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Geologic controls on movement of produced-water releases at US geological survey research Site A, Skiatook lake, Osage county, Oklahoma
Applied Geochemistry Applied Geochemistry 22 (2007) 2138–2154 www.elsevier.com/locate/apgeochem
Geologic controls on movement of produced-water releases at US geological survey research Site A, Skiatook lake, Osage county, Oklahoma James K. Otton
a,* ,
Robert A. Zielinski b, Bruce D. Smith c, Marvin M. Abbott
d
a
US Geological Survey, MS 939, Box 25046, Lakewood, CO 80225, USA US Geological Survey, MS 973, Box 25046, Lakewood, CO 80225, USA c US Geological Survey, MS 964, Box 25046, Lakewood, CO 80225, USA US Geological Survey, 202 NW 66th Street, Oklahoma City, OK 73116, USA b
d
Available online 13 May 2007
Abstract Highly saline produced water was released from multiple sources during oil field operations from 1913 to 1973 at the USGS research Site A on Skiatook Lake in northeastern Oklahoma. Two pits, designed to hold produced water and oil, were major sources for release of these fluids at the site. Produced water spills from these and other features moved downslope following topography and downdip by percolating through permeable eolian sand and colluvium, underlying permeable sandstone, and, to a lesser extent, through shales and mudstones. Saline water penetrated progressively deeper units as it moved through the gently dipping bedrock to the north and NW. A large eroded salt scar north of the pits coincides with underlying fine-grained rocks that have retained substantial concentrations of salt, causing slow revegetation. Where not eroded, thick eolian sand or permeable sandstone bedrock is near the surface, and vegetation has been little affected or has reestablished itself after the introduced salt was flushed by precipitation. The extent of salt-contaminated bedrock extends well beyond existing surface salt scars. These results indicate that one of the legacies of surface salt spills can be a volume of subsurface salinization larger than the visible surface disturbance. Ó 2007 Published by Elsevier Ltd.
1. Introduction Two petroleum production sites (A and B) on Skiatook Lake in northeastern Oklahoma (Fig. 1) are the subject of US Geological Survey investigations under the Osage-Skiatook Petroleum Environmental Research (OSPER) project. These studies, initiated in 2001, were designed to investi*
Corresponding author. E-mail address: [email protected] (J.K. Otton).
0883-2927/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2007.04.015
gate the environmental impacts of oil and gas production on local soils, bedrock, ground water and surface water, and the ecosystems they support (Keeland et al., 2003; Kharaka and Otton, 2003; Kharaka et al., 2005; Otton et al., 2005). The construction and filling of Lake Skiatook in the mid1980s expanded the scope of potential impacts to water supplies and a recreational fishery. Understanding the nature and extent of impacts at Site A and elsewhere can help land managers avoid or mitigate problems related to past oil production
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Fig. 1. Site location map. (a–c) Location of Sites A and B on Skiatook Lake, Osage County, Oklahoma. (d) False color aerial photo of the vicinity of Site A on Skiatook Lake. Arrow points to prominent salt scar at north end of the site. Photo derived from an October 2003 hyperspectral survey, courtesy of David Reister, Oak Ridge National Laboratory.
activities on lands that may be identified for future development. Thirteen auger holes were drilled to depths of 5–21 m at Site A to document the subsurface stratigraphy, characterize the subsurface distribution of saline bedrock by geochemical and geophysical means, and install ground-water monitoring wells. This report presents new lithologic, geochemical and geophysical data from these holes at Site A and results of a GEM-2 survey (a geophysical survey of the electrical conductivity in the shallow subsurface). The data are used to support a discussion of
geologic features that have influenced produced water movement across the topography and through the unconsolidated surficial sediments and bedrock. The geologic framework described herein is useful in understanding related papers in this volume. 2. Site description and production history Site A is located between two hills that form part of a narrow neck of land on the north side of Skiatook Lake (Fig. 1c). A white salt scar seen in a false color image (arrow, Fig. 1d) marks the north-cen-
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tral part of the site. This scar, which is only sparsely vegetated, is eroded to as much as 2 m below the original land surface. Seepage of brine from various areas of the salt scar forms halite crusts on surface rocks and on thin soil over bedrock. Grasses, forbs, and a few oak trees grow on 4 soil pedestals that remain within the eroded area (Fig. 2). Another
smaller salt scar is on the west side of the site upslope from the cove to the SW. A berm wall and a cut in the hillslope indicate that this salt scar is within and downslope from the trace of a small produced-water pit (old pit, Fig. 2). Open areas in the trees immediately SW, S and SE of the large salt scar are remnants of past oil field production
Fig. 2. Map of cultural and vegetative features at Site A, showing location of hand-auger soil profile hole (S1), 13 auger holes (01–13), and 2 cross sections (A–B and C–D). P-two pits. PU1 and PU2-old power unit sites.
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activities (Fig. 1d). Open areas elsewhere in Fig. 1d are natural meadows, sandstone outcrops, and other oil field-disturbed sites on the lease outside of Site A. A topographic divide between the two hills separates drainages that flow into the two coves. The lake level generally fluctuated from 215.8 to 217.6 m above sea level (asl) during the study, although during a few extreme rainfall events the lake rose to as much as 219.2 m and flooded the lower part of the salt scar. During low water levels, a pit from the post-1973 lease operations, rock ledges, and other features are exposed on the lake bottom in the cove to the NE (Fig. 2). A second post-1973 pit is in an open area onshore just south of this pit. Prior to the filling of Lake Skiatook, the local water table level was defined by the elevation of the closest adjacent perennial stream, Cedar Creek to the north, at about 202 m asl. This elevation is about 19 m below the elevation of the midpoint of the saddle at Site A. With the filling of Skiatook Lake, the local water table is now defined by the lake surface elevation which can be taken as the normal pool elevation of 217.6 m asl although there has been fluctuation as noted above. Site A is centrally located on an oil lease that occupies the north half of section 13, T22N, R10E in Osage County, Oklahoma (a section is 1.6 km on a side). About 19,000 m3 of oil were produced on the lease from 1913 to 1999, when production ceased (estimated from Bureau of Indian Affairs (BIA), Pawhuska, OK, unpublished lease record files). The volumes of produced water generated are not part of lease records, however some of the wells had extreme ‘‘water cuts’’ prior to being shut-in, with as little as a few tens of liters of oil per m3 of fluid being produced. Using a ratio of 10 m3 of produced water to 1 m3 of produced oil (Veil et al., 2004), about 190,000 m3 of produced water may have been generated at the site. Most of this production was likely processed at site ‘‘A’’, but post-1973 production was processed at a newer tank battery about 350 m to the SE. More recent brine production was reinjected to the subsurface. Remnants of production facilities remain at the site. A small tank battery, marked by remnants of 2–3 redwood tanks, is located in the southeastern part of the open area (Fig. 2). Based on local practices, these tanks likely included a separator, an oil storage tank, and a brine storage tank. Along the
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east edge of the site are remnants of a central power unit (bullwheel and concrete powerhouse foundation, ‘‘PU1’’ in Fig. 2) that supplied mechanical pumping action via long pump rods to oil wells on the lease. A large tank, observed in a 1936 aerial photo of the site, was located NE of this power unit. Its former position is presently marked by a barren, stained patch of sandstone. Another concrete foundation, probably a pump jack base or another power unit, is located to the SW (‘‘PU2’’ in Fig. 2). A former injection well is located south of this concrete foundation. In the central part of the site are two pits that temporarily stored produced fluids (two contiguous irregular oval areas labeled ‘‘P’’ in Fig. 2). The larger west pit stored produced water whereas the east pit stored crude oil. A notch along the north edge of the west pit was a release point for produced water to flow from the pit (shown by an arrow), which was a common practice prior to the 1970s. A trench extending from the area of the tank battery to the pits carried produced fluids to the pits. Soils surrounding the tanks, the trench, and the pits are variably saturated with weathered asphaltic hydrocarbon and scarred from saline water. Weathered sandstone blocks exposed around the tanks are coated with oil, variably bleached, and heavily stained with Fe oxides indicating possible dissolution, movement and re-precipitation of Fe by reducing, highly saline waters at the site. Although, the volumes of produced water from production operations are not known precisely, salinities were likely high as evidenced from nearby oil field operations. Produced waters from nearby wells are notably high in total dissolved solids (TDS) and Cl, as documented by analyses of waters from nearby oil wells (114,000–185,000 mg/ L TDS, Table 1 in Kharaka et al., 2003). A layer of highly saline water (110,000 mg/L TDS) preserved beneath the thick asphalt layer in the east pit indicates that the produced waters at this site were also highly saline (AP01, Table 1 in Kharaka et al., 2003). Oil field operations have kept the oak trees from encroaching on the open areas of the site. All operations at Site A ceased about 1973, but production continued on other parts of the lease until 1999. The 1936 aerial photograph shows that this large salt scar and other smaller salt scars were well developed within 23 a after production started. The smaller scars indicate that, in addition to the two pits, the tank battery, the tank to the east, and the two
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power units were significant points of release for produced waters and the salts associated with them. The present-day road was moved to its current position, apparently in response to erosion in the main salt scar. Remnants of an old road culvert remain in the salt scar.
the survey lines were fixed by GPS readings, often at monitoring well locations.
3. Methods
A color image of the composite high-frequency GEM-2 survey data for site A (Fig. 3) portrays the relative conductivity of the shallow soil and bedrock to a depth of investigation of about 3 m. Areas near and immediately downslope from the tank battery and the eastern power unit could not be surveyed because of the presence of abundant pipe, pump rods and other discarded metal. The highest conductivity readings were over deeply eroded parts of the large salt scar (see outline in Fig. 3), reflecting the near-surface, salt-saturated, weathered sandstone and fine-grained rocks. High conductivity was also observed at the head of the cove to the NE. The cove receives salt runoff from the large scar and also overlies the channel of the small stream that drained the northern part of the site prior to the filling of the lake. Salt-saturated thin alluvium and bedrock below the alluvium in the northeastern cove likely contribute to the high conductivity observed in the upper 3 m. Areas of moderately high conductivity include: (1) around the site of an old tank seen in the 1936 aerial photo (1 in Fig. 3); (2) south of the power unit in the SW part of the site near the location of the injection well; and (3) along the drainage leading down to the cove to the SW. However, the area immediately west and NW of the tank battery (4, Fig. 3) and south of the pits (the area along cross section line A–B, close to point A) shows low conductivity values. There, permeable (as inferred from porosity data, see below), near-surface, weathered sandstone is thick (2.6–4 m) and salts may have been largely flushed from the surface zone measured by the GEM-2 instrument (0–3 m deep). Elevated subsurface conductivity extends to the NW of the salt scar, as well as to the east between the salt scar and the tank showing that the extent of high subsurface salinities exceeds the salt-scar footprint and underlies large areas covered by oak forest.
