Geological Society of America Bulletin - Geochemistry...Becker et al. 1082 Geological Society of...

Geological Society of America Bulletin

doi: 10.1130/B30067.1 published online 29 March 2010;Geological Society of America Bulletin

S.P. Becker, P. Eichhubl, S.E. Laubach, R.M. Reed, R.H. Lander and R.J. Bodnar Cretaceous Travis Peak Formation, East Texas basinA 48 m.y. history of fracture opening, temperature, and fluid pressure:

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ABSTRACT

Quartz cement bridges across opening-mode fractures of the Cretaceous Travis Peak Formation provide a textural and fl uid inclusion record of incremental frac-ture opening during the burial evolution of this low-porosity sandstone. Incremen-tal crack-seal fracture opening is inferred based on the banded structure of quartz cement bridges, consisting of up to 700 cement bands averaging ~5 µm in thick-ness as observed with scanning electron microscope–cathodoluminescence. Crack-seal layers contain assemblages of aqueous two-phase fl uid inclusions. Based on fl uid inclusion microthermometry and Raman microprobe analyses, we determined that these inclusions contain methane-saturated brine trapped over temperatures ranging from ~130°C to ~154°C. Using textural crosscutting relations of quartz growth in-crements to infer the sequence of cement growth, we reconstructed the fl uid tem-perature and pore-fl uid pressure evolution during fracture opening. In combination with published burial evolution models, this reconstruction indicates that fracture opening started at ca. 48 Ma and above-hydrostatic pore-fl uid pressure conditions, and continued under steadily declining pore-fl uid pressure during partial exhuma-tion until present times. Individual frac-tures opened over an ~48 m.y. time span at rates of 16–23 µm/m.y. These rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings.

INTRODUCTION

The timing of fracture opening is com-monly inferred from crosscutting and abutting relationships and orientation (Engelder, 1985; Hancock, 1985; Pollard and Aydin, 1988). How-ever, these observations are rarely suffi cient to closely constrain deformation timing, and the age of most fractures is highly uncertain. Rates of fracture opening are unknown, despite the relevance of such information for constraining fracture growth models and for understanding the mechanical and hydrogeologic evolution of sedimentary basins. It is commonly assumed, without further examination, that fracture sets mark discrete structural events that are rapid relative to other changes in the rock such as ce-ment precipitation (Engelder, 1985; Lash and Engelder, 2007). Some modeling work, on the other hand, assumes that fracture growth in sedimentary rocks may occur over millions of years (Renshaw and Harvey, 1994; Olson, 2007; Olson et al., 2007). Despite the importance of fracture opening and sealing processes in gov-erning rock strength, anisotropy, and capacity to conduct fl uids, surprisingly little is known of the timing and conditions of fracture development in sedimentary basins. Less is known about the pore-fl uid pressure history of sedimentary ba-sins: While fl uid pressures have been estimated based on fl uid inclusions for stages of vein for-mation in sedimentary basins (Bodnar, 1990; Parris et al., 2003; Hanks et al., 2006) and ac-cretionary wedges (Hashimoto et al., 2003) and for strain fringes (Goldstein et al., 2005), we are not aware of any study that has provided direct measurements of a time–fl uid pressure record during opening of a single fracture.

Many opening-mode fractures (joints or extension fractures) in clastic sedimentary rocks are open and contain only minor or trace amounts of cement deposits. In this, they con-trast with cement-fi lled veins that have little to no remaining porosity from metamorphic and

hydro thermal environments. In moderate to deep burial in sedimentary basins, fractures tend to be partially cemented and have ample remain-ing fracture porosity. These fracture cement deposits can be tied to diagenetic cement se-quences of known age, and thus provide insight into fracture timing (Laubach, 1988; Eichhubl and Boles, 1998; Laubach, 2003; Perez and Boles, 2004). In addition, some cement deposits contain fl uid inclusion assemblages that can be correlated with a known thermal history to re-construct the timing and conditions of fracturing (Narr and Currie, 1982; Laubach, 1988, 1989, 2003; Eichhubl and Boles, 2000; Parris et al., 2003; Laubach et al., 2004a, 2004c; Hanks et al., 2006; Laubach and Diaz-Tushman, 2009). Other workers have used fl uid inclusions hosted in metamorphic fracture cement deposits to reconstruct pressure-temperature-time (P-T-t) paths during progressive metamorphism and deformation (e.g., Mullis, 1987; Vrolijk et al., 1988; Boullier et al., 1991; Mullis et al., 1994; Mullis, 1996; Xu, 1997; Boullier, 1999; Wagner and Cook, 2000; Hurai et al., 2002; Schulz et al., 2002; Urban et al., 2006; Nüchter and Stöckhert , 2008). However, metamorphic veins are typi-cally completely sealed and formed under higher temperatures, pore-fl uid pressure, and confi ning stress compared to mostly open frac-tures in diagenetic environments. Therefore, timing and conditions of fracture opening recon-structed for metamorphic veins may not apply to diagenetic environments in sedimentary basins.

The history of fractures that open under moderate to deep diagenetic conditions is re-corded by textural and compositional mark-ers within cement deposits that locally span fractures. As we describe here, these isolated deposits in otherwise open fractures are key to unlocking the history of these fractures. High-resolution scanning electron microscope (SEM)–cathodoluminescence (CL) images al-low a sequence of cement deposit growth to be established based on crosscutting relationships.

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GSA Bulletin; July/August 2010; v. 122; no. 7/8; p. 1081–1093; doi: 10.1130/B30067.1; 11 fi gures; Data Repository item 2010041.

*Current address: ExxonMobil Upstream Re-search Company, P.O. Box 2189, Houston, Texas 77252-2189, USA.

†E-mail: [email protected]

A 48 m.y. history of fracture opening, temperature, and fl uid pressure: Cretaceous Travis Peak Formation, East Texas basin

S.P. Becker1,*, P. Eichhubl1,†, S.E. Laubach1, R.M. Reed1, R.H. Lander2, and R.J. Bodnar3

1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924, USA2Geocosm LLC, 3311 San Mateo Drive, Austin, Texas 78738, USA3Department of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, Virginia 24061, USA

Published online March 29, 2010; doi:10.1130/B30067.1 on March 30, 2010gsabulletin.gsapubs.orgDownloaded from


Becker et al.

1082 Geological Society of America Bulletin, July/August 2010

We then apply microthermometry and Raman spectroscopy of fl uid inclusions hosted within these cement deposits to reconstruct the pressure-temperature-composition (P-T-X) his-tory of fl uids present during fracture opening. In combination with burial history information, we determine timing of fracture opening relative to the burial and fl uid migration history of the basin , and the rate of fracture opening.

GEOLOGIC SETTING

A thick sequence of Mesozoic and Cenozoic rocks was deposited in the gradually subsiding East Texas basin, part of the larger northern Gulf of Mexico basin, following Late Triassic to Middle Jurassic rifting (Buffl er et al., 1980; Pindell, 1985). The northern Gulf of Mexico basin is a structural province characterized by small normal faults, gentle bedding dips, and open folds. The largest structure of the East Texas basin is the Sabine Arch, a broadly north-trending, basement-cored anticline (Lau-bach and Jackson, 1990; Nunn, 1990) (Fig. 1). Uplift in the early Late Cretaceous and another episode of movement in the early Paleogene produced less than 300 m of uplift on the crest of the arch (Jackson and Laubach, 1991). We sampled a core from the Staged Field Experi-ment (SFE) Well No. 2 in the North Appleby Field, Nacogdoches County, Texas, located on the western fl ank of the Sabine Arch (Fig. 1). A relatively simple and well-preserved burial his-tory makes this area suitable for testing models of fracture growth and timing.

