ABSTRACT: Well and Seismic Correlation of Devonian Reef Sequences: An Example from the Canning Basin, Western Australia
SOUTHGATE, PETER N., JOHN M. KENNARD, M. JIM JACKSON, PHIL E. O'BRIEN, and MIKE J. SEXTON, Bureau of Mineral Resources, Canberra, Australian Capital Territory, Australia
Integrated sequence interpretation of well logs and seismic data provides a powerful tool for analyzing sedimentary successions in areas where interpretation of seismic data alone is ambiguous, and where well-seismic ties based on synthetic seismograms are equivocal. The method is based on the recognition of repeated wireline log trends both within and between wells, and is illustrated with an example of reef sequences in three wells (Yarrada, Meda, and Terrace) from the Lennard Shelf, northern Canning basin. Interpretations of these trends are constrained by well cuttings, cores and biostratigraphic data. Well interpretations are then extrapolated back to, and mapped, on seismic data.
Sequence boundaries are generally recognized by abrupt shifts or shoulders in wireline log patterns, and in most cases this reflects abrupt lithofacies discontinuities. Depositional systems within a sequence can be identified by more subtle log trends and the stacking patterns of individual log cycles (parasequences). Lowstand deposits are characterized by progradational to aggradational log cycles, and comprise either siliciclastic conglomerate and sandstone (incised braid-stream and deltaic deposits), carbonate conglomerates (slump and debris flow deposits), or bioclastic wackestones and packstones (shallow shelf deposits).
Transgressive deposits usually display a backstepping progradational pattern on gamma logs, culminating at a maximum gamma value. Progradational cycles comprise glauconitic, calcareous sandstone and sandy oolitic carbonates, and the interval of maximum flooding is represented by marine shales or shaly carbonates. Highstand deposits display a long term upward-decreasing gamma trend representing either progradational marginal slope ("clean" uniform gamma log, upward-increasing dipmeter), reef ("clean" uniform gamma log, erratic "bag of nails" dipmeter pattern), or platformal carbonates ("dirty" variable gamma log).
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)