Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico depth in a single well from the Ship Shoal area of the GOM. n this

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1 Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico Totten, Matthew W.; 1 Dixon, Mark; 2 and Hanan, Mark A. 2 1 Dept. of Geology, ansas State University, Manhattan, ansas Dept. of Geology & Geophysics, University of New Orleans, New Orleans, Louisiana 7148 ntroduction Abstract Although the distribution of mixed-layer clay minerals is geologically important for understanding the development of the Gulf of Mexico basin and technologically useful in preventing drilling and completion problems associated with expandable clays, there are few studies documenting clay mineral distribution in the subsurface Gulf of Mexico. Shale sections from nine wells within the South Timbalier protraction area (~18 square miles) were sampled at depths near known paleontological markers identifying the Miocene, Pliocene, and Pleistocene boundaries. The bulk clay mineralogy of each sample was determined by XRD, and is dominantly mixed-layer smectite and illite with minor kaolinite. The less than 1- micron fraction of each sample was further separated into four fractions by density. The mineralogies of these four fractions as determined by XRD are end-member smectite, smectite-rich mixed layer, illite-rich mixed layer, and end-member illite. The relative amounts of these clay mineral fractions do not vary systematically with age. However, the percentage of the illite-rich mixed layer fraction does increase with depth, at the expense of the smectite-rich fractions. This correlation decreases when depth is converted to temperature, based upon corrected well-log temperatures. The correlation of the illite-rich mixed layer fraction with depth, however, is not as strong in this multi-well study as the correlation reported from a single well in Ship Shoal using identical methods (Totten et al., 22). This is likely due to the fact that each well has a unique time-temperature history that controls the conversion to illite from mixed-layer clays. The amount of time rocks are exposed to increasing temperature is an important factor in the diagenesis of clay minerals. n addition, the presence of mixed-layer kaolinite in many of the smectite-rich fractions indicates a significant mineralogical variation not seen in the single well study. The distribution of clay minerals in the subsurface rocks of the Gulf of Mexico (GOM) is of interest for many reasons. One of the most compelling incentives is the possible affects of expandable clays on production and drilling operations. An abundant literature exists on clay mineral transformations in the Gulf, including the conceptual model of illitization of expandable smectite proposed by Burst (1969) and the seminal paper by Hower et al. (1976) that refined and popularized the idea of illite maturation with burial. n contrast to the plentiful literature on clay-mineral diagenesis, there have been few systematic studies investigating the geographic distribution of clay minerals in the GOM. The only study that investigates the distribution of clay minerals in the GOM that we are aware of focused on modern seafloor sediments (Devine, 1971). Studies that investigate the distribution of clays through time examined samples from a single well (e.g., Aronson and Hower, 1976; Boles and Franks, 1979). The focus of this study is to investigate the distribution of clay minerals from multiple wells and intervals in a large area of the Gulf of Mexico basin (GOM). n particular, we report the variation of illite-smectite clays between different locations, depths, and stratigraphic age. We have developed a method to physically separate these mineral species by density (Totten et al., 22) and have previously used this method to describe clay-mineral variations with Gulf Coast Association of Geological Societies Transactions, Volume 55,