The Lower Cretaceous Travis Peak Forma-tion was deposited during the thermal sub-sidence phase of basin evolution. In East Texas, the Travis Peak Formation is ~600 m thick, and the depth to the top of the formation ranges from 1790–2895 m. The formation consists of inter-bedded quartz arenite, subarkose, and mudstone; extensive quartz cementation has reduced per-meability of much of the formation to less than 0.1 mD (~10–6 cm/s for water) (Dutton, 1987; Jackson and Laubach, 1991). The presence of partially uncemented opening-mode fractures locally enhances permeability of this producing gas reservoir (Fig. 2) (Laubach, 1989, 2003).

Fractures measured in nine wells located on the Sabine Arch strike east-northeast; dis-persion in fracture strike is much wider than azimuths of east-northeast–trending maximum horizontal stress measured in the same wells, so fractures and in situ stress are imprecisely aligned (Laubach, 1988). The greatest principal stress is vertical, and the least principal stress, which is ~60% of the vertical stress (Laubach et al., 2004b), is generally oriented normal to regional fault trends and the continental mar-

gin (Laubach, 1988). Fracture dips are steep, about perpendicular to bedding. Regionally, macrofractures observed in the core are sub-divided into two groups based on their aspect ratios (Laubach, 1989): Tall, narrow fractures are found throughout the region, but wider and generally shorter fractures are more prevalent in the deepest samples. The fractures we examined are of the latter type. Fracture intensity increases with depth. At all depths, fractures show a wide aperture size distribution. Fractures having kine matic apertures (distance from wall rock to wall rock) of 0.1 mm or less are generally en-tirely sealed with quartz (Laubach, 2003). Frac-tures predominantly contain quartz, but other phases, primarily calcite to ferroan calcite and dolomite to ankerite, are also widespread in the rock mass and in fractures (Dutton and Land, 1988; Laubach, 1989). For this study, fractures containing quartz cement were collected from the upper Travis Peak Formation at a depth of 2999 m (9840 ft) (Fig. 2).

FRACTURE CEMENT PETROGRAPHY

Fracture cement textures were imaged using cathodoluminescence (CL) and backscattered electron (BSE) microscopy using a Phillips

XL30 SEM equipped with an Oxford Instru-ments MonoCL cathodoluminescence system. The SEM was operated at 12–15 kV and at large sample currents for panchromatic CL imaging. Color CL images were obtained by combin-ing three grayscale images produced using red, green, and blue fi lters. Signal amplifi cation dur-ing CL image acquisition was selected to obtain maximum contrast and signal-to-noise ratio. Contrast and saturation were further enhanced during digital image processing.

The fractures imaged for this study have kine-matic apertures ranging from ~800 to ~1200 µm and are partially cemented with quartz bridges, euhedral quartz, and carbonate (ankerite) ce-ment (Figs. 3–5). Bridges are cement columns consisting of isolated crystals or masses of crys-tals that connect both fracture walls and that are surrounded by remnant fracture porosity or later fracture cement (Fig. 3A) (Laubach et al., 2004c). The core of the quartz bridges frequently contains closely spaced planar assemblages of fl uid inclusions that parallel the fracture walls (Fig. 3B). This fl uid inclusion–rich core is sur-rounded by a layer of inclusion-poor clear quartz (e.g., upper part of bridge in Fig. 3B).

We selected two bridges for detailed tex-tural and fl uid inclusion analyses, referred to as

Mex

ia-Talco fault zone

East

Tex

as S

alt Basin

NAngelina FlexureElkhart-Mt E

nterprise

fault zone

Sabine Arch

South Arkansas

fault zone

Oklahoma

Texas

Arkansas

Louisiana

Texas

Tyler

Shreveport

0

0

100 miles

150 km

SFE2 Site

94°

33°

34°

32°

31°

95°96°

Figure 1. Location map for well SFE2 located on the western fl ank of the Sabine Arch in East Texas. Core samples of partially cemented fractures were collected at a depth of ~3 km in the upper Travis Peak Formation.



A 48 m.y. history of fracture opening

Geological Society of America Bulletin, July/August 2010 1083

bridges 9840p2 and 9840p5 (Figs. 3, 4, and 5). These bridges were collected from separate fractures that were ~8 cm in core depth apart. SEM-CL reveals that the inclusion-rich core of these bridges is composed of up to 700 individ-ual cement bands that range from 1 to 10 μm in thickness and are roughly parallel to the fracture walls (Figs. 4A, 4B, 5A, and 5B). Cement bands are, for the most part, parallel to one another, although single bands are observed to cut across earlier bands (e.g., left half of bridge 9840p5; Figs. 5A and 5C). At several locations within these bridges, zones composed of multiple par-allel bands are also observed to cut across earlier zones of parallel bands, forming angular uncon-formities within the cement sequence (Figs. 4B and 5B). These crosscutting relations dem-onstrate that cement bands form sequentially by repeated bridge breakage and cement pre-cipitation. Evidence for bridge breakage is also provided by inclusions of grain fragments that fi t broken grains along the fracture wall in size and shape (Fig. 5C). Cement bands observed in SEM-CL are also parallel to, and contain, the planar fl uid inclusion assemblages observed under the light microscope. Fluid inclusions located at the sample surface, and thus visible in SEM-CL, are for the most part located in the

center of cement bands (Figs. 4B and 5B). Be-cause planar fl uid inclusion assemblages follow cement bands, we infer that these assemblages refl ect and record repeated and sequential stages of fracture opening, bridge breakage, and ce-ment precipitation. Similar textures observed here have been described previously in quartz bridge cements in sedimentary basins (Lau-bach, 1988, 1989; Parris et al., 2003; Laubach et al., 2004a, 2004b, 2004c; Hanks et al., 2006; Laubach and Ward, 2006). These studies attrib-uted these cement textures to repetitive fracture opening characteristic of the crack-seal mecha-nism of fracture opening and sealing based on a

comparison to similar textures in metamorphic crack-seal veins (Ramsay, 1980). We follow this interpretation and refer to the inclusion-rich ce-ment in the core of bridges 9840p2 and 9849p5 as crack-seal cement.

Under SEM-CL, we observe that the layer of inclusion-poor cement that surrounds the crack seal core in both bridges 9840p2 and 9840p5 is free of crack-seal cement layers. Instead, the internal texture of this inclusion-poor cement is characterized by idiomorphic cement growth layers (Figs. 4A and 5A). Unlike crack-seal ce-ment layers, idiomorphic cement growth lay-ers vary in thickness from layer to layer, have

Figure 2. Vertical opening-mode fracture in Travis Peak sandstone from a depth of ~3 km from the SFE2 well (Fig. 1). The frac-ture walls are coated with diagenetic quartz cement. Abundant synkinematic quartz cement bridges are present, particularly to-ward the bottom tip of the fracture. Signifi -cant fracture porosity is preserved between cement bridges.

