Nanometer-scale pore structure and the Monterey Formation: A new tool to investigate silica diagenesis

Silica and its alteration play integral roles in many disciplines such as geology, chemical engineering, ceramics (including glasses), bioengineering, pharmaceuticals, electronics, mining, manufacturing, optics, petroleum industry, historical conservation, archaeology, gemology, and construction. In industries such as pharmaceuticals and nanometer-scale engineering, diatoms are of interest due to their three-dimensional (3-D) frustule structure, large surface area, punctae (pore) sizes, availability of large quantities (cultured and mined), mechanical properties, susceptibility to silica replacement, and ability to precipitate orderly spheres of silica from solutions undersaturated with silica (e.g., Sumper and Brunner, 2006; Halas, 2008; Kröger and Poulsen, 2008; Sun et al., 2017). Calcined diatomite products (e.g., filter production) and ceramics involve high-temperature alteration of silica (e.g., Pagliari et al., 2013; Dudina et al., 2019). Gem opals are composed of hydrated silica mineraloids occurring either as amorphous silica (opal-A) or a disordered mixture of cristobalite and tridymite (opal-CT and opal-C; e.g., Gaillou et al., 2008). These mineraloids along with diagenetic quartz also comprise siliceous hydrothermal deposits or sinters (e.g., Lynne et al., 2005). In biosiliceous depositional systems, amorphous biogenic silica (opal-A) is altered with increasing burial depth and temperature to a more-ordered opal-CT and eventually the crystalline end member, quartz (e.g., Iijima and Tada, 1981). This diagenetic sequence is well documented for the biosiliceous Miocene Monterey Formation of California (e.g., Behl, 1999).

Diatomites, related diatomaceous rock types, and their alteration products of the Miocene to lower Pliocene Monterey Formation and related formations (e.g., overlying Etchegoin Formation of the southern San Joaquin Basin) in California provide a unique opportunity to study silica diagenesis due to the thickness of the formation (up to ~3050 m or ~10,000 ft), broad areal distribution, abundant sample sources (extensive outcrops and cores), varying compositions (biosilica, organic matter, clay, carbonate, apatite, volcanic ash, silt, and sand), burial history, tectonic complexity, and economic importance (e.g., Bramlette, 1946; Behl, 1999). In its resource assessment, the U.S. Geological Survey reported that the quarries near Lompoc, California, could supply the world’s diatomite needs (assuming current consumption rates of ~3000 metric kilotons per year) for hundreds of years (Crangle, 2018). In California, diatomaceous reservoirs also hold more than 10 billion barrels of original oil-in-place (Ilderton et al., 1996). Additional hydrocarbon reserves occur in fractured reservoirs of diagenetically altered diatomaceous rocks, namely, porcelanites and cherts.

Similar to other “shale plays” or producible mudrocks, diatomaceous rocks have limited permeabilities (1–10 mD; Table 1; Stosur and David, 1976) and respond positively to hydraulic fracturing (Nelson et al., 1996). What distinguishes diatomaceous rock types from other mudrocks are their substantial storage capacity, with typical porosities of 50–70 vol%, and their susceptibility to silica alteration that reduces both porosity and permeability (Isaacs, 1984). During silica maturation or diagenesis, the opal-A porosity of 50–70 vol% is reduced to 30–40 vol% after opal-CT conversion and then to 10–20 vol% after conversion to quartz (Isaacs, 1984). The porosity ranges in Table 1 have been extended to include total porosity values for the study samples. Similarly, permeability is reduced during silica conversion from 1–10 mD for opal-A to <0.01–0.1 mD for opal-CT to <0.01 mD for quartz (Table 1; Isaacs, 1984). The temperature at which these transformations occur is controlled by composition and other factors that are summarized in Behl (1999); these are discussed further in the following section. At depths and regions with later stages of silica diagenesis (opal-CT and quartz), extensive fracturing can compensate for the reduction in porosity and permeability.

Diatomaceous and porcelanite reservoirs pose intrinsic development and production challenges, including, but not limited to, subsidence, well failures, production declines, small recoveries, and potential damage when exposed to thermal recovery methods (e.g., Strickland, 1985; Koh et al., 1996; Montgomery and Morea, 2001). Thermal recovery methods, such as steam injection, could theoretically induce silica transformation in diatomaceous or opal-A production intervals, with the potential for reservoir damage (e.g., permeability reduction). Experimental studies conducted on diatomaceous rocks have evaluated one or more of the following topics: fracture healing, oil recovery, water flooding, fines migration, permeability, compaction, and relative permeability (summarized in Ross et al., 2016). These experiments were conducted at a range of temperatures (ambient up to 230 °C), using different fluids (brines, crude oil, mineral oil, and simulated boiler effluent), and over durations from a day to several years. In these studies, silica alteration was often subtle, if it occurred at all, and many of the measurement techniques were not definitive. For example, several experiments on Field B samples (San Joaquin Basin) yielded an increase of 1 wt% opal-CT as determined by X-ray diffraction (XRD); however, this increase is due to a relative enrichment in opal-CT caused by the dissolution of more soluble minerals (such as opal-A, pyrite, and heavy carbonates) and not the formation of opal-CT under experimental conditions (Ikeda et al., 2007; Ross et al., 2008).

