ABSTRACT: Sequence Analysis of a Maastrichtian Shelf Margin Systems Tract on the Australian NW Shelf

ZIOLKOWSKI, VICTOR, Independent Consultant, Brisbane, Australia, NOEL GUPPY, Formerly Western Mining Corporation, Perth, Australia, and JAMES DURRANT,* Durrant and Associates, Darlington, West Australia

Thick marginal sandstones of early marine Maastrichtian age underlying porous Tertiary carbonates have been encountered on the southern flanks of the Sahul/Echo Syncline and the Londonderry High. In wells to the north, this sandstone facies is absent passing into marine shales of similar age. Intermediate wells in the area have encountered these shales overlying the Maastrichtian sandstones. A sequence stratigraphic study was undertaken to determine if the distribution of the reservoir sandstone overlain by sufficient sealing shale was predictable.

Using seismic data, an isochronous map was constructed of the Maastrichtian sequence consisting of a shelf margin systems tract which is overlain by transgressive systems tract marine shales. The lower sequence boundary shows evidence of downlap and erosion. The upper sequence boundary (Top Bathurst Island Group) indicates toplap and erosion. The transgressive systems tract has a tabular form that thins onto the shelf margin wedge in the south.

A zone corresponding to the distribution of a sufficient thickness of shelf margin wedge sandstone (reservoir) overlain by a sufficient thickness of transgressive marine shales (seal) was defined. Suitable trap types within the zone are structural drapes over deep seated fault blocks or depositional mounds (basin floor fans). Fault dependent structures would need to have a throw of less than 50 m to sustain seal integrity against the overlying their shale seal.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)