A 9840p2

B

B

200 μm

Figure 3. (A) Scanning electron microscope (SEM) backscattered electron (BSE) image of fractured Travis Peak sandstone containing quartz bridge 9840p2 (box). The host rock is primarily composed of quartz sand grains (darkest gray) and pore-fi lling dolomite cement (medium-gray rhombs). The fracture contains abundant synkine-matic (and possibly postkinematic) ankerite cement (light gray) that tends to overgrow broken pore-fi lling dolomite along the fracture wall. Gray-white speckled material is remnant drilling mud. Black indicates porosity fi lled by epoxy. Quartz bridge 9840p2 contains inclusions of synkinematic ankerite cement. (B) Transmitted light image of quartz bridge 9840p2. Numerous trails of pseudo secondary fl uid inclusion assemblages oriented parallel to the fracture walls are visible.



Becker et al.


diffuse boundaries, and can contain sharp cor-ners that refl ect the angular relations of crystal-lographic zones relative to the orientation of the thin section. We thus refer to this cement rim as lateral euhedral cement (mapped blue in Figs. 4C and 5C). Euhedral lateral quartz cement forming on crack-seal bridges is laterally con-tinuous with euhedral cement that forms a ce-ment layer up to 30 µm thick on quartz grains along the fracture walls between quartz bridges (Figs. 4C and 5C).

The length of crack-seal cement bands as observed under SEM-CL varies systematically: Shorter crack-seal cement layers are consis-tently crosscut by longer cement layers (Figs. 3B and 4B). In addition, lateral euhedral cement is generally narrower adjacent to longer crack-

seal cement layers and wider adjacent to shorter crack-seal cement layers. Based on these geo-metric relations, we interpret lateral euhedral cement to represent quartz overgrowth ce-ment that precipitates on the crack-seal cement bridge. The width of the lateral euhedral cement layer varies systematically across the bridges (Figs. 3C and 4C), suggesting that the lateral euhedral cement precipitates as the crack-seal cement bridge lengthens over time with increas-ing fracture opening. This systematic trend in lateral euhedral cement thickness is interrupted by late-stage single crack-seal cement bands that cut across lateral euhedral cement, with exception of an outermost layer of euhedral ce-ment (e.g., left part of bridge 9840p5; Fig. 5C). Growth of lateral euhedral cement concurrent

with crack-seal bridge growth is also indi-cated by crack-seal bands cutting across inner (earlier) layers of lateral euhedral cement but abutting against outer (later) lateral euhedral ce-ment (e.g., bottom of zones IV and V in bridge 9840p2; Fig. 4A).

Based on the crosscutting relations of zones of crack-seal cement, the length of crack-seal cement layers, the thickness of the lateral euhedral cement, and crosscutting relations be-tween lateral euhedral cement and crack-seal cement layers (Fig. 6), we reconstructed the growth sequence for the two cement bridges 9840p2 and 9840p5 (Figs. 4C and 5C). We limited this reconstruction to zones of crack-seal cement, rather than individual crack-seal cement layers, because the resolution of

CB

B

IIIIII IVIV

VV

I II

Ankerite

Quartz crack-seal cementFra

ctur

e w

all

Fra

ctur

e w

allQuartz

grain

Pore-fillingquartz cement

Pore-fillingankerite cement

40 μm

Abandoned bridge

Crack seal cement

Fluid inclusions

Lateral cement

Abandoned bridge

Lateral quartz cement

Bridge cem

ent zone boundary

A 9840p2

Fracture porosity


200 μm

200 μm

Figure 4. (A) Color scanning electron microscope–cathodoluminescence (SEM-CL) image of quartz bridge 9840p2. (B) Detail of boundary between crack-seal cement zones boundaries marked by angular unconformity between cement bands. (C) Map of quartz bridge 9840p2. Quartz grains are mapped in red, pore-fi lling quartz cement is yellow, and ankerite cement is orange. The crack-seal cement of the quartz bridges is divided into color-coded zones as described in the text. Lateral euhedral quartz cement forming as overgrowth on the bridge is blue.





CL images did not allow for mapping of the thinnest crack-seal layers. Bridge 9840p2 is mapped into eight such zones. The contacts be-tween zones labeled I and II in Figure 4C are marked by angular unconformities in growth zones, indicating a slight change in fracture opening direction. The contacts between zones II and III are largely concordant but marked by an increase in length of crack-seal cement layers for zone III. The contacts between zones IV and older cement, and between IV and V, are again marked by angular unconformities and an increasing length in crack-seal cement lay-ers (Fig. 4B). The three outermost zones on the right size of the bridge fi t to three zones on the left side based on band orientation and band length. Pairing these matching zones, we obtain fi ve growth zones consecutively ar-ranged from zone I at the bridge margins in-

ward to zone V, with zone I oldest and zone V youngest (Fig. 4C; see Fig. DR1 for a possible palin spastic reconstruction1). Thus, the age of crack-seal layers gets younger from the frac-ture walls toward the center of the bridge. The margin-to-center symmetry is only broken by zone IV, which formed within zone II cement, rather than within zone III cement (Fig. 4C). This asymmetry indicates that new cement layers do not consistently form within, or adjacent to, the most recent cement layer, but they can at times form within older crack-seal cement. Within zones, individual crack-seal cement layers

also generally get younger from the fracture walls to the bridge center, although some ce-ment layers also occur out of sequence, cutting across earlier formed parts of the crack-seal bridge. Bridge 9840p5 is mapped into four dis-tinct zones, where zone I is oldest, and zone IV is youngest (Fig. 5C). Unlike bridge 9840p2, these zones formed from the left of the bridge (as displayed in Fig. 5C) to the right. Some late crosscutting cement layers in zones I and II may be coeval with zone IV. No attempt was made to correlate zones I–IV in bridge 9840p5 to zones I–IV in bridge 9840p2.

Ankerite cement forms both as synkinematic crack-seal cement and as postkinematic cement partially fi lling remaining fracture space after quartz bridge cement (Figs. 3A, 4, and 5). Be-cause ankerite has no detectable CL emission, the internal texture of ankerite is obscure.

100 μm

A 9840p5

B

B

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200 μm

Lateral quartz cementGrain fragments

Fracture porosity

Fracture porosity

Ankerite

Quartz crack-seal cement

Fra

ctur

e w

all

Fra

ctur

e w

all

Quartzgrain

Pore-fillingquartz cement


I IIIII IVCrack seal cement

Lateral cement

Bridge cem

ent zone boundary200 μm

Fluid inclusions

Figure 5. (A) Color scanning electron microscope–cathodoluminescence (SEM-CL) image of quartz bridge 9840p5. (B) Detail of boundary between crack-seal cement zones boundaries marked by angular unconformity between cement bands. (C) Map of quartz bridge 9840p5. Color code of map is same as in Figure 4.

1GSA Data Repository item 2010041, palinspastic reconstruction of bridge 9840p2, Raman spectrum of the vapor bubble of inclusion p5ISO18 at room tem-perature, and fl uid inclusions, is available at http://www.geosociety.org/pubs/ft2009.htm or by request to [email protected].



Becker et al.


FLUID INCLUSION ANALYSIS

Petrography

Fluid inclusion assemblages hosted in the crack-seal core of cement bridges are com-posed of two-phase aqueous liquid-vapor fl uid inclusions that, at room temperature, contain ~5–7 vol% vapor and range from ~1 to ~15 µm in size (Fig. 3B). In comparison with the SEM-CL images, we note that the size of the in-clusions typically does not exceed the thickness of individual crack-seal layers. This observation is consistent with the interpretation of crack-seal mechanism of bridge growth where the inclu-sion size is limited by the aperture of the fracture opening increment. The length of fl uid inclusion trails agrees with the length of crack-seal cement layers as viewed in SEM-CL (Figs. 3–5). Lateral euhedral cement in bridges 9840p2 and p5 were found to be free of primary inclusions.

