--> Abstract: Applications of Sequence Stratigraphy to Permian Coal Measures, Cooper Basin, South Australia, by F. L. Weht and E. M. Johnstone; #91012 (1992).

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ABSTRACT: Applications of Sequence Stratigraphy to Permian Coal Measures, Cooper Basin, South Australia

WEHR, FREDERICK L., Exxon Production Research Co., Houston, TX, and E. M. JOHNSTONE and J. W. HUNT, Esso Australia Ltd., Sydney, Australia

The Cooper basin is a Permian-Triassic intracratonic rift formed during foreland deformation along the eastern margin of Australia. Its early fill includes a succession of coal-bearing alluvial and lacustrine deposits (Patchawarra Formation). The Patchawarra Formation accumulated during active faulting and varies in thickness from 0 to over 1500 ft within the southern Cooper basin. Understanding the mechanics of thickness change is essential in exploring for off-structural traps in the Patchawarra.

Within the study area, the upper Patchawarra Formation contains three low-ash, inertinite-rich coals of regional extent (>500 sq. miles), ranging in thickness from 20 to over 100 ft. The three intervals separated by the coals consist of fluvial channel sandstones, fine-grained floodbasin deposits, and thin coals. Within these intervals the largest fluvial channels tend to be concentrated in the lower portions. Intra-Patchawarra thickness changes from basin deeps to highs is accommodated by a combination of onlap and interval thinning. Both seismic and well-log data indicate that the regional coals approximate the surfaces of onlap. This pattern combined with the coal petrology suggest that the Patchawarra coals formed during prolonged periods of low subsidence and low depositional radients, separated by pulses of increased subsidence and clastic influx into the basin. The coals are thus considered to approximate depositional sequence boundaries, formed during depositional hiatuses and overlain by basinward shifts in onlap. These Patchawarra "sequences" appear to be the product of episodes of normal faulting and differential subsidence rather than externally forced (eustatic) base-level changes.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)