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Abstract

Studies of producing oil pools of the predominantly fluvial to shallow-lacustrine Eromanga Basin in central Australia indicate that capillary seals have a significant control on hydrocarbon occurrence and production. Oil pools sealed by the Callovian Birkhead Formation at the Bodalla South, Jackson, and Gidgealpa fields, and by the Berriasian Murta Formation at the Murteree Ridge fields, are filled to capillary seal capacity. Resultant upward leakage has taken place, filling oil pools higher up.

The major producing reservoir of the Eromanga Basin is the braided flu-vial Hutton Sandstone. The transition from this to the Birkhead Formation seal is marked by two features: (1) a change from braided fluvial to meandering fluvial and (2) a change from craton-derived (CD) sediment to volcanic-arc-derived (VAD) sediment. The CD and VAD sediments have Sm/Nd model ages of ~1500 and 800 m.y., respectively, suggesting a major provenance change. In the most prospective part of the basin, this provenance change and the resultant switch from quartz to lithic arenites control the location of the effective intra-Birkhead Formation seal. These lithic arenites are the same grain size as the underlying reservoir-quality quartz arenites; however, their pore-throat sizes have been reduced by diagenesis and the precipitation of pore-filling authigenic clays.

In the Murteree Ridge area, a lacustrine, upper shoreface, intra-Murta Formation reservoir is the primary producer. The reservoir that occurs beneath the Murta Formation is the McKinlay Member. Between the McKinlay Member reservoir and the intra-Murta Formation, reservoir vertical stacking of thin, fine-grained, shallow-lacustrine, quartz arenites occurs. This has produced tortuous vertical flow paths, or conduits, which have lower displacement pressures and higher permeabilities than the intervening siltstone layers. These paths have allowed upward migration of oil, by capillary leakage, beyond the McKinlay Member into the intra-Murta Formation reservoir over geological time. These paths have recently allowed the leakage of brine through this interpool seal during production, providing pressure maintenance to the otherwise confined higher oil pool from the very strong water drive of the lower pool.

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