As an alternative to compositional methods of detecting silica diagenesis and alteration, Ross et al. (2016) first proposed using nanometer-scale porosity to study silica alteration and detect subtle changes using both natural and post-experiment samples. “Silica diagenesis” refers exclusively to natural changes, including mineral conversion and more subtle changes (e.g., dewatering of opal-A). The phrase “silica alteration” includes both silica diagenesis and changes induced by human activity (e.g., steam injection). In the unreviewed conference paper of Ross et al. (2016), processed low-pressure nitrogen sorption isotherms provided nanometer-scale pore volume, surface area, and pore throat size distribution measurements. The nanometer-scale pore characteristics are unique for opal-CT–bearing samples compared to samples that lack opal-CT. The differences in the sorption responses are obvious even for samples with opal-CT compositions of only 1–3 wt% (XRD). In Ross et al. (2016), two distinct trends based on the nanometer-scale surface area versus nanometer-scale pore volume were identified for natural samples. These trends are also expressed in the nanometer-scale pore throat size distribution. This method was used to determine whether silica alteration occurred during the experiments (Ross et al., 2016). By using nitrogen sorption, larger-scale pore structures (e.g., pore throat diameters >300 nm) as detected by mercury injection are intentionally excluded as these larger porosity contributions mask the nanometer-scale pore structures associated with opal-CT.

In this work, we further develop this new sorption-based tool to detect and study silica alteration based on its expression in the nanometer-scale pore structure. Unlike Ross et al. (2016), this study focuses on natural diagenesis with a few samples that were altered in situ by adjacent steam-based enhanced oil recovery operations. Additional insights are provided by the inclusion of diagenetic quartz samples, high-resolution scanning electron microscopy (SEM) imaging, and additional compositional measurements. Interpretive schematics are proposed to facilitate the application of this tool. Quarry and reservoir samples ranging from relatively pure opal-A to diagenetic quartz serve to demonstrate this method. In this way, we validate the use of this sorption-based method as a tool to aid in the interpretation of the silica phases for natural, field, and experimental samples.

Biogenic silica is hydrous, amorphous silica, or opal-A, derived from living organisms such as diatoms, siliceous sponges, and radiolarians. Diatoms are algae that are predominantly planktonic and typically range in size from 10 to 200 µm in diameter (Crangle, 2018). Their namesake rock, diatomite, is composed of diatom frustules and other detritus that have been converted from a depositional ooze into rock via compaction. With increased burial depth and time, diatomites, diatomaceous mudstones, and other diatomaceous rock types are converted into porcelanites, cherts, and other diagenetic products depending on their initial composition (Bramlette, 1946). In this chapter, the terms “diatomite” and “porcelanite” are loosely applied to all diatom-sourced opal-A rock types (e.g., diatomaceous mudstone) and their diagenetic products (e.g., porcelanite), respectively. The term “chert” is applied to siliceous samples exhibiting conchoidal fractures regardless of mineralogy.

Rocks containing significant amounts of biogenic silica are unique in their response to increased temperature in that the hydrous, amorphous phase, opal-A, progressively dewaters, its structure organizes, and porosity decreases as it converts into opal-CT, a hydrous disordered mixture of cristobalite and tridymite. This diagenetic trend continues as opal-CT progressively becomes more ordered until it is converted into quartz, the dewatered, crystalline end member of silica diagenesis. This process and supporting literature are summarized in Behl (1999). During silica diagenesis, the pore structure of the rock is affected, reducing both porosity and permeability as it progresses from opal-A to quartz (Table 1). Reported alteration temperatures determined by field observations are ~35–56 °C for opal-A to opal-CT conversion and ~65–80 °C for opal-CT to quartz conversion (Table 1; e.g., Pisciotto, 1981; Keller and Isaacs, 1985). Using oxygen isotopes, temperature ranges of 17–56 °C have been reported for opal-A to opal-CT alteration and 31–110 °C for the opal-CT to quartz transition (Pisciotto, 1981; Matheney and Knauth, 1993). Botz and Bohrmann (1991) reported opal-A to opal-CT conversion temperatures as low as 0–4 °C in Antarctic deep-sea sediments. Whereas early studies such as that by Murata and Nakata (1974) observed the relationship between silica phase changes and burial depth (i.e., temperature), they also noted multiple silica types coexisting at the same depth, revealing that silica diagenesis is not controlled by temperature alone.

