Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, Northern Paradox Basin, Utah GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 4 2017 © 2017 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. SOURCE WITHIN THE SEAL—DISTRIBUTION AND IMPLICATIONS OF ORGANIC SHALE-BEARING STRINGERS WITHIN THE ONION CREEK DIAPIR, NORTHERN PARADOX BASIN, UTAH Samuel M. Hudson, Trevor Tuttle, and Matthew Wood GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Extensive folding of fractured carbonate and deformed shale beds in the Onion Creek diapir in the northern Paradox Basin near Moab, Utah. i 2018 President Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2018 President-Elect Peter Nielsen peternielsen@utah.gov 801.537.3359 2018 Program Chair Emily McDermott ekeller@utah.gov 801.537.3389 2018 Treasurer Zach Anderson zanderson@utah.gov 801.538.4779 2018 Secretary Christopher Kravits ckravitsgeo@gmail.com 2018 Past President Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Board UGA Committees Education/Scholarship Loren Morton lmorton@utah.gov 801.536.4262 Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Terry Massoth twmassoth@hotmail.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Membership Rick Ford rford@weber.edu 801.626.6942 Public Education Paul Jewell pwjewell@mines.utah.edu 801.581.6636 Matt Affolter gfl247@yahoo.com Publications Roger Bon rogerbon@xmission.com 801.942.0533 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017-2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committee UGA Representative Jason Blake blake-j@comcast.net 435.658.3423 UGA Newsletter Newsletter Editor Bob Biek bobbiek@utah.gov 801.537.3356 UGA Website www.utahgeology.org Webmasters Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Lance Weaver lanceweaver@utah.gov 801.403.1636 Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. Volume 4 2017 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. Earthquake Safety Committee Chair Grant Willis gwillis@utah.gov 801.537.3355 Douglas A. Sprinkel Utah Geological Survey 801.391.1977 GIW@utahgeology.org Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 4 2017 215 ABSTRACT The Onion Creek diapir is one of the best exposures of a dissected salt diapir in the world, offering a unique opportunity to better understand the internal character of heterolithic diapirs that are common in sedimentary basins worldwide. Large amounts of interbedded shale, carbonate, and evaporites are in- corporated into the diapir as stringers or boudins, and excellent three-dimensional exposure allows us to document the nature, size, deformation, and distribution of these stringers. Blocks range in size from single, disaggregated layers of dolomite to several meters of coherent stringers that contain multiple cycles of do- lomite-shale-evaporite and are upwards of 20 m thick and more than 100 m in observed length. The largest blocks are most commonly located along the margins of the exposed diapir, though stringers are common throughout the exposed caprock. In areas devoid of large stringers, there is more extensive deformation of the gypsum caprock, suggesting that the presence of stringers leads to a more heterolithic distribution of stress within the salt as it diapirically rises. These observations can help to better characterize similar diapirs elsewhere that are not well exposed at the surface. Black shale is present in all observed large stringers of the Onion Creek diapir. These shale beds are interpreted to have been deposited in a shallow, restricted marginal marine environment along with the in- terbedded carbonate and evaporite strata. Pyrolysis analysis of 13 samples from within the stringers shows a range of 2.56 to 60.22% total organic carbon (TOC), with an average value of 16.93%. These strata contain Type I/Type II hydrocarbon source facies, consistent with a restricted shallow marine environment. Tmax data suggest that these source rock facies have been exposed to sufficient thermal energy to generate hydro- carbons (average = 437o C), as evidenced by common hydrocarbon staining of intra-stringer carbonate strata and evaporite beds surrounding the stringers. Twelve additional samples were collected from these stained strata and pyrolysis analysis shows that all are enriched in free oil, as shown by elevated S1 peaks, high pro- duction index ratios, and TOC values of 0.64 to 1.66%. This hydrocarbon staining is found around stringers near the center of the exposed caprock, as well as stringers along the margins. Near the margins in particular, extensive alteration can be seen across tens of meters of evaporitic strata, showing that hydrocarbons are effectively generating within and migrating away from stringers fully encased in the anhydrite caprock of the Onion Creek diapir. This has important implications for potential seal integrity of diapiric caprocks, as well as providing a potential mechanism for caprock carbonate formation suggested by other researchers. Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, Northern Paradox Basin, Utah Samuel M. Hudson1, Trevor Tuttle1, and Matthew Wood2 1Department of Geological Sciences, Brigham Young University, Provo, UT 84602, USA; sam.hudson@byu.edu, trevortuttle15@icloud.com 2School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, USA; woodmatt52@gmail.com Citation for this article. Hudson, S.M., Tuttle, T., and Wood, M., 2017, Source within the seal—distribution and implications of organic shale-bearing stringers within the Onion Creek diapir, northern Paradox Basin, Utah: Geology of the Intermountain West, v. 4, p. 215–229. © 2017 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. www.utahgeology.org 216 Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 INTRODUCTION Salt diapirism is an important process in many of the world’s sedimentary basins, leading to complex syntec- tonic sedimentation patterns in surrounding salt with- drawal mini-basins. Whereas much work has focused on the effects of salt on surrounding strata, less has been done to better understand the internal dynamics of di- apiric bodies themselves. Important exceptions to this are dominantly from studies of the Zechstein salts of the Netherlands and diapirs in the Gulf of Mexico, both of which commonly have large stringers, rafted blocks, or layered evaporites (Williamson and others, 1997; Cal- lot and others, 2006; Van Gent and others, 2011; Fiduk and Rowan, 2012; Li and others, 2012; Strozyk and oth- ers, 2012; Reiche and others, 2014; Strozyk and others, 2014). The Onion Creek diapir, located near Moab, Utah (figure 1), offers a rare opportunity to study the detailed internal dynamics of a complex salt diapir in surface exposures. It is part of a series of northwest-southeast trending diapiric salt walls originating in the Pennsyl- vanian Paradox Formation within the Paradox Basin. The Onion Creek diapir is part of the larger Fisher Val- ley salt wall, and is the only part exposed at the surface. The Fisher Valley salt wall is the most proximal salt wall to the Uncompahgre uplift to the northeast (figure 1, Trudgill and Arbuckle, 2009), and diapirism was initi- ated by the deposition of the overlying Permian Cutler PARADOX BASIN UNCOMPAHGRE UPLIFT Paradox Salt Walls La Sal Intrusives UT CO AZ NM Salt Structures a CC - Cane Creek CV - Castle Valley DA - Dolores FV - Fisher Valley GV - Gypsum Valley HC - Hamilton Creek LV - Lisbon Valley MV - Moab Valley PV - Paradox Valley SR - Sinbad SV - Salt Valley MoabCC SV CV DA FV GV HCLV MV PV SR Onion Creek Diapir N 40 km200 UT CO NMAZ Fi gu re 2 Figure 1. Regional map showing trends of major salt structures associ- ated with the Paradox Basin and Un- compahgre uplift, Utah and Colora- do. The Onion Creek diapir (marked with a star) is associated with the Fisher Valley salt wall, located on the northern margin of the Paradox Ba- sin. The location of figure 2 is shown by the black line running from Fisher Towers in the northeast to the Nee- dles District of Canyonlands National Park in the southwest. Modified from Rasmussen and Rasmussen (2009). 217 Source Within the Seal—Distribution and Implications of Organic Shale-bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 Formation, which was shed off from the nearby uplift (Ohlen and McIntyre, 1965; Doelling, 1988; Barbeau, 2003; Trudgill and others, 2004; Kluth and DuChene, 2009; Trudgill, 2011; Venus and others, 2015). The di- apir exposes approximately 5 km2 of highly deformed Paradox Formation (McCalla, 2008). The Paradox Formation of the Hermosa Group was deposited in the Paradox Basin; a broad, shallow ma- rine basin nearly surrounded by highlands (Ohlen and McIntyre, 1965). Open marine conditions were com- mon, as evidenced by preserved limestone and shale beds (Hite and Buckner, 1981; Handschy and Dyer, 1987; Blakey and Knepp, 1989; Dickinson and Lawton, 2003). However, frequent eustatic sea level changes resulted in complex and cyclic depositional patterns (Hite and Buckner, 1981). This, combined with the semi-arid to arid climate (Guthrie and Bohacs, 2009), led to frequent basin restriction and deposition of appreciable amounts of evaporitic strata (Baker and others, 1933; Rasmussen and Rasmussen, 2009). Originally, there were 29 evap- orite-carbonate cycles of interbedded shale, carbonate, and evaporites (halite, gypsum, anhydrite, and potash) identified within the Paradox Formation (figure 2) (Hite 1960; Hite and Buckner, 1981; Trudgill and Arbuckle, 2009); however, as many as 80 stratigraphic sequences (cycles) are now identified (Rasmussen and Rasmussen, 2009). The exposed Paradox Formation at Onion Creek is dominated by gypsum with minor amounts of anhy- drite, but interbedded shales and carbonates of the Para- dox are common within the caprock as well. These more competent layers of strata within the diapir (shale and carbonate) are from within the Paradox Formation and thus are identified as stringers. They are differentiated from rafted blocks, which are from older or younger formations that have been entrained into the salt during diapirism (Talbot and Jackson, 1987; Reuning and oth- ers, 2009; Fiduk and others, 2014). Stringers within the Onion Creek diapir formed by the rupture and displace- ment of carbonate- and shale-rich beds as the Paradox Formation was diapirically deformed by sub-regional sediment loading. These stringers are preserved as bou- dins within the caprock ranging in size and expression from large, competent blocks of strata greater than 100 m in observed length (and possibly longer – exposure