2 Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico depth in a single well from the Ship Shoal area of the GOM. n this study we will contrast the results of that study with a wider distribution within the South Timbalier area. Methods Sample wells were chosen based upon their geographic distribution in the South Timbalier Protraction area (Fig. 1), offshore Louisiana, and the availability of well-cuttings at the University of New Orleans-Chevron Earth Science Laboratory. Depths were chosen using paleontological markers correlative to the Miocene, Pliocene, and Pleistocene boundaries as reported from public paleontological information database of the Minerals Management Service (available online at: homepg/pubinfo/pdfindex.html). Shale-rich intervals were chosen for sampling as indicated by well logs. Rock chips were handpicked out of well cuttings from 3 total sampling depths from 9 wells within South Timbalier. Sand-rich sections were skipped to avoid the possible local influence of diagenetic reactants from sandstone bodies within the shales. Shale chips were crushed using a mortar and pestle and passed through a 62 µm sieve. The coarser-grained material was removed by ultrasonically wet-sieving with a 1 µm micromesh sieve, the smallest size practical. The coarser than 1 µm fraction of shales is primarily detrital in origin (silt-sized quartz) and is not a major reactant in the /S reaction. Figure 1. Location map of South Timbalier Protraction area of multi-well study (shaded), with location of single-well study in Ship Shoal Protraction Block 97 (black square). Density separation method Traditional heavy liquids (tetrabromethane and bromoform) are not suited to separating clay minerals because of their tendency to strongly adsorb to the mineral surfaces. t is also difficult to wash these adsorbed molecules from the clays after exposure. n addition, smectite preferentially adsorbs these organic heavy liquids over other clays, which reduces the effective density difference between them and reduces the potential for separating them (Nelson 1985). These problems are solved using lithium metatungstate (LMT) as the heavy liquid (Totten et al., 22). We discovered the properties of LMT with respect to clays during our experience in separating heavy-minerals from shale (Hanan and Totten, 1996). LMT solutions can be adjusted using distilled water to between 1. and 3.4 (specific gravity). After mineral separation, LMT may be washed from the filtered samples using distilled water, recovered by evaporation at low temperatures (less than 1 o C), and reused. The final preparation for the density separations consisted of filtering the less than 1 µm pan fraction with a.45 µm acetate filter, then washing the material off the filter into a 5 ml centrifuge tube with LMT of the beginning density (2.3 g/cc). The mixture was suspended by vigorously shaking the tubes, followed by centrifuging at 3 rpm for one hour at a constant temperature. To minimize rafting (dense, sink material trapped in lighter, floating material) the float at the top of the tubes was re-suspended without disturbing the sink portion using a paddle made from a paper clip and spun with an electric drill. The tubes were then re-centrifuged for an additional hour. The entire LMT solution was frozen by placing the test tube in a liquid nitrogen bath. The thin layer of float material at the top of the tube was carefully removed by washing with distilled water into a.45 µm acetate filtering apparatus and recovered. The remaining LMT was then allowed to melt, and the liquid above the layer of sink material was slowly withdrawn using a micro-pipette and set aside. The 822

3 Totten et al. purpose for this was to minimize the amount of LMT remaining in the tube to dilute the next suspension. The thin layer remaining in the bottom of the test tube contains only material greater than 2.3g/cc. For the next separation, LMT of 2.4g/cc was added to the tube and the sink material of greater than 2.3g/cc was re-suspended in this fluid using an ultrasonic probe. The tube was centrifuged as before to isolate the material with a density greater than 2.4g/cc. The resulting float layer after this separation contains material less than 2.4g/cc and greater than 2.3g/cc. The sink layer contains only material denser than 2.4g/cc. This process was repeated using LMT with specific gravities of 2.7 and The final sink layer was recovered by filtering in a similar manner. This fraction contains the heavy mineral fraction (greater than 2.85g/cc). The result of the multiple density separations is four clay-mineral bearing density fractions: less than 2.3g/cc, 2.3 to 2.4g/cc, 2.4 to 2.7g/cc, and 2.7 to 2.85g/cc. Additional details of the separation method, including a test of its efficiency, are described in Totten et al. (22). X-Ray diffraction analyses The mineralogy of the density separates was determined by X-ray diffraction at the Microbeam Laboratory in the Department of Geology and Geophysics at the University of New Orleans. Oriented sample mounts were prepared after the method of Moore and Reynolds (1997). Air-dried and glycolated analyses of the same preparation were made on a Scintag XDS-2 diffractometer using Cu-alpha radiation, at 4kV and 2mA, scan range 2-4o, step-size.2o, and scan time of 2 seconds per step. Glycolation was achieved by placing samples in a glycol-saturated atmosphere for 24 hours at room temperature. Results We recognize that it is physically impossible to achieve a single mineral separate, however, our method is an efficient way to concentrate the major components of GOM shales. Figure 2 illustrates the four different mineral components separated from a single sample, with and XRD pattern of the un-separated clay-mineral fraction for comparison. Each separate has a distinct mineralogy, composed of primarily the mineral phases for which they are named below. As discussed later in this paper, the names of each density separate are not exclusively an accurate description of the mineralogy in every case in this study, as they are not uniquely mono-mineralic, and differ in many respects from the same density separates of the earlier study. The names of each separate are retained for easy comparison between the two studies. The four mineral separates are defined in the following manner after Totten et al. (22): EMS The less than 2.3g/cm3 density fraction is termed end-member smectite; SML The fraction with a density between 2.3g/cm3 and 2.4g/cm3 is termed smectite-rich mixed layer; ML The fraction between 2.4g/cm3 and 2.7g/cm3 exclusive of the fine-grained quartz, is termed illite-rich mixed layer; EM The fraction greater than 2.7g/cm3 (< 2.85g/cc) is termed end-member illite; The different amounts of each component for each sample location and depth are given in Table 1. The geologic stage is also listed in the table, as well as corrected temperature data derived from bottom-hole temperature data reported on the well logs. 823