Numerous field and laboratory studies investigated factors other than temperature that play a role in the alteration of silica. For example, Kastner et al. (1977) conducted laboratory experiments and determined that clays delay opal-A to opal-CT conversion, whereas carbonates accelerate the process compared to pure biosilica samples. This finding agrees with field observations (e.g., Murata and Nakata, 1974; Isaacs, 1982; Behl, 1992). Williams et al. (1985) propose that solubility rates impact the alteration process; solubility is controlled by factors such as surface area, particle size, and detrital composition. Kastner (1985) summarized the factors controlling silica diagenesis as being dominated by temperature, pH, and silica concentration in the aqueous phase, with a lesser influence attributed to the ionic strength and either the abundance of certain cations and anions or lack thereof. Hinman tested the role of organic matter (1990) and cations (1998) in the conversion process. In addition, exposure to high temperatures in laboratory experiments and thermal recovery methods (e.g., cyclic steam and steam flooding) in field applications have been documented as altering biogenic silica deposits (e.g., Kastner et al., 1977; Fassihi et al., 1982; Rice et al., 1995; Lore et al., 2002; Ross et al., 2008; Bloeser et al., 2013).

We demonstrate the nitrogen sorption method on samples from five oil fields in the San Joaquin Basin (Kern County, California) and a quarry in the Santa Maria Basin (Lompoc, California). Individual fields are identified by a letter, A through E, and the samples derived from them are identified by their field letter and depth of origin. For samples from Field C, the depths are obscured by replacing the first number with an X (e.g., C X539.5). These Field C samples were collected from a horizontal well with a wellbore depth range of 9.75 m (32 ft), whereas their true vertical depth range is 0.3 m (1 ft). Overall, the samples were collected at the surface (quarry) and various depths below 230 m (750 ft). In total, 59 samples represent the opal-A, opal-A to opal-CT transition, opal-CT, opal-CT to quartz transition, and fully converted quartz intervals. Although trends with depth are apparent within most wells, sample depths alone should not be used as an indicator of silica transformation between fields, within a field, and within an individual well. Their compositions range from pure diatomites to clay- and carbonate-rich diatomites; this affects the temperature at which silica mineral conversion occurs (Kastner et al., 1977). In addition, some samples may have been heated either directly or indirectly by steam recovery methods in adjacent wells and reservoir intervals. To represent induced alteration, a sample altered in situ by steam flooding with a maximum temperature of 156 °C (B 1498.7 I) was included. A detrital quartz sample (St. Peter Sandstone) was added for comparison to diagenetic quartz samples.

The methods used in this study include pore structure measurements, compositional analyses, and image-based characterization. This novel method of detecting silica alteration is based on nanometer-scale surface area, pore volume, and pore throat size distribution measurements derived from low-pressure nitrogen sorption isotherms. Other techniques such as mercury injection, XRD, and SEM imaging aided in the interpretation and comparison of the sorption data. All methods were applied to cleaned samples in which the oil and soluble organic matter had been removed via repeated toluene soaks at room temperature until the toluene remained clear.

Pore structure and its measures fall into two categories: total porosity and nanometer-scale porosity. Total porosity refers to the porosity values reported in Table 1; these values consist of the porosity accessed across the entire range of pore throat sizes from millimeter- to nanometer-scale diameters. Nanometer-scale porosity discussed in this chapter is limited to a subset of the total porosity connected by pore throats between 3.7 and 300 nm in diameter. This comparatively inaccessible porosity serves as an indicator of the silica phase by detecting the nanometer-scale pore structures associated with opal-CT.

Mercury injection porosimetry (MIP) generates pore throat size distributions (PSD) with diameters of 1 mm to either 4 or 8 nm. MIP measures the volume of mercury intruded into a sample as pressure increases. Throat size is then calculated using the Washburn (1921) equation, which relates pressure (P) to the radius (r) of idealized cylindrical pores:

where σ is the surface tension of mercury, θ is the contact angle between mercury and the pore wall, and k is a conversion factor for the units used. The system measures the volume of mercury filling pores accessed through throat sizes (cylinders) equal to or greater than the radius, r, at increasing pressures. It is important to note that the cylinder diameter is equivalent to the pore throat diameter and not the larger pore body diameter. The resulting PSD are plotted as incremental pore volume per gram of sample material for comparison with sorption-derived pore throat size distributions (Fig. 1). MIP measurements were performed by three different companies with a maximum pressure of 414 MPa (60,000 psi; equivalent to a pore throat diameter of ~3.9 nm), as well as in-house measurements made using a Micromeritics AutoPore IV 9500 with a maximum pressure of 228 MPa (33,000 psi; equivalent pore throat diameter of ~7.8 nm).

MIP pore throat diameters for the study samples are less than 2 µm. Opal-A samples such as Quarry 1 and B 794.3 commonly contain a single peak between 300 and 1000 nm in PSD plots (Figs. 1A–Fig. 1B). Opal-A samples with some opal-CT detected using XRD display a second peak at less than 200 nm (e.g., B 1420.5 and E 1224.6; Figs. 1C–Fig. 1D). For samples that have been fully converted to opal-CT (e.g., C X539.5; Fig. 1F), there is only one peak that is typically 100 nm or less in diameter. Rather than use the full PSD as measured using MIP, we focus on the nanometer-scale porosity associated with opal-CT as derived from nitrogen sorption isotherms.