The geology and stratigraphy of Site A were documented by surface mapping, shallow hand augering, shallow direct-push holes, and deeper hollow-stem auger and rotary drilling. Shallow soil samples (0–15 cm) were collected by hand auger across the site. Soil profiles through the surficial sediment were hand augered at two locations. Channels in colluvium were mapped from exposures in the large salt scar north of the pits, in the low, seasonally wet areas (originally smaller salt scars) SW of the pits, and in some shallow auger holes. Borehole gamma and borehole conductivity geophysical surveys provided additional stratigraphic information. Core was retrieved from nearly all auger, rotary, and direct-push holes. Thin sections of selected core were prepared for petrologic, mineralogic, and petrophysical studies. Quantitative petrologic data, including porosity, are based on point counts (n = 300). Further characterization of surficial sediments included grain-size and X-ray powder diffraction analyses. For the bedrock studies, data from 13 deeper auger holes are reported herein (01–13). Fig. 2 shows the location of the 13 drill holes and two cross section lines with respect to cultural and vegetation features at the site. Samples (100 g) were crushed to <3 mm, mixed with an equal weight of DI water, stirred vigorously, and allowed to stand overnight. The liquid phase was separated by centrifugation and filtration through a 0.45 lm filter membrane. Specific conductance and pH of the aqueous extracts were measured immediately and the solutions were submitted for measurement of dissolved Cl, Br and SO2 by ion chromatography, following the 4 procedure described by Zielinski et al. (2003). A multi-channel GEM-2 electromagnetic survey was conducted across the surface of Site A when Skiatook Lake was at a low water level in May 2005. This survey was an expansion of a previous, more limited GEM-2 survey (Haoping, 2005) and an older survey that was limited to a narrow area focused on the pits and the large salt scar (Smith et al., 2003; Otton et al., 2005). The end points of
4. Results and discussion 4.1. Surface geophysical survey
4.2. Surficial sediment and topography Unconsolidated surficial sediments, including eolian sand and colluvium, cover the weathered bedrock across much of the site. The eolian sand
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Fig. 3. Composite high frequency GEM-2 survey map showing subsurface (0–3 m, 0–10 ft) electrical conductivity. Base is a georeferenced 1995 aerial photo of Site A. The trace of the present-day road (dashed lines) and the location of 2 cross section lines (shown in Fig. 2) are shown for reference. Data in the NE cove were taken during a low-water period. Orange, red, and pink colors represent higher electrical conductivity. The salt scar underlies the pink color in the center of the figure (note outline of the scar). Location of the sources: 1- former location of a tank; 2- east-side power unit; 3- two pits.
varies in thickness from 0 to 1.5 m and is uniformly very fine grained and permeable (as inferred by its texture and drainage). Observations during rainstorms indicate that water quickly infiltrates the eolian sand; surface runoff is limited except in the scarred areas where eolian sand and colluvium are absent. The colluvium ranges from scattered sandstone pebble layers a few cm thick at or near the base of the eolian sand to thicker (0.3 m or more) layers of angular sandstone clasts in a sandy to clayey matrix, the latter occurring mostly in channels sitting on weathered bedrock. Where the matrix is clayey, the colluvium tends to retain salt as evidenced by salt crusts on exposed surfaces after a
rainstorm. The pre-erosion surface topography of Site A is portrayed in Fig. 4 (reconstructed in salt scar using the 4 soil pedestals) as is the distribution of the mapped colluvium and the position of channels inferred from the location of the mapped colluvium. Arrows show the inferred flow direction of produced water from the pits into the channels. Although, most produced water released from the pits flowed northward, it is inferred that some leaked from the south edge of the pits at the drainage divide and moved westward along a shallow channel immediately SW and west of the pits (Fig. 4). Salt derived from these pit-sourced produced waters, plus any salt from spills at the tank
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Fig. 4. Map showing topography of Site A and positions of the drainage divide (long-dashed line), two pits, and mapped colluvium. Probable locations of channels incised on the surface of the bedrock are shown by short-dashed lines and arrows.
battery and along the trench, likely contribute to the high subsurface conductivity measured along the drainage leading to the southwestern cove (Figs. 2 and 3). 4.3. Bedrock stratigraphic section and structure Bedrock at Site A is in the Pennsylvanian Wann Formation (Gardner, 1957), a deltaic and marginal marine sequence comprised of sandstone, mudstone, siltstone, shale and limestone. Although, not observed at Site A, the formation is known to contain limestone reefs with crinoid fossils that identify it as Middle Pennsylvanian (Missourian). Ecological studies indicate that the crinoid genera were likely adapted to muddy to firm substrates and turbid waters characteristic of the Middle
Pennsylvanian in the present-day mid-continent where crustal instability created local uplifts that contributed detritus to adjacent shallow seas (Ettensohn, 1980). The rapid lateral facies changes of the sedimentary rocks observed at Site A, and described below, may reflect this regional tectonic dynamics. Bedrock lithologies at the site include 5 units (1–5, see below) that vary substantially in porosity and permeability and thus strongly control movement of produced water. Lithologic logs from two drill holes (02, 07, Fig. 2) that penetrate these units are compared with other relevant data for these holes in Figs. 5 and 6. These holes were selected because hole 02 lies immediately downslope and north of the two pits and hole 07 lies 110 m downslope and downdip from the two pits in the direction that much of the
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Fig. 5. Data for hole 02 (see Fig. 2 for location) showing (a) lithologic log (legend to the right), (b) extractable Cl concentrations (mg/L) from crushed core samples, (c) porosity from thin section observations, (d) conductivity log (milliSiemens/m), and (e) gamma log (counts per second). Arrows on the gamma log indicate primary (lower) and secondary gamma spikes at the top of Unit 5 and the top of Unit 4, respectively. Gray line (Blue line in web version) represents the depth of the normal pool elevation for Lake Skiatook, the average current base level. Base level for this site prior to the filling of Lake Skiatook, Cedar Creek just to the north, would be about 21 m below the land surface at this well site (see depth scale).
brine moved. Photomicrographs for hole 07 are shown in Fig. 7 and a summary of the porosity data for Units 1–4 are plotted in Fig. 8. The 5 lithologic units are, in descending order: Unit 1. Weathered, but generally case-hardened, resistant sandstone that forms low outcrops at the southernmost and northwestern part of the site. This unit is in the upper part of the section in 3 holes (05, 12, 13, Fig. 2) on the NW side of the site. It underlies the hills on either side of the saddle where Site A is located. No petrographic or porosity data were measured for this unit; however, it is of limited areal distribution within the site and was not a significant conduit for produced water. Unit 1 is likely equivalent to the Clem Creek Sandstone Tongue of the Wann Formation (Gardner, 1957).
Unit 2. Widespread, weathered, very fine-grained sandstone and clayey sandstone. Petrographic analysis indicates an average porosity of about 24% (Figs. 5–8). Photomicrographs show variable amounts of Fe oxyhydroxide cement (Fig. 7a). In some sections, remnant dolomite cement is present, indicating that porosity may be caused by dissolution of dolomite during weathering. The presence of Fe produces a wide variety of red to orange staining. Thin clay partings and laminae produce a subhorizontal fabric (Fig. 7a). Unit 2 intertongues, in part, with fine-grained rocks of underlying Unit 3. Unit 3. Intertonguing, mostly weathered shale, sandy siltstone, sandy mudstone and sandstone. Unit shows rapid lateral facies changes and striking contrasts in porosity and permeability. No two holes have a per-
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Fig. 6. Data for hole 07 (see Fig. 2 for location) showing (a) lithologic log (see Fig. 5 for legend) (b) extractable Cl concentrations (mg/L) from crushed core samples, (c) porosity from thin section observations, (d) conductivity log (milliSiemens/m) and (e) gamma log (counts per second). The arrow on the gamma log indicates a gamma spike associated with the carbonaceous black shale at the top of Unit 5.
fectly correlative lithologic sequence (compare Figs. 5 and 6; also see Fig. 11a and b). Some unweathered shale is carbonaceous with thin coaly laminae and contains abundant pyrite. Fossil plant fragments are abundant on bedding surfaces. In several core holes, the greenish-gray weathered shale contains abundant gypsum, and some surface exposures in the large salt scar display abundant gypsum crystals. Shales have low (<1%) porosity whereas sandstones range from 15 and 20% porosity (Figs. 5–8). Clay partings are abundant in sandstones and sandy layers are abundant within the shales (Fig. 7b and c). The sandstone beds locally show worm burrows, indicating shallow water deposition. These
worm burrows also enhance vertical permeability. Sandstones are preferred conduits for flow of produced water through Unit 2. Unit 2 is the only part of the section exposed in the large salt scar. There, laterally discontinuous sandstone layers and a cross-bedded channel sand body are enclosed by weathered mudstone, siltstone and shale. One sandstone bed is deeply pitted, and variably stained by Fe oxides or bleached. Highly saline, reducing produced water moving through this sandstone probably dissolved cementing material and solubilized Fe, producing bleached areas. Soluble Fe re-precipitated locally where near-surface oxidation produced Festained areas. The contact between Units
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Fig. 8. Porosity values determined for samples from holes 02 and 07 by lithologic unit (n = 39).
Fig. 7. Thin section photomicrographs of core from hole 07 with unit designation, depth, and porosity values. The long dimension of each image is 2.4 mm. Blue color is tinted epoxy filling pore spaces.
