Date: Saturday, June 17, 2017
Time: 8:00 a.m. – 4:00 p.m.
Fee: $250.00 with registration, $350.00 field trip only (lunch included)
This unique opportunity to see a world class shale reservoir that is producing in the subsurface only a few miles away from the conference site.
The Woodford Shale of Oklahoma is mainly upper Devonian in age and sits unconformably upon the Hunton Group in the areas of the field trip. The unconformity is in the form of paleokarst and incised valley topography, which has a significant impact on sedimentation of the overlying transgressive Woodford Shale. The Woodford is traditionally subdivided into Lower, Middle, and Upper intervals.
A depositional model has been developed for this area based upon outcrop and subsurface stratigraphy. The model is dependent upon topographic relief on the Hunton unconformity surface. During sea level fall, and early rise, semi- to restricted basins can form which impede open ocean circulation and create oxygen deficiency, thus preventing organic matter from being oxidized and removed; thus it accumulates on the sea floor. With continued rise in sea level, eventually there is communication between restricted basin and open ocean waters, thus oxygenating the waters, and eliminating organic matter. Two 3D seismic surveys tend to support this model. Seismic amplitudes are inverted to acoustic impedances, which are then calibrated to rock-derived TOC. In both surveys there are ‘pockets’ of high TOC dispersed in the sediment column, but in greater abundance and distribution near the Hunton unconformity, indicating deposition and preservation in restricted basins on the unconformity surface.
Stop 1: This stop has been visited by many people over the years as it affords a good stratigraphic section, mainly of the upper Woodford. The studies by Galvis and Becerra are the most comprehensive studies completed at this outcrop. This stop one is located on the south flank of the Ardmore Basin, in southern Oklahoma.
The lower, unconformable contact of the Woodford with the underlying Hunton Group is well exposed; a distinctive green shale lies above the contact; this shale is traceable to several other outcrops in the area. The middle Woodford is covered at this site. The exposed Woodford is mainly the upper Woodford; just beneath the contact with the overlying Sycamore Limestone is a distinctive green shale that is traceable to several of the other outcrops in the area. Rock types vary from very fissile shale to hard chert beds, but in some sections, Radiolarians, which are precursors to the recrystallized chert, have retained their skeletal structure and are porous. Within the upper Woodford section, gamma ray response generally decreases as the proportion of chert beds increases. However, distinct gamma ray log intervals of decreasing upward, increasing upward, and blocky patterns are recognizable.
Various parameters were measured in about 600 beds to evaluate the fracture patterns at this outcrop. Fractures tend to be contained within chert beds but are generally absent in adjacent
shale beds. Two fracture sets are identified: Set 2 with fold axis about N45oE (regional stress field) and Set 1 about N15oW. Fracture frequency (# fractures per vertical ft. of beds) is directly proportional to the bed frequency. Fracture frequency (density) along a bed is directly proportional to the bed thickness and hard chert beds contain more fractures than soft shale beds for any given bed thickness. These properties can be related to the gamma ray response.
Stop 2: This is a popular stop to view Woodford fracture patterns, but to our knowledge, this outcrop has not been extensively studied until now. It has been studied by our students Richard Brito, Sayantan Ghosh, and Carolina Mayorga. This outcrop is on the south limb of the Arbuckle Mountains. Beds are steeply dipping and are overturned.
The I77 Road cut provides an excellent set of fractured Woodford. Five sets of fractures have been identified here, and their timing of formation has been deciphered. An aerial view, shot from a drone, shows the location of the I77 road cut and the nearby Arbuckle Wilderness Area, which was also studied in detail, both stratigraphically and structurally, but is no longer accessible for study or field trips. By walking west along I77, beneath the I35 Highway, another exposed Woodford section, which is referred to as I35 road cut is present, though it is not the same as Stop 1 and to our knowledge has never been documented. There is no place to park cars along I35, so one hikes over a small ridge on I77 to reach this outcrop, which also has been documented. An overhead Google view shows the two areas: East of fried pies shop is US 77D outcrop which is 1.3 kms to the northwest of the AWO. Beds at both places are overturned. In the AWO, the average strike and dip of exposed formations is approximately 130/60 (Right hand rule) in areas devoid of minor folds and 113/72 (Right hand rule) in the US-77D Outcrop.
