ED-XRF spectra were generated using Bruker Tracer IV spectrometers. ..... Robinson, K., 2012, The quantification and application of handheld energy-dispersive.
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Defining Linkages Between Chemofacies and Mechanical Stratigraphy in the Austin Chalk: Implications for Geomechanics and Induced Fracture Simulations Harry Rowe1*, Evan Sivil2, Chris Hendrix2, Santhosh Narasimhan1, Andy Benson1, Austin Morrell1, Gerardo Torrez1, Pukar Mainali1 1 Premier Oilfield Laboratories, 2Bureau of Economic Geology, UT Austin Copyright 2017, Unconventional Resources Technology Conference (URTeC) DOI 10.15530/urtec-2017-2668845 This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Austin, Texas, USA, 24-26 July 2017. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited.
Abstract Beyond the traditional use of descriptive sedimentology for assigning lithofacies, the added ingredient of chemostratigraphy provides unique supporting evidence for the assignment of facies (chemofacies) and the development of more robust paleoenvironmental and diagenetic interpretations. Chemostratigraphic investigation of several cores from Late Cretaceous Gulf Coast strata reveals chemofacies relationships in what is largely a twocomponent system (carbonate-clay). Micro-Rebound Hammer-Unconfined Compressive Strength (MRH-UCS) estimates, coupled with the chemofacies interpretation, outline predictable associations between facies and mechanical stratigraphy that yield insight toward understanding stratigraphically-controlled changes in rock strength that can be quantified and used as inputs for geomechanical models and completions simulations. This presentation focuses on 1) highlighting the simple element-mineral linkages, 2) developing an understanding of how mechanical stratigraphy changes as a function of chemofacies and the underlying cyclical drivers of clay-rich and clay-poor facies, and 3) introducing a stratigraphically-constrained model of rock strength behavior for Austin Chalk successions that can be used as partial input for induced fracture simulations. Cored strata from Austin Chalk wells represent a broad range of fine-grained lithologies that were used to develop geochemical and chemostratigraphic records of depositional and diagenetic change. Cores were sampled for major and trace element composition at a 2-inch vertical resolution using a Bruker Tracer XRF spectrometer. Raw x-ray spectra were calibrated using a reference suite that encompasses the range of elemental variability observed in the cores. Output from a hierarchical cluster analysis (HCA) of the elemental results was evaluated in terms of a partition index table (PIT), which was subsequently used to define relative elemental enrichments and assign characteristic chemofacies names. Samples representing the range of chemofacies assignments were analyzed for estimates of rock strength using a micro-rebound hammer, defining a general linkage between chemical and mechanical properties of the Austin Chalk. Introduction Coniacian- to Campanian-aged Austin Chalk Group strata of Texas and equivalent strata deposited along the Late Cretaceous Gulf Coast represent calcareous mudrock (chalk) successions that accumulated on the drowned lower Cretaceous Comanche platform (Figure 1). Investigations covering depositional systems, lithofacies, diagenesis, natural fracturing have been completed over the years (e.g., Stapp, 1977; Dravis, 1980; Haymond, 1991; Hovorka and Nance, 1994; Phelps et al. 2014, 2015), as have investigations into hydrocarbon potential (Scott, 1977; Grabowski, 1981, 1984), but only recently have chemostratigraphic studies been conducted on Austin Chalk and equivalent units (Hendrix, 2016). This investigation focuses on highlighting quantitative characterizations gained through developing a facies-scale (sub-log-scale) X-ray fluorescence (XRF) geochemical data set of the Austin Chalk. Specifically, because the Austin Chalk is mineralogically dominated by one mineral (calcite) that is largely
2 URTeC: 2668845 diluted by one mineral (illite), a simple chemofacies system based on clustering of major element results provides a codified spectrum of mineralogical change that can be compared with lithofacies interpretations and measurements of mechanical stratigraphy. This approach builds a more holistic and quantitative framework for defining and quantifying change in a cyclical chalk succession that can be used to refine geomechanical studies and completions models aimed at optimizing hydrocarbon extraction. Methods Multiple drill cores from South Texas and South Louisiana were studied. Cores were generally scanned at a 2- or 3inch sample interval. Focus is here placed on the integrated data sets and interpretations of Austin Chalk, and not on the details of specific cores; however, the results presented in the extended abstract were generated from a ~270foot-long succession of Austin Chalk equivalent from South