--> Abstract: Source Rock–Specific Calibration of Gas Formation in the North Sea: Gas-Source Correlation and Timing of Gas Formation, by Martin Schoell, Yongchun Tang, and Gavin Lewis; #90039 (2005)

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Source Rock–Specific Calibration of Gas Formation in the North Sea: Gas-Source Correlation and Timing of Gas Formation

Martin Schoell1, Yongchun Tang2, and Gavin Lewis3
1 Gas Consult International Inc, Berkeley, CA
2 California Institute of Technology, Covina, CA
3 ChevronTexaco North America, Houston, TX

Petroleum systems offshore Norway are complex with Upper Jurassic shales and Lower Jurassic Coals, respectively, at the top and the bottom of the mostly Middle Jurassic reservoir section. One of the key elements in the assessment of the Mid Norway petroleum systems is the identification of the gas source rock(s) and the relative timing of oil vs. gas formation. We applied a new isotope modeling technique in the Haltenbanken area, a complex rift system in Mid Norway. Isotope modeling calibrations revealed characteristically different delta13Cmethane-delta13Cethane calibration curves for gases from Are and Spekk formation, respectively. Most reservoir gases that are associated with Spekk-sourced oils are, surprisingly, coal gases from the deeper Åre Formation. Using a depth map of the Are formation we can identify gas kitchens of major gas fields in the area. The onset of gas formation precedes the onset of oil formation, however, significant gas formation requires depth of the Are of more than 6km and, therefore, follows oil formation. The general migration model derived from these findings is that oils charge reservoirs first and generally through lateral migration from the Upper Jurassic source to the Middle Jurassic reservoir and later, gases from the deeper Åre formation migrate mostly structurally controlled into the reservoirs that are already charged with oils. A risk for gas flushing can be predicted for reservoirs close to deep Åre kitchens.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005