3 and 4 is delineated by a spike in gammaray intensity on logs of about half of the holes (upper arrow, Fig. 5e; gamma spike at this position is missing in Fig. 6e). The stratigraphic position of this gamma spike corresponds to the base of the shales within Unit 3 and the upper limit of dolomite cement in Unit 4. Unit 4. Variably dolomite-cemented, generally unweathered sandstone with abundant clay partings and thin shale beds, especially in the lower half of the unit, that are commonly carbonaceous and have fossil plant-leaf detritus. Fossil-shell debris is locally abundant. Sandy dolomite forms a minor lithologic component. The porosity is highly variable, ranging from near zero in highly cemented sandstone and thin
shale beds to 31% in the lightly cemented sandstone (Figs. 5–7d, e, f, Fig. 8). Ferroan dolomite is the primary dolomite cement, as determined by SEM. Unit 4 is observed only in the subsurface at the site, but crops out as a dark-brown-weathered, resistant, cliff-forming ledge on south-facing slopes at the south end of the peninsula. Unit 5. Interbedded muddy sandstone, shale, siltstone and mudstone of generally low porosity (median less than 2%, Figs. 5–8). Sandstones are variably dolomite-cemented with as much as 10–14% porosity. Like Unit 3, no two holes have a similar lithologic sequence across the site. This unit is exposed at the surface below the cliff-forming ledges of Unit 4 in the southernmost parts of the peninsula. The base of Unit 4 and the top of Unit 5 is everywhere marked by a carbonaceous shale except in hole 10, the most southerly of the holes at Site A (Fig. 2). This shale produces a distinctive spike in the gamma log (lower arrow in Fig. 5e, the sole arrow in Fig. 6e) that aids the correlation. Pennsylvanian carbonaceous marine shales in the mid-continent are commonly U-enriched (Hyden and Danilchik, 1962). Based on the altitude of the shale with the distinctive gamma spike at the top of Unit 5 in several holes, the strike and dip of the bedrock section
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across Site A are estimated to be N25°E and 1°25 0 NW, respectively. The gentle dip and the local topography (hills to the SE and NW with a saddle in between) control the surface distribution of bedrock Units 1, 2 and 3 (Fig. 9). Unit 1 forms two outcrop bands that wrap around the two hills. The tank battery sits on the lower part of Unit 1 just above the contact with Unit 2. The sandstone-dominated Unit 2 forms a continuous ‘‘bridge’’ between the two hills. This permeable bedrock unit underlies the two pits and the trench that allowed produced water and oil to flow down to the two pits from the tank battery. Spilled produced water moving through the permeable overlying eolian sand and colluvium readily percolated into this unit. Unit 3, which contains abundant mudstone and shale, is exposed along the lower slopes on the flanks of the saddle. It underlies all of the large salt scar north of the pits. The tank on the east side of the site that was observable in the 1936 aerial photo, rested on this unit.
A linear segment of the shoreline along the southwestern cove aligns with the topographic low that trends about N45°E across the site and corresponds to the area where the peninsula forms a narrow neck (Fig. 1d; Fig. 9). This apparent lineament may mark a zone of preferential fracturing or possibly minor offset in the bedrock. Gardner (1957) mapped several short, NE-trending fault segments in the surrounding area. Vertical to subvertical fractures and faults are commonly observed in core; however, no deep holes were drilled along the trace of the lineament to determine if they are more abundant in this zone. Faults observed in core show slickensided surfaces and juxtapose lithologies of different types; however, the amount of slip cannot be determined. Vertical movement of these beds was not substantial (probably less than 1 m total) as indicated by correlations of lithologic contacts and geophysical markers. Fractures and faults may also provide important conduits for produced
Fig. 9. Bedrock geologic map of Site A and vicinity. Squares represent locations of all hand auger, auger and direct-push holes used to develop the map. The contact between Unit 1 and Unit 2 was mapped at the surface. Units 1, 2 and 3 are described in the text.
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water flow in bedrock. Further study of the fractures and faults in core is warranted. 4.4. Salt distribution Dissolved Cl concentration in 1:1 aqueous extracts of soil and bedrock samples ranges from <10 to several thousand mg/L (Figs. 5 and 6b, and unpublished data for other holes). Extractable Cl concentrations <150 mg/L are considered to be natural, based on extracts from a nearby reference drill core collected offsite and from highly cemented sandstone and deep shale samples from Site A. The conductivity (lS/cm)/Cl (mg/L) ratio of lab-prepared solutions of variable NaCl concentration is approximately 3.3; therefore, ratios >5.0 in sandstone extracts containing <150 mg/L extractable Cl and <150 mg/L extractable SO2 were 4 interpreted as an additional indication of minimal NaCl contamination. The Cl/Br mass ratio can be another indicator of produced water contamination. This ratio was determined in extracts containing greater than 0.2 mg/L Br. The mean Cl/Br ratio for all samples with extractable Cl >150 mg/L was 312 ± 37 mg/L. This compares to a Cl/Br mass ratio of 279 in the highly saline water just beneath the east pit and to an average value of 282 ± 36 for 8 modern produced waters collected from nearby wells in Osage County (Table 1 in Kharaka et al., 2003). 4.4.1. Extractable chloride in surficial sediments Twenty shallow (depth 0–15 cm) soil samples collected from eolian sand soil profiles across the site all had extractable Cl concentrations <24 mg/L. Extracts of 12 similar soil samples within the limits of the salt scar ranged from <24 mg/L near the edges of the scar to 2840 mg/L in alluvium within the scar. A 1.6 m vertical soil profile was extracted with a hand auger from one of the isolated soil pedestals within the salt scar (Fig. 10, S1 in Fig. 2). The profile shows extractable Cl < 10 mg/L in the upper 56 cm of eolian sand. In the intervals 56– 79 cm and 79–99 cm, the extractable Cl content increases progressively with increasing clay content. Maximum Cl concentrations of 1167 mg/L are in the 99–122 cm interval that includes basal clayey, clast-rich mixed sand and colluvium (99–109 cm) and underlying weathered sandy claystone of Unit 3 (109–122 cm). Below this interval mg/L drops to the 400–500 mg/L range through the bottom of the auger hole at 160 cm.
Fig. 10. Extractable Cl concentrations versus depth for a handaugered soil profile (S1, Fig. 2) located on a soil pedestal within the salt scar. Depth intervals are not equal.
Elevated soluble Cl in the basal clayey part of the surficial sediments indicates that NaCl-rich fluids probably moved along the contact with underlying clayey weathered bedrock. Soluble salt retained within this horizon was subject to some upward movement by capillary action from the underlying, more saline, weathered bedrock. A second hand-augered hole, located between the berm of the two pits and the road, encountered 74 cm of eolian sand overlying weathered, permeable sandstone bedrock of Unit 2. The sand was asphalt-cemented in the upper 10 cm. Extractable Clconcentrations ranged from 8.7 to 18.9 mg/L in this hole, despite its close proximity to the asphaltic oil-filled eastern pit that has a layer of highly saline water (110,000 mg/L TDS) just beneath the asphalt in the pit. 4.4.2. Stratigraphy and salt distribution in the bedrock across the site Lines of section in Figs. 2 and 3 are shown as lithologic and correlative log-stratigraphic cross sections in Fig. 11a and b. The cross sections also show concentrations of extractable Cl versus depth (ranges given in a color-bar format) and the positions of surface sources of produced water salts. Lithologic columns for these cross sections identify the upper, weathered portion of each columnar section in color and the lower, unweathered portion in black and white. The persistent gamma-ray spike associated with the black carbonaceous shale at the top of Unit 5 is shown with a red arrow, except
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Fig. 11. Cross sections showing lithologic logs, correlations of stratigraphic units, and extractable Cl concentrations at Site A. Locations for line of sections shown in Figs. 2 and 3. The solid and dashed horizontal red lines connect major and minor gamma marker horizons (gamma kicks shown with arrows) used for correlations (Figs. 5 and 6). Refer to Fig. 5 for the lithologic legend.
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in hole 10 (Fig. 11a), where it is absent. A solid red line shows the suggested correlation from hole to hole (dashed where inferred in Fig. 11a). The lesser gamma-ray spike in the upper part of Unit 3 is present in several, but not all, holes. An indicated correlation is delineated by a dashed red line. Depth of weathering of bedrock at Site A was defined as the last appearance of Fe oxyhydroxide mineral staining of the core or as the last appearance of the distinctive olive-green coloration of the clays. In some fine-grained core, the last appearance of Fe oxyhydroxide-filled fractures or partings was used. The depth of bedrock weathering ranges from about 4 m in hole 10 to about 11 m in holes 08 and 09. Bedrock in the northern parts of the site seems to be more deeply weathered to include all of Unit 2 and much of Unit 3. Removal of cement in the sandstones of Unit 2 and Unit 3 during weathering resulted in increasing the porosity of these rocks and facilitated movement of the saline water. Weathering of the shales and mudstones of Unit 3 may have made them more susceptible to penetration by highly saline water. The cross sections of Fig. 11a and b illustrate the complex intertonguing of sandstone and finegrained rocks between Units 2 and 3 and within Unit 3, the relative uniformity of most of Unit 4 across the site, the sharp basal contact of Unit 4, and the apparent complex intertonguing of sandstone and fine-grained units in Unit 5. Sandstones in Units 2 and 3, and to a lesser degree in Unit 4, are preferred conduits for produced water movement and subsequent salinization of bedrock. Because of the rapid lateral facies changes within the limited area of the site, there is insufficient data to show in much detail the 3-dimensional geometry of sandstone bodies in Unit 3, or zones of intense cementation in Unit 4, even with the relatively close spacing of holes. 4.4.3. Extractable chloride distribution in bedrock Concentration of extractable Cl in bedrock is relatively high in holes downdip and north of the two pits (Fig. 11a, holes 02, 01, 07). Downslope and west of the east tank (Fig. 2), bedrock in holes 09 and 08 also show high Cl (unpublished data, see also elevated conductivity in Fig. 3). Extractable Cl concentrations exceeding 1500 mg/L are largely restricted to the weathered shales of Unit 3 encountered in the holes near these two sources and the deeper sand units in the more distal holes 06 and 07 (Fig. 11b). Extractable Cl concentrations in
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the 600–1500 mg/L range are present in some samples from almost all 13 holes. The exceptions are hole 10 (Fig. 11a) which lies south of the major sources and may only be impacted by releases from the tank battery and hole 13 (Fig. 11b) which appears to be slightly beyond the west edge of the produced water plume generated by the two-pits source. Hole 06 may have received Cl salts from both the two pits and the east tank sources. Anomalous concentrations of extractable Cl (>150 mg/L) are also present at varying depths in other holes across the site, but the Cl does not generally penetrate to depths much below the base of Unit 4. In hole 02, Cl concentrations drop to background values in the uppermost part of Unit 4. Porosity measurements based on thin section examinations from hole 02 (Fig. 5) show that porosity is relatively high in the uppermost part of Unit 4, averaging about 20%, but decreases noticeably at 8.4 m and remains low for the next 1–1.3 m due to increased dolomite cementation. The abrupt decrease in porosity/permeability may have blocked or slowed downward penetration of saline water at that location. Farther downslope from the two pits, near hole 07, Cl concentrations remain high to a depth of 11.9 m (Fig. 6, Fig. 11a). If this salt is derived from the two pits, then salt contamination has penetrated stratigraphically deeper as it moved laterally downdip. At hole 10, updip from the two pits (Fig. 2), moderate concentrations of extractable Cl were observed to the base of Unit 2, as well as into Unit 4 to a depth of about 10.4 m (Fig. 11a). This hole is topographically lower and downdip from the tank battery and trench, the likely source, but lateral dispersion of salts originating from the two pits cannot be ruled out. 5. Produced water movement from the two pits at Site A – conceptual model Migration of salt from the two pits through surficial sediments and bedrock is inferred to have occurred in a sequence similar to that portrayed in Fig. 12a and b. Shortly after produced water releases began from the two pits, percolation was downward through the eolian sand, initially following the contact between the surficial sediments and the weathered permeable sandstone of Unit 2, possibly preferring the colluvial channels (Fig. 4). The water also moved into and downdip through permeable Unit 2 to contact fine-grained rocks of Unit 3.
