Application of Image Analysis in Assessment of Latrobe Group Sandstones Reservoir Properties, Gippsland Basin, Australia
Latrobe Group sandstones (Upper Cretaceous to Eocene) of the Australian Gippsland basin contain irregularly distributed dolomite-cemented beds which are a major cause of porosity reduction. Conventional methods of evaluating porosity and other reservoir properties in cement-free areas are often unreliable because of poor core recovery, mud-solids invasion, and variable connate-water salinity. Optical-imaging techniques were developed to determine porosity, permeability, cement fractions, and grain-size distributions of cemented and uncemented Latrobe sandstone samples. Image analysis was done before and after acid etching with dilute HCl to obtain dolomite cement volumes. The potential porosity or intergranular volume of the dolomite-cemented samples was then determined b summing the existing porosity and the volume of dolomite cement. This method has been used successfully to assess porosity and permeability of the uncemented, poorly consolidated parts of the reservoir, where undisturbed core material cannot be obtained for plug analysis.
Image-analysis porosity values and calculated permeabilities for the Latrobe samples studied compare favorably with nearby conventional plug analyses. The image-analysis porosities of cemented samples range from 2 to 12% and dolomite cement fractions range from 27 to 39%. Permeabilities ranging from 17 to 33 darcys were calculated after acid etching using the Kozeny-Carman relationship. Exceptionally high porosity, greater than 34%, and permeabilities greater than 10 darcys have been measured by conventional core-plug analysis in consolidated material where the cement is absent.
Image-analysis grain-size data can be useful for relating cementation to sediment fabric, as grains of both cemented and uncemented samples can be readily measured. All samples are generally poorly sorted with grains commonly etched and fractured. However, cemented samples are in the medium sand range with a bimodal distribution, whereas uncemented samples are in the fine-sand range and are characterized by a skewed, single-mode distribution.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.