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Diagenesis Of Eastern View Group Sediments, Bass Basin, Tasmania.

Zampatti, David

Honours Degree, 1991

University of Adelaide

Abstract

This project studied diagenetic processes occurring in Bass Basin sediments. Five wells forming an east-west profile across the Bass Basin were used to observe vertical and lateral variations in diagenetic processes.

The study identified two zones of silica cement, termed early silica cement and late silica cement. Early silica cement occurs in the region broadly defined between the initial surface and 4200 feet. Late silica cement occurs in the region below the 80oC to 95oC isotherms. The correlation suggests temperature is an important controlling mechanism in the initiation of late silica cementation.

An external source of silica is proposed for late silica cements where local pressure solution as a mechanism is not active and cementation is proceeding in zones depleted of labile silica-forming components (eg feldspar). Implied from these observations is the importance of silica transport as a soluble species and associated deep fluid movements within the basin.

Rapid compaction adjustments are observed in sediments following dissolution of carbonate cement. Inferred from this observation, is that secondary porosity resulting from dissolution of cement, and where minimal framework support exists, will be transient. Preservation of secondary porosity at depth is suggested as being more likely where dissolution occurs in the zone of late silica cementation where framework supporting cement is forming.

Carbonate cement is observed in all Bass Basin wells. A progression from dolomite to ferroan dolomite to ankerite is observed in various wells with increasing depth. A lateral correlation of the observed increase in iron (Fe2+) content with depth between adjacent wells has not been established, suggesting localised conditions control the Fe + supply.

Stable isotope values of carbon in carbonate indicate cements are sourced predominantly from the breakdown of organic material. A correlation between formation temperature and the oxygen isotope ratio strongly supports an "open system" for carbonate evolution where carbonates are continually adjusting and equilibrating to prevailing conditions.

Carbonate cement postdates early silica cementation in the sediment column as evidenced by etching of overgrowths and is observed throughout deeper sediments. Carbonate cement diminishes to minor occurrences into the late silica cementation zone. Observation of thin sections of deeper sediments indicate the possibility of minor ankerite development into the late silica cementation zone. The study has established the following broad diagenetic evolution within central Bass Basin sediments;

  1. Siderite deposition/formation,
  2. Early silica cementation,
  3. Carbonate emplacement,
  4. Carbonate dissolution, feldspar dissolution,
  5. Late silica cementation, kaolin formation.
Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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