Reflectance spectroscopic mapping of diagenetic ... reservoir rocks can facilitate identification

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AUTHORS

Brenda Beitler Bowen Department ofGeology and Geophysics, University of Utah,135 S. 1460 E., Salt Lake City, Utah 84112;present address: Department of Earth andAtmospheric Sciences, Purdue University, 550Stadium Mall Drive, West Lafayette, Indiana47907; bbowen@purdue.edu

Brenda Beitler Bowen is currently a postdoc-toral research associate at Central MichiganUniversity studying fluid-sediment interactionsin acid saline systems. She will begin as as-sistant professor of earth sciences at PurdueUniversity in the fall of 2007. She earned herB.S. and M.S. degrees at the University ofCalifornia, Santa Cruz, and her Ph.D. at theUniversity of Utah, studying the diagenesis ofthe Navajo Sandstone.

Brigette A. Martini Amber Scientific Inc.,8 B St. #1, Ashland, Oregon 97520; presentaddress: Riverside Research Institute, 2681Commons Blvd., Beavercreek, Ohio 45431

Brigette A. Martini is an independent remotesensing and geological consultant in Ashland,Oregon, and a part-time researcher for River-side Research Institute. She received her B.S.degree (1997) from the University of Arizona,emphasizing structure and tectonics, and herPh.D. (2002) from the University of California,Santa Cruz, emphasizing volcano-tectonicsand hyperspectral imaging. She spent the lastthree years in Australia working for HyVistaCorporation as a hyperspectral analyst andgeologist.

Marjorie A. Chan Department of Geologyand Geophysics, University of Utah, 135 S.1460 E., Salt Lake City, Utah 84112

Marjorie A. Chan is a professor of geologyand department chair at the University of Utah.She received her B.S. degree from the Universityof California, Davis, and her Ph.D. from theUniversity of Wisconsin, Madison. Her recentand current research focuses on Mesozoic sedi-mentology and stratigraphy on the ColoradoPlateau, with applications to eolian reservoirsand terrestrial iron oxide concretion analogs toMars.

Reflectance spectroscopicmapping of diageneticheterogeneities and fluid-flowpathways in the JurassicNavajo SandstoneBrenda Beitler Bowen, Brigette A. Martini,Marjorie A. Chan, and William T. Parry

ABSTRACT

Multiple episodes of fluids migrating through the Jurassic Navajo

Sandstone have resulted in abundant and spatially variable diage-

netic mineral changes. Depending on fluid chemistry, flow events

have produced or removed varying amounts of iron oxides, clays,

and carbonates with distinctive spectral reflectance signatures that

can be used to map spatial heterogeneities in diagenetic mineralogy

and paleofluid-migration pathways (including hydrocarbons and

groundwaters). Field and laboratory reflectance spectroscopy shows

that the common diagenetic minerals in the Navajo Sandstone have

diagnostic visible, near-infrared, and short-wave infrared spectral

characteristics in the 0.352.5-mm range. Comparisons of (1) geo-chemical data, (2) in-situ reflectance spectroscopy, and (3) airborne

imaging spectroscopy for zones of variably altered Navajo Sandstone

in southernUtah show that themineralswithin alteration facies have

distinctive spectral signatures. Reflectance spectroscopic mapping

provides a method for evaluating the effects of diagenesis and fluids

in this well-exposed reservoir sandstone. Reservoir heterogeneity

in many eolian sandstones is largely controlled by diagenetic pro-

cesses that can be difficult to evaluate on outcrop to reservoir scales

(approximately tens to hundreds of meters to several kilometers).

Imaging spectroscopy allows for the evaluation of mineralogy varia-

tions on these scales. The patterns of authigenic iron oxide, clay, and

carbonate removal and precipitation trace the paths of different

episodes of fluid flow and sandstone alteration. Mineral variations

occur as kilometer-scale reaction fronts related to structural fluid

GEOHORIZONS

AAPG Bulletin, v. 91, no. 2 (February 2007), pp. 173190 173

Copyright #2007. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received November 22, 2005; provisional acceptance March 2, 2006; revised manuscriptreceived June 12, 2006; final acceptance August 22, 2006.

DOI:10.1306/08220605175

conduits and as 100-m (330-ft)-scale changes that follow stratigra-

phy. These spectroscopic techniques provide important tools for

reservoir evaluation, and the patterns observed serve as an analog

to understanding regional diagenetic patterns in other subsurface

eolian reservoirs.

INTRODUCTION

Understanding fluid movement through sedimentary reservoirs is

critical to predicting the distribution of natural resources. Themove-

ment of subsurface fluids controls the distribution and evolution of

valuable hydrocarbons, economicmineral deposits, and fresh water.