is two-dimensional and may be oblique to the longest axis of these bodies), to concentrated layers of cobble-sized clasts of carbonate. Stringers have been observed else- where in both outcrop (Jackson and others, 1990) and the subsurface (Talbot and Jackson, 1987; Warren, 2006; Van Gent and others, 2011; Strozyk and others, 2012), and are a common phenomenon in salt bodies world- wide (Fiduk and Rowan, 2012; Reiche and others, 2014). The excellent exposure of the Onion Creek diapir en- ables detailed description of the character, deformation, and distribution of these stringers within the exposed caprock. Black shale beds of the Paradox Formation are en- riched in preserved organic carbon, with previous stud- ies reporting total organic carbon (TOC) values as high as 20% from nearby wells (Tischler, 1995; Nuccio and Condon, 1996; Van Buchem and others, 2000; Guthrie and others, 2004; Rasmussen and Rasmussen, 2009). Association of evaporites and black shale is common in both shelfal and intracratonic settings such as the Para- dox Basin, the Williston Basin (North Dakota), the Ea- gle-Gypsum basin (Colorado), and the Cretaceous stra- ta along the Gulf Coast (Krumbein, 1951; Sloss, 1953; Zharkov, 1983). The organic-rich shales of the Paradox Formation are incorporated into the stringers of the Onion Creek diapir, and hydrocarbon staining due to the migration of free oil is common within both the stringers and the evaporitic strata surrounding them. This observation has potentially important implications for seal integrity of caprock facies, as well as local impli- cations for hydrocarbon generation and migration on a limited scale, and perhaps on the formation of caprock dolomite. METHODS Five transects were described throughout the diapir, and thirteen sections were measured along these tran- sects where stringers were identified (figure 3A). Ob- servations made along these transects include presence or absence of stringers, stringer size, lithologic compo- sition, relative levels of deformation, and their location relative to the edge of the diapir. The location of each observation was recorded using a Trimble differential GPS unit, with sub-meter accuracy for all gathered 218 Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 points. A basic geospatial model of surface observations was built using Petrel software. Two Paradox Formation carbonate samples were collected from within the larg- est two stringers and analyzed for mineralogical content using the Bruker D2 Phasor x-ray diffraction (XRD) instrument. Samples were run for 25 minutes using a Lynxeye Silicon strip detector from 5 to 65° 2-theta at a step increment of 0.02°. A copper-sealed tube was used, and samples were front-loaded for analysis. Quantifica- tion was done by Rietveld analysis using the DIFFRAC. TOPAS software package. Thirteen black shale samples were collected from within the stringers of the Onion Creek diapir, and 12 samples were taken from hypothesized hydrocar- bon-stained strata from both the stringers and the gyp- sum caprock surrounding these stringers. In order to obtain total organic and carbonate carbon, as well as source type and relative maturity, these samples were analyzed by whole-rock pyrolysis and total organic carbon/carbonate carbon (TOC+CC) techniques at Brigham Young University using the Wildcat Technol- ogies HAWK Workstation. A standard pyrolysis+TOC method was used, where the sample was heated from 300⁰ C to 850⁰ C at a rate of 25⁰ C/minute during the pyrolysis stage, and from 300⁰ C to 850⁰ C at a rate of 25 ⁰C/minute during the oxidation stage (Espitalie and WHITE RIM SANDSTONE U N IN D IF FE RE N TI AT ED C U TL ER G RO U P PARADOX FORMATIONHERMOSA GROUP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOENKOPI FORMATION _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ SHAFER LIMESTONE Honaker Trail Formation Ismay Zone Desert Creek Zone Akah Zone Barker Creek Zone Alkali Gulch Zone HONAKER TRAIL FORMATION HONAKER TRAIL FORMATION HONAKER TRAIL FORMATION + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Paradox Salt HONAKER TRAIL FORMATION ORGAN ROCK FORMATION CEDAR MESA SANDSTONE LOWER CUTLER BEDS 289.9 Ma 308 Ma 270-280 Ma PE RM IA N PE N N SY LV A N IA N TR IA SS IC Pa ra do x F m He rm os a G ro up Fisher TowersNeedles District NortheastSouthwest Figure 2. Composite strati- graphic column of Pennsylva- nian and Permian strata of the study area, near Moab, Utah. See figure 1 for column loca- tion. Modified from Hintze and Kowallis (2009) and Venus and others (2015). 