Seismic stratigraphic interpretation of megasequence and sequence framework in the Upper Cambrian through Devonian formations in the central part of the Darling Basin, western New South Wales, Australia

Abstract

Upper Cambrian through Devonian formations in the central part of the Darling Basin, western New South Wales were investigated using seismic data, complemented by well and outcrop information. Detailed seismic stratigraphic interpretation of four main second-order seismic megasequence buildups in the central-northern Blantyre and southern Pondie Range sub-basins, shows that they are constructed of seventeen principal third-order seismic sequences. The four seismic megasequences are characterized as follows: Seismic megasequence A corresponds to the Upper Cambrian through Middle Ordovician strata and is equivalent to the Scropes Range Formation or Mutawintji Group, Seismic megasequence B corresponds to uppermost Silurian through Lower Devonian strata and is equivalent to the Winduck Interval, Seismic megasequence C corresponds to upper Lower Devonian through lower Middle Devonian strata and is equivalent to the Snake Cave Interval and Seismic megasequence D corresponds to upper Middle Devonian through Upper Devonian strata and is equivalent to the Ravendale Interval. Five regional seismic-stratigraphic boundaries have been identified in the three seismic sections to show the continuity of the Upper Cambrian through Devonian formations throughout the central-northern Blantyre and southern Pondie Range sub-basins. All seismic-stratigraphic boundaries are based on good continuous markers, with strong amplitudes throughout the whole sub-basins.Detailed sedimentological analysis was applied using lithologic samples, wireline-log and limited biostratigraphic information, to aid in the development of a paleodepositional environment for the Upper Cambrian through Devonian sequence which displays ten different sedimentary facies successions. Seismic facies analysis was also applied to some of the seismic lines, comparing well log observation with reflection configuration and other seismic characteristics for the Upper Cambrian through Devonian successions. The seismic facies patterns show continuous, semicontinuous and discontinuous reflections with moderate amplitude and high to moderate frequency. The sedimentary sequence from the well data can also be recognized in the seismic lines, with features such as subparallel to divergent high amplitude reflections, parallel to subparallel medium to low amplitude reflectors with high-amplitude and high-continuity reflections, sediment waves with hummocky clinoforms and sometimes high-angle clinoforms, indicating fluvial channels complex and shallow channel-fill complex. As a result, a sequence stratigraphic framework was constructed successfully, consisting of nine seismic sequences. Six of these contain three systems tracts (lowstand, transgressive to highstand) and two sequence surfaces (transgressive surface and marine flooding surface). Another seismic sequence is composed of two systems tracts, a transgressive systems tract and a highstand systems tract separated by a maximum flooding surface. The other two, within the Blantyre 1 well and the Pondie Range 1 well, consist of one stratal pattern that has a highstand systems tract capped by a seismic sequence boundary.