In eolian sandstones, reservoir quality (porosity and permeability)

and heterogeneity are largely controlled by fluid-related diagenetic

processes. Several potential methods exist for evaluating the history

of past fluid flow in sedimentary basins, including analyses of fluid

inclusions (Goldstein, 2001;Mark et al., 2005), numericalmodeling

(Bethke, 1989; Garven, 1995; Person et al., 1996), and geochem-

istry of diagenetic minerals and textures (Morad et al., 2000; Kyser

andHiatt, 2003; Boles et al., 2004; Eichhubl et al., 2004). Although

all of these methods provide valuable information about the his-

tory of subsurface fluid-sandstone interactions, they typically do

not allow for outcrop to basin-scale mapping of fluid-related alter-

ation. The use of reflectance spectroscopy in areas of exhumed

reservoir rocks can facilitate identification of alteration-related min-

erals and mapping of their spatial distribution on an outcrop scale

that is likely analogous to alteration patterns expected in similar

units in the subsurface.

TheNavajo Sandstone is one of the largest known erg (dune sea)

deposits on Earth (Blakey et al., 1988). With favorable reservoir

characteristics conducive to interstitial fluid flow, the sandstone has

had a complex history of water-rock interactions related to the mi-

gration of multiple generations of fluids with varying chemistry

approximately over the last 200 m.y. and continues to be an im-

portant reservoir for both hydrocarbons and water. For example,

the Covenant field in the Sevier overthrust belt in central Utah is a

major new discovery, with potential reserves that are estimated at

75200 million bbl (Brown, 2005a). This new field could poten-

tially be analogous to the very successful Anschutz Ranch East field

in the WyomingUtah thrust belt, where the Nugget Sandstone,

the northward correlative unit of the Navajo Sandstone, has pro-

ducedmore than288MMBOand5.1 tcf of gas (Chidsey andMorgan,

2005). The continued recognition of the Navajo Sandstone and other

eolian units worldwide as productive reservoirs underscores the im-

portance of understanding fluid-flow pathways and the diagenetic his-

tory of this formation.

Chemical reactions in the Navajo Sandstone have resulted in

diageneticmineral facies that reflect the effects of fluid flow through

the sandstone. The characteristic types of chemical alteration in the

sandstone suggest hydrocarbon-induced reactions (e.g., Schumacher,

William T. Parry Department of Geologyand Geophysics, University of Utah, 135 S.1460 E., Salt Lake City, Utah 84112

William T. Parry is a professor emeritus ofgeology and geophysics at the University ofUtah. He was formerly an associate professorof geosciences at Texas Tech University, Lubbock,Texas. He received his B.S. and M.S. degreesand his Ph.D. in geological engineering from theUniversity of Utah. His research interests aregeochemistry and mineralogy related to faultsand ore deposits.

ACKNOWLEDGEMENTS

Acknowledgment is made to the donors ofthe Petroleum Research Fund of the AmericanChemical Society and the Bureau of LandManagementGrand Staircase-Escalante Na-tional Monument for support of this research(grants to M. A. Chan and W. T. Parry). Wethank Jens Ormo and Olga Prieto Ballesterosat the Centro de Astrobiologa (Instituto Na-cional de Tecnica Aeroespacial) in Madrid,Spain, for the Micro-Fourier transform infraredanalyses. AAPG Bulletin editor Carol Christo-pher and reviewers F. G. Ethridge andS. C. Stover provided constructive commentsto improve the manuscript during revisions.

174 Geohorizons

1996). Based on the diagenetic facies that are observed,

the most common chemical reactions in the Navajo

Sandstone involve the following:

Reduction of iron bleaching the sandstone

Hematite 3:75 H 0:25 CH4 2:25 H2O 2 Fe2 0:25 HCO3 1

Alteration of K-feldspar to kaolinite and illite

K feldspar 0:5 H2OH K 0:5 Kaolinite 2 Quartz 2

K feldspar 0:67 H 0:67 K 0:33 Illite 2 Quartz 3

Precipitation and dissolution of carbonate cement

CalciteH Ca2 HCO3 4

Oxidation and precipitation of iron oxide from solution

0:25 O2 Fe2 1:5 H2O Goethite 2 H 5

2 Goethite HematiteH2O 6

Reaction 1 involves methane as a proxy for a

hydrocarbon-reducing agent and reaction 5 involves

molecular oxygen as an oxidizing agent. Reactions 14

consume hydrogen ions, and reaction 5 produces the

hydrogen ion. The hydrogen ion produced in reaction 5

may drive reactions 24. Reactio