ABSTRACT: Prospective Lowstand Fan Systems in EP 129 Canning Basin, Western Australia

DIEKMAN, LEN J., Petroleum Securities Australia Ltd, New South Wales, Australia

Due to the lack of any additional large and conventional exploration targets being formed within EP 129, Canning basin, Western Australia after the assessment of an extensive exploration database, and in the recognition of the necessity to bring into production larger and more productive oil fields within EP 129 to maintain the commercial viability of the existing production operations, alternative prospects were sought with the potential to host large oil reserves.

Consequently, sequence stratigraphic studies commenced early in 1988 quickly focused upon the Upper Devonian and Lower Carboniferous sequences because (1) the seismic reflection characteristics of these beds were recognized as being similar to those in other basins that host large hydrocarbon reserves, (2) within the area studied these sequences have been typed as the source beds for the existing oil discoveries and, (3) there is both geological and geophysical evidence for the presence of siliciclastic reservoir lithologies within these sequences that otherwise are dominated by the presence of carbonate lithologies and that were deposited within a larger hybrid carbonate-siliciclastic system. Studies began by the regional mapping of sequence boundaries and are now concerned with fina izing prospect definition prior to drilling.

The application of cost-effective sequence stratigraphic analysis uncovered six new play types: basin floor fans, slope fans, transgressive backstepping and highstand prograding carbonate plays, incised valley plays, and shingled toe-of-slope turbidite plays, the most prospective of which is considered to be basin floor fan play which, unique among the other play types, more fully satisfies the essential hydrocarbon entrapment requirements of seal, reservoir, and access to mature source in conjunction with potential high reserves.

The relationship between the presence of basin floor fans and the basinwide sequence stratigraphic framework enables the sequence stratigraphic interpretation to be used as an effective predictive tool

for exploration. The diagnostic seismic signature of basin floor fans is that of a locally continuous, high amplitude, externally mounded and internally transparent anomaly overlying and downlapping two major, basinwide downlap surfaces or type I sequence boundaries of late Famennian age. These chronostratigraphic boundaries are correlative with global eustatic events. Seismic attribute displays enable easier identification and better definition of the basin floor fan geometry. Similar, but smaller, mounded anomalies are observed overlying type I sequence boundaries of Tournaisian age.

The interplay between sea level changes, sequence stratigraphy and tectonism is demonstrated by the observed variations in stacking patterns of depositional systems within an evolving rift and post-rift sequence both through time (i.e., vertically) and across the basin topography (i.e., laterally). The distribution and type of carbonate lithofacies are highly sensitive to relative sea level. Carbonates deposited during sea level rises and highstands formed progradational and aggradational sequences whereas the main basin filling, sequences were deposited during sea level lowstands as large fan complexes overlain by overlying prograding complexes.

The application of sequence stratigraphic methods in EP 129 has not only uncovered new and exciting play types and prospects, it has also resolved many old problems of stratigraphic correlation, which has led to a better understanding of basin geology and basin evolution.

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