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Beyond Orgas- BP’s New Predictive Model for Biogenic and Thermogenic Gas Expulsion from Source Rocks

Mark Osborne and Tony Barwise
BP Exploration, Sunbury on Thames, UK

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- very 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.

Recent production from unconventional gas plays and integrated technical studies have provided a flood of new information and insights about source rocks and petroleum expulsion. In light of this data, BP has made improvements to the Orgas 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.

With reference to the 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. 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.
2) A revised model for gas adsorption and retention. The new model is critical for the correct evaluation of gas in place in 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.

Collectively these three modifications produce more realistic assessments of gas expelled from (and retained within) source rocks

[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.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012