Field Trip, Please!

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JVB’s Hydrodays – 2012

             by S.T. Paxton 

Halihan, Ross, and Osborn provide a historical and hydrogeological context for the Arbuckle-Simpson aquifer and Byrd’s Mill Spring, visible in the background.




Evaluating Oklahoma’s Gas Shales in the Field

Stan Paxton (U.S. Geological Survey, USGS Oklahoma Water Science Center) and Brian Cardott (Oklahoma Geological Survey, or OGS), with assistance from OGS’s Michelle Summers and Sue Crites, provide day trips for interested parties to examine Oklahoma’s gas shales in the field (figure 1). The Arbuckle Mountains area of south-central Oklahoma is a frequent destination. Participants examine the physical properties of the Woodford Shale (Upper Mississippian – Lower Devonian) at locations across the Arbuckle Anticline and south into the Criner Hills of the Ardmore Basin. Typically, the trips will also include brief stops to look at the qualities of the Sylvan Shale (Upper Ordovician) and the Caney Shale (Upper Mississippian) on the northern limb of the Arbuckle Anticline. Technical detail about the shale at each of the field stops is available in OGS Open File Report 2-2008 (entitled “Oklahoma Gas Shales Field Trip”).



Figure 1 – The tip of the red staff (upper right of photograph) indicates the position (yellow dashed line) of the basal contact of the Upper Devonian-Lower Mississippian Woodford Shale with the underlying Lower Devonian Hunton Group limestone. This outcrop is located on the north limb of the Arbuckle Anticline along US 77-D in the Arbuckle Mountains. The beds are slightly overturned, revealing the underside of chert beds in the Woodford Shale (chert beds are light gray in color, black dots) and the Hunton Group limestone (yellow-brown, area of white dots shown in the central-right part of the image). The vertical succession includes a 1) limestone breccia (g) filling fractures in the uppermost Hunton Group limestone beds (white dots), 2) soft, laminated, yellow-brown shale (x), 3) poorly sorted greenish sandstone containing glauconite and rare gravel (tip of red staff), 4) dark-gray shale with molds and casts of brachiopods and pelecypods (y), and finally, 5) dark gray to black shale with thin chert beds (black dots). The unconformity surface occurs at the base of the greenish sandstone (tip of red staff). A strong gamma-ray response, typical of black shales, occurs about 3.5 feet (or 1.1 meters) from the top of the Hunton Group limestone. The basal contact of the Woodford Shale, known as the Acadian Unconformity, is a regionally extensive surface throughout much of North America.

(Photograph by Sue Crites, OGS Staff)_________________________________________________________________________

Field trip participants usually meet the afternoon before departure to the field to discuss trip logistics, safety, and to receive a series of lectures. The intent of the pre-trip lectures is to provide participants with the technical background necessary to gain maximum benefit from a day in the field looking at shale. The lectures include (1) overview of the field trip guidebook; (2) introduction to the geology of the greater Arbuckle Mountains area; (3) review of the field trip stops; (4) overview of Woodford and Caney Shales; (5) overview and geological interpretation of full-scale spectral gamma-ray logs for shales; and (6) application of vitrinite reflectance to Woodford gas-shale plays in Oklahoma.

A major objective of the field trip is to observe properties of shale in outcrop with emphases on mineralogy and rock type (chert versus fissile shale), color, organic matter, shale density, bedding (fissility), and fractures and joints (figures 2, 3). Ideally, thermally mature gas shale contains abundant oil-prone organic matter and is brittle. The physical properties of gas shale, which are important to the effectiveness of a hydraulic fracturing program, are strongly related to the proportions of the different rock types that make up any given gas shale. Proportions and distributions of silt, clay, and chemical sediments (particularly diagenetic silica or quartz) influence the susceptibility of gas shales to fracturing. Shale without a significant proportion of chemical sediment tends to exhibit ductile behavior. Such shales respond less effectively to hydraulic fracturing.


Figure 2 – Slightly overturned beds of Woodford Shale are inspected by field-trip participants along US 77-D in the Arbuckle Mountains of south-central. The character of the Woodford Shale in this outcrop helps participants to distinguish bedding planes, fracture surfaces, and lithology (beds of chert versus fissile shale). This locality is also instructive for convincing fieldtrip participants that cycles in the internal lithology (or rock types) of the Woodford Shale correspond closely to a synthetic vertical gamma-ray profile constructed for this outcrop. The high-frequency gamma-ray cycles from this location can be correlated with a synthetic gamma-ray log constructed for a Woodford Shale outcrop located in the spillway of the Lake Classen YMCA Camp (about 1.5 miles or 2.4 kilometers away).

(Photograph by Sue Crites, OGS Staff)

Figure 3 – The purpose of this stop is to acquaint participants with the nature and scale of joints, fractures, and faults exposed in a nearly vertical standing wall of Woodford Shale along US 77-D. The field team inspects faulting and breccia associated with a faulted kink band developed in the bedding planes of the Woodford Shale (fault = arrow, kink = k). Fracture orientation and fracture density at this and other locations is observed to vary directly with the lithologies (or rock types) that make up the Woodford Shale. The scale of fracture density can change over short vertical distances depending on bed thickness and proportions of brittle chert and ductile fissile shale present in the succession.