219 Source Within the Seal—Distribution and Implications of Organic Shale-bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 others, 1977; Peters and Cassa, 1994). OBSERVATIONS Stringers were identified throughout the Onion Creek diapir from the margins to the center and high in the exposed caprock to the lowest observable point. The stringers observed in the diapir range from concen- trated exposures of disarticulated cobble-sized clasts to relatively undeformed interbedded stringers with con- tinuous bedding that are as much as 100 m long and greater than 20 m thick. Six large, intact stringers were identified (figure 3B), with many smaller stringers of interbedded carbonate and black shale throughout the exposed caprock. Stringers with intact bedding planes average between 2 to 3 m thick and 2 to 10 m long par- allel to bedding. Some of the stringers are highly folded (figure 4A) whereas others are relatively undeformed, with well-preserved parallel bedding planes (figure 4B). Throughout the diapir and across the spectrum of stringer sizes, there is common preservation of cyclic packages of shale, carbonate, and gypsum. Carbonate strata are the dominant facies represented in the string- ers. The carbonate observed in outcrop is not limestone, as has been reported at depth (Rasmussen and Rasmus- sen, 2009), but is dolomitic limestone and dolomite (fig- Salt Carbonate Shale 1 2 3 4 5 6 Transects Edge of salt A B Figure 3. Aerial view of the Onion Creek diapir. (A) Black lines show transects of the Onion Creek diapir that are a part of this study, and the approximate edge of salt is marked (pink dashed line) where it is in contact with younger strata of the domi- nantly the Permian Cutler Formation. (B) Pie charts represent locations of measured sections along transects, with the relative abundance of evaporite, carbonate, and shale shown. The six large stringers dis- cussed later are marked 1 through 6, with stringer 6 being more interior to the diapir whereas 1 through 5 are at or near the mar- gins of the diapir. 220 Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 ure 5A). XRD analysis of two carbonate samples from different stringers show high dolomite content of 63% and 98%, respectively (figure 5B). The sample contain- ing lower dolomite content (63%) also contains clastic material, with 26% orthoclase, 8% muscovite, and 3% quartz. These carbonates are often fractured, but prima- ry bedding is discernable and sedimentary structures can be observed. In multiple stringers, microbial tex- tures are observed, as shown in figure 5C. Thirteen large stringers (greater than 10 m in length) were measured as part of this study (figure 3B). Of these, six are larger than 50 m in length (figure 3B). Five of the six largest stringers are located along the margins of the diapir, either at the edge of salt or very close to the contact. The only other stringer larger than 50 m in length is located in the southwestern interior of the diapir (figure 3B, section 6). Stringers at or near the edge of the diapir show coherent bedding, and beds are steeply dipping and roughly aligned with the edge of the diapir, though internal folding is often prevalent in these stringers. The very large stringer in the interior of the diapir, in contrast, shows less deformation of beds and is near-horizontal in orientation. Composition var- ies within the largest six stringers, with four being dom- inated by carbonate strata and two being dominated by black shale. In general, shale is better preserved in the largest stringers with smaller stringers being dominated by dolomite. Though stringers of various sizes are common throughout the diapir, there are large sections of the BA Gypsum Deformed Stringer Vertical Stringer Figure 4. Examples of stringers within the Onion Creek diapir showing: (A) extensive folding of fractured carbonates and deformed shales, and (B) a large carbonate-dominated stringer near the southwest margin of the diapir. GPS antenna shown in the images is 2 m in height. 221 Source Within the Seal—Distribution and Implications of Organic Shale-bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 exposed caprock that are devoid of significant string- er material. In these areas, it is common to find highly banded and deformed gypsum (figure 6). These sec- tions of gypsum can form resistant outcrops near the margins and within the interior of the exposed caprock. Proximal to the stringers, in contrast, caprock gypsum is commonly more massive to weakly bedded. It is common to find hydrocarbon staining associat- ed with the preserved Paradox Formation black shales in the Onion Creek diapir. Staining is typically rusty brown to yellow to black and is found both within the stringers and surrounding gypsum caprock. In all large stringers observed, staining and the associated live oil is focused along fractures within dolomite beds, as well as within the surrounding deformed gypsum caprock (fig- ure 7). In smaller stringers, staining of dolomite is com- mon as well, though to a lesser extent. In contrast to the large stringers, staining in small stringers is commonly restricted to fractured dolomite, and does not stain the surrounding gypsum. Whole-rock pyrolysis analysis was performed on the 12 samples collected from black shale within the large stringers of the Onion Creek diapir. Pyrolysis was also performed on the 13 samples collected from stained car- bonates within the stringers and stained caprock gypsum proximal to the stringers. All samples showed apprecia- ble TOC values, ranging from 0.64% to 60.22% (table 1). Samples collected from Paradox black shale within in the large stringers had consistently higher TOC values, rang- ing from 2.56% to 60.22%, with a mean of 16.93%. Samples collected from the stained beds are easily distinguished based on much lower TOC values, with a range of 0.64% to 1.71% and a mean of 1.06%. Additionally, the two sam- ple sets can be differentiated by the proportionately higher S1 (free