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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
2004. The American Association of Petroleum Geologists. All rights reserved.
Textural and sequence-stratigraphic controls on sealing capacity of Lower and Upper Cretaceous shales, Denver basin, Colorado
Sally J. Sutton,1 Frank G. Ethridge,2 William R. Almon,3 William C. Dawson,4 Kimberly K. Edwards5
1Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523; [email protected]
2Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523; [email protected]
3ChevronTexaco Exploration Technology Company, 4800 Fournace Place, Bellaire, Texas 77401; [email protected]
4ChevronTexaco Exploration Technology Company, 4800 Fournace Place, Bellaire, Texas 77401; [email protected]
5Jehn Water Consultants, Inc. Denver, Colorado 80231; [email protected]
AUTHORS
Sally J. Sutton received a B.S. degree from the University of Michigan and a Ph.D. from the University of Cincinnati. She was a research associate at the University of Texas at Austin and is now an associate professor at Colorado State University. Her research interests include shale petrology and geochemistry, clastic diagenesis, fluid and rock interaction, and chemical weathering.
Frank G. Ethridge obtained his B.S. degree from Mississippi State University in 1960, his M.S. degree from Louisiana State University in 1966, and his Ph.D. from Texas A
M University in 1970. He is a full professor of geology at Colorado State University. His present research interests include estimating reservoir shale seal capacity for exploration and risk analysis; testing sequence-stratigraphic concepts using outcrop, subsurface, and experimental data; and formation and architecture of modern and ancient fluvial deposits.
William R. Almon earned his Ph.D. in geology from the University of Missouri. He received an M.S. degree in petroleum engineering from Tulsa University and his A.B. degree in chemistry from Washington University (St. Louis). His 29-year career includes positions in research, applied technology, and exploration at Cities Service, Anadarko, Texaco, and ChevronTexaco. His research interests include sequence stratigraphy and fine-grained marine siliciclastic depositional systems, as well as sedimentary geochemistry and diagenesis.
W. C. Dawson is a research geologist for ChevronTexaco. He received a Ph.D. in sedimentary petrography (University of Illinois at Champaign-Urbana) in 1984. His work history includes the Illinois Geological Survey, Eason Oil Company, and Texaco Research. His current interests include seal characterization, shale sedimentology, and reservoir diagenesis. He is a member of AAPG, SEPM, and the International Association of Sedimentologists.
Kimberly K. Edwards is a water resource consultant for Jehn Water Consultants, Inc. in Denver, Colorado. She received an M.S. degree in geology from Colorado State University in 1999 and previously worked with Texaco Exploration and Production Technology Department (EPTD) as an intern. She is a member of the National Ground Water Association and is a professional geologist in the state of Utah.
ACKNOWLEDGMENTS
The authors thank William Lawrence for his assistance in the field and lab; Douglas McCarty, Victor Drits, B. J. Katz, J. Jones, and G. Vardillos for assistance with clay mineral and organic geochemistry analyses; Robert J. Weimer and Jim Jones for assistance with core access; John Neasham for the mercury injection measurements; and Keith Etchells for assistance with field work. Financial support was provided by Texaco EPTD and an AAPG Grant-in-Aid to Kim Edwards. We also thank Jack C. Pashin, Wan Yang, and an anonymous reviewer for their thorough and very helpful reviews.
ABSTRACT
Shale units can be important barriers to fluid flow in sedimentary basins and commonly serve as seals to petroleum reservoirs. Little is known, however, about the controls on shale permeability. Consequently, variation in seal competency is one of the greatest risk factors associated with petroleum exploration.
Here, we examine possible controls on sealing capacity in two Cretaceous marine shale units in the Denver basin, Colorado. Sealing capacity, as determined by mercury injection–capillary pressure analysis, is compared to several textural and compositional parameters and to sequence-stratigraphic setting. These two shale units display highly variable sealing capacity, even between some adjacent samples. This suggests that variability in some small-scale shale characteristics may strongly influence sealing capacity. The best seals are generally in transgressive systems tracts, especially within or immediately below condensed sections.
Textural characteristics of shale appear to be especially important in determining sealing capacity. In particular, well-sorted pore-throat sizes and well-developed bedding-parallel preferred orientation of flattened organic matter particles strongly favor high sealing capacity. High degrees of bioturbation degrade sealing capacity, possibly by disrupting preferred orientation and by increasing variability in grain size and hence in pore-throat sorting. Preferred orientation of matrix clays parallel to bedding also appears to increase with increasing sealing capacity, but is probably less important than the preferred orientation of organic matter.
Compositional characteristics are generally less important than textural characteristics in determining sealing capacity in these shale units. Neither silt content nor cement content appears to be important to sealing capacity in these shale units. Total organic carbon is generally high in samples with good sealing capacity, but can be either high or low where sealing capacity is poor.
Overall, the variables that most strongly favor high sealing capacity, pore-throat sorting, organic matter bedding-parallel preferred orientation, and low bioturbation, are most likely in anoxic, deep-water settings, hence, the association between good seals and condensed sections.
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