Nitrogen sorption is used to characterize the nanometer-scale porosity subset indicated by the arrows in Figure 1, thereby excluding contributions from larger pore structures observed in MIP data. Sorption isotherms indicate the amount of gas, in this case, nitrogen, adsorbed onto a sample as the relative pressure increases, followed by the amount of desorption as the relative pressure decreases at a constant temperature (Fig. 2A). The resulting isotherms are plotted as unitless relative pressure versus nitrogen volume (cm³/g) at standard temperature and pressure (STP). For these low-pressure sorption measurements, the amount of nitrogen gas depends upon physical adsorption onto pore surfaces (surface area) and capillary condensation (pore filling; Barrett et al., 1951). In this study, nanometer-scale surface area (nSA), pore volume (nPV), and pore throat size distributions (nPSD) were calculated using the Barrett-Joyner-Halenda (BJH) method (Barrett et al., 1951) as these values best align with the MIP data as shown in Figure 1. An “n” signifying nanometer was added to pore characterization measures derived from sorption data. This distinguishes them from total porosity measures that include contributions from macroscale porosity.

The BJH method determines the desorption rate of nitrogen from cylindrical pores based on the Kelvin equation using the relative pressure portion of the isotherm greater than 0.35 (Barrett et al., 1951). The de Boer method (de Boer et al., 1966) was used to calculate the thickness of adsorbed nitrogen on the pore walls. A subsequent paper by Joyner et al. (1951) compared the nSA, nPV, and nPSD from BJH analyses to MIP measurements and found good agreement. This is consistent with comparisons of Monterey Formation samples as shown in Figure 1. The only exceptions occur in Field C samples with comparatively high pore volumes (≥0.025 cm³/g) for porosity accessed through pore throat diameters of 30–100 nm (Fig. 1F). Nitrogen did not completely fill the pore volume accessed through these pore throat sizes during the sorption measurements, affecting both the nPSD and nPV values. In light of this, the BJH pore volumes or nPV for Field C samples are less than their actual volume. The impact of this is addressed in the Synthesis and Discussion section. The isotherm characteristics and resulting nPSD differ depending upon the opal-CT content (as listed in the legend) as shown in Figure 2. The hysteresis loop (area between isotherms) and subsequent nPSD are distinct for all samples containing opal-CT compared to samples lacking opal-CT (e.g., B 794.3; Fig. 2B). This is true even when the amount of opal-CT is minimal (e.g., 3 wt% for B 1420.5; Fig. 2C).

A brief description of the methodology follows (Ross et al., 2016). Shaped mini cores (~0.8 cm diameter and 0.5–1.5 cm long) were prepared from cleaned samples using 1500 grit sandpaper (Supplemental Material Fig. S1¹). A low-temperature outgassing procedure with a maximum temperature of 50 °C was performed before sorption measurements to remove adsorbed gases and water. The low temperature minimizes the potential for silica alteration and irreversible dewatering of opal mineraloids while providing a stable, repeatable starting point for sorption measurements. For the isotherm measurements, 70 points were collected on both adsorptive and desorptive branches to improve the data quality (i.e., generate closed isotherms) and its subsequent fit with MIP data (e.g., Fig. 1). The desorptive isotherms were processed using the BJH method. The resulting distributions or nPSD are plotted as pore throat diameter in nanometers versus dv(d), in which dv(d) is the differential pore volume with respect to pore throat diameter in cm³/g/nm (Fig. 2B). In these plots, dv(d) values diminish as throat size increases given equal pore volumes. In this way, dv(d) plots accentuate smaller nanometer-sized pore structures as needed for this application. All nitrogen sorption measurements were performed using Quantachrome Autosorb iQ3 systems. The Supplemental Material (see ^{footnote 1}) contains comprehensive information on method development, sample preparation, outgassing procedure, isotherm acquisition, data processing, plot options, and quality control as well as the steps necessary for alignment with MIP data as shown in Figure 1. It should be noted that there are a few examples in the literature of sorption measurements on porcelanites. Differences between the samples and measurement settings complicate direct comparisons with this work. These studies are discussed in the Supplemental Material.