oil) signal in the stained beds (table 1). Though the level of preserved organic carbon is lower within the samples of migrated material, the ratio of S1/(S1+ S2), oth- erwise known as the production index (PI), is much higher for these samples (figure 8). DISCUSSION Stringer Character and Distribution We interpret the observed stringers within the On- ion Creek diapir to be intact blocks of Paradox Forma- A C B Figure 5. Carbonates within the Onion Creek diapir. (A) Intensely folded bedding of Paradox Formation dolomite. Although diagenetically altered, microbial textures are still preserved. (B) XRD analysis of Paradox Formation carbon- ate samples within the stringers show dolomitic composi- tion, whereas the Paradox carbonates are limestone at depth nearby, suggesting diagenetic alteration (Rasmussen and Rasmussen, 2009). (C) A thin section from within one of the large stringers shows preserved microbial texture of the do- lomite. 222 Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 tion strata that have moved as coherent bodies into the diapir. The interbedded nature of strata is interpreted to be the original depositional bedding of the Para- dox. Cyclic packages of carbonate, shale, and gypsum are best observed in fresh washes cut into the side of the exposed caprock. In more weathered outcrops, car- bonate facies are most easily observed, whereas the in- terbedded shale and gypsum can be less obvious due to their less resistant nature. Although the carbonate strata observed within the exposed diapir are domi- nantly dolomite rather than the limestone observed at depth (Rasmussen and Rasmussen, 2009), the inter- bedded nature and preserved sedimentary structures within these carbonate rocks (figure 5) leads us to con- clude that they are altered original Paradox strata. The cause of this alteration to dolomite within the stringers is likely infusuion of meteoric waters due to uplift and exposure of the Onion Creek caprock. These interbed- ded stringer dolomites are different than the brecciat- ed, structureless lateral caprock dolomite exposed in- frequently around the margins of the diapir (figure 9). Lateral caprock dolomite forms at depth in response to hydrocarbon presence and the associated sulfate-reduc- ing bacteria (Giles and others, 2012; Jackson and Lewis, 2012), and has been described along the flanks of the nearby Castle Valley salt wall (figure 1) by Giles and others (2012). A 15-m-long, 1-m-thick lateral caprock dolomite is present along the northern margin of the Onion Creek diapir in direct contact with both the ad- jacent Permian Cutler strata and the diapiric salt. The absence of primary sedimentary structures or bedding, the high degree of brecciation, and the position along the edge of the diapir are all consistent with what is seen at the Castle Valley salt wall to the south, leading to clear differentiation between primary stringer dolomite and lateral caprock dolomite at the Onion Creek diapir. Whereas large stringers were identified throughout the diapir, the largest and most continuous stringers are generally located near the margins. Some of these stringers are as much as tens of meters of cohesive, rel- atively undeformed strata, and can be least 100 m long. The concentration of larger stringers along the margins of the diapir suggest that this part of the diapir may have experienced less extensive deformation as compared to the more highly deformed stringers observed in the in- terior parts of the diapir. Additionally, the highly de- formed, interior stringers appear to be aligned roughly with the edge of salt in many places. Though many of the largest stringers are located at or near the margins of the diapir, one of the larger stringers observed was found approximately 500 m from the edge of the diapir along the southern transect (figure 3B, section 6). This stringer is a large (~95 m in observed length), laterally continuous stringer containing several cyclic packages. This exception points to the fact that stringers of signif- icant size can be present within the interior of a diapir as well. Future work at other diapirs is important to val- idate these trends. As previously mentioned, all stringers exhibit de- formation; however, there is a wide range in the degree Figure 6. Large exposure of highly banded and deformed gypsum. This locality is far from any sizable stringers and is typical of the high level of deformation seen in areas devoid of stringers. GPS pole in image is 2 m tall. 223 Source Within the Seal—Distribution and Implications of Organic Shale-bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 of deformation. The deformation correlates well with stringer size. The smallest identified stringers are dis- crete clusters of aligned, fist-sized fragments of dolo- mite. These are common and present throughout the diapir. In contrast, only modest amounts of brittle de- formation are seen in the core of larger stringers, with deformation becoming more evident towards the edg- es. There is a spectrum of intermediate deformation between these two end members, and smaller coherent stringers transitioning laterally into shattered string- ers are common in the more central parts of the diapir. Regardless of stringer size, fracturing and destruction of