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Fig. 12. Conceptual diagrams depicting the probable history of produced water movement from the two pits. (a) Early movement and geologic controls on produced water movement at Site A. (b) Salt movement later in the site history after the deeply eroded salt scar formed. Red arrows indicate directions of ground water movement.
Saline waters emerged as downslope seeps to the NW where Unit 2 thins and the surficial sediments are also thin. Some of the saline water may have
entered Unit 2 sands on the NW flank of the drainage (Fig. 12a). Once surface vegetation was killed by saline water, headward erosion expanded the salt
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scar upslope. The 1936 aerial photo of the site indicates that the salt scar grew to about 80% of its present-day extent within the first 23 a of production. Headward erosion seems to have been limited by the position of the Unit 2-Unit 3 contact in the shallow subsurface (Fig. 9) except for two narrow gullies that have eroded southward toward the two pits (Fig. 2). With time (Fig. 12b), salt moved progressively deeper into the bedrock, penetrating Unit 3 sandstones and fine-grained rocks and ultimately into and through Unit 4 to the top of Unit 5 at the position of hole 07 (Fig. 12b). Deeper penetration is deflected downslope by the gentle dip of the sequence and the strong horizontal fabric in the bedrock (clay partings and highly cemented horizons in sandstones). Fractures probably facilitated the overall downward movement perhaps most notably in the trace of the lineament. Weathered shale in the upper part of Unit 3 may have initially acted as an aquitard that later became salt saturated. Substantial salt penetration of the shale in the upper part of the section is present in all holes on the site, possibly the result of 3 different mechanisms: (1) lateral movement of saline produced water into the shales through intertonguing permeable sand bodies several cm to 10 s of cm thick (Fig. 11a and b) to thin sandy laminae less than a few mm thick (Fig. 7c); (2) vertical movement along fractures through shale; and (3) increased weathered shale permeability resulting from clay flocculation and shrinkage in response to introduction of highly saline water (van Olphen, 1977; Yariv and Cross, 1979) although X-ray mineralogy of these weathered shales suggest that kaolinite dominates the clay phases with lesser mixed illite-smectite clay present thus this effect may be limited (Cyndi Rice, USGS, unpublished data, 2007). 6. Conclusions Surface and subsurface geological, geochemical, and geophysical studies to determine the distribution of high salinity at Site A indicate that 4 sources related to past oil field operations likely contributed produced water salts to soils and bedrock at Site A: (1) two pits originally emplaced to store produced water and hydrocarbon (major source); (2) a tank originally located near hole 09, as seen in a 1936 aerial photo, and the nearby central power unit (major source);
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(3) a tank battery and trench upslope from the two pits (minor source) and (4) an injection well or another power unit on the SW part of Site A (minor source). The subsurface extent of salt contamination from the two pits (item 1 above) is not constrained by the data in the direction north and NW of holes 07 and 06 as the contamination may continue downdip and along fractures to deeper horizons. Salt-contaminated sediments and bedrock likely extend beneath Skiatook Lake following the now-submerged alluvial channel of the drainage to the north. Some salt contamination also extends under Skiatook Lake in the cove to the SW. Salt moving downslope from the two pits following channels in the colluvium and salt water stored in the small pit just upslope from the SW cove are likely sources. Produced water from these sources flowed down-section and down-dip to the NW and north following pathways of favorable porosity and permeability. Preferred pathways include: (1) the contact between the base of the eolian sand and colluvium and the underlying bedrock, notably where channels at the contact may have focused some produced water flow; (2) the contact between the permeable weathered sandstone of Unit 2 and the uppermost, less permeable, weathered shale of Unit 3; (3) sandstone bodies within Unit 3; (4) lightly cemented horizons within the dolomitic sandstone in Unit 4, and (5) sandy lenses within all shales and siltstones. Prior to filling of Skiatook Lake the local base level was set by the level of Cedar Creek to the north at about 202 m asl (lower blue line, Fig. 11b) and that may have controlled downward movement of salt. However, with the filling of Skiatook Lake, the base level has risen to about 218 m asl (upper blue line, Fig. 11b) which would slow downward movement, perhaps limiting it to that caused by density differences between the saline plume and the local ground water. After cessation of oil field operations at Site A, salt contamination has been subjected to flushing by precipitation (about 80 cm/a in this part of Oklahoma) and surface runoff. Much of the salt has likely been removed from permeable sandstones of Unit 2. However, a transition from highly concentrated brine to relatively fresh precipitation allows dispersion of any smectitic clays present and subsequent reduction in permeability of clay-rich parts of the section. This may promote retention of salt in fine-grained rocks of Unit 3 that now floor the salt
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scar. Salt is being removed through surface runoff from the salt scar however. Kharaka et al. (2005) and Thordsen (2006, USGS, unpublished data) estimate that the amount of salt removed from the salt scar by surface runoff during storm events from salinity and flow volume measurements at a weir at the north end of the salt scar is about 500 kg/a. Salt now retained in bedrock below the presentday level of Skiatook Lake is much less likely to be discharged to the surface streams and carried away. The relatively high salinity of surface materials within the nearly barren salt scar contrasts with the low salinity and more robust vegetation on eolian-sand-capped soil pedestals within the scar. Remediation of the salt scar must successfully isolate the saline fine-grained rocks of Unit 3 from the root zone. The soil pedestal at S1 (Fig. 2) with about 1 m of eolian sand (0.6 m essentially salt-free, Fig. 10) supports grasses. Other soil pedestals at the site support grasses, forbs and small oak trees. Further research is necessary to establish what thickness of eolian sand or similar permeable material may be necessary to form a capillary break to isolate the saline bedrock from root zones. References Ettensohn, F.R., 1980. Parrogassizocrinus: systematics, phylogeny and ecology. J. Paleontol. 54, 978–1007. Gardner, W.E., 1957. Geology of the Barnsdall area, Osage County, Oklahoma. Norman, Oklahoma. M.S. Thesis, Univ. Oklahoma. Haoping, H., 2005. Depth of investigation for small broadband electromagnetic sensors. Geophysics 70, G135–G142. Hyden, H.J., Danilchik, W., 1962. Uranium in some rocks of Pennsylvanian age in Oklahoma, Kansas, and Missouri. US Geol. Surv. Bull. No. 1147-B. Keeland, B.D., McCoy, J.W., Otton, J.K., 2003. Effects of produced water and hydrocarbon releases on vegetation at Site A of the Osage-Skiatook petroleum research project, Osage County, Oklahoma. In: Sublette, K.S., (Ed.), Program and papers, 10th Ann. Internat. Petroleum Environmental Conf., Houston, Texas, November 11–14, 2003. http://ipec.utulsa.edu/ Conf2003 /Papers/ keeland_mccoy_otton_105. pdf.
Kharaka, Y.K., Otton, J.K. (Eds.), 2003. Environmental impacts of petroleum production: initial results from the OsageSkiatook petroleum environmental research sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260. Kharaka, Y.K., Thordsen, J.J., Kakouros, Evangelos, Abbott, M.A., 2003. The fate of inorganic and organic chemicals in produced water from the Osage-Skiatook petroleum environmental research sites, Osage County, Oklahoma. In: Y.K. Kharaka, J.K. Otton (Eds.), Environmental Impacts of Petroleum Production: Initial Results from the Osage-Skiatook Petroleum Environmental Research Sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260, 57–84. Kharaka, Y.K., Thordsen, J.J., Kakouros, Evangelos, Herkelrath, W.N., 2005. Impacts of petroleum production on ground and surface waters: results from the Osage-Skiatook petroleum environmental research Site A, Osage County, Oklahoma. Environ. Geosci. 12, 127–138. Otton, J.K., Zielinski, R.A., Smith, B.D., Abbott, M.A., Keeland, B.D., 2005. Environmental impacts of oil production on soil, bedrock, and vegetation at the US Geological Survey OsageSkiatook petroleum environmental research Site A, Osage County, Oklahoma. Environ. Geosci. 12, 73–87. Smith, B.D., Bisdorf, R.J., Horton, R.J., Otton, J.K., Hutton, R.S., 2003. Preliminary geophysical characterization of two oil production sites, Osage County, Oklahoma- Osage-Skiatook petroleum environmental research project. In: Y.K. Kharaka, J.K. Otton (Eds.), Environmental impacts of petroleum production: initial results from the Osage-Skiatook petroleum environmental research sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260, 43–56. van Olphen, H., 1977. An introduction to clay colloid chemistry for clay technologists geologists and soil scientists. John Wiley & Sons, New York. Veil, J.A., Puder, M.G., Elcock, D., Redwick, R.J., Jr., 2004, A white paper describing produced water from production of crude oil, natural gas, and coal bed methane. Argonne National Laboratory online publication prepared for US Department of Energy, National Energy Technology Laboratory, http:// www.ead.anl.gov/pub/doc/ProducedWatersWP0401.pdf. Yariv, S., Cross, H., 1979. Geochemistry of colloid systems. Springer-Verlag, New York. Zielinski, R.A., Rice, C.A., Otton, J.K., 2003. Use of soil extracts to define the extent of brine-impacted soils and bedrock at the Osage-Skiatook petroleum environmental research ‘‘B’’ site, northeastern Oklahoma. In: Y.K. Kharaka, J.K. Otton (Eds.), Environmental impacts of petroleum production: initial results from the Osage-Skiatook petroleum environmental research sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260, 127–136.