The term “silica group” is nomenclature devised by the authors to organize samples within this study and facilitate discussions about contrasting silica states or phases between these groups. In total, 29 samples were assigned membership in one of five silica groups based on their silica composition as determined using quantitative XRD data and Fourier transform infrared spectroscopy (FTIR) data. (Composition methodologies are presented in the following section.) These silica groups are: (1) opal-A, (2) opal-A to opal-CT transition, (3) opal-CT, (4) opal-CT to quartz transition, and (5) diagenetic quartz (Table 2). The silica groups are ordered in terms of their relative maturity with respect to silica diagenesis, progressing from immature opal-A to intermediate opal-CT and ending with diagenetic quartz. All but one silica group, diagenetic quartz, contain multiple silica phases (Table 2). For the opal-A group, no opal-CT was detected. The opal-A to opal-CT transition group contains more opal-A than opal-CT, with opal-CT content ranging from 1 to ~20 wt% (Table 2). Quartz in the opal-A and opal-A to opal-CT transition groups was interpreted to be depositional. This was confirmed via SEM with energy dispersive X-ray spectroscopy (EDS) capabilities as presented in the Image-Based Characterization section. The presence of nanometer-scale peaks in MIP data supports the XRD detection of opal-CT for samples containing 1–3 wt% opal-CT (Fig. 1C, arrow). Opal-CT group samples contain more opal-CT than opal-A, with opal-CT values of 58–83 wt% (Table 2). The quartz content for the opal-CT group is in line with the depositional quartz values in the less mature silica groups. Samples assigned to the opal-CT to quartz transition group have no residual opal-A, an opal-CT content between 60 and 78 wt%, and significant amounts of quartz (17–32 wt%; Table 2). FTIR served to distinguish further between opal-CT and opal-CT to quartz transition group samples; this is presented in the following Composition section. Samples assigned to the diagenetic quartz group contain no opal-A, no opal-CT, and an abundance of quartz (92–93 wt%; Table 2). The majority of the quartz is diagenetic, having formed in situ from biogenic silica; 2 to 14 wt% of the quartz may be depositional based on the detrital quartz content of less mature silica groups (Table 2). These silica groups were consistent within reservoir intervals for individual wells.

Thirty samples lacking quantitative XRD data were classified based on XRD spectra, FTIR spectra, quantitative XRD measurements on adjacent core samples (similar samples within a meter (1–3 ft) of each other in the same well and stratigraphic unit), MIP data (e.g., Fig. 1), SEM texture, and SEM imaging of grain mounts. The Supplemental Material includes information about silica group classification for samples lacking quantitative XRD data.

Compositions of the study samples were determined using XRD (quantitative and spectral) and FTIR. In addition, EDS spectra provide elemental compositions; this image-based method is presented in the following section. Foremost, quantitative XRD served to establish the mineral composition and silica groups of 29 powdered samples (Tables 2–3). These samples and measurements were collected and studied over a 20 yr period. Given the discrete nature of the various projects as well as the time span, the XRD processing methods differ between projects. For example, service companies and internal laboratories at several petroleum companies performed quantitative XRD analyses. Each company had their own proprietary method for converting XRD spectra into quantitative mineralogies, which was further complicated by the presence of amorphous phases. These different sources, calibration methods, and standards complicate direct comparisons of the weight percentages between sample sets. In addition, most of the quantitative data either did not include methodology information or referenced inaccessible proprietary reports. All data sets lack reported error ranges except for references to the previously mentioned proprietary reports. The weight percentages listed in Table 3 are those reported for each data set. One data set includes a comment that decimal values do not imply greater precision, and zero values indicate that the mineral abundance is below detection level. This comment should be applied to all the reported quantitative XRD data in Table 3.

The quantitative XRD results list each silica phase separately, opal-A, opal-CT, and quartz (Table 3). XRD data for Field E reported another silica phase, opal-C; these values were added to the opal-CT weight percentages. Clays, carbonates, and other minerals are composite values. Clay speciation was not performed on all samples. For samples with clay mineralogy, illite and mixed-layer illite-smectite were the dominant clays, with minor contributions of chlorite and kaolinite in some cases. For the mixed-layer clays, smectite comprised 20–50 wt%. Carbonate minerals (calcite, dolomite, siderite, and ankerite) were reported. The remaining minerals, grouped together as “other” in Table 3, include plagioclase, potassium feldspars, gypsum, pyrite, and less common minerals.

FTIR spectra were used to distinguish between samples in opal-CT and opal-CT to quartz transition silica groups. In-house FTIR spectra were generated using a Nicolet iS50 spectrometer with a diamond attenuated total reflectance (ATR) accessory that collects FTIR spectra from 4000 to 350 cm⁻¹. The peak locations and characteristics identify various bond types associated with particular minerals. A silica peak at 623 cm⁻¹ occurs in samples from Field C (Table 3; Fig. S9); this peak is attributed to opal-C, a disordered cristobalite (Lippincott et al., 1958; Gadsden, 1975). Progressive opal-CT ordering along with the opal-C peak is reported in a cristobalite standard and after experiments conducted at 300 °C for up to 180 d using pure diatomites (Rice et al., 1995). This peak was only observed in the opal-CT to quartz transition group and was used to distinguish those samples from the opal-CT group (Table 3). Another silica band occurs at 694 cm⁻¹ in the diagenetic quartz samples (Table 3; Fig. S9; Gadsden, 1975).