Bridge 9840p5 hosts some inclusions larger than crack-seal layers, up to 50 µm, with liquid-to-vapor ratios similar to the crack-seal–parallel fl uid inclusion assemblages. These large inclu-sions tend to occur along bridge-parallel trends where the preexisting crack-seal growth has

been disturbed. Although petrographic evidence is ambiguous, we interpret these inclusions as primary or pseudosecondary inclusions formed along reentrants or deep etch pits, respectively, in analogy with mechanisms of fl uid inclusion trapping described by Roedder (1984). We do not consider these large inclusions useful for interpreting the evolution of quartz bridges be-cause they could not be petrographically corre-lated to a specifi c crack-seal layer or generation of lateral growth, and they were thus omitted from our study.

Methodology

Segments of natural cement-lined and bridged fractures from core SFE2 were im-pregnated with clear epoxy, and 2.5 × 5 cm sec-tions were prepared into permanently mounted double-polished, 40–50-µm-thick thin sections. Bridges for fl uid inclusion analysis were chosen for completeness, optical quality, and the abun-dance and quality of unambiguous fl uid inclu-sion assemblages. The thin sections were then broken into ~1 cm2 pieces that contained only the quartz bridge of interest. Microthermometry was performed using a U.S. Geological Survey

(USGS)–style gas-fl ow heating/freezing stage mounted on an Olympus microscope equipped with a 40× objective (N.A. = 0.55) and 15× ocu-lars. The heating/freezing stage was calibrated at 374.1°C and 0.0°C using synthetic pure H

2O

fl uid inclusions, and at –56.6°C using synthetic CO

2 fl uid inclusions. Multiple homogenization

temperatures were measured from each fl uid inclusion assemblage following Goldstein and Reynolds (1994) to ascertain that measurements within individual crack-seal increments were internally consistent. Liquid-vapor homogeni-zation temperatures (Th) of individual inclusions within fl uid inclusion assemblages were deter-mined to ±0.5°C by thermal cycling using tem-perature steps of 1°C (Goldstein and Reynolds, 1994). Final ice melting temperatures (Tm) were determined to ±0.1°C in the same manner. The eutectic melting temperature of fl uid inclusions was not specifi cally measured; however, the ap-proximate temperature was observed in some inclusions in order to estimate the overall chem-ical system. Fluid inclusion microthermometry analyses were performed prior to SEM imag-ing to avoid localized heating from the electron beam and possible reequilibration of fl uid inclu-sions by inelastic stretching and/or leakage of fl uid inclusions.

The composition and pressure of vapor bubbles within fl uid inclusions were deter-mined using Raman spectroscopy following the technique and calibration of Lin et al. (2007) to measure the methane pressure within a fl uid inclusion bubble at room temperature and to calculate the pressure at trapping conditions. Previous studies have determined methane con-centrations in aqueous CH

4-bearing inclusions

using Raman spectroscopy on fluid inclu-sions heated to the homogenization tempera-ture in a Linkam™ heating stage (Leng et al., 1996; Guillaume et al., 2003). Our need to pre-serve petrographic relationships between the delicate fracture cement and the wall rock for subsequent SEM examination required larger samples, which prevented us from applying this technique. We therefore determined the methane pressure within inclusion bubbles us-ing Raman spectroscopy at room temperature and calculated the density of the vapor bubble using equations of state. We used equations of state of Duan et al. (1992) and Duan and Mao (2006) for the CH

4 system and the H

2O-NaCl-

CH4 system, respectively, to estimate concen-

trations of methane within the different fl uid phases and their densities at room temperature. We then calculated the bulk composition of the inclusions using the mass balance approach of Bodnar (1983). Pressures within inclusions at trapping temperatures were then calculated using the equation of state of Duan and Mao

grain

grain

I III IIIII IIIIVV VI

1

1

2

2

3

3

4

4

5

5

B

A

D

C

fractureporosity

kinematic fracture aperture

Figure 6. Schematic evolution of a crack-seal cement bridge illus-trating textural relations used for crack-seal bridge reconstructions. Roman numerals indicate sequence of zones of crack-seal cement (red shades); Arabic numerals indicate time-equivalent layers of lateral euhedral cement growth (blue shades). Letters indicate relative age relations. (A) Crosscutting of crack-seal cement layers; (B) angular unconformity between adjacent crack-seal cement zones; (C) length of crack-seal cement band and thickness of lateral euhedral cement layer; and (D) jump in length of crack-seal cement band across adjacent zones due to change in accretion direction of crack-seal cement (arrows).





(2006) and an iterative technique. Details of this technique and the Raman methodology are given in the Appendix.

Fluid Inclusion Microthermometry

Homogenization temperatures (Th) for fl uid inclusions hosted in quartz bridges from sam-ples 9840p2 and 9840p5 are plotted in Figures 7A and 7B and tabulated in Tables DR1 and DR2 (see footnote 1). Temperature variation within any fl uid inclusion assemblage was generally less than 2–3°C, and often less than 1°C. Average fl uid inclusion assemblage temperatures fall in a range between 125°C and 155°C and track systematic temperature changes during growth of crack-seal layers . In bridge 9840p2, temperatures systemati-cally decrease from 154°C to 132°C from the left fracture wall to ~50% of the length of the bridge, followed by a slight temperature

increase to ~80% of the length of the bridge (Fig. 7A). Analyses were not performed to the right of this location due to some dark mate-rial underneath the quartz, either remnant drill-ing mud or wall-rock material, which made it impossible to optically resolve inclusions. In bridge 9840p5, temperatures systematically increase from ~130°C to ~150°C from the left fracture wall to ~45% of the length of the bridge, followed by a decrease to ~132°C at ~85% of the length of the bridge (Fig. 7B).

In addition to fl uid inclusion assemblages, we measured single/isolated inclusions that were part of a recognizable fl uid inclusion as-semblage from which only one homogeniza-tion temperature could be obtained. Reporting single inclusion measurements would not typi-cally be considered appropriate technique in a fl uid inclusion study. However, single inclusion Th values follow the systematic trend of Th val-ues as a function of relative position along the

bridges obtained from fl uid inclusion assem-blage measure ments, suggesting that most of the single inclusion measurements are tracking the same temperature trend as the fl uid inclusion assemblages (Figs. 7A and 7B).

Final ice melting temperatures were found to be consistent in the range of –11 ± 1°C for fl uid inclusion assemblages and single inclu-sions found within crack-seal growth layers of bridges 9840p2 and 9840p5 (Figs. 7C and 7D) (Tables DR1–DR2 [see footnote 1]). This corre-sponds to salinities of ~15% ± 1% NaCl equiva-lent, and these values are consistent with Travis Peak Formation water salinities of 170,000 ppm total dissolved solids (TDS) (17 wt%) (Dutton et al., 1993). The eutectic melting temperature of many of these inclusions was observed near –49°C, indicating the composition of the fl uid to be a calcic brine. This is consistent with the presence of signifi cant amounts of coeval anker-ite within the same fractures (Figs. 3 and 4).

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!!

0.0 0.2 0.4 0.6 0.8 1.0

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160

Th

(°C

)

! FIAsSingles

A

!

