AAPG Bulletin, v. 85, no. 4 (April 2001), pp. 621–635 621 Simulation of potassium feldspar dissolution and illitization in the Statfjord Formation, North Sea Geoffrey Thyne, Bernard P. Boudreau, Mogens Ramm, and Ruth Elin Midtbø ABSTRACT Prior work has suggested that potassium mobility is an important control on the formation of late-stage illite in deep reservoirs of the North Sea. We developed a one-dimensional finite-difference re- active transport model to simulate the concurrent dissolution of K- feldspar and illitization of kaolinite in Statfjord Formation reser- voirs. The kinetics of K-feldspar dissolution is set to be faster than diffusion, consistent with the observed behavior and the proposi- tion that the reaction was transport-limited. K-feldspar dissolution produced dissolved potassium and silica that diffused and then pre- cipitated to form illite and quartz. Areas that had higher initial re- actant (K-feldspar) produced higher solute concentrations and formed chemical gradients having areas that had lower initial feld- spar content. In typical North Sea cases this means mass transfer would be from sandstone to shale. The diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration. Simulations of an oil-filled reservoir showed that oil emplace- ment did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone. Preservation of preillitization permeability was favored in water-filled reservoirs that had slower illite formation where diffusion could export po- tassium from the sandstone before precipitation. Similar patterns of transport and precipitation were predicted for silica from K- feldspar dissolution. The results of the model showed good agree- ment with field data from other North Sea reservoirs. INTRODUCTION The Statfjord Formation serves as the major reservoir in several petroleum fields of the North Sea at burial depths ranging from 2 Copyright 2001. The American Association of Petroleum Geologists. All rights reserved. Manuscript received June 1, 1999; revised manuscript received May 19, 2000; final acceptance August 16, 2000. AUTHORS Geoffrey Thyne  Colorado School of Mines, Golden, Colorado, 80401; gthyne@mines.edu Geoffrey Thyne earned his Ph.D. in geology from the University of Wyoming in 1990. Since 1996 he has held the position of research associate professor in the Department of Geology and Geological Engineering at Colorado School of Mines and regularly consults in the petroleum industry. He was an assistant professor at California State University-Bakersfield (1991–1996) and a research geochemist for Arco Oil and Gas in Plano, Texas (1979–1986). His research interests continue to focus on the mysteries of siliciclastic diagenesis. His current research and consulting projects involve the quantitative prediction of the effects of diagenesis on the reservoir quality. Bernard P. Boudreau  School of Ocean and Earth Sciences, University of Southampton, European Way, Southampton, SO14 3ZH, United Kingdom As of August 2000, Bernard Boudreau occupies the position of Professor and Chair in Benthic Processes in the School of Ocean and Earth Science of Southampton University (United Kingdom). Boudreau received his Ph.D. from Yale University in 1985 and held various academic posts at the University of British Columbia and Dalhousie University (Canada) before moving to the United Kingdom. He is the author of more than 45 scientific papers and two textbooks. His interests center on modeling diagenesis, sediment-water exchange, and benthic processes in general. Mogens Ramm  Norsk Hydro E&P, Postboks 200, 1321 Stabekk, Vækerø, Oslo, Norway Mogens Ramm earned his Dr. Scient. in diagenesis and reservoir quality prediction at the University of Oslo in 1991. He is currently with the Norsk Hydro E&P division. Since 1999 he has been working as project leader in the North Sea exploration group. After finishing his Cand. Scient. degree in 1986, he spent five years at the University of Oslo, on VISTA (the Norwegian Academy of Science and Letters