Geologic controls on movement of produced-water releases at US geological survey research Site A, Skiatook lake, Osage county, Oklahoma James K. Otton
a,* ,
Robert A. Zielinski b, Bruce D. Smith c, Marvin M. Abbott
d
a
US Geological Survey, MS 939, Box 25046, Lakewood, CO 80225, USA US Geological Survey, MS 973, Box 25046, Lakewood, CO 80225, USA c US Geological Survey, MS 964, Box 25046, Lakewood, CO 80225, USA US Geological Survey, 202 NW 66th Street, Oklahoma City, OK 73116, USA b
d
Available online 13 May 2007
Abstract Highly saline produced water was released from multiple sources during oil field operations from 1913 to 1973 at the USGS research Site A on Skiatook Lake in northeastern Oklahoma. Two pits, designed to hold produced water and oil, were major sources for release of these fluids at the site. Produced water spills from these and other features moved downslope following topography and downdip by percolating through permeable eolian sand and colluvium, underlying permeable sandstone, and, to a lesser extent, through shales and mudstones. Saline water penetrated progressively deeper units as it moved through the gently dipping bedrock to the north and NW. A large eroded salt scar north of the pits coincides with underlying fine-grained rocks that have retained substantial concentrations of salt, causing slow revegetation. Where not eroded, thick eolian sand or permeable sandstone bedrock is near the surface, and vegetation has been little affected or has reestablished itself after the introduced salt was flushed by precipitation. The extent of salt-contaminated bedrock extends well beyond existing surface salt scars. These results indicate that one of the legacies of surface salt spills can be a volume of subsurface salinization larger than the visible surface disturbance. Ó 2007 Published by Elsevier Ltd.
1. Introduction Two petroleum production sites (A and B) on Skiatook Lake in northeastern Oklahoma (Fig. 1) are the subject of US Geological Survey investigations under the Osage-Skiatook Petroleum Environmental Research (OSPER) project. These studies, initiated in 2001, were designed to investi*
Corresponding author. E-mail address: [email protected] (J.K. Otton).
0883-2927/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2007.04.015
gate the environmental impacts of oil and gas production on local soils, bedrock, ground water and surface water, and the ecosystems they support (Keeland et al., 2003; Kharaka and Otton, 2003; Kharaka et al., 2005; Otton et al., 2005). The construction and filling of Lake Skiatook in the mid1980s expanded the scope of potential impacts to water supplies and a recreational fishery. Understanding the nature and extent of impacts at Site A and elsewhere can help land managers avoid or mitigate problems related to past oil production
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Fig. 1. Site location map. (a–c) Location of Sites A and B on Skiatook Lake, Osage County, Oklahoma. (d) False color aerial photo of the vicinity of Site A on Skiatook Lake. Arrow points to prominent salt scar at north end of the site. Photo derived from an October 2003 hyperspectral survey, courtesy of David Reister, Oak Ridge National Laboratory.
activities on lands that may be identified for future development. Thirteen auger holes were drilled to depths of 5–21 m at Site A to document the subsurface stratigraphy, characterize the subsurface distribution of saline bedrock by geochemical and geophysical means, and install ground-water monitoring wells. This report presents new lithologic, geochemical and geophysical data from these holes at Site A and results of a GEM-2 survey (a geophysical survey of the electrical conductivity in the shallow subsurface). The data are used to support a discussion of
geologic features that have influenced produced water movement across the topography and through the unconsolidated surficial sediments and bedrock. The geologic framework described herein is useful in understanding related papers in this volume. 2. Site description and production history Site A is located between two hills that form part of a narrow neck of land on the north side of Skiatook Lake (Fig. 1c). A white salt scar seen in a false color image (arrow, Fig. 1d) marks the north-cen-
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tral part of the site. This scar, which is only sparsely vegetated, is eroded to as much as 2 m below the original land surface. Seepage of brine from various areas of the salt scar forms halite crusts on surface rocks and on thin soil over bedrock. Grasses, forbs, and a few oak trees grow on 4 soil pedestals that remain within the eroded area (Fig. 2). Another
smaller salt scar is on the west side of the site upslope from the cove to the SW. A berm wall and a cut in the hillslope indicate that this salt scar is within and downslope from the trace of a small produced-water pit (old pit, Fig. 2). Open areas in the trees immediately SW, S and SE of the large salt scar are remnants of past oil field production
Fig. 2. Map of cultural and vegetative features at Site A, showing location of hand-auger soil profile hole (S1), 13 auger holes (01–13), and 2 cross sections (A–B and C–D). P-two pits. PU1 and PU2-old power unit sites.
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activities (Fig. 1d). Open areas elsewhere in Fig. 1d are natural meadows, sandstone outcrops, and other oil field-disturbed sites on the lease outside of Site A. A topographic divide between the two hills separates drainages that flow into the two coves. The lake level generally fluctuated from 215.8 to 217.6 m above sea level (asl) during the study, although during a few extreme rainfall events the lake rose to as much as 219.2 m and flooded the lower part of the salt scar. During low water levels, a pit from the post-1973 lease operations, rock ledges, and other features are exposed on the lake bottom in the cove to the NE (Fig. 2). A second post-1973 pit is in an open area onshore just south of this pit. Prior to the filling of Lake Skiatook, the local water table level was defined by the elevation of the closest adjacent perennial stream, Cedar Creek to the north, at about 202 m asl. This elevation is about 19 m below the elevation of the midpoint of the saddle at Site A. With the filling of Skiatook Lake, the local water table is now defined by the lake surface elevation which can be taken as the normal pool elevation of 217.6 m asl although there has been fluctuation as noted above. Site A is centrally located on an oil lease that occupies the north half of section 13, T22N, R10E in Osage County, Oklahoma (a section is 1.6 km on a side). About 19,000 m3 of oil were produced on the lease from 1913 to 1999, when production ceased (estimated from Bureau of Indian Affairs (BIA), Pawhuska, OK, unpublished lease record files). The volumes of produced water generated are not part of lease records, however some of the wells had extreme ‘‘water cuts’’ prior to being shut-in, with as little as a few tens of liters of oil per m3 of fluid being produced. Using a ratio of 10 m3 of produced water to 1 m3 of produced oil (Veil et al., 2004), about 190,000 m3 of produced water may have been generated at the site. Most of this production was likely processed at site ‘‘A’’, but post-1973 production was processed at a newer tank battery about 350 m to the SE. More recent brine production was reinjected to the subsurface. Remnants of production facilities remain at the site. A small tank battery, marked by remnants of 2–3 redwood tanks, is located in the southeastern part of the open area (Fig. 2). Based on local practices, these tanks likely included a separator, an oil storage tank, and a brine storage tank. Along the
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east edge of the site are remnants of a central power unit (bullwheel and concrete powerhouse foundation, ‘‘PU1’’ in Fig. 2) that supplied mechanical pumping action via long pump rods to oil wells on the lease. A large tank, observed in a 1936 aerial photo of the site, was located NE of this power unit. Its former position is presently marked by a barren, stained patch of sandstone. Another concrete foundation, probably a pump jack base or another power unit, is located to the SW (‘‘PU2’’ in Fig. 2). A former injection well is located south of this concrete foundation. In the central part of the site are two pits that temporarily stored produced fluids (two contiguous irregular oval areas labeled ‘‘P’’ in Fig. 2). The larger west pit stored produced water whereas the east pit stored crude oil. A notch along the north edge of the west pit was a release point for produced water to flow from the pit (shown by an arrow), which was a common practice prior to the 1970s. A trench extending from the area of the tank battery to the pits carried produced fluids to the pits. Soils surrounding the tanks, the trench, and the pits are variably saturated with weathered asphaltic hydrocarbon and scarred from saline water. Weathered sandstone blocks exposed around the tanks are coated with oil, variably bleached, and heavily stained with Fe oxides indicating possible dissolution, movement and re-precipitation of Fe by reducing, highly saline waters at the site. Although, the volumes of produced water from production operations are not known precisely, salinities were likely high as evidenced from nearby oil field operations. Produced waters from nearby wells are notably high in total dissolved solids (TDS) and Cl, as documented by analyses of waters from nearby oil wells (114,000–185,000 mg/ L TDS, Table 1 in Kharaka et al., 2003). A layer of highly saline water (110,000 mg/L TDS) preserved beneath the thick asphalt layer in the east pit indicates that the produced waters at this site were also highly saline (AP01, Table 1 in Kharaka et al., 2003). Oil field operations have kept the oak trees from encroaching on the open areas of the site. All operations at Site A ceased about 1973, but production continued on other parts of the lease until 1999. The 1936 aerial photograph shows that this large salt scar and other smaller salt scars were well developed within 23 a after production started. The smaller scars indicate that, in addition to the two pits, the tank battery, the tank to the east, and the two
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power units were significant points of release for produced waters and the salts associated with them. The present-day road was moved to its current position, apparently in response to erosion in the main salt scar. Remnants of an old road culvert remain in the salt scar.
the survey lines were fixed by GPS readings, often at monitoring well locations.
3. Methods
A color image of the composite high-frequency GEM-2 survey data for site A (Fig. 3) portrays the relative conductivity of the shallow soil and bedrock to a depth of investigation of about 3 m. Areas near and immediately downslope from the tank battery and the eastern power unit could not be surveyed because of the presence of abundant pipe, pump rods and other discarded metal. The highest conductivity readings were over deeply eroded parts of the large salt scar (see outline in Fig. 3), reflecting the near-surface, salt-saturated, weathered sandstone and fine-grained rocks. High conductivity was also observed at the head of the cove to the NE. The cove receives salt runoff from the large scar and also overlies the channel of the small stream that drained the northern part of the site prior to the filling of the lake. Salt-saturated thin alluvium and bedrock below the alluvium in the northeastern cove likely contribute to the high conductivity observed in the upper 3 m. Areas of moderately high conductivity include: (1) around the site of an old tank seen in the 1936 aerial photo (1 in Fig. 3); (2) south of the power unit in the SW part of the site near the location of the injection well; and (3) along the drainage leading down to the cove to the SW. However, the area immediately west and NW of the tank battery (4, Fig. 3) and south of the pits (the area along cross section line A–B, close to point A) shows low conductivity values. There, permeable (as inferred from porosity data, see below), near-surface, weathered sandstone is thick (2.6–4 m) and salts may have been largely flushed from the surface zone measured by the GEM-2 instrument (0–3 m deep). Elevated subsurface conductivity extends to the NW of the salt scar, as well as to the east between the salt scar and the tank showing that the extent of high subsurface salinities exceeds the salt-scar footprint and underlies large areas covered by oak forest.
