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Reservoir, Theif And Seal Potential Of Aptian Radiolarite In The Timor Sea.

Davies, Krista Leigh

Honours Degree, 1995

University of Technology, Sydney


The Aptian Radiolarite is a concern with respect to breached seal and thief risk in the Timor Sea. This study aims to find the characteristics which determine the role the unit will play in a given area.

The Radiolarite was deposited during the early post-rift period, following the breakup of Gondwana in the Late Jurassic (Oxfordian). Deposition occurred at a transgressive maximum from the Mid Aptian to Early Albian, as part of an overall transgressive sequence tract. Thickness variations follow the basin architectural trends of the time. The unit has a distinctive log response allowing relatively simple discrimination of the unit on a suite of wireline logs. The Radiolarite shows three log patterns within this general trend, which are controlled by the nature of the bounding Intra-Aptian and Intra-Albian unconformities and depth-related facies variations. In deeper wells the unit shows continuous deposition with the overlying unit while in shallower areas the unit is truncated by the Intra-Albian unconformity.

The unit is an extremely fine grained chert with several stages of silica cementation, some calcite cement and minor clay. Sample analysis led to the division of the unit into three facies types (A, B & C).

Sediment bypass and depth-dependant current-reworking controlled the distribution of clays and the thickness of the unit. Early cementation in some areas protected the unit from mechanical compaction and induced early brittle fracturing on a small scale. Burial history modelling suggests this early stage of compaction occurred during the Mid Cretaceous. Carbonaceous matter catalysed silica dissolution, allowing compaction by stylolitization. Loading in the deeper areas where there was no protection by silica cement, compacted the unit by mechanical compaction alone, increasing the heterogeneity of the Radiolarite. With later sediment loading, large vertical fractures formed.

The integration of burial history plots and facies analysis has facilitated the interpretation of the unit away from the control wells and its likely behaviour as a reservoir, seal or thief zone. Parameters that control its behaviour include the depth during the Mid-Cretaceous, rate of burial and timing of maximum sediment loading, degree of late structuring and the proximity to the deep basin.

Depth conversion of exploratory seismic lines will allow the calculation of burial history and the depth to the unit at key times which will ultimately allow prediction of the nature of the unit prior to drilling.

Australian School of Petroleum



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