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Late Proterozoic Hydrocarbon Potential And Its Association With Diapirism In Blinman #2, Central Flinders Ranges, South Australia.

Cooper, Andrew Mcgregor

Honours Degree, 1991

University of Adelaide


In early 1991, Blinman #2 drilled through 2031 metres of sediments of the late Adelaidean Umberatana Group an the flank of the Blinman Diapir. Three small methane shows occurred in fracture zones in the Tapley Hill Formation.

Two source intervals were delineated. An interval corresponding to the Tindelpina Shale Member was intersected between 1550-1620 metres with maximum T.O.C. values of approximately 1.2%. A similar interval from 865-1025 metres overlies an anhydrite cemented breccia interpreted to represent reworked extrusive diapiric breccia. This episode of salt movement resulted in the rapid deepening of secondary peripheral sinks and the formation of anoxic conditions suitable for the preservation of organic matter. The effect of syn-depositiona1 diapirism is also evident throughout the cored -section as numerous slumps and turbidites of diapirically sourced sediments.

Analysis of kerogen isolated from the sediments indicates an equivalent vitrinite reflectance of approximately 2%, placing the source rocks just beyond the oil generation window. The lack of maturation is due to diapir re-activation and the resultant `buoyancy' during the Delamerian Orogeny preventing deep burial of the sediments.

The continued movement of the diapir has also resulted in a very complex history of brecciation and cementation. Cement phases include a relic microstromatolitic dolomite rim with bitumen, and saddle dolomite. In general, where fracture porosity is present, gas also occurs.

Synthetic seismograms indicate a strong reflector at the base of the Tapley Hill Formation with weak amplitudes representing various other stratigraphic boundaries.

This study shows diapirism is directly related to all facets of hydrocarbon accumulation in the Blinman area implying further exploration should be concentrated on the diapirs of the Central Flinders Ranges with Martins Well providing a promising target.

Australian School of Petroleum



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