The geology and stratigraphy of Site A were documented by surface mapping, shallow hand augering, shallow direct-push holes, and deeper hollow-stem auger and rotary drilling. Shallow soil samples (0–15 cm) were collected by hand auger across the site. Soil profiles through the surficial sediment were hand augered at two locations. Channels in colluvium were mapped from exposures in the large salt scar north of the pits, in the low, seasonally wet areas (originally smaller salt scars) SW of the pits, and in some shallow auger holes. Borehole gamma and borehole conductivity geophysical surveys provided additional stratigraphic information. Core was retrieved from nearly all auger, rotary, and direct-push holes. Thin sections of selected core were prepared for petrologic, mineralogic, and petrophysical studies. Quantitative petrologic data, including porosity, are based on point counts (n = 300). Further characterization of surficial sediments included grain-size and X-ray powder diffraction analyses. For the bedrock studies, data from 13 deeper auger holes are reported herein (01–13). Fig. 2 shows the location of the 13 drill holes and two cross section lines with respect to cultural and vegetation features at the site. Samples (100 g) were crushed to <3 mm, mixed with an equal weight of DI water, stirred vigorously, and allowed to stand overnight. The liquid phase was separated by centrifugation and filtration through a 0.45 lm filter membrane. Specific conductance and pH of the aqueous extracts were measured immediately and the solutions were submitted for measurement of dissolved Cl, Br and SO2 by ion chromatography, following the 4 procedure described by Zielinski et al. (2003). A multi-channel GEM-2 electromagnetic survey was conducted across the surface of Site A when Skiatook Lake was at a low water level in May 2005. This survey was an expansion of a previous, more limited GEM-2 survey (Haoping, 2005) and an older survey that was limited to a narrow area focused on the pits and the large salt scar (Smith et al., 2003; Otton et al., 2005). The end points of
4. Results and discussion 4.1. Surface geophysical survey
4.2. Surficial sediment and topography Unconsolidated surficial sediments, including eolian sand and colluvium, cover the weathered bedrock across much of the site. The eolian sand
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Fig. 3. Composite high frequency GEM-2 survey map showing subsurface (0–3 m, 0–10 ft) electrical conductivity. Base is a georeferenced 1995 aerial photo of Site A. The trace of the present-day road (dashed lines) and the location of 2 cross section lines (shown in Fig. 2) are shown for reference. Data in the NE cove were taken during a low-water period. Orange, red, and pink colors represent higher electrical conductivity. The salt scar underlies the pink color in the center of the figure (note outline of the scar). Location of the sources: 1- former location of a tank; 2- east-side power unit; 3- two pits.
varies in thickness from 0 to 1.5 m and is uniformly very fine grained and permeable (as inferred by its texture and drainage). Observations during rainstorms indicate that water quickly infiltrates the eolian sand; surface runoff is limited except in the scarred areas where eolian sand and colluvium are absent. The colluvium ranges from scattered sandstone pebble layers a few cm thick at or near the base of the eolian sand to thicker (0.3 m or more) layers of angular sandstone clasts in a sandy to clayey matrix, the latter occurring mostly in channels sitting on weathered bedrock. Where the matrix is clayey, the colluvium tends to retain salt as evidenced by salt crusts on exposed surfaces after a
rainstorm. The pre-erosion surface topography of Site A is portrayed in Fig. 4 (reconstructed in salt scar using the 4 soil pedestals) as is the distribution of the mapped colluvium and the position of channels inferred from the location of the mapped colluvium. Arrows show the inferred flow direction of produced water from the pits into the channels. Although, most produced water released from the pits flowed northward, it is inferred that some leaked from the south edge of the pits at the drainage divide and moved westward along a shallow channel immediately SW and west of the pits (Fig. 4). Salt derived from these pit-sourced produced waters, plus any salt from spills at the tank
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Fig. 4. Map showing topography of Site A and positions of the drainage divide (long-dashed line), two pits, and mapped colluvium. Probable locations of channels incised on the surface of the bedrock are shown by short-dashed lines and arrows.
battery and along the trench, likely contribute to the high subsurface conductivity measured along the drainage leading to the southwestern cove (Figs. 2 and 3). 4.3. Bedrock stratigraphic section and structure Bedrock at Site A is in the Pennsylvanian Wann Formation (Gardner, 1957), a deltaic and marginal marine sequence comprised of sandstone, mudstone, siltstone, shale and limestone. Although, not observed at Site A, the formation is known to contain limestone reefs with crinoid fossils that identify it as Middle Pennsylvanian (Missourian). Ecological studies indicate that the crinoid genera were likely adapted to muddy to firm substrates and turbid waters characteristic of the Middle
Pennsylvanian in the present-day mid-continent where crustal instability created local uplifts that contributed detritus to adjacent shallow seas (Ettensohn, 1980). The rapid lateral facies changes of the sedimentary rocks observed at Site A, and described below, may reflect this regional tectonic dynamics. Bedrock lithologies at the site include 5 units (1–5, see below) that vary substantially in porosity and permeability and thus strongly control movement of produced water. Lithologic logs from two drill holes (02, 07, Fig. 2) that penetrate these units are compared with other relevant data for these holes in Figs. 5 and 6. These holes were selected because hole 02 lies immediately downslope and north of the two pits and hole 07 lies 110 m downslope and downdip from the two pits in the direction that much of the
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Fig. 5. Data for hole 02 (see Fig. 2 for location) showing (a) lithologic log (legend to the right), (b) extractable Cl concentrations (mg/L) from crushed core samples, (c) porosity from thin section observations, (d) conductivity log (milliSiemens/m), and (e) gamma log (counts per second). Arrows on the gamma log indicate primary (lower) and secondary gamma spikes at the top of Unit 5 and the top of Unit 4, respectively. Gray line (Blue line in web version) represents the depth of the normal pool elevation for Lake Skiatook, the average current base level. Base level for this site prior to the filling of Lake Skiatook, Cedar Creek just to the north, would be about 21 m below the land surface at this well site (see depth scale).
brine moved. Photomicrographs for hole 07 are shown in Fig. 7 and a summary of the porosity data for Units 1–4 are plotted in Fig. 8. The 5 lithologic units are, in descending order: Unit 1. Weathered, but generally case-hardened, resistant sandstone that forms low outcrops at the southernmost and northwestern part of the site. This unit is in the upper part of the section in 3 holes (05, 12, 13, Fig. 2) on the NW side of the site. It underlies the hills on either side of the saddle where Site A is located. No petrographic or porosity data were measured for this unit; however, it is of limited areal distribution within the site and was not a significant conduit for produced water. Unit 1 is likely equivalent to the Clem Creek Sandstone Tongue of the Wann Formation (Gardner, 1957).
Unit 2. Widespread, weathered, very fine-grained sandstone and clayey sandstone. Petrographic analysis indicates an average porosity of about 24% (Figs. 5–8). Photomicrographs show variable amounts of Fe oxyhydroxide cement (Fig. 7a). In some sections, remnant dolomite cement is present, indicating that porosity may be caused by dissolution of dolomite during weathering. The presence of Fe produces a wide variety of red to orange staining. Thin clay partings and laminae produce a subhorizontal fabric (Fig. 7a). Unit 2 intertongues, in part, with fine-grained rocks of underlying Unit 3. Unit 3. Intertonguing, mostly weathered shale, sandy siltstone, sandy mudstone and sandstone. Unit shows rapid lateral facies changes and striking contrasts in porosity and permeability. No two holes have a per-
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Fig. 6. Data for hole 07 (see Fig. 2 for location) showing (a) lithologic log (see Fig. 5 for legend) (b) extractable Cl concentrations (mg/L) from crushed core samples, (c) porosity from thin section observations, (d) conductivity log (milliSiemens/m) and (e) gamma log (counts per second). The arrow on the gamma log indicates a gamma spike associated with the carbonaceous black shale at the top of Unit 5.
fectly correlative lithologic sequence (compare Figs. 5 and 6; also see Fig. 11a and b). Some unweathered shale is carbonaceous with thin coaly laminae and contains abundant pyrite. Fossil plant fragments are abundant on bedding surfaces. In several core holes, the greenish-gray weathered shale contains abundant gypsum, and some surface exposures in the large salt scar display abundant gypsum crystals. Shales have low (<1%) porosity whereas sandstones range from 15 and 20% porosity (Figs. 5–8). Clay partings are abundant in sandstones and sandy layers are abundant within the shales (Fig. 7b and c). The sandstone beds locally show worm burrows, indicating shallow water deposition. These
worm burrows also enhance vertical permeability. Sandstones are preferred conduits for flow of produced water through Unit 2. Unit 2 is the only part of the section exposed in the large salt scar. There, laterally discontinuous sandstone layers and a cross-bedded channel sand body are enclosed by weathered mudstone, siltstone and shale. One sandstone bed is deeply pitted, and variably stained by Fe oxides or bleached. Highly saline, reducing produced water moving through this sandstone probably dissolved cementing material and solubilized Fe, producing bleached areas. Soluble Fe re-precipitated locally where near-surface oxidation produced Festained areas. The contact between Units
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Fig. 8. Porosity values determined for samples from holes 02 and 07 by lithologic unit (n = 39).
Fig. 7. Thin section photomicrographs of core from hole 07 with unit designation, depth, and porosity values. The long dimension of each image is 2.4 mm. Blue color is tinted epoxy filling pore spaces.