Powder XRD spectra were collected using a Rigaku MiniFlex 600 (Cu Kα source at 40 kV and 15 mA with 0.01° steps from 3° to 90° 2θ) on select samples lacking quantitative XRD data to determine if an amorphous phase is present. Processing and semiquantitative phase identification for crystalline samples were performed using PDXL 2.8.4 software. Example XRD and FTIR spectra representing each silica group are available in the Supplemental Material as Figures S8 and S9.

Images and elemental data were generated using SEMs with EDS capabilities. SEM samples included grain mounts, polished epoxy-impregnated blocks, and polished epoxy-impregnated petrographic thin sections. Grain mounts were prepared by adhering silt- to sand-sized pieces onto conductive carbon tape. For polished mounts and thin sections, either wet or dry vacuum impregnation was performed using dyed epoxy cured under ambient conditions. The imaging surfaces of impregnated samples were progressively polished using 600 and 1500 grit sandpaper followed by 6 and 1 µm diamond grit. All samples were sputter coated with gold to improve conductivity and affixed to pin mounts using one of the following: conductive carbon tape, silver paste, or Crystalbond^TM. Either carbon or silver paint was applied on the sides of polished blocks and thin sections to improve the conductivity between the mount and the imaging surface of the sample.

Low-resolution textural images were collected on polished impregnated samples using a backscattered electron (BSE) detector on a JEOL JSM-5600LV SEM with an EDAX Sapphire Si(Li) EDS and GENESIS software. These BSE images were collected at 500× with a horizontal field of view of 287 µm and a pixel width ranging from 0.14 to 0.45 µm, depending on the pixel dimensions of the individual images. In SEM BSE images of polished surfaces, grayscale values are relative to the average atomic number, where dense minerals (such as pyrite) are brighter than less dense materials (such as clays and epoxy). Elemental compositions of selected regions are determined using characteristic X-ray counts in the EDS spectra. In this way, grayscale values within an individual image can be related to composition. Compositions of detrital grains were determined in situ in this manner.

SEM textures as viewed at low magnification on polished surfaces are distinct for samples predominantly composed of opal-A (opal-A and opal-A to opal-CT transition groups), samples consisting of mostly opal-CT (opal-CT and opal-CT to quartz transition groups), and diagenetic quartz group samples (Fig. S7; Table 2). These textures were used to assign silica groups for samples lacking quantitative XRD data, as discussed in the Supplemental Material.

Imaging of grain mounts was conducted using an FEI Magellan 400 XHR SEM and a TFS Apreo S LoVac SEM with a Bruker Quantax XFlash 6 | 60 EDS and Quantax Esprit 2.1 software. Most images were collected at magnifications of 2000×–35,000×, yielding pixel widths of 2–36 nm using the D ring of the concentric backscattered (CBS) electron detector at 5 kV and 0.8 nA. This detector and settings generated quality images and minimized the otherwise extensive charging observed with other detectors, even using accelerating voltages as low as 1 kV. The purpose of these images is to characterize the morphology of the nanometer-scale porosity that contributes to the sorption measurements. Opal-CT lepispheres and other bladed morphologies are of particular interest.

The nanometer-scale porosity of diatomaceous rocks and their alteration products was characterized using nitrogen sorption–derived measures, namely, pore volume (nPV, volume accessed through nanometer-sized pore throats), surface area of this pore volume (nSA), and the distribution of nanometer-scale pore volume with respect to pore throat diameter (nPSD). Sorption-based data exclude contributions from pores accessed via pore throats greater than 300 nm in diameter. These nanometer-scale pore characteristics were evaluated along with composition and silica morphology as viewed in high-resolution SEM images.

The plot of nPV in cm³/g versus nSA in m²/g reveals two distinct data trends (Figs. 3A and 3C). The “small nSA trend” exhibits a slight increase in nSA with increasing nPV, whereas the “large nSA trend” shows a substantial increase in nSA as nPV increases. The gray reference lines in the graphs are both adjacent and parallel to the actual data trends to avoid obscuring data. The small nSA trend is much more linear (R² = 0.80) than the large nSA trend (R² = 0.38). These data are plotted by silica group (Figs. 3A–3B) and sample source (Figs. 3C–3D). Measured nSA and nPV values are listed in Table 4 for silica group samples with quantitative XRD data and Table 5 for interpreted silica groups or silica groups determined using other methods.

Within each nSA trend, the samples plot by silica group with few exceptions (Figs. 3A–3B). Samples along the small nSA trend, from small to large nPV, include detrital quartz (SS, near the origin), diagenetic quartz, opal-A, and opal-CT to quartz transition group samples, including all of the quarry, Field C, and Field D samples as well as a subset of other field samples (Fields A, B, and E; Fig. 3). The large nSA trend includes opal-A to opal-CT transition and opal-CT group samples from Fields A, B, and E (Fig. 3); these three fields contribute samples to both trends. The large and small nSA trends intersect at ~0.1 cm³/g nPV and ~20 m²/g nSA; samples in this area could belong to either trend. E 1363.7 has alternating brown and gray laminations of sufficient width that mini cores represent individual lamina. The variability of this sample is shown by the right blue triangles in Figure 3C.