(Photograph by Sue Crites, OGS Staff)______________________________________________________________________

A secondary objective of the trip is to relate outcrop properties to synthetic spectral gamma-ray profiles previously compiled for the outcrops visited during the field trip. A tool used for measuring the gamma-ray response at an outcrop is a portable spectrometer tuned to detect and quantify gamma-rays associated with potassium, uranium, and thorium (figure 4). This instrument has a handheld detector (three inches in diameter) that is placed against the face of the outcrop. Data (counts) are collected for 60 seconds per measurement. The counts for each measurement are representative of a hemispherical volume of shale, one-half cubic meter in volume, behind the surface of the outcrop. Data output from the spectrometer includes potassium in percent, and uranium and thorium concentrations, in parts per million. Conventionally, the potassium contribution to the total gamma-ray response is proportional to the amount of clay minerals in a shale (such as illite). The uranium contribution to the gamma-ray response is an indicator of organic matter content of a shale. Thorium is commonly associated with clay minerals and some heavy minerals.


Figure 4 – A gamma-ray spectrometer was used to quantify the amount of potassium, uranium, and thorium in the outcrops of gas shale. The hand-help detector (to the right of the orange field notebook) is held in contact with the shale formation and data is accumulated for 60 seconds. The yellow ribbons are separated by 6 inches (or about 15 centimeters) and mark the locations for taking measurements. The data for each measurement is displayed in the window of the console. The shale above is the Caney Shale exposed along Phillips Creek in the northern Arbuckle Mountains.

(Photograph by S.T. Paxton, USGS)_________________________________________________________________________

Study of shale in outcrop may appear to be of limited value because of the effects of surface weathering. However, weathering of shale in outcrops actually provides insight to the mineralogy and texture of shale that may not be readily apparent in subsurface core. Surface weathering tends to bring out subtle compositional and textural differences in shales which enhance the visibility of bedding contacts, laminations, and organic matter in many shales. Moreover, the occurrence and distribution of fractures, fracture lengths, and joints observed in outcrop is associated with the mineralogy and shale rock types present in the outcrop. Clay-rich formations, such as the Sylvan Shale, weather quickly and uniformly in outcrop exposures. In contrast, shale formations containing discrete concentrations of diagenetic (chemical) silica, carbonate, and phosphate minerals dispersed in the silt- and clay-rich groundmass tend to exhibit differential weathering patterns that are best seen in outcrop (figure 5).


Figure 5 – Concentric phosphate nodules are common to the upper part of the Woodford Shale in the Arbuckle Mountains. Note the chert layer (c) below the nodule (p) is deformed. This observation, along with other indicators in the outcrop, indicates that the phosphate nodules formed early in the history of the Woodford Shale (at or near the sediment-water interface) while the chert layers in the seafloor were still relatively soft.

(Photograph by S.T. Paxton, USGS)_____________________________________________________________________

Gamma-ray measurements, taken every 6 inches (about 15 centimeters) from the base to the top of an outcrop, yield a high-resolution vertical gamma-ray response indicative of cyclical patterns of shale deposition. The cycles, defined by alternating layers of detrital silt, clay, organic matter, and chemical sediments (diagenetic silica, carbonate, and phosphate) are interpreted as responses to changing sea-level, sedimentation rate, and/or chemical oxidation-reduction conditions in the depositional environment of the ancient ocean. Gamma-ray patterns collected from the outcrops in the Arbuckles and Criner Hills can be correlated between the field-trip stop locations. Provisionally, the gamma-ray response patterns also can be correlated into the subsurface of the Ardmore and Arkoma basins and onto the Cherokee Platform.

Evaluation of shale exposures, particularly with the aid of a handheld gamma-ray spectrometer, can improve the description of rock properties that have a direct bearing on gas-shale prospecting. Because gamma-ray profiles from an outcrop can be correlated to subsurface geophysical logs, knowledge about gas-shale properties determined from an outcrop can help define the physical and chemical conditions responsible for the origin and quality of gas-shale on a regional basis (figures 6, 7). Walking through a vertical succession of gas shales in outcrop while armed with a synthetic gamma-ray log and data for total organic carbon, organic matter type, density, and mineralogy associated with the key gamma-ray markers in the shale proves to be very instructive to geoscientists and engineers responsible for evaluation of gas-shale productivity. Field-trip participants who are responsible for geosteering of gas shale lateral wells are particularly enthusiastic about the benefits of seeing gas-shale in outcrop.


Figure 6 – One highlight of the field trip is a hike up through a complete vertical section of Woodford Shale exposed along a stretch of Henry House Creek on the southern limb of the Arbuckle Anticline. The outcrop, 230 feet in vertical thickness, contains fissile shale and some chert beds in the lower third of the outcrop, fissile shale in the middle third, and chert beds with some fissile shale and phosphate nodules in the upper third. The image above highlights part of the middle portion of the Woodford Shale. Because the middle portion of the Woodford is characteristically fissile and contains few chert beds, the middle portion of the Woodford weathers quickly relative to the lower and upper parts of the section. As a result, the middle interval is rarely exposed in outcrop. The lithology around the area of the white dot (above) is shown in greater detail in Figure 7.

(Photograph by Sue Crites, OGS Staff) 

 Figure 7 – The middle portion of the Woodford Shale along Henry House Creek in the Arbuckle Mountains of south-central Oklahoma is highly fissile. In addition, the middle portion of the Woodford at this locality contains very few chert beds. As a result of those properties, the style of bedding and natural fracturing differs markedly from the adjacent portions of the section. Two thin and incompletely developed chert beds are indicated by the arrows. Comparison of the above photograph to the chert-bed rich section in Figure 2 (inset, rotated and cropped) highlights the pronounced lithological variation common to the Woodford Shale in and between outcrops.

(Photograph by Sue Crites, OGS Staff)_________________________________________________________________________


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