Distribution of porosity and permeability in dolomite and non- dolomitized carbonate facies- analysis with a Minipermeameter

Abstract

The core from well 7030/03-U-01, drilled on the eastern part of Finnmark Platform includes mixed siliciclastic-carbonate and carbonate sediments of the Falk and Ørn formations of the Gipsdalen Group. The carbonate sediments represent various environments from sabkha associated with evaporites and lagoon to open marine. The majority of the facies have been more or less affected by dolomitization. The aim of the study was to examine permeability distributions in dolomite and non- dolomitized carbonate facies by performing minipermeameter analysis on the complex carbonate rocks. The analytical approach included sedimentary core description for facies distribution and dolomitization, thin section analysis for porosity types, and minipermeability analysis using a portable TinyPerm II. The core was broadly divided into porous/permeable and tight intervals, and porosity types and distributions were recorded. Small-scale measurements down to 5 cm spacing were performed on the porous/permeable intervals. By comparing the results for similar facies groups a dependency of early dolomitization on reservoir quality is verified for the majority of the facies, as dolomitization creates porosity and pore-connectivity. Highest permeability is related to sucrosic dolomite texture, while low permeability is due to pore-filling anhydrite and tightly spaced microdolomite. In the undolomitized intervals, high permeability is related to vuggy porosity and primary interparticle porosity in the packestone/grainstone facies, whereas low permeability is commonly associated with compaction and calcite cementation. The study has shown that the miniperm values are 2-3 orders of magnitude higher than conventional core plug permeability in the relatively tight intervals, and 2- 5 times higher in the permeable zones, following the same trends as the plug measurements. Therefore the miniperm data are applicable to show permeability trends, rather than for estimating absolute Darcy values. The miniperm measurments in the permeable carbonate intervals are highly variable, reflecting heterogeneous porosity distributions. In contrast, siliciclastic- rich intervals with dolomitic sandstone show relatively uniform and high minipermeability values throughout, reflecting a homogeneous interparticle porosity distribution.