Shale Facies and Seal Variability
William R. Almon, William C. Dawson and Craig W. Stichtenoth
Fine-grained lithofacies are dominant components of deep-marine depositional systems, but limited systematic study of fine-grained siliciclastic lithofacies results in the incomplete understanding of deep-marine petroleum systems. Most significantly, these fine-grained strata are baffles and barriers to fluid flow. Petrologic analyses of Tertiary-aged samples reveal the common occurrence of 6 shale types in deep-marine stratal packages: 1) well-laminated organically-enriched shales; 2) slightly silty, weakly laminated shales; 3) moderate to very silty, weakly laminated shales; 4) distinctly mottled very silty shales; 5) very silty shales and argillaceous siltstones; and 6) calcareous shales and claystones. Each shale type represents a limited range of depositional and geochemical conditions. Shale types 1, 2 and 6 have significantly greater critical seal pressures (10% non-wetting saturation) relative to shale types 3 and 4. Shale type 5 has the lowest sealing potential.
These shale facies vary systematically in terms of sequence stratigraphy and exhibit strong correlations with seal capacity suggesting that textural parameters have a direct effect on seal capacity. Silt-poor (< 10%) transgressive shales typically have excellent (7,000 −10,000 psia) to exceptional (>10,000 psia) critical seal pressures. Increased percentages of silt-sized detrital grains inhibit mechanical compaction and allow preservation of relatively large-diameter pore throats, which typify highstand and lowstand shales. Well-developed laminar fabrics, increased content of organic matter, and/or early marine carbonate cementation can enhance seal capacity. Bioturbation disrupts primary fabrics and thereby degrades seal character. Thin calcareous (condensed) shales are prone to fracturing. Stacking patterns inherent to deep-marine depositional systems can result in considerable (several hundred feet) vertical separation between a lowstand reservoir and the overlying seal resulting in the formation of a thick “waste zone.” Log-derived parameters generally lack significant ability to accurately predict critical leak pressures in deep-water seal samples.
An improved understanding of potential sealing facies, in terms of sedimentology and stratigraphy, offers valuable insight for both the exploration and development geologist. Excellent seal character may be exhibited by transgressive shales found below a sequence boundary. In contrast, silty shales and argillaceous siltstones found in lowstand units overlying the sequence boundary may exhibit moderate to poor seal potential. In exploration, recognizing the stratigraphic position and measureable seal capacity of specific stratigraphic facies may assist in the estimation of potential hydrocarbon columns and predict the possibility of waste zones between anticipated reservoir and seal facies. In field development scenarios, a more precise understanding of the stratigraphic position of the most effective sealing facies and their faults intersects may allow greater appreciation of fault seal risk throughout the field area.
In summary, seal character is related to shale texture and fabric, content of detrital silt, early marine diagenesis (carbonate cementation), and stratigraphic position. Each seal type has a different compaction rate, described by porosity-depth and porosity-effective stress relationships, porosity versus permeability, and capillary pressure distributions. Ongoing analyses of shale data have begun to provide a compelling argument for textural control of seal character induced by high frequency sedimentary cycles.
William R. Almon is a research geologist for Chevron Energy Technology Company,
Houston, TX. He earned his Ph. D in geology and a M. A. in chemistry from the University of
Missouri (Columbia). He received an M.S. in petroleum engineering from the University of
Tulsa and an M.A. in geology and a B.A. in chemistry from Washington University (St.
Louis). His 34-year career includes positions in research management, applied technology
and exploration at Cities Service, Anadarko, Texaco and Chevron. His research interests
include sequence stratigraphy and siliciclastic depositional systems, as well as sedimentary
geochemistry and diagenesis. He has twice been a recipient of AAPG’s Levorsen award. He
is a member of AAPG, SEPM, GCAGS, and the SPE.
William C. Dawson is a research geologist for Chevron Energy Technology Company,
Houston, TX. He earned B.S. and Ph.D. degrees Champaign-Urbana and an M.S. degree in
geology from University Texas at Arlington. His professional experience includes: Illinois
State Geological Survey; Fossil Petroleum Corporation; Eason Oil Company; Texaco
Research. His research interests include shale sedimentology, seal characterization,
sequence stratigraphy, reservoir diagenesis. He is a former associate editor of the AAPG
Bulletin and a recipient of AAPG’s Levorsen award. He is a member AAPG, SEPM, GCAGS,
and the International Association of Sedimentologists.
Craig W. Stichtenoth is an exploration geologist for Chevron Energy Technology Company
in Houston, TX. He earned a B.A. degree in geology from Miami University, and an M.S.
degree from Bowling Green University. His professional experience includes exploration and
development activities in onshore and offshore basins of the US, and in West Africa, the
Middle East, and elsewhere. He is a member of AAPG.