Each silica group plots with unique nSA and nPV ranges except for four samples (Figs. 3A–3B). Three opal-A samples, A 1563.6, B 795.1, and D 1484.9, occur next to opal-CT to quartz transition group samples. These three samples correspond to the three green inverted triangles next to blue circles having a nPV range from 0.23 to 0.24 cm³/g (Fig. 3B). One of the opal-A to opal-CT transition samples, E 1224.6, plots adjacent to an opal-A sample, B 794.3 (nPV ~0.11 cm³/g and nSA ~30 m²/g; Fig. 3B). Two other samples that warrant further scrutiny are B 1498.1 I and E 823.9 in that they were both altered by steam operations. B 1498.1 I is classified as an opal-A sample, whereas E 823.9 is classified as opal-CT to quartz transition (Table 5). Being unique, these samples have their own symbol (Figs. 3A–3B). These six samples are further examined in the Synthesis and Discussion section.

The two nSA trends are also expressed in the BJH pore throat size distributions (nPSD) in that samples in the large nSA trend have distinct nPSD compared to samples in the small nSA trend. In Figure 4, nPSD and nPV-nSA plots for select samples represent each silica group and nSA trend. SEM images included in Figure 4 are presented in the following section. For the nPSD plots in Figure 4, the distributions are plotted as pore throat diameter in nanometers versus dv(d), in which dv(d) is the differential pore volume with respect to pore throat diameter in cm³/g/nm. The vertical scales for dv(d) differ between plots as a single data range cannot adequately depict nPSD for all silica groups.

All samples (except for one, E 823.9) have a peak between 3.7 and 3.9 nm in nPSD plots (Fig. 4; Fig. A1). This is the only peak for some opal-A and all quartz (both diagenetic and depositional) group samples. Ross et al. (2016) investigated the magnitude of the first peak and its implications using comparisons of samples before and after various experimental conditions. A summary of these findings is provided in the Supplemental Material (see ^{footnote 1}). The remainder of this section focuses on the second peak and its development.

The nPSD of opal-A silica group samples fall into two categories: (1) distributions with only a single peak and (2) distributions with a small second peak. Quarry 1 is an example of a single-peak nPSD (Fig. 4A). Opal-A group samples with only a single peak (at ~3.8 nm) include all three quarry samples and five samples from Fields A and D (Table 5; Figs. A1A, A1C, and A1D). The nPSD of the remaining 19 opal-A group samples exhibit the initial development of a second peak in addition to the first peak. The second peak opal-A example, A 1563.6, has increased pore volumes accessed through pore throat diameters of 10–100 nm, generating a small, broad second peak or hump (Fig. 4B; Table 5). This hump is observed for the remaining opal-A silica group samples from Fields A, B, and D (Tables 4–5; Figs. A1A–A1D). The one exception, A 1596.8, has a small, sharp second peak between 4 and 10 nm (Table 4; Fig. A1D). In addition, B 794.3 differs from the other samples in that the baseline deviation is a ramp rather than a broad hump (Fig. A1A). The difference in nPV between opal-A group samples with single and double peaks is demonstrated in the nPV-nSA plots of Quarry 1 and A 1563.6 (Figs. 4A–4B). The small second peak contributes to the nPV with only a slight increase in nSA.

Samples in the opal-CT to quartz transition silica group have the greatest nPV on the small nSA trend and exhibit broad, better-developed peaks at 10–100 nm in the nPSD as demonstrated by C X540.4 (Fig. 4G). The small broad hump that occurs for many opal-A group samples corresponds to the broad hump with well-defined peaks observed for the opal-CT to quartz transition samples. Although similarly located, the magnitude (height) of the peaks for opal-CT to quartz transition samples is greater than that for the opal-A samples (Tables 4–5; Figs. 4B and 4G; Figs. A1A–A1D and A1I–A1J). Comparison of the second peak opal-A example, A 1563.6, and the opal-CT to quartz transition example, C X540.4, reveals the difference in the nanometer-scale pore structure between these small nSA trend samples as shown in the nPSD and nPV-nSA plots (Figs. 4B and 4G). Diagenetic quartz samples have single peak distributions, whereas the detrital quartz sample lacks this peak (Fig. 4H; Figs. A1K–A1L).

In contrast, samples on the large nSA trend have well-developed second peaks between 4 and 10 nm as demonstrated in the opal-A to opal-CT transition and opal-CT group examples (Figs. 4C–Fig. 4F). Although the pore throat size range is the same, opal-CT group samples exhibit much larger peaks as well as greater nPV and nSA (Figs. 4E–Fig. 4F) compared to opal-A to opal-CT transition samples (Figs. 4C–Fig. 4D; Table 4). The opal-CT silica group exhibits the most variability in nPSD and nPV-nSA plots (Fig. 3A; Fig. A1). This variability is observed between samples and within a single sample, as shown by the three samples representing individual lamina from E 1363.7 in Figures 3C (blue right triangles) and A1H. The “cherty” opal-CT lamina with conchoidal fractures, E 1363.7 ch, is unique in that the first and second peaks overlap significantly. The implications of this laminated sample are presented in the Synthesis and Discussion section. Other samples were consistent in their response, as presented in the Supplemental Material (Quality Control).