!

! !!

!

!

!!

!!

!

!

!!

0.0 0.2 0.4 0.6 0.8 1.0

120

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B

!!!!!

!!

!!

!!!!!!!!

!!!

0.0 0.2 0.4 0.6 0.8 1.0

1012

1416

1820

Normalized distance

Sal

inity

(w

t% N

aCl e

quiv

alen

t) ! FIAsSingles

C

!! !

!

!

!!!

!

!

!

0.0 0.2 0.4 0.6 0.8 1.0

1012

1416

1820

Normalized distance

! FIAsSingles

D

IVV II IIVIIIIII I II III IV

C)

Figure 7. (A–B) Fluid inclu-sion homogenization tempera-ture versus distance for fl uid inclusion assemblages (FIAs) and single inclusions hosted in quartz bridges 9840p2 (A) and 9840p5 (B). Distances “0.0” and “1.0” represent left and right fracture walls, respectively. Ro-man numerals indicate growth zones as mapped in Figures 4C and 5C. (C–D) Salinities of fl uid inclusions in bridges 9840p2 and 9840p5 are very uniform across the bridges.



Becker et al.


Fluid Inclusion Timing

According to the petrographic relation-ships and timing interpreted from the SEM-CL imagery (Figs. 4C and 5C), homogenization temperatures of fl uid inclusion assemblages and single inclusions are sequentially rearranged in Figure 8 according to the relative age of ce-ment deposits. Fluid inclusion homogenization temperatures of bridge 9840p2 systematically decrease from ~154°C to ~132°C (Fig. 8A). Within bridge 9840p5, temperatures follow a systematic temperature increase from ~130°C to ~150°C, followed by a decrease to ~134°C (Fig. 8B). We interpret these trends in fl uid inclusion homogenization temperatures as a record of the temporal evolution in fl uid tem-perature during burial and exhumation.

Methane Concentrations of Fluid Inclusions and Trapping Pressures

Methane pressures of fl uid inclusion assem-blages and single inclusions at room tempera-ture range from 5.6 MPa to 3.9 MPa, following a trend of decreasing pressure with decreasing homo genization temperature (Fig. 9; Tables DR1 and DR2 [see footnote 1]). Methane con-centrations range between ~1800 and ~2800 ppm and represent trapping pressures from ~27 MPa to ~55 MPa (Tables DR1 and D2 [see foot-note 1]). In Figure 10, minimum trapping pres-sures for bridges 9840p2 and 9840p5 are plotted against homogenization temperature on a phase diagram of the H

2O-NaCl-CH

4 system, display-

ing isopleths of methane dissolved in a 15 wt% NaCl solution. The methane concentration of fl uid inclusions in both bridges decreases sys-tematically with homogenization temperature, traversing a path along the liquid-vapor surface in the H

2O-NaCl-CH

4 system from higher to

lower pressure (Fig. 10). The lowest pressures of ~27–33 MPa obtained for the coolest and young-est fl uid inclusions (Tables DR1–2) compare well to modern-day reservoir pressures. Pore-fl uid pressures in the vicinity of the SFE2 well are hydrostatic (Bartberger et al., 2002), which corresponds to a fl uid pressure of ~32 MPa at the sample depth of 3 km. This suggests that the pressure calculated from fl uid inclusions is within 15% of actual reservoir pressures. This error likely refl ects uncertainties in the equa-tions of state used to model the P-V-T-X proper-ties of phases present at room temperature and of the bulk inclusions. The systematic decrease in methane pressure at room temperature with homogenization temperature shown in Figure 9 is based on experimentally calibrated measure-ments only and is thus not subject to errors from equation of state calculations.

DISCUSSION

Fracture Opening and Cement Bridge Formation

Based on the crosscutting and overlapping growth relations between lateral euhedral quartz cement and crack-seal cement layers (Fig. 6), we propose the following model of fracture opening and cement bridge formation: Following each opening increment of the fracture, the ensu-ing gap in the cement bridge is fi lled by quartz precipitating on the freshly created fracture sur-faces, trapping fl uid inclusions as the two ce-ment growth layers meet at the center of the gap. Lateral euhedral quartz growth simultaneously accumulates along the margins of the bridge, widening the bridge over time. This euhedral lateral cement is subsequently disturbed by the repeated cracking events. Because the bridge

thickens with time, later and younger crack-seal cement layers are longer than earlier ones. A distinctive pattern develops where older crack-seal growth increments are both shorter in length and have accumulated more lateral growth than younger crack-seal growth increments. The lat-eral euhedral cement width is thus primarily a function of time available for cement growth.

The largely consistent sequence layers within zones of crack-seal cement suggests that bridges crack predominantly within or adjacent the most recent crack-seal cement layer. In metamorphic vein cement, the sequential formation of crack-seal cement across veins has been referred to as syntaxial or antitaxial cement for cement grow-ing toward the vein center or toward the walls, respectively (Ramsay, 1980). The formation of out-of-sequence crack-seal cement layers, as observed in both bridges 9840p2 and 9840p5, has been described from quartz cement bridges

120

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Th

(°C

)

Relative age

Th

(°C

)

120

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!

!

!

!

!!

!

!!!!!!!

!!

!

!!!!!!

!!!!!!

!!!!!!!!

!!

!!

9840p2A

I II III IV V

!

!

!!

!

!!!!

!

!

!

!!

!!!!

!

!!

!!!!!

9840p5B

I II III IV

Figure 8. Plots of homogenization tem-perature versus relative age for (A) bridge 9840p2 and (B) bridge 9840p5 arranged in sequence of growth zones (Roman nu-merals). The variation in temperature systemati cally changes with time from ~154°C to ~134°C for bridge 9840p2, and from ~130°C up to ~150°C and back down to ~134°C for bridge 9840p5.

!

!

!!

!!

!

!!

!!

!

!

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ssur

e (M

Pa)

!!

!!

!

130 135 140 145 150 155

130 135 140 145 150 155

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5.5

6.0

Temperature (°C)

Pre

ssur

e (M

Pa)

9840p5! FIAs

Singles

9840p2! FIAs

Singles

Figure 9. Room-temperature bubble pres-sure determined from Raman spectroscopy versus homogenization temperature, Th, for bridges 9840p2 and 9840p5. The pres-sure (and by extension amount) of methane contained with each bulk inclusion varies systematically with Th from lower amounts at lower temperatures to higher amounts at higher temperatures. FIA—fl uid inclusion assemblage.





previously and compared to ataxial fi ber growth (Laubach et al., 2004a, 2004c). Ataxial or stretched fi ber growth has been described from crack-seal veins displaying no consistent growth direction (syntaxial or antitaxial; Passchier and Trouw, 1996), and it appears to be characteristic of crack-seal cement bridges that form under dia-genetic conditions (Laubach et al., 2004c). Out-of-sequence cement growth suggests that older sections of a bridge are sometimes more prone to bridge failure than the most recent cement layer. Thus, a single crack-seal cement bridge may display both syntaxial and antitaxial growth at different locations (Laubach et al., 2004c). The underlying cause for this behavior is not known.

Trapping Conditions of Fluid Inclusions

We interpret the consistent covariation be-tween Th and methane composition to repre-sent trapping under methane saturation as the P-T conditions within the reservoir systemati-cally change over time during burial and uplift.