3 and 4 is delineated by a spike in gammaray intensity on logs of about half of the holes (upper arrow, Fig. 5e; gamma spike at this position is missing in Fig. 6e). The stratigraphic position of this gamma spike corresponds to the base of the shales within Unit 3 and the upper limit of dolomite cement in Unit 4. Unit 4. Variably dolomite-cemented, generally unweathered sandstone with abundant clay partings and thin shale beds, especially in the lower half of the unit, that are commonly carbonaceous and have fossil plant-leaf detritus. Fossil-shell debris is locally abundant. Sandy dolomite forms a minor lithologic component. The porosity is highly variable, ranging from near zero in highly cemented sandstone and thin
shale beds to 31% in the lightly cemented sandstone (Figs. 5–7d, e, f, Fig. 8). Ferroan dolomite is the primary dolomite cement, as determined by SEM. Unit 4 is observed only in the subsurface at the site, but crops out as a dark-brown-weathered, resistant, cliff-forming ledge on south-facing slopes at the south end of the peninsula. Unit 5. Interbedded muddy sandstone, shale, siltstone and mudstone of generally low porosity (median less than 2%, Figs. 5–8). Sandstones are variably dolomite-cemented with as much as 10–14% porosity. Like Unit 3, no two holes have a similar lithologic sequence across the site. This unit is exposed at the surface below the cliff-forming ledges of Unit 4 in the southernmost parts of the peninsula. The base of Unit 4 and the top of Unit 5 is everywhere marked by a carbonaceous shale except in hole 10, the most southerly of the holes at Site A (Fig. 2). This shale produces a distinctive spike in the gamma log (lower arrow in Fig. 5e, the sole arrow in Fig. 6e) that aids the correlation. Pennsylvanian carbonaceous marine shales in the mid-continent are commonly U-enriched (Hyden and Danilchik, 1962). Based on the altitude of the shale with the distinctive gamma spike at the top of Unit 5 in several holes, the strike and dip of the bedrock section
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across Site A are estimated to be N25°E and 1°25 0 NW, respectively. The gentle dip and the local topography (hills to the SE and NW with a saddle in between) control the surface distribution of bedrock Units 1, 2 and 3 (Fig. 9). Unit 1 forms two outcrop bands that wrap around the two hills. The tank battery sits on the lower part of Unit 1 just above the contact with Unit 2. The sandstone-dominated Unit 2 forms a continuous ‘‘bridge’’ between the two hills. This permeable bedrock unit underlies the two pits and the trench that allowed produced water and oil to flow down to the two pits from the tank battery. Spilled produced water moving through the permeable overlying eolian sand and colluvium readily percolated into this unit. Unit 3, which contains abundant mudstone and shale, is exposed along the lower slopes on the flanks of the saddle. It underlies all of the large salt scar north of the pits. The tank on the east side of the site that was observable in the 1936 aerial photo, rested on this unit.
A linear segment of the shoreline along the southwestern cove aligns with the topographic low that trends about N45°E across the site and corresponds to the area where the peninsula forms a narrow neck (Fig. 1d; Fig. 9). This apparent lineament may mark a zone of preferential fracturing or possibly minor offset in the bedrock. Gardner (1957) mapped several short, NE-trending fault segments in the surrounding area. Vertical to subvertical fractures and faults are commonly observed in core; however, no deep holes were drilled along the trace of the lineament to determine if they are more abundant in this zone. Faults observed in core show slickensided surfaces and juxtapose lithologies of different types; however, the amount of slip cannot be determined. Vertical movement of these beds was not substantial (probably less than 1 m total) as indicated by correlations of lithologic contacts and geophysical markers. Fractures and faults may also provide important conduits for produced
Fig. 9. Bedrock geologic map of Site A and vicinity. Squares represent locations of all hand auger, auger and direct-push holes used to develop the map. The contact between Unit 1 and Unit 2 was mapped at the surface. Units 1, 2 and 3 are described in the text.
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water flow in bedrock. Further study of the fractures and faults in core is warranted. 4.4. Salt distribution Dissolved Cl concentration in 1:1 aqueous extracts of soil and bedrock samples ranges from <10 to several thousand mg/L (Figs. 5 and 6b, and unpublished data for other holes). Extractable Cl concentrations <150 mg/L are considered to be natural, based on extracts from a nearby reference drill core collected offsite and from highly cemented sandstone and deep shale samples from Site A. The conductivity (lS/cm)/Cl (mg/L) ratio of lab-prepared solutions of variable NaCl concentration is approximately 3.3; therefore, ratios >5.0 in sandstone extracts containing <150 mg/L extractable Cl and <150 mg/L extractable SO2 were 4 interpreted as an additional indication of minimal NaCl contamination. The Cl/Br mass ratio can be another indicator of produced water contamination. This ratio was determined in extracts containing greater than 0.2 mg/L Br. The mean Cl/Br ratio for all samples with extractable Cl >150 mg/L was 312 ± 37 mg/L. This compares to a Cl/Br mass ratio of 279 in the highly saline water just beneath the east pit and to an average value of 282 ± 36 for 8 modern produced waters collected from nearby wells in Osage County (Table 1 in Kharaka et al., 2003). 4.4.1. Extractable chloride in surficial sediments Twenty shallow (depth 0–15 cm) soil samples collected from eolian sand soil profiles across the site all had extractable Cl concentrations <24 mg/L. Extracts of 12 similar soil samples within the limits of the salt scar ranged from <24 mg/L near the edges of the scar to 2840 mg/L in alluvium within the scar. A 1.6 m vertical soil profile was extracted with a hand auger from one of the isolated soil pedestals within the salt scar (Fig. 10, S1 in Fig. 2). The profile shows extractable Cl < 10 mg/L in the upper 56 cm of eolian sand. In the intervals 56– 79 cm and 79–99 cm, the extractable Cl content increases progressively with increasing clay content. Maximum Cl concentrations of 1167 mg/L are in the 99–122 cm interval that includes basal clayey, clast-rich mixed sand and colluvium (99–109 cm) and underlying weathered sandy claystone of Unit 3 (109–122 cm). Below this interval mg/L drops to the 400–500 mg/L range through the bottom of the auger hole at 160 cm.
Fig. 10. Extractable Cl concentrations versus depth for a handaugered soil profile (S1, Fig. 2) located on a soil pedestal within the salt scar. Depth intervals are not equal.
Elevated soluble Cl in the basal clayey part of the surficial sediments indicates that NaCl-rich fluids probably moved along the contact with underlying clayey weathered bedrock. Soluble salt retained within this horizon was subject to some upward movement by capillary action from the underlying, more saline, weathered bedrock. A second hand-augered hole, located between the berm of the two pits and the road, encountered 74 cm of eolian sand overlying weathered, permeable sandstone bedrock of Unit 2. The sand was asphalt-cemented in the upper 10 cm. Extractable Clconcentrations ranged from 8.7 to 18.9 mg/L in this hole, despite its close proximity to the asphaltic oil-filled eastern pit that has a layer of highly saline water (110,000 mg/L TDS) just beneath the asphalt in the pit. 4.4.2. Stratigraphy and salt distribution in the bedrock across the site Lines of section in Figs. 2 and 3 are shown as lithologic and correlative log-stratigraphic cross sections in Fig. 11a and b. The cross sections also show concentrations of extractable Cl versus depth (ranges given in a color-bar format) and the positions of surface sources of produced water salts. Lithologic columns for these cross sections identify the upper, weathered portion of each columnar section in color and the lower, unweathered portion in black and white. The persistent gamma-ray spike associated with the black carbonaceous shale at the top of Unit 5 is shown with a red arrow, except
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Fig. 11. Cross sections showing lithologic logs, correlations of stratigraphic units, and extractable Cl concentrations at Site A. Locations for line of sections shown in Figs. 2 and 3. The solid and dashed horizontal red lines connect major and minor gamma marker horizons (gamma kicks shown with arrows) used for correlations (Figs. 5 and 6). Refer to Fig. 5 for the lithologic legend.
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in hole 10 (Fig. 11a), where it is absent. A solid red line shows the suggested correlation from hole to hole (dashed where inferred in Fig. 11a). The lesser gamma-ray spike in the upper part of Unit 3 is present in several, but not all, holes. An indicated correlation is delineated by a dashed red line. Depth of weathering of bedrock at Site A was defined as the last appearance of Fe oxyhydroxide mineral staining of the core or as the last appearance of the distinctive olive-green coloration of the clays. In some fine-grained core, the last appearance of Fe oxyhydroxide-filled fractures or partings was used. The depth of bedrock weathering ranges from about 4 m in hole 10 to about 11 m in holes 08 and 09. Bedrock in the northern parts of the site seems to be more deeply weathered to include all of Unit 2 and much of Unit 3. Removal of cement in the sandstones of Unit 2 and Unit 3 during weathering resulted in increasing the porosity of these rocks and facilitated movement of the saline water. Weathering of the shales and mudstones of Unit 3 may have made them more susceptible to penetration by highly saline water. The cross sections of Fig. 11a and b illustrate the complex intertonguing of sandstone and finegrained rocks between Units 2 and 3 and within Unit 3, the relative uniformity of most of Unit 4 across the site, the sharp basal contact of Unit 4, and the apparent complex intertonguing of sandstone and fine-grained units in Unit 5. Sandstones in Units 2 and 3, and to a lesser degree in Unit 4, are preferred conduits for produced water movement and subsequent salinization of bedrock. Because of the rapid lateral facies changes within the limited area of the site, there is insufficient data to show in much detail the 3-dimensional geometry of sandstone bodies in Unit 3, or zones of intense cementation in Unit 4, even with the relatively close spacing of holes. 4.4.3. Extractable chloride distribution in bedrock Concentration of extractable Cl in bedrock is relatively high in holes downdip and north of the two pits (Fig. 11a, holes 02, 01, 07). Downslope and west of the east tank (Fig. 2), bedrock in holes 09 and 08 also show high Cl (unpublished data, see also elevated conductivity in Fig. 3). Extractable Cl concentrations exceeding 1500 mg/L are largely restricted to the weathered shales of Unit 3 encountered in the holes near these two sources and the deeper sand units in the more distal holes 06 and 07 (Fig. 11b). Extractable Cl concentrations in
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the 600–1500 mg/L range are present in some samples from almost all 13 holes. The exceptions are hole 10 (Fig. 11a) which lies south of the major sources and may only be impacted by releases from the tank battery and hole 13 (Fig. 11b) which appears to be slightly beyond the west edge of the produced water plume generated by the two-pits source. Hole 06 may have received Cl salts from both the two pits and the east tank sources. Anomalous concentrations of extractable Cl (>150 mg/L) are also present at varying depths in other holes across the site, but the Cl does not generally penetrate to depths much below the base of Unit 4. In hole 02, Cl concentrations drop to background values in the uppermost part of Unit 4. Porosity measurements based on thin section examinations from hole 02 (Fig. 5) show that porosity is relatively high in the uppermost part of Unit 4, averaging about 20%, but decreases noticeably at 8.4 m and remains low for the next 1–1.3 m due to increased dolomite cementation. The abrupt decrease in porosity/permeability may have blocked or slowed downward penetration of saline water at that location. Farther downslope from the two pits, near hole 07, Cl concentrations remain high to a depth of 11.9 m (Fig. 6, Fig. 11a). If this salt is derived from the two pits, then salt contamination has penetrated stratigraphically deeper as it moved laterally downdip. At hole 10, updip from the two pits (Fig. 2), moderate concentrations of extractable Cl were observed to the base of Unit 2, as well as into Unit 4 to a depth of about 10.4 m (Fig. 11a). This hole is topographically lower and downdip from the tank battery and trench, the likely source, but lateral dispersion of salts originating from the two pits cannot be ruled out. 5. Produced water movement from the two pits at Site A – conceptual model Migration of salt from the two pits through surficial sediments and bedrock is inferred to have occurred in a sequence similar to that portrayed in Fig. 12a and b. Shortly after produced water releases began from the two pits, percolation was downward through the eolian sand, initially following the contact between the surficial sediments and the weathered permeable sandstone of Unit 2, possibly preferring the colluvial channels (Fig. 4). The water also moved into and downdip through permeable Unit 2 to contact fine-grained rocks of Unit 3.
