Porosity Reducing (And Enhancing) Mechanisms In The Gippsland Basin - A Study Of Three Contrasting Areas: Blackback, Flounder And Tuna.
Honours Degree, 1992
University of Adelaide
Five wells were chosen from the Gippsland Basin in S.E. Australia to illustrate the controls on porosity in the Upper Cretaceous reservoir sequences.
Thirty two samples were taken from cores in Tuna #4, Flounder #1 and Blackback #1, Terakihi #1 and Hapuku #1, the majority corning from medium to coarse fluvial sands. Six samples of sidewall core and two of ditch cuttings were taken from Blackback #1 Side Track #2. Thin sections were made and described of all the samples, from which eleven samples were chosen for SEM analysis and ten for CL analysis. The bulk mineralogy of all the samples was tested using X-ray diffraction.
A key part of the petrological analysis was the description of Intergranular Volume (IGV), the sum of porosity and authigenic cements as a percent of the rock. Study of IGV allows assessment of timing and relative importance of compaction and cementation on porosity. Complications arose in assessing the relative contribution of etching of framework grains by carbonate cement to IGV. This study has compared the IGVs in each hole with the presence of cements and clays, and then looked for similarities in trends, both down-hole and between the diagenetic processes that were observed. These observations were then compared to log and core plug data.
Two trends in porosity were observed:
- A "normal" decrease in porosity with depth (Tuna, Flounder).
- high but irregular porosity values at depth (Blackback, Terakihi and Hapuku).
Two main diagenetic associations were observed:
- Feldspar dissolution, quartz cement, kaolin cement, feldspar alteration with moderate illite cement,
- K-feldspar preservation and overgrowths, followed by poikilotopic carbonate cementation.
Other diagenetic sequences observed but not necessarily ascribed to either association include glauconite cement and alteration, siderite cement and pyrite.
These diagenetic sequences are dependant upon whether the pore-water was saturated with respect to potassium during the early stages of diagenesis. If they were under-saturated, then the first sequence took place. The dissolution of feldspars was followed by quartz, kaolin and illite cement and minor to moderate late stage carbonate. Although there is a "normal" loss of porosity with depth, the IGVs observed do not go below 20%. The IGVs increase almost proportionally with the introduction of carbonate. This suggests that without the presence of carbonate cement there is about a 15% loss in porosity due to compaction. Early carbonate cement that preserved high IGVs and prevents this diagenetic sequence is uncommon.
The second diagenetic sequence is associated with pore-water saturated with respect to potassium during early diagenesis. This sequence was found in Blackback, Terakihi and Hapuku. These samples also showed evidence of marine conditions during early diagenesis in the form of authigenic and altered glauconite. This sequence appears to have halted at the stage of feldspar overgrowths. The main binding material in the Blackback area is the carbonate cement. Even samples where there is no evidence of carbonate ever having been present show a halt to diagenetic development at the K-feldspar overgrowth stage. Samples with substantial carbonate cement have a high IGV, showing cementation to be an early event. It may be the presence of this early cement that has protected the surrounding rock from compaction and associated porosity loss, judging from the lack of authigenic clays, the through-flow of ground-water.