Coexist ing liquid-rich and vapor-rich inclu-sions were not observed within bridges 9840p2 or 9840p5, which would indicate immiscible trapping under methane saturation (Roedder, 1984). Instead, we infer bridge growth and fl uid inclusion trapping under water-saturated reservoir conditions that are in pressure com-munication with a free-gas phase. Pressure communication between these phases would ensure that any fl uid inclusions trapped in the water phase would remain methane-saturated as P-T conditions declined in the reservoir during exhumation, and gas exsolved from the liquid phase. Under methane-saturated conditions, homogeni zation temperatures obtained for these inclusions correspond to trapping tem-peratures. We thus interpret these fl uid inclu-sion tem peratures and pressures as a record of the temperature and pressure evolution of the reservoir. Based on the relative ages of fl uid inclusions determined from bridge mapping (Figs. 4, 5, and 8), we infer that the temperature of the fl uids varied systematically during burial

and exhumation, increasing during burial from ~130°C to the lower ~150–155°C range during the early stages of fracturing, and decreasing to ~132°C during exhumation. The fl uid pressure changed from ~50% of lithostatic overpressure at maximum burial to near-hydrostatic condi-tions at present times (Fig. 10).

The observed systematic trends in fl uid tem-perature suggest that this temperature record corresponds to the temperature evolution of the host formation. This systematic trend is in contrast to thermal pulses observed in some faults and connected fracture systems through alternating higher- and lower-temperature fl uid inclusion assemblages (Vrolijk et al., 1988; Eichhubl and Boles, 2000).

Fracture Timing and Opening Rate

We correlated the trapping temperature records of bridges 9840p2 and 9840p5 with a burial his-tory model for the East Texas basin by Dutton (1987) to date the onset and end of fracture open-ing. Based on the preserved sedimentary record on and off the Sabine Uplift, this basin history model predicts that the sampled core interval reached maximum burial conditions of ~3450 m at around 41 Ma, with subsequent uplift to present depth of 3000 m (Fig. 11). Maximum temperature conditions, predicted by Dutton (1987) based on present-day geothermal gradi-ents and models of organic maturity evolution, are around 150°C for the stratigraphic horizon of samples 9840p2 and 9840p5. This estimate

0 50 100 150 200

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ssur

e (M

Pa)

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!

!

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ssur

e (M

Pa)

!!

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thra

te s

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lity

limit

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thra

te s

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lity

limit

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3000 ppm

2500 ppm

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static

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dient

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2000 ppm

1500 ppm1000 ppm

Litho

static

gra

dient

Hydrostatic gradient

9840p2

9840p5

! FIAsSingles

! FIAsSingles

A

B

Figure 10. Pressure-temperature (P-T) phase diagrams showing isopleths from 500 to 3000 ppm in the H2O-NaCl-CH4 system for the 15 wt% NaCl-CH4 pseudobinary system (Duan and Mao, 2006). The P-T points of minimum trapping condi-tions of fl uids inclusions hosted in (A) bridge 9840p2 and (B) bridge 9840p5 are plotted. These points follow a straight trend along the liquid vapor surface from ~3000 ppm to ~1900 ppm over a temperature range of ~150°C to ~134°C, suggesting trapping under methane-saturated conditions. See text for discussion. FIA—fl uid inclusion assemblage.

120 80 60 40 20 0

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Time (Ma)

Tem

pera

ture

(°C

)

Dep

th (

km)

100

80 4

0

32

10

9840p2

Durationof fractureopening

9840p5

42 m.y.

48 m.y.

Figure 11. Burial history for samples 9840p2 and 9840p5 in well SFE2 (Fig. 1), adapted from the burial history for the nearby Ashland SFOT No. 1 well (Dutton, 1987). Temperature scale is based on present-day thermal gradient and is considered applicable for last 90 m.y. (Dutton, 1987). Inferred duration of fracture opening is based on the fl uid inclusion temperature record in bridges 9840p2 and 9840p5.



Becker et al.


compares well to the maximum tempera tures of 150–155°C obtained independently in this study using fl uid inclusions. This correspondence be-tween the two estimates in maximum burial tem-perature confi rms our interpretation that the fl uid inclusion temperatures recorded the temperature evolution of the host rock rather than of pulses of hot upward moving fl uid out of thermal equilib-rium with the host rock.

A correlation of the fl uid inclusion tem-perature trend from 154°C to 132°C in bridge 9840p2 (Fig. 9A) with the burial temperature evolution of Dutton (1987) suggests that frac-ture opening started at 42 Ma and continued to the present day (Fig. 11). Similarly, the ob-served temperature trend from 130°C to 150°C and to 132°C in bridge 9840p5 (Fig. 9B) cor-responds to fracture opening starting at 48 Ma and continuing to the present day (Fig. 11). With a fi nal kinematic aperture of ~670 µm over a du-ration of 42 m.y., bridge 9840p2 records a frac-ture opening rate of ~16 µm/m.y. With a fi nal kinematic aperture of ~1100 µm over a duration of 48 m.y., bridge 9840p5 records an average opening rate of ~23 µm/m.y.

Because these estimates of onset and end of fracture opening and derived rates of fracture opening depend only on the temperatures of the earliest and latest inclusions, as well as the maximum temperatures contained in the fl uid inclusion record, they do not depend on the de-tailed reconstruction of crack-seal cement incre-ments, which we consider interpretative (Figs. 4C and 5C). These estimates also do not depend on the sequence of individual crack-seal growth increments within mapped growth zones. The estimated onset of fracture opening at 48 Ma does depend on the slope of the burial trend in Figure 11 and the geothermal gradient used in the burial model. The slope of the burial trend at 48 Ma is constrained by the sedimen-tary record, which is preserved to ca. 50 Ma (Dutton, 1987). The maximum burial tempera-ture predicted by the geothermal gradient used in the burial model is within 5°C and thus in close agreement to the maximum tempera-ture recorded in the fl uid inclusions for both bridges. For a heating rate of 2.7°C/m.y. used in the burial model of Figure 11, an uncertainty of 5°C in the temperature evolution model im-plies an uncertainty of 2 m.y. for the onset of fracture opening. The estimate of termination in fracture opening at near-present times is based on the similarity of the youngest fl uid inclusion temperatures with present-day reservoir tem-peratures and on the observation of the young-est crack-seal cement layers terminating at the outer edge of the lateral euhedral cement.

A study by Parris et al. (2003) on the tim-ing of gas generation and migration during

development of the Brooks Range, Alaska, fold-and-thrust belt used fl uid inclusions hosted in synkinematic cement bridges to constrain timing of deformation in Triassic to Jurassic sediments. These workers derived a 15 m.y. time interval for the formation of a fracture system by relating the P-T-X evolution of pre-, syn-, and postkine-matic fl uid inclusions to a burial history. Hanks et al. (2006) similarly used fl uid inclusions hosted in synkinematic cement bridges from the Brooks Range to document the opening of multiple fractures over a 35 m.y. interval. While these workers reported durations comparable to the ~48 m.y. interval observed in our study, their results applied to the opening of multiple fractures rather than the opening duration of an individual fracture as presented in this study. In addition, our systematic analyses of fl uid inclu-sion assemblages relative to their position along synkinematic bridge cements not only provide the beginning and end of fracture formation, but also the opening rate of an individual fracture.

