Abstract We propose and validate methods for risk analysis of fault-bounded hydrocarbon traps in exploration. We concentrate on cross-fault leakage and consider lateral seals due to (1) juxtaposition and (2) high capillary-entry-pressure fault rock (membrane seal). We conclude that stochastic methods for fault seal analysis are essential, due to the large number of structural and stratigraphic parameters and the uncertainties. Central to the methods proposed is a Monte Carlo simulation which models geometrical and stratigraphic uncertainty. Multiple Allan maps (fault-parallel cross-sections) are produced and analysed for juxtaposition and shale gouge ratio (SGR). For validation, known discoveries with independently observed hydrocarbon–water contacts (IHWC) have been back-analysed. We present two case studies in this paper, and an additional 40 case studies are summarized (four public domain and 36 confidential case studies). The model outputs were compared with the IHWC. Juxtaposition analysis with no SGR contribution gives the smallest error. The inclusion of any fault rock seal mechanisms (such as SGR) matches or increases predicted hydrocarbon column heights compared to juxtaposition and gives larger errors. We conclude there is no reason to include fault rock membrane seals in exploration prospect risking.

Abstract: The rift basins of Thailand exhibit remarkable diversity of fault displacement patterns, fault length–displacement characteristics and mapped fault patterns during late rift, and post-rift, stages. These patterns reflect influences by: (1) zones of strength anisotropy in the pre-rift basement; (2) syn-rift fault patterns on post-rift faults; (3) spatial stress deflection, commonly related to irregularities in major fault profiles, and the basement–sediment interface; (4) temporal stress rotation, usually related to changes in the regional plate setting; and (5) varying strength properties (strain hardening or softening) of fault zones during their life. These influences created strongly segmented boundary faults, and long, low-displacement post-rift fault trends. The former are commonly strongly over-displaced, while the latter can be strongly under-displaced with respect to their length compared with typical length:displacement distributions. Seismic interpretation of multi-rift fault patterns requires 3D data to identify the complexities, otherwise the linkage pattern between deeper and shallower faults, and the changing fault strike-directions with depth, may be incorrectly mapped. Incorrect identification of fault patterns as breached relay structures may also arise. Oblique extension, the influence of pre-existing trends and stress rotation in multi-phase rifts provides a more comprehensive explanation for the observed features than the strike-slip interpretation of previous studies.

Abstract Many of the important Gulf of Thailand reservoirs are fluvial sandstones within the early to late Miocene. The fluvial sandstones vary considerably with respect to channel size, orientation, and sinuosity, making accurate reservoir characterization difficult as many of them are below seismic resolution. The stratigraphic architecture of the Miocene to Pleistocene succession in The Gulf of Thailand has been investigated by integrating seismic geomorphology, well logs, and biostratigraphic data. The Oligocene to early Miocene depocenter was controlled by synrift faulting and was adjacent to the large basin bounding faults. Oligocene lacustrine sediments are overlain by an early Miocene fluvial succession consisting of sinuous, broad (average width 2 km) northwest–southeast channel belts in the basin center. Channel belts became straighter and narrower (0.65 km) and change orientation to northeast–southwest in the middle Miocene when the main depocenter shifted eastward after the main phase of rifting ceased. Tidal creeks observed on seismic images supports biostratigraphic data that indicate a marine influence in the middle Miocene, the incursion coming from the northeast. Wide (1 km), northwest–southeast sinuous channels again dominate in the post-rift succession that comprises the top middle Miocene through Pleistocene. The general temporal variations indicate that tectonics was the main control on channel morphology until late early Miocene, whereas, in the middle Miocene short-lived marine incursions are present locally.