary porosity, developed in sandstones lacking evidence of extensive dolomitization, results from the dissolution of clay matrix. This contributes significantly to Mackerel. Hapuku, Kingfish and Halibut reservoir quality. Feldspar and rock fragment dissolution and alteration, as well as intra-clast fracturing, also contribute minor secondary porosity. Secondary porosity may subsequently be reduced by authigenic kaolinite (with or without illite) and rare chlorite filling, quartz cementation, framework restructuring and compaction, such as is typical of Bream, deeper Barracouta, Snapper, Tuna and other deeper arenite strata. Chemical modelling of the interaction between dolomite and aqueous fluids indicates that large volumes of fluid are necessary to effect dolomite precipitation and dissolution. Variation in pH and Pco2 related to fluid migration and subsurface mixing are evidently the controlling variables. Carbon and oxygen stable isotope data are compatible with dolomite precipitation from largely non-marine fluids containing organically sourced carbon species. Systematic lithological, spatial, depth and/or thermally related patterns of diagenetic modification are recognized on a basin-wide scale. In view of the significant role diagenetic processes have played in the evolution of hydrocarbon reservoirs, an understanding of their relationship to fluid migration and organic maturation is important to future exploration. In conjunction with knowledge of basin tectonics, thermal and sedimentological history, this understanding is fundamental to developing models which predict porosity trends and reservoir quality in the Gippsland and other sedimentary basins.