The ~48 m.y. duration of fracture opening contrasts with the common interpretation of fractures forming over geologically short times (Engelder, 1985; Eichhubl and Boles, 1998; Lash and Engelder, 2007). Although driving mechanisms responsible for fracture opening are varied and include changes in pore-fl uid pressure, overburden, and tectonic stresses, rates of change in loading stress are likely to refl ect regional tectonic loading conditions and strain rates. The fracture opening rates observed in this study may be characteristic of passive-margin settings undergoing slow exhumation, whereas faster rates would be more common in areas of rapid crustal deformation and high sedimentation rates.

Fluid Composition and Flow

The calculated rates of fracture opening and cementation are based on the assumption that the fracture system is in thermal equilibrium with the host formation. We justify this assump-tion by the agreement between maximum tem-peratures obtained from fl uid inclusion analyses (this study) and independently obtained burial models (Dutton, 1987), and by the absence of thermal pulses in the fl uid inclusion record. This assumption is also consistent with the constant salinity of fl uid inclusions across the cement bridges, and with fl uid inclusion salin-ity values equivalent to modern TDS values of 170,000 ppm (Dutton et al., 1993). It should be noted that fl uid compositional data such as the salinity measurements, if used in isolation, do not exclude advective transport of heat into the fracture system (Eichhubl and Boles, 2000): De-pending on the scale of heat transfer and of com-

positional variations within the basin, advective transport of heat may not result in changes in fl uid composition. Vice versa, slow infl ux of fl uids of different salinity may not result in ad-vective heat transport.

Available oxygen isotopic data of quartz ce-ment and formation water of the Travis Peak Formation suggest that cements precipitated in isotopic and thermal equilibrium with the host formation. Dutton (1987) reported a δ18O value of +20‰ standard mean ocean water (SMOW) for bulk authigenic quartz overgrowths in the upper 220 m of the Travis Peak Formation us-ing a method of analyzing bulk rock powders of varying quartz overgrowth abundance and ex-trapolating to 100% overgrowth. Laubach et al. (1995) analyzed fracture-lining quartz crystals from 9840 ft (2999 m) depth in core SFE2 and obtained a δ18O composition of +18.3‰ SMOW. This value was interpreted to have been contaminated by wall-rock material, since other isotopic analyses of diagenetic fracture cements from deeper in the core range from +21.5‰ to +22.9‰ SMOW. Williams et al. (1997) reported δ18O secondary ion mass spectrometry (SIMS) microanalyses of 15–20 µm spots in pore-fi lling quartz overgrowth cement from the Travis Peak Formation: values averaged 26‰ ± 3.5‰ SMOW. Compared to the values measured by Laubach et al. (1995), we interpret the heavier SIMS δ18O values to refl ect earlier precipitation of pore-fi lling cement under lower tempera-tures prior to fracturing. Taking δ18O values of +20‰ SMOW of the fracture-lining cements to be representative of quartz cement bridges, precipitation at temperatures of 130°C to 154°C require a δ18O formation water composition of +0.25‰–2.70‰ SMOW (Sharp and Kirschner, 1994). This agrees with analyses of modern-day Travis Peak Formation waters that range from +0.6‰ to +3.2‰ SMOW (Kreitler et al., 1984; Dutton and Land, 1988), suggesting that the fl uids precipitating the fracture cements were in isotopic equilibrium with the formation water in the host rock, in the temperature range indi-cated by the fl uid inclusion analyses.

Following our model of thermal equilibrium between fracture space and the surrounding host formation and the absence of thermally detect-able advective heat transport into the fracture system, we interpret quartz precipitation within fractures to result from local silica transfer be-tween adjacent host rock and fracture. This in-terpretation is consistent with current models for the precipitation of pore-fi lling quartz cement in sandstones under burial diagenetic conditions, where silica is derived locally from dissolution at grain to grain contacts and possible clay min-eral diagenetic reactions (Bjørkum et al., 1998; Lander and Walderhaug, 1999).





Fracture Opening Relative to Fluid Pressure Evolution

Fluid overpressure, recorded by the sys tematic change in methane concentration over time, may be related to hydrocarbon generation during the last segment of burial from 48 until 42 Ma (Fig. 11). Timing estimates for dry gas genera-tion in source rocks for Travis Peak reservoirs include ca. 57 Ma for underlying Bossier shale in East Texas (Dutton, 1987), and ca. 45 Ma for underlying source rocks of the Cotton Val-ley Group in Louisiana (Herrmann et al., 1991). These dates are similar to the estimate for the onset of fracture formation (Fig. 11).

Therefore, we propose a model in which res-ervoir charge occurred near maximum burial, contributing to an increase in pore-fl uid pressure and initiating fracture formation. During uplift, pressure was continuously released from the system, causing methane to exsolve from pore water into a free-gas phase, and thus maintain-ing methane saturation in the aqueous phase. While this was occurring, synkinematic quartz bridge growth continued in fractures fi lled with methane-saturated brines derived from ambient formation water in adjacent pore space, trap-ping fl uid inclusions at continually decreasing temperatures and methane concentrations, and reaching hydrostatic conditions in recent times.

CONCLUSIONS

In fractured Travis Peak sandstone in the East Texas basin, fl uid inclusions hosted in synkinematic quartz bridge cements record the P-T-X evolution of pore fl uids present during crack-seal fracture opening and cementation. Two quartz cement bridge deposits displaying evidence for crack-seal–style deformation were studied from two largely open fractures sampled from a modern-day depth of 3 km in the upper section of the Travis Peak Formation. Combined SEM-CL mapping and fl uid inclusion analyses indicate a systematic change in temperature and methane concentration during opening of indi-vidual fractures. Temperatures varied accord-ing to burial depth from ~130°C to ~150°C to ~134°C. Methane concentrations systemati-cally varied with homogenization temperature of fl uid inclusions, and they record a decrease in pressure from ~50% of lithostatic at maximum burial to hydrostatic conditions over time. Based on previously published burial histories, trap-ping conditions of fl uid inclusions suggest that opening of these fractures started at ca. 48 Ma and proceeded under elevated pore-fl uid pres-sure through maximum burial at 42 Ma and sub-sequent uplift to modern-day temperatures and hydrostatic pore-fl uid pressures. Independent

estimates of dry gas generation in Cotton Val-ley and Bossier source rocks overlap with onset of fracturing in the Travis Peak Formation and suggest that fl uid overpressure may have been generated during reservoir charge. Individual fractures opened over an ~48 m.y. time span at rates of 16–23 µm/m.y. These slow opening rates suggest that fractures may remain open, and thus hydraulically conductive, over geologi-cally extended periods in deep basinal passive-margin settings.

ACKNOWLEDGMENTS

Our work on structural diagenesis is supported by grant DE-FG02-03ER15430 from Chemical Sci-ences, Geosciences and Biosciences Division, Offi ce of Basic Energy Sciences, Offi ce of Science, U.S. Department of Energy, by the Jackson School of Geo-sciences at the University of Texas, and by sponsors of the Fracture Research & Application Consortium. We thank GSA Bulletin reviewers Peter Vrolijk, Quentin Fisher, Anne-Marie Boullier, Stephen Cox, and Asso-ciate Editor John Walsh for their helpful comments and suggestions. Publication was authorized by the Director, Bureau of Economic Geology, The Univer-sity of Texas at Austin.

