25th International Meeting on Organic Geochemistry IMOG 2011

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Beyond Orgas- BP’s new predictive model for biogenic and
thermogenic gas expulsion from source rocks
Mark Osborne, Tony Barwise
BP Exploration Operating Co Ltd, Sunbury-on-Thames, United Kingdom (corresponding
author:Mark.Osborne@uk.bp.com)
BP‘s ‗Orgas‘ kinetic scheme, was created in the
1980‘s-early 1990‘s to predict the oil and gas volume
expelled from different source rock
organofacies[1,2,3]. The global kinetic model
assigned kinetic parameters based on gross
depositional environment and stratigraphic age-
useful in areas of low geochemical knowledge
including frontier exploration. The published scheme
has been implemented in most commercially available
basin modelling tools, becoming widely used
throughout the petroleum industry to the present day.
While Orgas is still considered to be technically
robust and pragmatic, BP has recently made
improvements to the scheme to provide enhanced
pre-drill predictions of the volume, composition and
physical properties of expelled petroleum fluids. This
information is important for understanding the likely
quality, value and volume of petroleum fluid that may
be present in a basin or prospect, for both
conventional or unconventional play types (e.g. shale
gas, coal bed methane, biogenic).
With regards to modelling of gas expulsion, the
modifications to our proprietary scheme fall into three
main categories:
1) Modelling of both biogenic (bacterial) and
thermogenic gas generation and expulsion. Our work
suggests biogenic gas is generated from typical
source rocks, as well as leaner sediments. Biogenic
and thermogenic gas should not be modelled
separately- they are part of the continuum of fluid
types that are generated during burial of most source
rocks (Fig 1).
2) A revised model for gas adsorption and retention.
Both adsorption of gas on kerogen and retention in
porosity are important factors controlling whether gas
remains trapped in a source rock or is expelled from
it. The old Orgas scheme considered only adsorption.
The new model is critical for the correct evaluation of
shale gas plays.
3) Prediction of the isotopic composition of the
expelled gases. As biogenic and then thermogenic
gas is generated and becomes mixed in the source,
the expelled gas isotopic composition systematically
evolves in a predictable manner. This model is
consistent with global observations from isotube data
collected from exploration wells.
Collectively these three modifications produce more
realistic assessments of gas expelled from (and
retained within) source rocks
Fig. 1. An example of biogenic and thermogenic
expulsion rates and their variance with source rock
thermal stress
References
[1] Pepper AS and Corvi PJ. Simple Kinetic Models of
Petroleum Formation: Oil and Gas Generation from
Kerogen. Marine and Petroleum Geology 12 (1995):
291-319.
[2] Pepper AS and Corvi PJ. Simple Kinetic Models of
Petroleum Formation: Modelling an Open System.
Marine and Petroleum Geology 12 (1995): 417-452.
[3] Pepper AS and Dodd TA. Simple Kinetic Models of
Petroleum Formation Part II: oil-gas cracking. Marine
and Petroleum Geology 12 (1995): 321-340.
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