Louisiana. Geochemistry: Energy-Dispersive X-ray Fluorescence (ED-XRF) ED-XRF spectra were generated using Bruker Tracer IV spectrometers. Two instruments were used--one for major element analysis plus Ba, V, and Cr, the second for trace element analysis. The major elements were analyzed at 15kV and 35A with a vacuum system. Trace elements were analyzed at 40kV and 11A using a Ti-Al energy filter. Prior to analysis the slabbed core face was scrubbed vigorously with an industrial-grade brush attached to a drill press. The core was labeled every two or three inches with small Avery labels (#5412). Major element analysis was undertaken for 60 seconds; however, in the event that the raw x–ray spectrum revealed an abundance of sulfur greater than 2.5%, the sample was re-scrubbed, dried, and reanalyzed. Before trace element analysis the core slab face was dunked and lightly scrubbed with a wet towel, then dried and analyzed. It was deemed imperative that the final major element analysis occur before trace analysis in order to confirm that the original sulfur-rich (calcium sulfate salt) brine film was removed. Trace element analysis time was 60 seconds. X-ray spectra were subsequently calibrated with reference materials (Rowe et al., 2012, 2017). X-ray diffraction was undertaken on select sample powders in order to ascertain bulk mineralogical compositions. Bulk powders drilled from the back of the core were ground in a mortar and pestle and front-loaded into sample holders for analysis on a Bruker D2 XRD (Cu-K radiation; 0.02 ˚/sec; 2-65˚ 2θ). Hierarchical Cluster Analysis (HCA), Elemental Characteristics of Clusters, and Chemofacies Assignment In data mining, hierarchical clustering is a method of sample grouping—usually presented in terms of an elemental heat map and sample dendrogram, demonstrating sample ranking by similarity. While the cluster-selection process may appear subjective, adhering to a method that first pre-selects clusters based on an elemental heat map (not shown), followed by use of the HCA dendrogram, provides an added level of objectivity. In order to decide which sample groups (clusters) should be combined (for agglomerative), or where a cluster should be split (for divisive), a measure of dissimilarity between sets of observations is required (Ward, 1963; Templ, 2003; Templ et al., 2008; Rowe et al., 2017). This measure is achieved by use of an appropriate metric (a measure of distance between pairs of observations). The primary goal of HCA is to partition a multivariate data set into meaningful groups--in the present case these groups are ultimately considered chemofacies. The primary outcome of HCA is a number of sample clusters that maximize the sample similarity within each cluster and maximize the sample differences between clusters. In a vertical succession characterized by discrete lithologies, the inter-cluster differences are well-defined; however, in many Austin Chalk successions, the inter-cluster differences are smeared and overlapping because the lithologies fall along a mineralogical dilution pathway from near pure chalk (100% calcite) to calcareous mudstones characterized by variably elevated clay content but ultimately dominated by their calcite content. Chemofacies are ultimately defined through ranking the elemental abundances of each cluster, thereby identifying the major clusterdefined chemical signatures. This process is best undertaken by creating a partition index table (Phillips, 1991). Micro-Rebound Hammer and the Estimation of Rock Strength Micro-Rebound Hammer estimates of unconfined compressive strength (MRH-UCS) were undertaken at a 6-inch sample resolution using a Proceq Equotip (Brooks et al., 2016). It has been demonstrated that such measurements provide suitable approximations to direct measurements of UCS (Verwaal and Mulder, 1993; Aoki and Matsukura,
3 URTeC: 2668845 2008). Conversion of Leeb units to UCS units (MPa) was undertaken using a previously-derived empirical relationship (Zahm and Enderlin, 2010).
Figure 1: Paleogeographic map of southern end of Late Cretaceous Western Interior Seaway and northern Gulf of Mexico (Early Campanian) (Blakey, 2013). Austin Chalk Trend of South Texas, Louisiana, and Mississippi is superimposed.
Results XRF Geochemistry and Chemostratigraphy Placing textural characteristics and minor mineral occurrences aside, the mineralogical spectrum observed in the Austin Chalk is largely characterized as calcite diluted by variable clay content (Figure 2). The plot of %Al (clay proxy) versus %Ca (calcite proxy) demonstrates that most samples in an Austin Chalk succession from South Louisiana range 20-40%Ca (50-100% calcite), and are largely diluted by an aluminum-bearing mineralogical endmember that XRD work has demonstrated, barring volcanic contributions, is usually illitic in composition. The narrow range in the Ca-Al trend, and its proximity to the calcite-clay dilution line reflect the minimal importance of contributions from other minerals.