bedding increases towards the edges of all observed stringers. This is consistent with the interpretation that these are boudins of strata that have been stretched and deformed as entrained blocks during diapiric deforma- tion of Paradox strata (Jackson and others, 1990; War- ren, 2006; Strozyk and others, 2012). Lastly, although stringers were identified throughout the diapir, there are large areas within the diapir where stringers are absent. These areas are dominated by band- ed, highly deformed gypsum that is very different than the often massive, structureless gypsum seen in proxim- ity to the stringers (figure 6). The increased deformation in gypsum-dominated sections suggests that there is heterogeneity of strain within the diapir, driven by the heterogeneity of the lithologies moving upwards during diapirism. This may indicate that the stringers provide added rigidity to the diapir as it rises and may even, to a degree, reduce internal deformation during diapiric rise. Further work, both observational and experimental, can be done to test this hypothesis, which may add to our understanding of the more detailed processes that occur within deforming salt bodies. A B Figure 7. (A) Black to yellow staining associated with migrated hydrocarbons within fractured carbonates of a large stringer near the margin of the Onion Creek diapir (stringer 3), and (B) black to rusty staining resulting from migrated hydrocarbons within the evaporites surrounding a boudin block of dolomite near stringer 2. See figure 3B for stringer locations. 224 Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 Hydrocarbon Generation and Migration within the Diapir and Implications The black shale beds of the Paradox Formation are extremely enriched in organic carbon, as evidenced by high TOC values from samples within the stringers that average nearly 17%. High hydrogen index (HI) values are consistent with the restricted marine depositional environment invoked for the Paradox Formation, with marine microbial preserved organic types (Type I and Type II kerogens) seen in the sample suite (figure 8). Tmax data, while not as accurate as vitrinite reflectance or other thermal indicators, suggest that samples taken from the diapir are within the early oil window, with most values between 435 and 460 o C (Tissot and oth- ers, 1987). All samples collected from black shale beds within the stringers are interpreted to be from the Akah or Barker Creek zones of the Paradox Formation (Ras- mussen and Rasmussen, 2009), emplaced as part of the Onion Creek diapir during initial diapiric rise. This is consistent with data from subsurface wells and outcrop from within the Paradox Formation. TOC values of Type II marine source rock facies that are consistent- ly above 1%, and occasionally as much as 20%, have been reported at multiple levels and multiple locations at depth within the basin (Tischler, 1995; Nuccio and Condon, 1996; Van Buchem and others, 2000; Guthrie and Bohacs, 2009; Rasmussen and Rasmussen, 2009). Based on the similarity of organofacies, the characteris- tics of these strata have not significantly changed as the result of stringer emplacement; the black shale within the stringers of the Onion Creek diapir are classified as excellent marine source rock facies capable of generat- ing hydrocarbons in the past. Within the fractures of all large and many smaller stringers, hydrocarbon staining is interpreted to be ev- Sample ID TOC-Total Organic Carbon (Weight %) S1 Free Oil (mgHC/g rock) S2 Kerogen Yield (mgHC/g rock) S3 (mgCO2/g) Tmax-Maturity (°C) CC-Carbonate Carbon (Weight %) GOC-Generative Organic Carbon (Weight %) NGOC-Non- generative Organic Carbon (Weight %) PI-Production Index (S1/S1+S2) HI-Hydrogen Index (mgHC/gTOC) OI-Oxygen Index (mgCO2/gTOC) Primary Shale Samples T 4.13 20.26 7.78 125.83 0.45 439 0.63 11.38 8.88 0.06 621 2 T4 S13 C 20.23 6.36 94.90 2.37 435 0.70 8.75 11.48 0.06 469 11 T4 S13 B 19.44 6.92 103.85 1.48 435 0.31 9.48 9.95 0.06 534 7 T4 S13 A 20.32 7.67 122.10 0.71 440 0.19 11.07 9.25 0.06 600 3 T 4.12 10.92 2.81 50.70 0.77 433 0.15 4.58 6.34 0.05 464 7 T 4.7 60.22 17.99 356.31 4.56 437 0.37 32.00 28.22 0.05 591 7 T4.11 2.56 0.33 10.47 0.28 435 12.04 0.93 1.63 0.03 409 11 T 1.2 16.99 2.75 107.39 0.79 438 0.22 9.41 7.59 0.03 632 4 T 4.3 7.25 0.73 45.25 0.25 431 5.64 3.92 3.33 0.02 624 3 T 4.14 18.01 2.80 127.40 0.73 439 0.45 11.09 6.92 0.02 707 4 T 1.1 8.00 0.31 28.09 1.25 439 0.33 2.46 5.53 0.01 351 15 T 4.5 6.14 0.25 20.51 0.61 437 1.80 1.79 4.36 0.01 333 9 T 4.6 9.80 0.38 55.49 0.37 437 2.83 4.77 5.03 0.00 566 3 Secondary Hydrocarbon Stained Samples 03 SC 15 1.23 0.12 0.40 0.17 374 17.85 0.05 1.17 0.24 32 14 T 1.7 0.64 0.17 0.55 0.20 446 0.20 0.07 0.57 0.23 85 31 T 1.9 1.71 0.10 0.37 0.30 428 2.45 0.05 1.66 0.21 21 17 T 2.1 0.66 0.08 0.34 0.19 439 0.23 0.04 0.62 0.19 51 28 T 1.8 1.12 0.10 0.43 0.13 426 0.36 0.06 1.06 0.19 38 11 T 4.2 1.66 0.09 0.42 0.37 449 0.67 0.06 1.60 0.17 25 22 T 4.1 1.15 0.07 0.35 0.29 442 7.35 0.05 1.10 0.17 30 25 T 1.5 0.65 0.14 0.77 0.25 441 0.26 0.09 0.56 0.16 118 38 T 1.6 0.65 0.11 0.60 0.24 441 0.44 0.07 0.57 0.15 93 37 T 1.10 0.72 0.08 0.50 0.26 453 18.30 0.06 0.66 0.13 70 36 T 1.3 1.59 0.17 1.58 0.30 438 3.47 0.16 1.43 0.10 99 18 T 1.12 0.94 0.08 1.21 0.33 460 0.75 0.13 0.82 0.06 128 35 Table 1. Whole-rock pyrolysis data for 25 samples collected from the Onion Creek diapir. Thirteen samples were taken from shales within the stringers (primary shale samples), whereas 12 samples were taken from altered carbonates and evaporites (secondary hydrocarbon-stained samples). 