Validation of Fault Seal Mechanisms in the Timor Sea: An Outcrop and Subsurface Perspective
Fifth International Conference on Fault and Top Seals
Assessing exploration risk requires an analysis of a range of issues including trap, seal, charge then considering reservoir issues. When risking 3-way structures across-fault juxtaposition and/or membrane seal are key issues. Trap preservation related to fault reactivation may also need be assessed. Considerable work has been done by a number of workers (Vrolijk 2016) to calibrate membrane sealing. Most notably back calculating the pressure capacity of the Shale Gouge Ratio (SGR) algorithm, to
... (SGR) algorithm, to allow forecasting of columns. Importantly, this back-fitting of SGR and seal capacity has been conducted on single "best" technical models with no direct modelling of uncertainty (Yielding 2002). The methods we use considers across-fault leakage from potential hydrocarbon reservoirs, and consider lateral seals due to • Juxtaposition • Juxtaposition combined with impermeable/high capillary entry pressure fault rocks such as clay gouge or other fault related materials (membrane seal). Monte Carlo simulation is used to model both geometrical and stratigraphic uncertainty. The Monte Carlo analysis produces multiple 3D Allan maps normal to fault plane models (Allan 1989) which are analysed for juxtaposition leak points and shale gouge ratio (SGR). To validate the proposed methods, multiple case studies of successful discoveries have been analysed using back testing -for these case studies the model outputs were compared with the independently observed hydrocarbon water contact (IHWC) obtained from drilling. Stochastic juxtaposition analysis with no contribution from fault rock seal or SGR gives the smallest error in estimation of the IHWC. In general, the application of SGR methods in reservoirs with moderate Vshale artificially increases predicted column heights and enhances pre-drill chance of success. In well run risking processes these large columns are generally discounted through other geologic risk factors. When shorter columns are found, they are often "explained" by issues of charge or trap breach. It is well known that fault and stratigraphic uncertainties are significant and need to be explicitly included in the modelling of fault seal risk and inferred column heights. Example Outcrop Mapping of Fault Rock Whilst geologist commonly use cross sections to describe structures and traps it is vital in fault seal analysis to consider the strike variability of fault rock properties. When considering a strike outcrop in shallow to moderate dipping sediments it is reasonable to assume that over 10's of meters, along strike, that the throw and stratigraphy are consistent. Using a systematic approach to catalogue both how thick but also how thin the fault rocks are. As can be seen in Figure 1 a strike section of a fault that fault rock varies in thickness dramatically over the 10's of centimetres scale.