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AAPG Bulletin, V.
Using calibrated shale gouge ratio to estimate hydrocarbon column heights
1Badleys, North Beck House, North Beck Lane, Hundleby, Spilsby, Lincolnshire, PE23 5NB, United Kingdom; email: petebadleys.co.uk
2Badleys, North Beck House, North Beck Lane, Hundleby, Spilsby, Lincolnshire, PE23 5NB, United Kingdom
3Badleys, North Beck House, North Beck Lane, Hundleby, Spilsby, Lincolnshire, PE23 5NB, United Kingdom
Peter Bretan received a B.Sc. degree (with honors) from Kingston University in 1981, followed by a Ph.D. in structural geology from Imperial College, London. Before joining Badleys in 1995, he worked as a research geologist and seismic interpreter with the Fault Analysis Research Group at Liverpool University. At Badleys, his main tasks include software training, technical support, and consulting.
Graham Yielding received a B.A. degree in natural sciences from Cambridge University in 1979, followed by a Ph.D. in geophysics in 1984. He then worked for Britoil in Glasgow as a seismic interpreter before joining Badleys in 1988. His current interests include fault-seal analysis, fault populations, and fracture prediction.
Helen Jones joined Badleys in 1989. Having originally trained, worked, and published as a biologist, Helen then swapped sciences to geology to provide technical research and support to ongoing project work. In addition, she is the author of the manuals and online documentation for the TrapTester/FAPS software.
The authors are grateful to Stephen Dee and Peter Boult for their comments on early versions of this manuscript. Laurel Goodwin, Fred Dula, Russell Davies, and Jim Handschy are thanked for their constructive reviews.
Fault-zone composition, estimated using the shale gouge ratio (SGR) algorithm, can be empirically calibrated with pressure data to define depth-dependent seal-failure envelopes relating SGR to fault-zone capillary entry pressure (FZP) by the equation: FZP (bar) = 10 (SGR/27 − C). C is 0.5 for burial depths less then 3.0 km (9850 ft), C is 0.25 for burial depths between 3.0 and 3.5 km (9850–11,500 ft), and C is 0 where the burial depth exceeds 3.5 km (11,500 ft).
The seal-failure envelope provides a method to estimate the maximum height of a hydrocarbon column that can be supported by the fault. Leakage of hydrocarbons across a fault occurs when the buoyancy pressure exceeds the capillary entry pressure of the fault and is not confined to the crest of the structure or even to where the SGR value is lowest.
Established calibration diagrams based on across-fault pressure differences have overgeneralized the relationship between increasing SGR and increasing pressure support. Calibration diagrams based on buoyancy pressure show that gas and oil data exhibit a correlation between increasing SGR and increasing buoyancy pressure but only between SGR values of 20 and 40%. No increase in the strength of a seal is present, as reflected by an increase in maximum supportable buoyancy pressure, at SGR values greater than about 40% for both gas and oil data. Column heights do not continue to increase in the SGR range 50–100%.
Estimating hydrocarbon column heights using seal attributes depends upon the geologic input to the model, in particular, pressure data, volumetric shale fraction, and the precision of the three-dimensional mapping of reservoir geometry in the vicinity of the fault.
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