4 Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico S Q,Q EMS 3 Relative ntensity 25 2 SML ML 15 1 EM 5 Whole clay o 2 theta Figure 2. X-Ray diffractograms (glycolated) of 4 mineral separates from sample 19, ST 196, 941 foot depth. Separates are labeled as defined in the text, and confirm the validity of the separation methodology. Bottom XRD trace is for unseparated, whole-rock clay mount of the same sample (also glycolated). = kaolinte, S = smectite, = illite, Q = quartz. Discussion llitization The results of this multi-well study from South Timbalier are similar in a number of respects to our previously reported results from a single well in Ship Shoal. llitization occurs at the expense of smectite-rich layers as expected from the multitude of previous studies. Figure 3 illustrates this for the illite-rich mixed-layer component (ML) of this study. The percentage of ML does indeed correlate with depth (correlation coefficient r=.62) and is comparable to the previous single well study in Ship Shoal (r=.89). Both studies support the well-publicized control of depth on illitization. Although, many diagenetic variables are represented by depth, variations in geothermal gradient, burial history, rate of deposition, proximity to salt bodies, and fluid migration trends are minimized by examining sample in one small area (i.e., from one wellbore). The lower correlation coefficient in the multi-well study relative to the single-well study indicates less consistency with depth over a wide area. Simply stated, a single well has a more consistent diagenetic history than do a group of wells of a large area. We examined the correlation between illitization and bottom-hole temperature, which is generally considered the primary control on illitization (Boles and Franks, 1979). This is not seen in this study, as the correlation between the percentage of the illite-rich mixed-layer component (ML) and calculated bottom-hole temperature is less (r=.47). The overall diagenetic history each sample experienced during burial must be better reflected with depth than just current bottom-hole temperature alone. The current bottom-hole temperatures may not necessarily reflect the temperature histories that were driving clay-mineral transformations. 824

5 Totten et al. Table 1. Sample location, depth, age, and density separation data. Sample % EMS % SML % ML % EM Depth Age OCS-G# STBlock Plio 615#F Plio 615#F Mio 615#F Pleist 2927 # Plio 2927 # Plio 2927 # Mio 2927 # Pleist 1559 # Plio 1559 # Plio 1559 # Plio 1559 # Plio 1247# Plio 1247# Plio 1248#C Plio 1248#C Mio 1248#C Plio 196 # Plio 196 # Plio 1265 # Plio 1265 # Plio 1265 # Plio 1265 # Pleist 1575 # Pleist 1575 # Plio 1575 # Plio 1575 # Plio 1575 # Pleist 2154 # Plio 2154 # Plio 2154 #2 314 Smectite Although the percentage of ML present increases during diagenesis, this fraction does have a consistent mineralogy in each sample as seen in Figure 4. The mineralogy of the ML fraction is consistently an illite-dominated mixed-layer clay regardless of depth encountered. There are distinct differences in both the end-member smectite (EMS) and the smectite-rich mixed-layer (SML) fractions between the Ship Shoal single well study and the multiple-well study reported here. The most obvious difference in this study is the much larger variation in the EMS component. We were initially surprised by the many samples (seven) containing over 15% of this fraction, as the same fraction in our single well study averaged 5 %, and had a maximum value in the shallowest sample at 12%. Close inspection of the EMS fraction shows two different populations. One set of samples has a low percentage of this fraction, and parallels the results described in the Ship Shoal single-well study 825

6 Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico % ML Depth 8 r = Figure 3. Percentage of the illite-rich mixed layer component (ML) versus depth of all South Timbalier samples. Linear best-fit regression line has a correlation coefficient of r= Q,Q 4 S Relative ntensity o 2 theta Figure 4. X-Ray diffractograms (glycolated) of typical ML separates from several samples. Mineralogy is consistent, even though the percentage of the ML fraction varies between samples. = kaolinte, S = smectite, = illite, Q = quartz. 826

