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 The intracratonic, supra-subduction zone setting of the Thailand rift basins caused the rifts to evolve in several ways differently from other intracratonic rifts. Key differences include: widespread occurrence of low-angle normal faults; basin inversion alternating with rifting; diachronous initiation and cessation of rifting; rapid post-rift subsidence; and extensive, low-displacement post-rift faults. These characteristics are related to hot, weak continental lithosphere, rapid evolution of the plate boundaries and stresses during the Cenozoic, and the history of subduction and accretion. Low-angle normal faults impacted the Sirikit Field by controlling the location of fluvio-deltaic reservoirs interfingering lacustrine shales (source and seal). Basin inversion alternating with syn-rift section is detrimental to prospectivity of the southern half of the Phitsanulok Basin by destroying syn-rift structure, and halting hydrocarbon maturation early in the basin history. The diachronous timing of basin development resulted in large, gas-dominated hydrocarbon accumulations in the eastern Gulf of Thailand basins, but was detrimental for younger basins that lack the post-rift section, and the greater variety of petroleum systems and traps that post-rift basins offer. The extensive low displacement–length ratio post-rift faults form significant hydrocarbon traps in the Pattani and North Malay basins. They represent a structural style not usually found in post-rift basins.