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Fig. 12. Conceptual diagrams depicting the probable history of produced water movement from the two pits. (a) Early movement and geologic controls on produced water movement at Site A. (b) Salt movement later in the site history after the deeply eroded salt scar formed. Red arrows indicate directions of ground water movement.
Saline waters emerged as downslope seeps to the NW where Unit 2 thins and the surficial sediments are also thin. Some of the saline water may have
entered Unit 2 sands on the NW flank of the drainage (Fig. 12a). Once surface vegetation was killed by saline water, headward erosion expanded the salt
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scar upslope. The 1936 aerial photo of the site indicates that the salt scar grew to about 80% of its present-day extent within the first 23 a of production. Headward erosion seems to have been limited by the position of the Unit 2-Unit 3 contact in the shallow subsurface (Fig. 9) except for two narrow gullies that have eroded southward toward the two pits (Fig. 2). With time (Fig. 12b), salt moved progressively deeper into the bedrock, penetrating Unit 3 sandstones and fine-grained rocks and ultimately into and through Unit 4 to the top of Unit 5 at the position of hole 07 (Fig. 12b). Deeper penetration is deflected downslope by the gentle dip of the sequence and the strong horizontal fabric in the bedrock (clay partings and highly cemented horizons in sandstones). Fractures probably facilitated the overall downward movement perhaps most notably in the trace of the lineament. Weathered shale in the upper part of Unit 3 may have initially acted as an aquitard that later became salt saturated. Substantial salt penetration of the shale in the upper part of the section is present in all holes on the site, possibly the result of 3 different mechanisms: (1) lateral movement of saline produced water into the shales through intertonguing permeable sand bodies several cm to 10 s of cm thick (Fig. 11a and b) to thin sandy laminae less than a few mm thick (Fig. 7c); (2) vertical movement along fractures through shale; and (3) increased weathered shale permeability resulting from clay flocculation and shrinkage in response to introduction of highly saline water (van Olphen, 1977; Yariv and Cross, 1979) although X-ray mineralogy of these weathered shales suggest that kaolinite dominates the clay phases with lesser mixed illite-smectite clay present thus this effect may be limited (Cyndi Rice, USGS, unpublished data, 2007). 6. Conclusions Surface and subsurface geological, geochemical, and geophysical studies to determine the distribution of high salinity at Site A indicate that 4 sources related to past oil field operations likely contributed produced water salts to soils and bedrock at Site A: (1) two pits originally emplaced to store produced water and hydrocarbon (major source); (2) a tank originally located near hole 09, as seen in a 1936 aerial photo, and the nearby central power unit (major source);
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(3) a tank battery and trench upslope from the two pits (minor source) and (4) an injection well or another power unit on the SW part of Site A (minor source). The subsurface extent of salt contamination from the two pits (item 1 above) is not constrained by the data in the direction north and NW of holes 07 and 06 as the contamination may continue downdip and along fractures to deeper horizons. Salt-contaminated sediments and bedrock likely extend beneath Skiatook Lake following the now-submerged alluvial channel of the drainage to the north. Some salt contamination also extends under Skiatook Lake in the cove to the SW. Salt moving downslope from the two pits following channels in the colluvium and salt water stored in the small pit just upslope from the SW cove are likely sources. Produced water from these sources flowed down-section and down-dip to the NW and north following pathways of favorable porosity and permeability. Preferred pathways include: (1) the contact between the base of the eolian sand and colluvium and the underlying bedrock, notably where channels at the contact may have focused some produced water flow; (2) the contact between the permeable weathered sandstone of Unit 2 and the uppermost, less permeable, weathered shale of Unit 3; (3) sandstone bodies within Unit 3; (4) lightly cemented horizons within the dolomitic sandstone in Unit 4, and (5) sandy lenses within all shales and siltstones. Prior to filling of Skiatook Lake the local base level was set by the level of Cedar Creek to the north at about 202 m asl (lower blue line, Fig. 11b) and that may have controlled downward movement of salt. However, with the filling of Skiatook Lake, the base level has risen to about 218 m asl (upper blue line, Fig. 11b) which would slow downward movement, perhaps limiting it to that caused by density differences between the saline plume and the local ground water. After cessation of oil field operations at Site A, salt contamination has been subjected to flushing by precipitation (about 80 cm/a in this part of Oklahoma) and surface runoff. Much of the salt has likely been removed from permeable sandstones of Unit 2. However, a transition from highly concentrated brine to relatively fresh precipitation allows dispersion of any smectitic clays present and subsequent reduction in permeability of clay-rich parts of the section. This may promote retention of salt in fine-grained rocks of Unit 3 that now floor the salt
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scar. Salt is being removed through surface runoff from the salt scar however. Kharaka et al. (2005) and Thordsen (2006, USGS, unpublished data) estimate that the amount of salt removed from the salt scar by surface runoff during storm events from salinity and flow volume measurements at a weir at the north end of the salt scar is about 500 kg/a. Salt now retained in bedrock below the presentday level of Skiatook Lake is much less likely to be discharged to the surface streams and carried away. The relatively high salinity of surface materials within the nearly barren salt scar contrasts with the low salinity and more robust vegetation on eolian-sand-capped soil pedestals within the scar. Remediation of the salt scar must successfully isolate the saline fine-grained rocks of Unit 3 from the root zone. The soil pedestal at S1 (Fig. 2) with about 1 m of eolian sand (0.6 m essentially salt-free, Fig. 10) supports grasses. Other soil pedestals at the site support grasses, forbs and small oak trees. Further research is necessary to establish what thickness of eolian sand or similar permeable material may be necessary to form a capillary break to isolate the saline bedrock from root zones. References Ettensohn, F.R., 1980. Parrogassizocrinus: systematics, phylogeny and ecology. J. Paleontol. 54, 978–1007. Gardner, W.E., 1957. Geology of the Barnsdall area, Osage County, Oklahoma. Norman, Oklahoma. M.S. Thesis, Univ. Oklahoma. Haoping, H., 2005. Depth of investigation for small broadband electromagnetic sensors. Geophysics 70, G135–G142. Hyden, H.J., Danilchik, W., 1962. Uranium in some rocks of Pennsylvanian age in Oklahoma, Kansas, and Missouri. US Geol. Surv. Bull. No. 1147-B. Keeland, B.D., McCoy, J.W., Otton, J.K., 2003. Effects of produced water and hydrocarbon releases on vegetation at Site A of the Osage-Skiatook petroleum research project, Osage County, Oklahoma. In: Sublette, K.S., (Ed.), Program and papers, 10th Ann. Internat. Petroleum Environmental Conf., Houston, Texas, November 11–14, 2003. http://ipec.utulsa.edu/ Conf2003 /Papers/ keeland_mccoy_otton_105. pdf.
Kharaka, Y.K., Otton, J.K. (Eds.), 2003. Environmental impacts of petroleum production: initial results from the OsageSkiatook petroleum environmental research sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260. Kharaka, Y.K., Thordsen, J.J., Kakouros, Evangelos, Abbott, M.A., 2003. The fate of inorganic and organic chemicals in produced water from the Osage-Skiatook petroleum environmental research sites, Osage County, Oklahoma. In: Y.K. Kharaka, J.K. Otton (Eds.), Environmental Impacts of Petroleum Production: Initial Results from the Osage-Skiatook Petroleum Environmental Research Sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260, 57–84. Kharaka, Y.K., Thordsen, J.J., Kakouros, Evangelos, Herkelrath, W.N., 2005. Impacts of petroleum production on ground and surface waters: results from the Osage-Skiatook petroleum environmental research Site A, Osage County, Oklahoma. Environ. Geosci. 12, 127–138. Otton, J.K., Zielinski, R.A., Smith, B.D., Abbott, M.A., Keeland, B.D., 2005. Environmental impacts of oil production on soil, bedrock, and vegetation at the US Geological Survey OsageSkiatook petroleum environmental research Site A, Osage County, Oklahoma. Environ. Geosci. 12, 73–87. Smith, B.D., Bisdorf, R.J., Horton, R.J., Otton, J.K., Hutton, R.S., 2003. Preliminary geophysical characterization of two oil production sites, Osage County, Oklahoma- Osage-Skiatook petroleum environmental research project. In: Y.K. Kharaka, J.K. Otton (Eds.), Environmental impacts of petroleum production: initial results from the Osage-Skiatook petroleum environmental research sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260, 43–56. van Olphen, H., 1977. An introduction to clay colloid chemistry for clay technologists geologists and soil scientists. John Wiley & Sons, New York. Veil, J.A., Puder, M.G., Elcock, D., Redwick, R.J., Jr., 2004, A white paper describing produced water from production of crude oil, natural gas, and coal bed methane. Argonne National Laboratory online publication prepared for US Department of Energy, National Energy Technology Laboratory, http:// www.ead.anl.gov/pub/doc/ProducedWatersWP0401.pdf. Yariv, S., Cross, H., 1979. Geochemistry of colloid systems. Springer-Verlag, New York. Zielinski, R.A., Rice, C.A., Otton, J.K., 2003. Use of soil extracts to define the extent of brine-impacted soils and bedrock at the Osage-Skiatook petroleum environmental research ‘‘B’’ site, northeastern Oklahoma. In: Y.K. Kharaka, J.K. Otton (Eds.), Environmental impacts of petroleum production: initial results from the Osage-Skiatook petroleum environmental research sites, Osage County, Oklahoma. US Geol. Surv. Water-Resour. Invest. Rep. 03-4260, 127–136.