Anthony Gartrell

Ages varying from Late Triassic to Early Jurassic have been proposed by different authors for the onset of rifting in the Northern Carnarvon Basin. Seismic sections from the Exmouth Sub-basin and outer Exmouth Plateau demonstrate significant growth strata associated with displacement on normal faults starting at least at the base of the R. rhaetica zone (Rhaetian). This tectonic event coincides with a marked change in sequence architecture and a large landward shift (~300 km) of the paleo-shoreline to the vicinity of the Rankin and Alpha Arch trends. Rapid creation of accommodation in the inboard narrow rift basins (Exmouth, Barrow and Dampier sub-basins) is suggested to be the most likely cause of this major transgression. The preferential development of associated carbonate build-ups during the Rhaetian on the footwall side of active tilted fault blocks provides additional evidence for the onset of significant extensional faulting occurring during this time. An earlier more subtle initiation phase of rifting, however, is interpreted during the Norian, from around the middle part of the H. balmei biozone time, above which a change in stratigraphic architecture from aggrading to retrograding occurs. The observed structural and stratigraphic transitions can be related to typical phases of evolution described in many rift basins around the world. The work highlights the importance of integrating regional structural geology, sequence stratigraphy and depositional systems observations to provide robust constraints for basin evolutions.

Three-dimensional coupled deformation and fluid-flow numerical modeling, charge-history analysis, and seismic imaging of inferred leakage-related geobodies are integrated to investigate the response of a complex set of Jurassic trap-bounding normal faults to extensional reactivation and to assess hydrocarbon upfault seepage on the Laminaria High (Timor Sea, Australian North West Shelf). Fluid inclusion data are consistent with the presence of paleo-oil columns below the current accumulations in the Laminaria and Corallina fields. Evidence for other partially breached (current and paleo-oil column) as well as breached (dry with paleo-oil column) closures across the region implies that active and widespread seepage took place after the time of initial oil charge. The distribution of current and paleo-oil zones, and the location of inferred hydrocarbon leakage indicators defined on 3D seismic data, correlates with the prediction of fault-seal effectiveness based on modeled strain distribution. Within the geologic framework of the Laminaria High area, this distribution suggests that when sufficient reactivation shear strain is accumulated by reservoir faults, ductile deformation might give way to brittle failure in the top seal, allowing active flow pathways to develop and upfault seepage to take place from the reservoir to thief zones or the seafloor. The observations emphasize that strain and upfault fluid-flow partitioning is constrained by prereactivation fault size, lateral fault-tip distributions, and the presence of fault jogs inherited from successive episodes of growth processes. These elements can explain the complex distribution of paleo- and preserved oil columns in the study area and further support Cenozoic tectonic activity as being the first-order control on trap breaching and hydrocarbon seepage in this region.

A Play Based Exploration (PBE) approach has been applied to the Brunei onshore, principle, second and third offshore agreement areas (OAA) of the Neogene (Serravalian to recent) deltaic sequences of the Baram - Balbac Basin operated by Brunei Shell Petroleum (BSP). Previous studies, utilizing a global sequence framework, are now considered to have underestimated the influence of local autocyclic forcing, manifested through high sedimentation rates, extensional delta-top tectonics and mutli-phase crustal shortening. Integration of shelf margin trajectories, biostratigraphy and structural kinematics has enabled a revised tectono-stratigraphic framework consisting of 10 major 3rd order sequences to be established in the Late Miocene to Present section. Regional compressional pulses are at the origin of some of these sequences, with syntectonic sequences showing onlap terminations on the flanks and/or thinning on the top of inverted structures and significant dislocations in the shelf edge trajectory. A rigorous reassessment of the biostratigraphic framework has necessitated a revision of the sequence definitions and the synethsis of these observations is detailed. Two sequence boundaries are now calibrated to two major global eustatic falls (11.7 and 5.73 Ma). The 11.7 Ma event, a period of tectonic quiescence, is linked to a ~15 km basinward shift of the shelf edge trajectory without any clear sub-aerial erosion on the emerged shelf. The 5.73 Ma event, also a period of tectonic quiescence, is linked to a more modest basinward shift of the shelf edge, but accompanied by widespread canyon incision along most of the margin, and a major incision of the fluvial system feeding the Champion Delta. Establishing a chronostratigraphic framework in deltaic sequences deposited on active margins requires a combination of pragmatism and caution. The same sequence may show very different stratal patterns and inferred systems tract along the same margin.