Figure 2: Bi-plot of %Al (clay proxy) and %Ca (calcite proxy) from a South Louisiana Austin Chalk succession. Samples define a dilution spectrum between calcite- and clay-rich endmembers. Color-coding of samples is from hierarchical cluster analysis and partition index table workflow outlined in the Methods section and tabulated below (Table 1). The thin black line denotes the calcite-clay dilution line, with the average gray shale value denoted by the 8.8%Al intercept (Wedepohl, 1971, 1991).
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Figure 3: A) Bulk elemental chemostratigraphy of South Louisiana Austin Chalk succession showing dominance of %Ca (calcite proxy), punctuated by %Al (clay proxy) that define lithological cyclicity. B) The Ca/Al ratio (calcite/clay proxy), demonstrating variation in the range, and frequency of clay-rich and clay-poor cycles through the stratigraphic interval.
Dilutional effects of clays in the otherwise carbonate-dominated Austin Chalk are observed stratigraphically in the calcite and clay proxies (Figure 3A), and the calcite/clay ratio proxy (Figure 3B) illustrates the range in calcite-clay cycle distribution and thickness for the South Louisiana succession. Whilst the cycle frequency and thickness change throughout the succession, the relative abundances of mineral components that create the cyclicity is largely invariant. Table 1: Partition Index Table of the seven clusters (chemofacies) defined using hierarchical cluster analysis. Chemofacies are ranked relative to calcite content; high-to-low is left-to-right. Ranking represents the element average in the cluster relative to the average in the entire data set.
Hierarchical Cluster Analysis and Chemofacies The XRF-based geochemical sample set (1407 samples) was subdivided into seven groupings (clusters) using hierarchical cluster analysis (HCA). The resulting clusters were analyzed through elemental ranking, and ordered
5 URTeC: 2668845 from the highest to the lowest calcite cluster, “Clust-1” to “Clust-7”, respectively (Table 1). The seven clusters are referred to as chemofacies, as they describe the chemical characteristics of the strata that potential can be used to infer changes in the environment of deposition and/or diagenesis. The stratigraphic expression of the chemofacies defined through HCA and elemental ranking within each cluster reveal a complex cyclicity of deposition (Figure 4), largely dictated by the relative contributions of calcite and clay.
Figure 4: A) Partition index values for four major elements (Ca, Fe, Si, Al) in each of the seven clusters (chemofacies) from Table 1. These values represent the average concentration of each of the elements in each of the seven chemofacies. Note enrichment of Fe, Si, and Al as Ca depletes from Chemofacies #1 to #7. Also, note decoupling of average Al from Si toward more clay-rich chemofacies, potentially suggesting a loss of quartz in the most argillaceous endmembers (Chemofacies #6-#7). B) Stratigraphic distribution of each chemofacies, n=number of samples in each chemofacies.
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Figure 5: A) Micro-Rebound Hammer estimates of Unconfined Compressive Strength (MRH-UCS) plotted for the upper 100 feet of the Austin Chalk succession, their relationship with the Ca/Al ratio (proxy for calcite/clay), and their linkage to the HCA-defined chemofacies listed in Table 1 and presented stratigraphically in Figure 4. Note that Ca/Al is plotted inversely so that Ca/Al peaks are more easily correlated with peaks in MRH-UCS. Also note that 1) peaks in MRH-UCS and Ca/Al generally, but not always, correspond with high-calcite chemofacies (#1-#4), and 2) troughs in MRH-UCS and Ca/Al generally correspond with high-clay chemofacies (#5-#7). Remember that chemofacies assignments and Ca/Al are at the 2-inch resolution, whereas the MRH-UCS estimates are at the 6-inch resolution.
