Sequence Stratigraphic Distribution of Coaly Rocks: Fundamental Controls and Paralic Examples

Abstract

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.