An Experimental Study into the Effect of Water Pressure on Maturation and Hydrocarbon Generation: Implications for Modelling Hydrocarbon Generation in Geological Basins

A.D. Carr^1,6, C.E. Snape², W. Meredith², C. Uguna², T.L. Leith³, I.C. Scotchman⁴, and R.C. Davis^5
1Advanced Geochemical Systems, 1 Towles Fields, Burton-on-the-Wolds, LE12 5TD, UK
Email: [email protected]
²Nottingham Fuel and Energy Centre, School of Chemical, Environmental and Mining Engineering, University of Nottingham, NG7 2RD, UK
³Statoil, P.O. Box 273, N-7501. Stjørdal, Norway
⁴Statoil, Statoil House, 11A Regent Street, London, SW1 4ST, UK
⁵Woodside Energy Ltd., 240 St. Georges Terrace, Perth 6000, Western Australia
⁶British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

For the last twenty five years petroleum geochemists and basin modellers have argued that maturation and hydrocarbon generation are not influenced by pressure. Chemical theory however indicates that endothermic volume expansion reactions such as maturation and hydrocarbon generation are influenced by the system pressure. Is it possible that the chemical theory is correct and that geochemists and basin modellers have been incorrect in their assertion that pressure does not control hydrocarbon generation in geological basins? This paper presents the results of an experimental investigation into the effects of water pressure on maturation and hydrocarbon generation processes.

Using a pressure vessel rated up to 500 bar (ca. 7000 psi) at 350^oC, two source rocks have been studied: a Type II Kimmeridge Clay from Dorset, UK, and a Type III Wyodak sub-bituminous coal from the Argonne Premium Coal sample bank, USA. Previous work on the replication of hydrocarbon generation in the laboratory have used either anhydrous, vapour pressure (MSSV and gold bag) or hydrous conditions, in which the vapour phase is a significant component prior to starting the experiment. In the system designed for this study, the vessel can be filled completely with water thereby replicating conditions in the geological basin prior to hydrocarbon generation. The source rocks were heated to 310^oC and 320^oC for between 7 and 24 hours under atmospheric pressures with inert gas, steam and (distilled) liquid water pressure to examine the effects of the phase and pressure on hydrocarbon generation. The results clearly indicate that the presence of pressurized liquid water with no steam results in the reduction of both bitumen and gas generation compared with either steam or anhydrous conditions. Vitrinite reflectance is also retarded by the presence of pressurized water, and combined the results indicate that hydrocarbon generation is a thermodynamically and not a kinetically controlled process in geological basins. In addition the results clearly show that the use of Arrhenius first order kinetic models to model maturation and hydrocarbon generation does not obey the first law of thermodynamics under subsurface conditions.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands