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Depositional Environment And Diagenetic History Of The Neocomian Section In The Exmouth And Southern Barrow Sub-Basins (North West Shelf).

Novosell, Naomi Ann

Honours Degree, 1995

University of Adelaide

Abstract

The diagenetic history of the Neocomian section in both the Exmouth and southern Barrow Sub-Basins was studied using a selection of 235 thin sections from 23 wells. The information gained from the petrography of these thin sections was linked with core logs, wireline logs and seismic sections to add to the depositional history of the Carnarvon Basin.

The Barrow delta system was deposited over a series of horsts and grabens formed in a rift system with associated structural highs and troughs. Characteristics of this sequence is the interbedded nature of silty matrix rich layers with "clean" sandstone layers. There is a difference in the glauconite content of the Barrow Group in the Macedon and Pyrenees fields (1% to 16%) as compared to Novara #1 and Serrurier #1 where it occurs in trace amounts only.

Unconformably overlying this unit is the Zeepaard Formation. This unit was influenced by mechanical compaction and cementation. The main cements are kaolinite, illite and quartz. Siderite and ca1cite cement is rare to non existent. The matrix content in the thin sections is low and this indicates winnowing of clays as the feldspar content is similar or greater than that in all the other rock units studied.

The Birdrong Formation unconformably overlies the Zeepaard Formation. Matrix contents detected in thin section vary. The cements siderite, calcite, pyrite, illite and chlorite are the main contributors to the reduction in porosity for this sequence.

The most striking aspect of the Mardie Greensand is the high incidence of pelletal and cement glauconite. Two types of siderite cement as well as pyrite, smectite and illite occlude the pore spaces. An organic hump seen in the XRD spectrum for samples from this unit together with the presence of pyrite and glauconite pellets, suggests a shallow water environment with reworked sediments as the model for formation for this unit.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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