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An Evaluation Of Seals, Reservoirs And Fault Sealing Potential In The Eyre Sub-Basin, Great Australian Bight.

Quinn, David Martin Thomas

Honours Degree, 1999

University of Adelaide


A study of the hydrocarbon-sealing potential of seals, reservoirs and faults was performed in the Eyre Sub-Basin (Great Australian Bight) to assess the quality of the sealing lithologies as intersected at the Jerboa #1 well, and to predict the likely impact of faults breaching the sealing horizons. Many of the deep half-graben blocks within the rift sequence of the Bight Basin exhibit similar structural geometries and rift-fill morphologies. This study aims to provide a tool for predicting the fault sealing potential away from the Jerboa #1 well using the geochemical, petrophysical and fault attribute data as a guide.

Mercury injection capillary pressure (MICP) analysis was used to evaluate the sealing potential of the Platypus, Ceduna, Borda, Neptune, Echidna and Loongana formations. This analysis provided data on probable hydrocarbon column heights supported by measurements from formations within mapped sequences. Rock samples were taken from sidewall cores of the Jerboa #1 well. Samples were analysed by X-ray diffraction to group them on the basis of mineralogy. Thin section analysis was used to characterise the reservoir interval.

A preliminary fault seal study was performed using software called ?Triangle?. This technique provides a way of assessing the juxtaposition and fault-seal potential at various fault throws. This analysis provided insight into the fault-sealing problem predicted to be the reason behind failure of the Jerboa #1 well.

MICP suggests the Late Jurassic-Early Cretaceous Echidna Formation has the greatest seal capacity (potential oil column height of 1623m) of strata analysed during this study. However, the formation with the second highest potential column height and the best fault sealing attributes was the Early Cretaceous Lower Borda Formation, with a calculated oil column height of 1432m and a shale gouge ratio (SGR) between 0.9 and 1.0. This suggests the potential for fault sealing via clay smearing processes is very high. The Echidna Formation seal exists lower in the rift fill succession where reservoir intervals are thin and of poor quality.

The Early Cretaceous Neptune Formation contains the best reservoir lithology. The clean sands show high porosities and permeabilities as measured by MICP and Image Analysis. Threshold pressures for this reservoir zone were between 0.1 and 3.3 psi (brine/hydrocarbon system).

Mineralogy by XRD confirmed the groupings by formation. Smectite is abundant in the Upper and Lower Borda Formations. A change from potassium feldspar dominance to plagioclase indicates a change in rift provenance from dominantly intra-rift fill, to more regionally-derived sediment fill, at a depth of 2058m.

The principal fault sealing process is clay smearing due to the high volume of clay material in the faulted sealing lithologies. Fault seal analysis indicates the presence of low shale gouge ratios on some fault planes. These may form ?leaky windows? allowing cross-fault hydrocarbon migration on some throw values.

This study demonstrates the presence of viable seals within the sequences penetrated by Jerboa #1. These sealing units are relatively uniform and can be extrapolated beyond the well by the use of seismic mapping techniques. The sealing units have varying degrees of fault-sealing capacity and have the potential to seal and maintain an extensive hydrocarbon column up to 1623m thick. Analysis of the reservoir interval shows that the reservoir is currently pressurised and that the seal is intact, as no re-compaction of the rocks has been noted from the thin section analysis after losing the initial hydrocarbon charge. This suggests that the reservoir could hold another hydrocarbon charge. The Jerboa #1 well failed initially due to major faulting of the sequence. The faults have subsequently resealed by the deposition of cements on the fault plane under the influence of solute rich, formation water migration. The potential exists within other areas of the Bight Basin for similar scenarios.

Australian School of Petroleum



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