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 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.

Abstract Postcharge fault reactivation may cause fault seal breach. We present a new methodology for assessment of the risk of reactivation-related seal breach: fault analysis seal technology (FAST). The methodology is based on the brittle failure theory and, unlike other geomechanical methods, recognizes that faults may show significant cohesive strength. The likelihood of fault reactivation, which is expressed by the increase in pore pressure (Δ P ) necessary for fault to reactivate, can be determined given the knowledge of the in-situ stress field, fault rock failure envelope, pore pressure, and fault geometry. The FAST methodology was applied to the fault-bound Zema structure in the Otway Basin, South Australia. Analysis of juxtaposition and fault deformation processes indicated that the fault was likely to be sealing, but the structure was found to contain a residual hydrocarbon column. The FAST analysis indicates that segments of the fault are optimally oriented for reactivation in the in-situ stress field. Microstructural evidence of open fractures in a fault zone in the subsurface in an offset well and an SP (self-potential) anomaly associated with a subseismic fault cutting the regional seal in the Zema-1 well support the interpretation that seal breach is related to fracturing.

Abstract This pilot study indicates that estimating paleostress orientations and magnitudes from seismic data, through analysis of fault slip data obtained using three-dimensional restoration techniques, is possible, and the results generated are consistent with regional observations. The results suggest that the stress regime responsible for late Miocene fault activity in the vicinity of the Skua oil field in the Timor Sea differs from the present-day stress regime. An extensional stress regime, having the maximum principal stress axis (σ 1 ) oriented vertically, the intermediate principal stress axis (σ 2 ) oriented approximately east-west, the minimum principal stress (σ 3 ) oriented approximately north-south, and a stress ratio ( R ) of about 0.3, was calculated for the late Miocene. In contrast, measurements of the present-day stress field indicate a transtensional stress regime in which σ 2 is vertical, σ 1 is horizontal and trends east-northeast-west-southwest, σ 3 trends north-northwest-south-southeast, and R = 0.8. Estimation of the magnitudes of the principal stresses indicate that the differential stress operating in the late Miocene was similar to the present, but that greater mean stress in the present-day stress state results in a lowering of reactivation risk with time. These results are consistent with regional observations of widespread late Tertiary extensional faulting, with decreasing fault activity to the present day. The work also suggests that the majority of hydrocarbon leakage associated with fault reactivation in this region is less likely to be associated with the present-day stress regime than with the paleostress regime.