Along both the I77 and I35 road cuts, the contact between the Hunton Group and overlying Woodford is clearly exposed. Sitting directly on the Hunton surface is a tight, conglomeratic sandstone, which is overlain by a dark transgressive shale, then lighter colored, more siliceous shale. Whether these beds are lower, middle, or upper Woodford has not been ascertained. A outcrop gamma ray log was run along the I35 side of outcrop. TOC values in addition to the measured stratigraphic section. Several different rock types are present, and TOC values range from 0.6wt. % to 16.9wt. %. An outcrop gamma ray log was also run on the I77 outcrop, which is correlative with the I35 gamma ray log. Rocks collected from the I35 side of the outcrop were measured for Hardness. The tendency is for rocks of high hardness to correlate with rocks of low outcrop gamma ray response. For any given bed thickness, brittle (BRI), cherty strata have higher frequency of occurrence of fractures than do softer, ductile (DUC), clay-shale strata. Fracture aperture measurements were taken in Set 2 fractures at AWO location. Aperture is generally greater when fracture frequency (fractures per m) decreases.
Five (5) fracture sets were documented along the I77 outcrop face. Fractures were also measured from Arbuckle Wilderness Area (AWO) and compared with the I77 outcrop. The same fracture sets are found in both areas. Based on the restored view of the horizontal bedding, cross-cutting and length, presence and absence/type of cement and knowing the event timings the fracture formation timing can be constrained. The Set 1a fractures, which are the longest and highly bitumen filled are the earliest (Pre chesterian-Morrowan Orogeny. The set 1b fractures formed either around the same time or a bit later during early Chesterian/Morrowan orogeny before there was too much erosion and lower pressure that could sustain bitumen generation. Set 1c stylolites developed around the early to mid-stage of the orogeny. This also compacted some of the Set 1b fractures. Set 3 and subsequently developed between late Chesterian/Morrowan to the Mid Virgilian Arbuckle orogeny. Set 2 was most likely during Mid Virgilian (parallel to folds).
Stop 3: Across from the McAlester Cemetery Quarry is also a popular place to view a complete Woodford stratigraphic section, though rubble covers much of the ground surface. A Google image shows the distribution of the different contacts of the lower Woodford with the Hunton, between the lower and middle Woodford, between the middle and upper Woodford, and the Upper Woodford and overlying Sycamore Limestone.
Fractures are prevalent on bedding planes. Many of the fractures are filled with bitumen, which highlights the fracture patterns. Fracture aperture was also measured at the Quarry. Most of the aperture trends are lognormal or exponential, which are both characteristic sizes. These non-power law agrees with no repeated opening. This also indicates a barrier in fracture growth. The stratigraphy of the quarry was measured, described, and a full outcrop gamma ray log was obtained. Various lithologies were identified which comprise the almost 400 ft. of stratigraphy at this location. Hardness measurements were also obtained from samples at closely spaced intervals. Hardest rocks are chert (except where the radiolarians have retained their skeletal structure) and the softest Rocks are massive siliceous shale and laminated siliceous shale.
Chemostratigraphic (elemental composition of rocks) analysis reveals considerable detail in elemental composition. For examples, spikes of high Sr and Ca indicate limestone, and dolomite when Mg spikes occur at depths along with Ca and Sr. Low Si and Si/Al and high K, Al, Ti, and Zr indicate clay minerals, quartz and/or feldspars (detrital minerals). High Si/Al, and low K, Al, Ti, and Zr indicate biogenic quartz. High P indicates zones with P/Ca concretions. High Mo and V are indicative of reducing or anoxic oceanic conditions.
Target audience: Anyone who likes to walk, petrophysicists, petroleum engineers and geologists, historians and environmentalists.
Attire: Casual clothes and walking or hiking shoes
What to bring: Sun screen, insect repellant and a hat/cap.
Exertion: Transportation by passenger van. Walking on uneven surface.