General linkages between mechanical stratigraphy and mineral stratigraphy are revealed by comparing the MRH estimates of rock strength and Ca/Al (calcite/clay proxy) for the uppermost 100 feet of the South Louisiana Austin Chalk succession (Figure 5). Peaks in MRH-UCS estimates correspond with peaks in Ca/Al, indicating the overarching significance of calcite to the rock strength properties of the formation, and the calcite-clay cyclical controls on the variations in rock strength. Because the mineralogy (without reference to lithofacies textural descriptions) appears to play a dominant role in controlling rock strength, defining the distribution and thickness of weaker, more ductile clay-rich intervals in the overall carbonate-rich succession may provide a stronger framework for quantifying sub-log-scale lithological variability that ultimately refines geomechanical models and completions strategies derives from those models. Further work on quantifying the textural components of the Austin Chalk, including the evaluation of natural fractures, the relative importance of the spectrum from wispy seams to definitive stylolite fabrics, and relative importance of laminated and bioturbated intervals is required.
7 URTeC: 2668845 Conclusions The facies cyclicity of Austin Chalk, although variable stratigraphically and across the broad region of deposition, underpins unique aspects of how these strata behave under induced fracturing, and ultimately, how they are best completed as hydrocarbon resources. Fundamental characterization of chemofacies and mechanical facies, in addition to traditional lithofacies evaluation, is instrumental for generating a more quantitative framework for understanding and predicting rock strength characteristics that impact geomechanical perspectives on sampling and modeling and the associated completions strategies. This is believed to be particularly important in successions where facies-scale variability is below the resolution of downhole well logs. Acknowledgments HR, ES, and CH appreciate the support from the industry associates of the Bureau of Economic Geology’s Reservoir Characterization Research Laboratory (PI Charlie Kerans), and the State of Texas Advanced Resources Recovery program (PI William Ambrose). The endless assistance from the BEG Core Research Center Staff is deeply appreciated: Nathan Ivicic, Brandon Williamson, Bill Molthen, and Rudy Lucero. Several BEG people were instrumental in their support and assistance: Stephen Ruppel, Robert Loucks, Gregory Frébourg, Jarred Garza, JD Grillo, Adam Tuppen, Nathan Kaldin, Molly McCreary, and Gregg Stephens. The support and funding from Dean Sharon Mosher and the Jackson School of Geosciences to the BEG XRF Geochemistry Lab is deeply appreciated. Bruker Corporation continues to interact on instrumental advances and technical support. The author thanks Nestor D. Phillips II, for his MS thesis and his chemofacies insights. References Aoki, H., and Y. Matsukura, 2008, Estimating the unconfined compressive strength of intact rocks from Equotip hardness: Bulletin of Engineering Geology and the Environment, v. 67, p. 23–29. Blakey, R., 2013, Key Time Slices of North American Geologic History, CD-ROM. Brooks, D., Janson, X., and Zahm, C., 2016, The Effect of Sample Volume on Micro-Rebound Hammer UCS Measurements in Gulf Coast Cretaceous Carbonate Cores. GCAGS Journal 5, 189-202. Dravis, J.J., 1980, Sedimentology and Diagenesis of the Upper Cretaceous Austin Chalk Formation, South Texas and Northern Mexico: Doctoral Dissertation, Rice University. Grabowski, G.J., Jr., 1981, Source-rock potential of the Austin Chalk, Upper Cretaceous, southeastern Texas: Gulf Coast Association of Geological Societies Transactions, v. 31, p. 105-113. Grabowski, G.J., Jr., 1984, Generation and Migration of Hydrocarbons in Upper Cretaceous Austin Chalk, southcentral Texas; in Palacas, J. G., ed, Petroleum geochemistry and source-rock potential of carbonate rocks; American Association of Petroleum Geologists Studies in Geology 18, p 97-115. Haymond, D., 1991, The Austin Chalk – An Overview, Houston Geological Society Bulletin, v. 33, no. 8, p. 27-31. Hendrix, C., 2016, Chemolithofacies of the Upper Cretaceous Buda Formation and Austin Chalk Group, SouthCentral Texas: A Product of Integration of Lithologic and Chemical Data. MS Thesis, The University of Texas at Austin, 139pp. Hovorka, S.D., and Nance, H.S., 1994, Dynamic Depositional and Early Diagenetic Processes in a Deep-Water Shelf Setting, Upper Cretaceous Austin Chalk, North Texas, Transactions of the Gulf Coast Assocation of Geological Societies, v. 44, p. 269-276. Phelps, R.M., Kerans, C., Loucks, R.G., Da Gama, R.O.B.P., Jeremiah, J., and Hull, D., 2014, Oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the Cretaceous (ValanginianCampanian) passive margin, northern Gulf of Mexico, Sedimentology, v. 61, p. 461-496.
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