APPENDIX

Methodology of Raman Analyses

Raman analyses on fl uid inclusions were con-ducted at the Vibrational Spectroscopy Laboratory in the Department of Geosciences at Virginia Tech us-ing a JY Horiba LabRam HR (800 mm) spectrometer, with 2400 grooves/mm gratings and a slit width of 400 µm. The confocal aperture was set from 150 to 350 µm based on inclusion size. Excitation was pro-vided by a 514.53 nm Laser Physics 100S-514 Ar+ laser . The laser output was 50 mW at the source and <10 mW at the sample. The detector was an electroni-cally cooled open electrode charge-coupled device (CCD). Due to the nonlinear behavior of the mono-chromator, the spectrometer position was calibrated in the spectral region of interest to optimize accuracy in the peak position determination. Two emission lines from a neon (Ne) calibration lamp that was perma-nently fi xed within the optical path of the microscope were recorded simultaneously with each Raman spec-trum of the CH4 ν1 symmetric stretching band (Fig. DR2 [see footnote 1]). The position of each measured Raman line was determined after baseline correction using parameters for Gaussian/Lorentzian peak fi t-ting described by Lin et al. (2007). The 2851.38 and 2972.44 cm–1 Ne lines (relative to the 514.529 nm Rayleigh line of the Ar ion laser, in air) were used for calibration. Lin et al. (2007) estimated that the error in CH4 pressure determined with this technique varies from about ±1.5 bars at 20 bars pressure to about ±6 bars at 600 bars.

Calculation of Methane Concentrations and Trapping Pressure

The input data used to estimate methane concen-trations include the homogenization temperature, sa-linity of the aqueous phase, and pressure of the vapor bubble at room temperature. The liquid phase at room temperature is assumed to be adequately approxi-

mated by the system H2O-NaCl-CH4, and the vapor phase is assumed to be pure methane (examination by Raman spectroscopy confi rms this assumption). The pressure of the methane vapor bubble was deter-mined at room temperature (22°C) using the method developed by Lin et al. (2007) that relates methane peak position to pressure. Pressure data for bridges 9840p2 and 9840p5 are plotted against homogeniza-tion temperature in Figure 9. The density of the vapor bubble (g/cm3) is then calculated using an equation of state for pure CH4 (Duan et al., 1992). The equi-librium concentration of CH4 in the liquid phase, and the density of the liquid phase at 22°C are calculated using an equation of state for H2O-NaCl-CH4 (Duan and Mao, 2006). The bulk density of the inclusion is initially approximated based on P-V-T-X data of the H2O-NaCl system (Shibue, 2003; Spivey et al., 2004), using the homogenization temperature and sa-linity as input parameters.

Using estimates of the densities of the phases in the inclusion at room temperature, an approximate bulk density, and the concentrations of methane in each phase, we determine the ratios of all the phases at room temperature following the mass-balance tech-nique of Bodnar (1983). The total mass of the inclu-sion is the sum of the masses of the vapor bubble and liquid phase:

mI = mB + mL, (1)

where mI is mass of the bulk inclusion, mB is mass of the bubble at 22°C, and mL is mass of the liquid at 22°C. All masses are in units of grams to be con-sistent with density units of g/cm3. Given that den-sity is related to mass and volume according to the relationship:

ρ = m

v, (2)

where ρ is density, m is mass, and v is volume, Equa-tion 1 is transformed to:

ρIvI = ρBvB + ρLvL. (3)

The volume of the inclusion is assumed to be 1 cm3 to simplify the calculation. In practice, the actual volume is irrelevant because the calculation involves volume fractions occupied by the different phases at room temperature. Since the volume occupied by the liquid phase is the difference between the bulk (total) volume and the bubble volume, Equation 3 becomes:

ρI = ρBvB + ρL(1− vB). (4)

Equation 4 is then rearranged to solve for the volume of the vapor bubble:

vB =(ρI − ρL)

(ρB − ρL). (5)

The volume of the liquid phase is given by:

vL = 1− vB. (6)

The mass of the pure methane vapor bubble is then:

mB = vBρB. (7)

The mass of methane dissolved in the liquid phase is given by:

mmeth_liq = vLρL Xmeth_liq, (8)



Becker et al.


where mmeth_liq is the mass of methane dissolved in the liquid phase, and Xmeth_liq is the mass fraction of meth-ane in the liquid phase. The bulk methane concentra-tion of the inclusion is then given by:

Xmeth_bulk = 1E6(mmeth_liq + mB)

(vIρI ), (9)

where Xmeth_bulk is the bulk concentration of methane in the inclusion in ppm. The vI term in the denominator can be omitted, given that we are assuming a unit vol-ume of 1 cm3 for the bulk inclusion. The H2O-NaCl-CH4 equation of state of Duan and Mao (2006) may then be used to determine the homogenization pres-sure corresponding to the bulk methane concentration, homogenization temperature, and salinity.

The method described here provides an approxi-mation of the bulk methane concentration and mini-mum trapping pressure of the inclusion; however, these values will be too high because the initial bulk inclusion density based on the H2O-NaCl system used in the calculation underestimates these values for methane-bearing inclusions. The effect is that the bubble volume at room temperature is overestimated, which results in an overestimate of both methane concentration and homogenization pressure. This re-sults in bulk trapping pressure overestimates in this study from ~6 MPa for inclusions with lower-room-temperature bubble pressures (~4 MPa) to ~31 MPa for the inclusions with higher-room-temperature bubble pressures (~5.5 MPa).

The error in trapping pressure and inclusion density resulting from the initial assumption that the inclusion bulk density is approximated by the H2O-NaCl sys-tem can be corrected using an iterative technique. The H2O-NaCl-CH4 equation of state of Duan and Mao (2006) is used to calculate the homogenization pres-sure given the bulk concentration calculated by Equa-tion 9, the homogenization temperature, and the bulk salinity. The equation of state also calculates a density at the homogenization conditions. This new bulk den-sity will be greater than that calculated initially based on the system H2O-NaCl, and it will be greater than the true bulk density of the inclusion because it was based on a bulk methane concentration that is too high. However, the new bulk density estimate will be closer to a value that is internally consistent for the P-V-T-X properties predicted by the H2O-NaCl-CH4 equation of state between the measured room-temperature bubble pressure, calculated room-temperature liquid-phase density, and homogenization temperature of the inclu-sion than the value used initially based on the system H2O-NaCl. This new density is then substituted for ρI in Equations 5 and 9, keeping all other parameters constant, and calculating a new bulk methane concen-tration. This revised methane concentration estimate is then used to calculate a new homogenization pressure and density. Both values will be lower than the pre-vious estimate and closer to the internally consistent values. Repeated calculations will alternate between decreasingly higher and lower densities until the bulk methane concentration converges to a constant value. This ensures that the measured vapor bubble pressure, calculated phase ratios, calculated bulk composition, and calculated bulk density are internally consistent and that they agree with values predicted by the H2O-NaCl-CH4 equation of state for an inclusion of that composition trapped at the measured homogenization temperature and estimated trapping pressure.

We performed this calculation using a FORTRAN90 computer model based on published equations of state referenced already. In principle, these calculations are possible using a combination of programs available

for online use at http://www.geochem-model.org/models/h2o_ch4_nacl/; however, in practice, this is tedious and time consuming. We will provide the FORTRAN90 code to interested researchers upon request. It has been tested on Linux (32 and 64 bit) using GNU FORTRAN (gcc 4.x confi gured with gfortran support) and Intel™ FORTRAN compilers, and on 32 bit Windows platforms using the most cur-rent MinGW and MSYS tools (gcc 4.4). However, the code should run on any platform with a working FORTRAN90 compiler.

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