225 Source Within the Seal—Distribution and Implications of Organic Shale-bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 idence of migrated hydrocarbons. This staining is also prevalent in the gypsum caprock surrounding many stringers, including all large stringers. Staining is black to rusty colored to yellow colored along the fractures of dolomite stringers and within the groundmass of the gypsum caprock (figure 7). Hydrocarbon staining of strata within the Onion Creek diapir is similar in ap- pearance to other instances of hydrocarbon migration associated with salt diapirs, such as documented within and proximal to the La Popa diapir in Mexico (Hud- son and Hanson, 2010). TOC values from these stained samples are much lower than values reported from pri- mary black shale within the stringers (table 1), but val- ues are above 0.5%, which is still considered significant source rock potential (Peters and Cassa, 1994), confirm- ing that there are hydrocarbons present. Production in- dex (PI) values (ratio of free oil to total hydrocarbons) in the stained samples are elevated in comparison with the primary black shales. This can be the result of two things: (1) higher relative thermal maturation (the most common use of this indicator), or (2) the presence of migrated hydrocarbons (Tissot and Welte, 1984; Peters and Cassa, 1994). Both interpretations are based on the assumption that primary kerogens have cracked and or- ganic carbon is increasingly being stored in pore space. Tmax results from both the primary shale samples and the stained samples suggest that all samples have expe- rienced a similar thermal history and are within the ear- 0 100 200 300 400 500 600 700 800 900 1000 0 50 100 150 200 H YD R O G EN I N D EX (H I, m g H C /g T O C ) OXYGEN INDEX (OI, mg CO2/g TOC) Type I Oil Prone Type II (usu. marine) Oil Prone M ixed Type II / III Oil / Gas Prone Type III Gas Prone Type IV Gas Prone Figure 8. Modified Van Krevelen diagram (van Krevelen, 1950), showing a clear division between primary black shale samples collected from within the stringers of the Onion Creek diapir (green squares) and samples collected from the fractured dolomite stringers and the surrounding evaporite caprock (red diamonds). Samples from stringers of Paradox black shale are classified as Type I/Type II source rock facies, whereas migrated hydrocarbon samples are classified as a Type III/Type IV source rock facies. The low hydrogen index (HI) values for the migrated hydrocarbon samples are not indicative of a terrestrial-rich organic source, as might be the case for a primary source rock, but rather the lean HI is the result of thermal cracking of kerogens, as shown by the high production index (PI) ratio (table 1). Figure 9. One of several examples of lateral cap rock dolo- mite observed on the northern side of the Onion Creek dia- pir. It is characterized by a highly brecciated texture that is in contrast with dolomitic textures that are common within the stringers, such as shown in figure 5. 226 Source Within the Seal—Distribution and Implications of Organic Shale-Bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 ly oil window. Given the observed field relationships of these samples and the primary Paradox shale they are associated with, we believe that local migration of hy- drocarbons from within the black shale beds of large stringers is occurring within the Onion Creek diapir. This is further supported by depletion of HI within sec- ondary stained samples in comparison to primary black shale samples (figure 8). Evidence that hydrocarbons can be generated lo- cally in quantities sufficient for expulsion and migra- tion has several important implications for exploration around salt bodies. Around other similar salt diapirs, which incorporate organic-rich strata as stringers or rafted blocks, there is a possibility for false indicators of significant hydrocarbon accumulations. This could be as simple as hydrocarbon shows within a juxtaposed reservoir; while these stringers are generating enough free oil for migration to occur, there is insufficient ma- terial within even the largest stringers to form an eco- nomic accumulation. Less obviously, we propose that local generation of hydrocarbons from stringers within the caprock of a diapir could lead to local precipitation of caprock dolomite. Work by Giles and others (2012) suggests that the presence of anaerobic sulfate-reducing bacteria, commonly associated with hydrocarbons, is necessary for the precipitation of a carbonate caprock, thus implying that the presence carbonates is an indica- tor of a working petroleum system. If local generation and migration of hydrocarbons occurred in association with stringers along the margin of a diapir, it is possible that this could lead to the precipitation of a carbonate caprock in the absence of a large-scale working petro- leum system. Thus, the presence of a carbonate caprock alone could lead to a false characterization