Chapter 6: Basin-scale Migration-fluid Flow, Sealing, and Leakage-seepage Processes, Gippsland Basin, Australia
G. W. O’Brien, L. M. Goldie Divko, P. R. Tingate, M. L. Harrison, J. Hamilton, K. Liu, M. Campi, J. Miranda, 2013. "Basin-scale Migration-fluid Flow, Sealing, and Leakage-seepage Processes, Gippsland Basin, Australia", Hydrocarbon Seepage: From Source to Surface, Fred Aminzadeh, Timothy B. Berge, David L. Connolly
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The migration architecture of the Gippsland Basin, Australia, is dominated by two highly connected, filled-to-spill fill and spill (fill-spill) chains — the northern chain (gas dominated) and the southern chain (oil dominated) — that extend east–west across the basin and link at its far western nearshore part, forming a convergent chain that then extends onshore. The reservoir units across the basin are sealed by smectite-rich marine claystones that have very high seal potential within the central basin but become less effective toward the basin flanks and onshore. Decreasing top-seal potential along the convergent fill-spill chain onshore has localized the formation of a 25-km-long zone of leakage and seepage; here, leaking hydrocarbons are expressed as gas chimneys, as natural seeps, and as a prominent zone of shallow uranium enrichment. Active seepage, documented by a combination of chimney mapping and water-column geochemical sniffer data, also occurs in several areas offshore — mostly along the basin margins at the conjunction of well-developed migration fairways and zones of failing top and fault seal. Fluid inclusion and migration-modeling data reveal that the first major hydrocarbon charge in the basin, including that into the giant gas fields that dominate the northern fill-spill chain, was oil; this charge appears to have filled the traps to spill point, probably in the Late Miocene. Gas subsequently entered many of these traps in the Pliocene and displaced the oil, pushing it farther along the fill-spill chains. A lack of gas charge into the eastern portion of the southern fill-spill chain preserved the early oil charge along that trend. The integration of basin-scale fluid-flow modeling with assessments of seal integrity, charge history, and leakage-seepage processes provides a powerful, generic, predictive approach for assessing not only the petroleum systems and hydrocarbon prospectivity of a basin but also its ultimate CO2 geostorage potential.
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With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.