Abstract Past environments of equatorial SE Asia must have played a critical role in determining the timing and trajectory of early human dispersal into and through the region. However, very few reliable terrestrial records are available with which to contextualize human dispersal events. This circumstance, coupled with a sparse archaeological record and the likelihood that much of the archaeological record is now submerged, means we have an incomplete understanding of the role that geography, climate and environment played in shaping human pre-history in this region. From a review of the literature, we conclude that there must have been a substantial environmental barrier resulting in a genetic separation between east and west Sundaland that persisted even though a terrestrial connection was present for most of the Pleistocene. This barrier is likely to be a north–south corridor of open non-forest vegetation, and its existence may have encouraged the rapid dispersal of early humans through the interior of Sundaland and on to Sahul. We conclude that more reliable terrestrial palaeoenvironmental records are required to better understand the links between past environments and dispersal events. We highlight avenues of particular research value, such as focusing on eastern Sumatra, western/southern Borneo and the islands in the Java Sea, where the purported savanna corridor most probably existed, and including edaphic factors in palaeovegetation modelling.

Abstract A 630-km-wide continental shelf characterized by mixed carbonate and siliciclastic sedimentation formed during the Quaternary across the Bonaparte Basin, NW Australia. During this time interval (~2.6 million years), shelf-margin and slope deposits were disconnected from the inner shelf and hinterland by the 200-km-wide, low-gradient Malita intrashelf basin. In this study, two-dimensional (2D) and three-dimensional (3D) seismic, well log, and core data were used to determine the relative importance of allogenic and autogenic controls on the stratigraphic architecture of shelf-edge and slope deposits at multiple timescales. This work has determined that Quaternary sea-level variations (glacio-eustasy) provided a primary control on the stratigraphic evolution of shelf-margin and slope deposits. The early Quaternary period was marked by the aggradation and progradation of a carbonate margin under global sea-level highstand conditions. The onset of high-amplitude sea-level fluctuations at the Mid-Pleistocene Transition (ca. 0.9 Ma BP) enhanced the development of a mixed clastic-carbonate margin and slope system. During the late Quaternary, long-duration sea-level falls and lowstands and high rates of terrigenous sediment supply resulted in stacked fourth-and fifth-order systems tracts in the form of prograded shelf-margin and slope wedges. Conversely, rapid, high-amplitude, fourth-and fifth-order transgressions between these time intervals enhanced the aggradation of carbonate buildups at the shelf edge. Hence, high-frequency sea-level changes resulted in reciprocal sedimentation similar to many other mixed depositional systems of the late Quaternary. However, the main locus of carbonate and mixed deposition across the Bonaparte Basin shelf margin and slope varied spatially at longer times scales. Indeed, conventional seismic data have revealed that the third-order systems tracts at two separate locations in the Bonaparte Basin (eastern and northwestern shelf-margin) show stratigraphic asymmetry (rimmed carbonate margin vs. shelf-margin and slope progradation), which reversed during the late Quaternary. Our results suggest that this reversal in the locus of carbonate vs. mixed sedimentation was related to the shift of the detrital feeder system (the Malita tidal valley) during a major sea-level fall of the late Quaternary (tentatively ascribed to the ca. 0.6 Ma BP lowstand). This study illustrates the importance of both allogenic and autogenic parameters in controlling the stratigraphic architecture of shelf-margin and slope deposits at multiple timescales, in a very wide, mixed carbonate and clastic depositional setting.

Abstract The purpose of this Seals Atlas is to present the microstructural, petrophysical, and geomechanical properties of selected examples of cap rocks and fault seals for use as analogs in the prediction of seal capacity or containment potential. Similar atlases exist; however, this is the first such atlas to focus specifically on the characteristics of cap rocks. The atlas is primarily based on extensive mercury injection capillary pressure (MICP) analyses, but also includes thin section, XRD, grainsize distribution, SEM/EDS, and 'V shale' data. The samples included in this atlas are a result of APCRC and CO2CRC (Cooperative Research Centres) research programs focusing on top and intraformational seals and some fault seals (cataclasites) throughout Australia and New Zealand. The hydrocarbon/carbon dioxide seal examples are grouped by basin localities and further distinguished by formation, well, then depth. Where multiple examples are available, a range of lithologies and MICP data are included in the sample selection. This atlas also can be used in an evaluation of integrated seal potential for prospect risking and reservoir management.

Abstract Low-permeability reservoirs in which gas is the regionally continuous phase (gasifers) occur over large areas in the Alberta Basin in Canada and the Rocky Mountain basins in the United States. These tight-gas reservoirs have also been called “deep basin” and “basin-centered” gas systems and contain very large resources of natural gas. Observation and theory show that a gasifer, or a regional low-permeability gas system, can be developed in a four-stage process: genesis, transition, steady state, and imbibition. This process involves the generation, migration, and leakage of gas, accompanied by the regional dewatering of the system, even in the siltstones and shales.The genesis stage contains both conventional gas pools, early in the development of the gasifer, and unconventional gas pools later. Late genesis gas pools are characterized by tall gas columns, with normal downdip apparent gas-water contacts. These tall gas columns generate enough capillary pressure to drain very low-permeability reservoirs and establish gas as the continuous fluid over a very large part of the basin. At this point, the gasifer is developed. The transition stage has normal and underpressured gas, with tall columns that crosscut waterlines on pressure-versus-elevation plots. In the steady-state stage, gas is underpressured, and the tall gas columns have updip gas-water contacts. The imbibition stage marks the decline of the gasifer and is characterized by shorter, underpressured gas columns and underpressured waterlines. Laboratory experiments based on a simple capillary tube model support the four-stage development of the regional low-permeability gas system and defined both a normally pressured and overpressured gas-water system in the genesis stage. These experiments demonstrated that the mechanism for the underpressuring of the gasifer in the transition and steady-state stages was gas leakage, which confirmed the conclusions based on capillary theory. The combination of the empirical approach using pressure-versus-elevation plots and the capillary theory with the laboratory experiments leads to several interesting concepts. For example, a regional low-permeability gas system can be viewed as a source rock undergoing primary migration. Gas generation may be thermal or biogenic. Therefore, this four-stage process would also apply to the shallow biogenic gasifers in the Milk River and Horseshoe Canyon formations in southern and central Alberta. The genesis stage will contain some moveable water, and there will be an overprint of structural and stratigraphic traps with higher water production downdip. Reservoirs in the genesis stage will have variable water saturations, and therefore, relative permeability should be a concern. A basin may have any one of these stages well developed, or all four may be present at various levels of development, as is the case for the Alberta deep basin. By knowing the stages, the gasifer can be defined, and an effective exploration strategy can be developed.