Significant volumes of terrigenous organic
matter can be preserved to form coals only when and where the overall increase
in accommodation approximately equals the production rate of peat. Accommodation
is a function of subsidence and base level. For mires, base level is very
specifically the groundwater table. In paralic settings, the groundwater
table is strongly controlled by sea level and the precipitation/evaporation
ratio. Peat accumulates over a range of rates, but always with a definite
maximum rate set by original organic productivity and space available below
depositional base level (groundwater table).
Below a threshold accommodation rate (nonzero),
no continuous peats accumulate, due to falling or low groundwater table,
sedimentary bypass, and extensive erosion by fluvial channels. This is
typical of upper highstand, lowstand fan, and basal lowstand-wedge systems
tracts. Higher accommodation rates provide relatively stable conditions
with rising groundwater tables. Mires initiate and
©Copyright
1997. The American Association of Petroleum Geologists. All rights reserved.
1Manuscript
received September 7, 1995; revised manuscript received November 25, 1996;
final acceptance May 5, 1997.
2Exxon
Production Research Company, 3120 Buffalo Speedway, Houston, Texas 77096.
3Exxon
Production Research Company, 3120 Buffalo Speedway, Houston, Texas 77096.
Present address: Conoco, Inc., P.O. Box 2197, Houston, Texas 77252.
We
benefited from the input and assistance of many people. Of special assistance
were the teams involved in collaborative studies with Esso Australia and
Esso Malaysia: P. Moore, M. Sloan, J. Emmett, B. Burns, A. Partridge, S.
Creaney, Hanif Hussein, R. Hill, R. Lovell, and M. Feeley. We also thank
the Rock Springs team: R. Beauboeuf, P. McLaughlin, W. Devlin, A. Carroll,
Y. Y. Chen, G. Grabowski, Jr., K. Miskell-Gerhardt, M. Farley, R. Webster,
and J. Schwalbach. Group members D. Curry and J. Yeakel were always helpful.
We also enjoyed
and profited from many discussions of these concepts with our colleagues
outside Exxon: C. Diessel, R. Boyd, K. Shanley, B. Zaitlin, P. McCabe,
M. Hendricks, A. Cohen, and M. Kirschbaum.
We thank the
reviewers of company reports, whose careful comments on several generations
of this work improved it: S. Creaney, M. Feeley, J. Van Wagoner, F. Wehr,
J. Yeakel, and A. Young. F. Weber and J. Zullig provided extensive management
support. K. Linke translated our sketches into the fine figures herein.
We value all the help.