of the basin source rock potential or the migration pathways of hy- drocarbons within a productive basin if the carbonate caprock formed from local generation and migration rather than regional thermal maturation of a source rock interval. Lastly, the observation that hydrocarbons are free- ly migrating through gypsum caprock, similar to what was documented at the La Popa diapir, Mexico, (Hud- son and Hanson, 2010), has important implications for the sealing capacity of diapiric caprock, regardless of the inclusion of stringers. Salt is commonly considered to be a near-perfect seal, but evidence is increasingly showing that within the altered caprock of a diapir, the more brittle nature of the salt can lead to banding, frac- turing, and potential migration pathways, as seen in the Onion Creek diapir. We interpret hydrocarbon migra- tion within the Onion Creek caprock to happen only at a local scale, but these observations show that vertical migration within diapiric caprock should be considered as a risk in other basins where large quantities of hydro- carbon are being generated at depth, and salt is often called upon to be a lateral and/or vertical seal. CONCLUSIONS Cyclic packages of carbonate, shale, and salt are commonly found throughout the Onion Creek diapir. These packages preserve, even after deformation and emplacement into a salt diapir, the original interbedded nature of the Paradox Formation. This suggests that al- though diapirism is an intense deformational process, more coherent stratal packages can maintain structural integrity in the form of boudins. Banded, highly-de- formed gypsum in areas without notable stringers may help to explain the relatively undeformed nature of the large stringers, suggesting that strain is differentially partitioned into less competent evaporite-rich layers. These observations suggest that the internal mechan- ics of diapirs are perhaps less chaotic than is often as- sumed, in particular, where interbedded non-evaporitic strata are present. Spatially, there are trends within the Onion Creek diapir that are significant. Larger, less de- formed stringers are more common near the margin of the diapir. This implies that either: (1) there was more interbedded material originally in this part of the stra- ta, which is assumed to be the youngest, stratigraphi- cally highest Paradox strata exposed, or (2) that rigid, interbedded strata was concentrated near the margins during the process of diapiric rise and caprock forma- tion. Both hypotheses have different implications for di- apiric mechanics, with the latter suggesting a more cha- otic diapiric process and the former suggesting a more layered, organized model. The Onion Creek diapir pres- ents an excellent, accessible exposure of a heterolithic diapiric salt body that can serve as an analog for subsur- face heterolithic salt bodies elsewhere. Whereas seismic 227 Source Within the Seal—Distribution and Implications of Organic Shale-bearing Stringers within the Onion Creek Diapir, northern Paradox Basin, Utah Hudson, S.M., Tuttle, T., and Wood, M. Geology of the Intermountain West 2017 Volume 4 data is capable of imaging large stringers in the subsur- face, outcrop observations offer important details at the sub-seismic scale. The frequency of both large and small stringers observed throughout the Onion Creek diapir suggests that stringers should be expected to be present throughout similar salt bodies, even when not detected by geophysical methods. Whereas larger stringers may be more commonly encountered along the margins, significant intervals of non-evaporitic strata are com- mon regardless of relative location within the diapir. In addition to adding to our understanding of the distribution and character of stringers in diapirs, the in- clusion of organic-rich strata in the stringers of the On- ion Creek diapir gives us important insight into hydro- carbon resource evaluation. Diapiric salt is considered a near-perfect, impermeable seal; however, within the caprock strata of the Onion Creek diapir, migration of hydrocarbons is associated with every large and many of the smaller stringers of organic-rich shale studied, as evidenced by the presence of hydrocarbon-stained strata within and surrounding the stringers. This has important implications for similar, “dirty salt” dia- pirs in the Gulf of Mexico and elsewhere. Beyond this, the prevalence of hydrocarbons within the caprock of the Onion Creek diapir suggests that fluid migration through caprock should be a consideration in all diapir- ic systems, whether stringers are present or not. ACKNOWLEDGMENTS The authors would like to acknowledge Kate Giles (University of Texas El Paso), Mark Rowan (Rowan Consulting), Richard Langford (University of Texas El Paso), Thomas Hearon (EOG Resources), and Carl Fi- duk (Fiduk Consulting) for discussion that helped to improve this manuscript. Involvement with the Insti- tute of Tectonic Studies at University of Texas El Paso and the opportunity to discuss this work with both the researchers and sponsors of this consortium was invaluable. Comments and suggestions from Frank Strozyk (Aachen University) on an earlier draft of this work were very much appreciated as well. 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