V. Bianchi

Paleo-environmental reconstruction of Lopingian (upper Permian) sediments in the Galilee Basin, Queensland, Australia

ABSTRACT The recent review of the Lopingian (upper Permian) stratigraphic framework of the Galilee Basin, prompted a reconsideration of the paleo-environments of deposition. This study interpreted the distribution of sedimentary facies from geophysical logs across the basin complemented by detailed logging from four key wells (GSQ Tambo 1-1A, OEC Glue Pot Creek 1, CRD Montani 1 and GSQ Muttaburra 1). Seven facies associations were identified: terrestrial fluvial, floodplain, lacustrine and mire; and paralic to marine estuarine shoreline, delta and restricted marine. Coal measures (mire facies) are best developed in the northeastern margin of the basin, whereas the southern Springsure Shelf was dominated by marine conditions throughout the Lopingian, only developing terrestrial facies towards the very uppermost Lopingian. The ‘Colinlea Sandstone equivalent’ was deposited in a fluvial system, with tidal influence exhibited in the southern part of the basin, which decreases further north as lacustrine environments become common. The regional transgression represented by the Black Alley Shale can be mapped into the central part of the basin, but based on new exploration data its northern extent is more limited than previously thought. The ‘Burngrove Formation equivalent’ and Bandanna Formation represent a southerly prograding fluvial-deltaic system during the regional regression in the upper part of the Lopingian.

Evidence for marine influence in the Lower Jurassic Precipice Sandstone, Surat Basin, eastern Australia

ABSTRACT The Lower Jurassic (Sinemurian to Pliensbachian) Precipice Sandstone is the oldest formation and a regional aquifer in the Surat Basin, straddling southeast Queensland and northern New South Wales. Time-equivalent units are recorded in the Eromanga (Poolowanna Formation) and Clarence-Moreton (Ripley Road /Helidon Formation) basins. The Precipice Sandstone is generally considered to consist of braided channel deposits, which upwards become increasingly sinuous or meandering and ultimately give way to a lacustrine succession—the Evergreen Formation. Marine influence has been proposed by some, for parts of the Evergreen Formation. The transition from the Precipice Sandstone to the conformably overlying Evergreen Formation is gradual and diachronous leading to stratigraphic nomenclature inconsistency around the boundary. During sedimentological description of the Precipice Sandstone core from one of QGC Ptys ground water wells (Woleebee Creek GW4) ‘tidal’ features (flaser and wavy bedding, synaeresis cracks, clay drapes and Lockeia traces) were recorded. Biostratigraphy samples contained ‘brackish’ or coastal margin flora supporting the tidal interpretation. Although not sampled for biostratigraphy, similar ‘tidal’ facies have been noted where the Precipice Sandstone crops out in the Carnarvon Gorge section of Carnarvon National Park (Queensland), ∼200 km from the Woleebee Creek GW4 well, suggesting widespread development of a tidal system or systems. The Precipice Sandstone can be divided into an upper and lower unit; each has a distinct sedimentological and electric or wireline-log character, which are described and have been mapped in over 750 wells across the Surat Basin.

A seismic investigation into the geometry and controls upon alluvial architecture in the Walloon Subgroup, Surat Basin, Queensland

ABSTRACT The sedimentology of the Walloon Subgroup (WSG) has been extensively studied; however, gaps exist in our understanding of the successions alluvial architecture and the mechanisms controlling its complex internal organisation. Successful coal-seam gas development in the Surat Basin requires the construction of predictive facies models, which in turn necessitates a fulsome understanding of the geometry and controls on the spatial and temporal distribution of alluvial sub-environments. To improve our models of WSG facies, this study employs an open-source high-resolution 3D seismic dataset available on the western limb of the Surat Basin. Integration of core, wireline and seismic data has resolved the geometries of four discrete alluvial architectural elements, representing simple channel, channel belt, crevasse splay complexes and peat-mire sub-environments. Channel belts were found to be 1600–2000 m wide, simple channels 400–800 m in width and crevasse splays averaging 3.5 × 5.5 km. Coal bodies mapped from seismic attribute extractions were found to be 4.2 km2 on average. The high-resolution dataset has also yielded insight into the geological controls governing the spatial and temporal distribution of these sub-environments, explaining, in part the mechanisms responsible for the complex internal distribution of facies within the WSG. In places within the study area, the WSGs sedimentary organisation appears to be initiated by the rejuvenation of deep-seated tectonic features, the expression of which is propagated upward via the mechanics of compensational stacking.

Conditioning of Stratigraphic Forward Modelling Using Geostatistics

The combination of Stratigraphic Forward Modelling and geostatistical technique can be used to improve the quality and comprehensiveness of the sedimentary reservoir model then reduce the costly data collection procedures to understand the reservoirs. The Stratigraphic forward modelling (SFM) is a process-based method comprising s realistic complexities and heterogeneities of its subsurface sedimentary architecture and may be applied in generating an enhanced model where limited well and seismic data available. The proposed method in this study is to generate a large SFM-basin scale then identify a field-scale location within this and applying some relevant transformation factor that matches the field conceptual deposition. Grainsize distributions may be inferred from well logs and compared against the grain size model in the SFM. Set of differences or residuals can be analyzed spatially and modeled using geostatistical techniques. Adding the modeled residuals to the SFM grain size will ensure that the hard data is honored. It is expected that more reasonable and defendable model characterization can be generated along with the main co-existed uncertainties and risks that can be captured and analyzed. The proposed methodology will incorporate the best advantage of each technique: process-based method for geological realism and Geostatistic methods for data conditioning.

Mapping a coastal transition in braided systems: an example from the Precipice Sandstone, Surat Basin

ABSTRACT The Precipice Sandstone is traditionally interpreted as a braided fluvial deposit that transitions upwards into meandering channel deposits responding to a rise in base level that eventually deposits the overlying alluvial to lacustrine Evergreen Formation. This study found sedimentary evidence of tidal to marine influence within the Precipice Sandstone coincident with avulsion and diversion of the system from southward to northward-flowing channels as the system was transgressed. The north-flowing channels are interpreted to debouch into a shallow restricted marine embayment with tide and wave influence, which provides an alternative insight into this unit and suggests a Lower Jurassic north or northeasterly marine connection. The Precipice Sandstone is a regional aquifer, in places hosts hydrocarbons and has been considered as a storage unit for CO2 geosequestration. Outcrop analogues can provide geometries to accompany facies interpreted from sedimentary structures that are observable in core, to assist in characterising reservoir heterogeneity.