7 Totten et al. (Totten et al., 22). The second set of samples with a significantly higher component of the low-density clay shows very different character as seen by the X-Ray diffractograms. Figure 5 shows XRD patterns of this fraction from the low percentage samples, which compares to the XRD results of the Ship Shoal well. Figure 6 illustrates the XRD patterns for samples with higher EMS percentages. The most obvious difference is an increase in intensity of the kaolinite peak. Our earlier tests of the LMT method on pure kaolinite standards from the Clay Mineral Repository suggest that kaolinite should sink in densities >2.5g/cc. The presence of a persistent kaolinite peak in these samples suggests a mixed-layer kaolinite-smectite clay, which might be expected to have a lower density than mixed-layer illite-smectite based on theoretically derived densities. Weaver (1989) reports significant randomly interstratified kaolinite/smectite from Georgia and the Coastal Plain of the United States and suggests that they are probably more abundant than generally realized. A significant increase in this component within the EMS would explain the increased percentage of this fraction compared to the samples without significant /S. The large variability in the amount of the EMS component could also explain the unpredictable swelling behavior observed during drilling operations in this area. The amount of this component is not observed by XRD of the entire clay fraction but is only clearly exposed in some of the lighter fractions after density separation using LMT. The kaolinite peak (2 theta =12.4) is more readily discernable in the EMS and SML fractions as compared to the ML, EM, and the non-separated sample shown in Figure 2. Note in Figure 2 that the kaolinite peak is nearly obscured in the non-separated sample, yet this sample contained 59% of the smectite/kaolinite-rich fractions (EMS & SML). 45 S 4,Q 35 3 Sample Number Relative ntensity o 2 theta Figure 5. X-Ray diffractograms (glycolated) of EMS fraction of typical samples with a low-weight percentage (< 1%) of this fraction. Note the relatively low kaolinite peak in these samples. = kaolinte, S = smectite, = illite, Q = quartz. 827

8 Diagenesis of Mixed-Layer Clay Minerals in the South Timbalier Area, Gulf of Mexico 45 S 4,Q 35 3 Sample Number Relative ntensity o 2 theta Figure 6. X-Ray diffractograms (glycolated) of EMS fraction of typical samples with a highweight percentage (>2%) of this fraction. Note the relatively large kaolinite peak in these samples when compared to Figure 5. = kaolinte, S = smectite, = illite, Q = quartz. Neither of the smectite-rich components (EMS or SML) correlates with depth or temperature. The smectite-rich mixed-layer component varies considerably in percentage, and many samples have a distinct kaolinite component comparable to the EMS fraction. The combined smectite component (EMS plus SML) decreases with depth, in an inverse relation to the illite-rich clay component. Conclusions The general trend of the clay-minerals in shales from South Timbalier is toward an increased illite-rich mixed-layer component with increased depth, consistent with many previous studies. The correlation with modern bottom-hole temperature is not as strong. Likewise, the correlation between illitization and depth is not as strong in this multi-well study as previously reported in a single well in Ship Shoal. Clay-mineral diagenesis is a complex reaction dependent on many variables. Single-well studies limit many of these variables, perhaps exaggerating the apparent control of depth on clay-mineral reactions. An interesting result of this study is the variability of the smectite-rich clays. This result is not obvious using standard clay mineral XRD techniques, but is apparent using the separation method outlined in this study. The large variation in these expandable clays could explain the unpredictable behavior of these rocks during drilling operations. 828

9 References Totten et al. Aronson, J.L., and Hower, J., 1976, Mechanism of burial metamorphism of argillaceous sediment: 2. Radiogenic argon evidence: Geological Society of America Bulletin, 87, p Boles, J.R., and Franks, S.G., 1979, Clay diagenesis in Wilcox sandstones of southwest Texas: implications of smectite diagenesis on sandstone cementation: Journal of Sedimentary Petrology, v. 49, p Burst, J.F., 1969, Diagenesis of Gulf Coast clayey sediments and its possible relation to petroleum migration: American Association of Petroleum Geologists Bulletin, 53, p Devine, S.B., 1971, Mineralogical and geochemical aspects of the surficial sediments of the deep Gulf of Mexico: PhD dissertation, Louisiana State University, Baton Rouge, Louisiana, 161 p. Hanan, M.A., and Totten, M.W., 1996, Analytical techniques for the separation and SEM identification of heavy minerals in mudrocks: Journal of Sedimentary Research, 66, p Hower, J., Eslinger, E.V., Hower, M.E., and Perry, E.A., 1976, Mechanism of burial metamorphism of argillaceous sediment: 1. Mineralogical and chemical evidence: Geological Society of America Bulletin, 87, p Minerals Management Service Paleontological Database available online at: pdfindex.html Moore, D.M., and Reynolds, R.C., Jr., 1997, X-Ray diffraction and the identification and analysis of clay minerals: Oxford University Press, New York, 378 p. Nelson, T.A., 1995, Density separation of clay minerals: Unpublished M.S. thesis, Oregon State University, Corvallis, Oregon, 59 p. Totten, M.W., Hanan, M.A., Mack, D., and Borges, J., 22, Characteristics of mixed-layer smectite/illite density separates during burial diagenesis: American Mineralogist, v. 87, p Weaver, C.E., 1989, Clays, muds, and shales: developments in sedimentology: v. 44, New York, Elsevier, 819 p. 829

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