Three-dimensional (3-D) coupled deformation and fluid-flow numerical modeling are used to simulate the response of a relatively complex set of trap-bounding faults to extensional reactivation and to investigate hydrocarbon preservation risk for structural traps in the offshore Bonaparte Basin (Laminaria High, the Timor Sea, Australian North West Shelf). The model results show that the distributions of shear strain and dilation as well as fluid flux are heterogeneous along fault planes inferring lateral variability of fault seal effectiveness. The distribution of high shear strain is seen as the main control on structural permeability and is primarily influenced by the structural architecture. Prereactivation fault size and distribution within the modeled fault population as well as fault corrugations driven by growth processes represent key elements driving the partitioning of strain and up-fault fluid flow. These factors are critical in determining oil preservation during the late reactivation phase on the Laminaria High. Testing of the model against leakage indicators defined on 3-D seismic data correlates with the numerical prediction of fault seal effectiveness and explains the complex distribution of paleo- and preserved oil columns in the study area.

Based on comparisons between structural histories and the distribution of current and paleo-oil accumulations, it is proposed that the partitioning of postrift strain between faults in relation to trap geometry was critical in determining oil preservation during Neogene fault reactivation in the Timor Sea. Most of the trap-bounding faults in the region have been reactivated; however, the distribution of postrift displacements is heterogeneous and depends heavily on rift-phase fault size, location, and interaction with nearby faults. Preferential localization of postrift strain onto larger faults in the population resulted in the partial protection of some fault-bound traps with favorable geometries, but promoted breaching of others. Oil columns tend to be preserved where the crest of the trap is bound by a fault segment that has accommodated relatively low postrift displacements (less than about 60 m [196 ft]) during reactivation, typically where smaller rift faults are overlapped by larger rift faults. Complete loss of oil column is generally observed where the crest of the trap is bound by a typically large fault with high postrift displacements (greater than about 60 m [196 ft]). Where faults with high postrift displacements are located downdip of the trap crest, hydrocarbon columns are preserved down to the depth of the intersection between this fault and the top reservoir horizon. A simple trap integrity model based on these observations was found to be largely consistent with a database of 69 drilled traps in the region. The mechanisms and models discussed in this study are likely to apply to other petroleum systems where fault reactivation represents a risk to hydrocarbon preservation.

A strong relationship between the distribution of Late Miocene to recent (post-rift) strain, trap geometry and remaining oil columns is recognised in the heavily re-activated Laminaria High area. Preferential localisation of post-rift strain onto larger faults in the population resulted in the smaller faults accommodating progressively less strain with time. The strain localisation process appears to have protected some fault-bound traps with favourable geometries from breaching during fault re-activation, but promoted breaching of others. Where faults with high postrift strain are located at the crest of a trap, oil columns are not preserved. In contrast, where high strain post-rift faults are located down-dip of the trap crest, oil columns are preserved down to the depth of the critical high strain fault-top reservoir intersection. These observations suggest that a relatively simple assessment of the basic structural geometry of a trap may provide a first order approximation of trap integrity risks and could also be predictive of preserved hydrocarbon column heights.

Analog models were used to investigate the influence of cross-trending basement faults on inverted rift systems similar to those located in the Makassar Straits. Multilayer models composed of sand, silicone putties, and a silicone-plasticine mixture were constructed so that laterally offset rift basins would form on either side of a preexisting basement fault. Transfer zones comprising a series of en echelon normal faults that connect downward with the basement fault formed to link the laterally offset rift basins during the extension phase. Subsequent oblique inversion of the models resulted in a general progression starting with broad folding, followed by the development of small-scale inversion anticlines associated with reactivated rift faults, to thrust faulting, and finally, to the formation of cross-trending wrench faults. Rift fault reactivation and wrench fault development were prominent in the transfer zones because of the orientation of the constituent faults (oblique to the shortening direction) and their connection with the underlying basement fault. The addition of an intrasedimentary detachment layer, which represents a thick overpressured shale present in the vicinity of the economically important Mahakam delta, allowed gravitational collapse and extensional faulting to occur on structural highs. The cross-trending basement fault was again the locus for inversion structures, both above and below the detachment layer in the latter case, which may have influenced the distribution of petroleum in the region.