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AAPG Bulletin, V.
Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability
1Consultant, 3822 W. Clay Street, Houston, Texas, 77019; email: [email protected]
2Geocosm LLC, 6280 McNeil Drive #604, Austin, Texas, 78729; email: [email protected]
3Geocosm LLC, 6280 McNeil Drive #604, Austin, Texas, 78729; email: [email protected]
Sal Bloch's main technical interests are predrill prediction of reservoir quality, reservoir quality assessment, and sandstone petrology. He received his Ph.D. from George Washington University, Washington, D.C., in 1978. Sal recently left Texaco to become a geological consultant. Prior to joining Texaco in 1997 he was employed by Norsk Hydro (consultant, 1995-1996), Arco (principal research geologist, 1982-1995), and the University of Oklahoma (geologist/adjunct professor, 1978-1982). He was a co-instructor in an SEPM Short Course on "Reservoir Quality Assessment and Prediction in Clastic Rocks" and a co-editor of AAPG Memoir 69, Reservoir Quality Prediction in Sandstones and Carbonates. He served six years as associate editor for the AAPG Bulletin and is currently an AAPG Distinguished Lecturer.
Rob Lander's research involves diagenetic and petrophysical modeling of sandstones. In 2000 he cofounded Geocosm, where he is a scientific advisor. He obtained a Ph.D. in geology from the University of Illinois in 1991 and worked for Exxon Production Research from 1990 to 1993. In 1993 he joined Rogaland Research in Stavanger, Norway, and cofounded a spin-off company, Geologica, where he held the position of technical director at the time of his departure in 2000.
Linda Bonnell received a Ph.D. in geology from the University of Illinois in 1990. After doing postdoctoral research at Washington University and Rice University, she took a position at Rogaland Research in Stavanger, Norway. From 1996 until 2000, she worked as a senior staff geologist for Geologica in Stavanger, Norway. In 2000, she cofounded Geocosm in Austin, Texas, where she specializes in reservoir quality prediction and characterization.
Sal Bloch gratefully acknowledges Norsk Hydro's support (1995-1996) for his work on chlorite coats and Texaco's support for his work on the hydrocarbon emplacement and overpressure effects. Discussions with Ruth Elin Midtbo and Mogens Ramm (Norsk Hydro) during the course of this project and their review of an early version of the manuscript were very helpful. We are also grateful to Tom Dreyer and John Gjelberg (Norsk Hydro) for their sedimentologic interpretation of core from the key well discussed in the section on chlorite coats and to Johannes Rikkje (Norsk Hydro) for the backscattered electron images of chlorite-coated sands. Comments by Bill Almon (Texaco) on the effect of grain coating are greatly appreciated. AAPG reviewers Richard Larese and James Schmoker offered numerous suggestions that significantly improved the article. We also wish to express our appreciation to Norsk Hydro and Texaco, Inc. for their permission to publish this article.
Porosity and permeability generally decrease with increasing depth (thermal exposure and effective pressure); however, a significant number of deep (>4 km [approximately 13,000 ft]) sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability. Anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology (composition and texture), age, and burial/temperature history. In sandstones containing anomalously high porosities, such porosities exceed the maximum porosity of the typical sandstone subpopulation.
Major causes of anomalous porosity and permeability were identified decades ago; however, quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature. The focus of this article is on quantification and predictability of three major causes of anomalously high porosity: (1) grain coats and grain rims, (2) early emplacement of hydrocarbons, and (3) shallow development of fluid overpressure.
Grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital-quartz grains. Currently, prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets. In the absence of adequate empirical data, sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality. Such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability (Begin page 302) in a given play or prospect. These constraints include thermal history and sandstone grain size and composition.
The overall effect of hydrocarbon emplacement on reservoir quality is controversial. It appears that at least some cements (quartz and illite) may continue to precipitate following emplacement of hydrocarbons into the reservoir. Our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify, prior to drilling, the potential impact of hydrocarbon emplacement on porosity and permeability.
The best-case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited Tertiary or Quaternary sandstones. Our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths. The models also indicate that the potential for porosity preservation is greatest in ductile-grain-rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks. The case for significant porosity preservation associated with fluid overpressures in pre-Tertiary basins, however, is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss.
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