Opal-CT–bearing samples are found in both the large and small nSA trends, with increased nPV and nSA associated with increased opal-CT content for each silica group along each trend (Figs. 3A–Fig. 3B). Although samples containing opal-CT are included in both trends, the sorption data alone do not indicate why some opal-CT–bearing samples exhibit nPSD peaks between 4 and 10 nm (large nSA trend) while others have broad peaks between 10 and 100 nm (small nSA trend; Figs. 4C–Fig. 4G).

SEM imaging of grain mounts was performed on samples selected based on their silica group membership (Tables 4–5; and, if measured, composition in Table 3), location in the nPV-nSA plot (Fig. 3), and nPSD behavior (Fig. 4; Fig. A1). A few samples, such as E 823.9 and B1498.7 I, were chosen because of their ambiguous silica group classification and lack of quantitative XRD data. SEM photomicrographs representing each of the silica groups are shown with corresponding nPSD and nPV-nSA plots in Figure 4. All of the SEM images in Figures 4 and S11 (see ^{footnote 1}) were collected at 20,000× magnification with a horizontal field width (HFW) of 7.46 µm and a pixel size of 3.6 nm. Figures A2 and S12 contain SEM images collected at multiple magnifications.

Samples in the opal-A silica group consist of biogenic silica (intact and broken frustules), other minerals such as clays and carbonates, and detrital grains. For some samples, dissolution is observed on biogenic silica with large surface areas (e.g., thin and intricate structures; Fig. 4A, arrow) and not on frustules with smaller surface areas (e.g., thick and robust structures). No evidence of opal-CT formation was found for opal-A group samples except for A 1596.8. In A 1596.8, bladed half spheres ~200–300 nm in diameter occur on silica substrates (Fig. A2A, arrows).

Opal-A to opal-CT transition group samples are similar to opal-A group samples, composed primarily of unaltered particles with more evidence of selective dissolution. Isolated regions exhibit evidence of opal-CT precipitation. For E 1225.3, isolated lepispheres ~250 nm in diameter are visible (Fig. 4D, arrow). In comparison, E 1224.6 contains opal-CT as rare coalescent mats, with little evidence of distinct lepispheres, covering most of the image in Figure 4C. Equivalent regions with evidence of opal-CT formation were located in Field B members of the opal-A to opal-CT transition group, with B 1421.3 as an example (Fig. A2C, arrow).

Opal-CT group samples differ in that opal-CT lepispheres and coalescent mats dominate, whereas opal-A remnants are rare, if present. In the two opal-CT examples, two morphologies are apparent (Figs. 4E–Fig. 4F, arrows). For the opal-CT samples from Field A (A 1009.7), lepispheres are ~0.75–1.25 µm in diameter (Fig. 4E). These lepispheres have extremely fine, closely spaced, short blades and appear fuzzy or fibrous at lesser magnifications. For the other morphology in E 1326.5, well-developed lepispheres range from 1 to 2 µm with typical penetrating bladed morphologies (Fig. 4F, arrow).

Opal-CT to quartz transition group samples are primarily composed of lepispheres between 3 and 8 µm in diameter. Quartz, identified by its crystal habit, protrudes between the blades of the lepispheres, yielding a nubby surface (instead of a bladed surface) as shown for C X540.4 (Fig. 4G, arrow). Charging and other artifacts were an issue at magnifications required to identify quartz crystal habit; these high-magnification images are not shown. In contrast, diagenetic quartz samples consist of 1–3 µm spherical clusters composed primarily of subhedral to euhedral quartz crystals (e.g., C X509.8; Fig. 4H, arrow).

Characterization of the nanometer-scale pore structure provides a new tool for interpreting natural silica diagenesis and alteration by thermal recovery methods in outcrop, reservoir, and laboratory samples. As presented in this section, a nanometer-scale subset of the total porosity is tied to the morphologic manifestation of opal-CT precipitates. This allowed for the detection of subtle and initial changes as well as complete conversions with respect to silica alteration. Composition-defined silica groups can be determined based on nanometer-scale pore structure alone. As with any method, verification procedures should be in place such that interpretations are supported by other methods. The Applications section reveals five observations based on the study samples. The remainder of this section includes opportunities that apply to field and experimental work, as well as potential future directions for this method and its application.

The presence and morphology of opal-CT is captured in the nanometer-scale porosity measures. For example, opal-A to opal-CT transition samples E 1224.6 and E 1225.3 contain similar amounts of opal-CT, 18.5 and 17 wt%, respectively (Table 3). Comparison of the BJH measures reveals similar nPV values, with the sample containing less opal-CT, E 1225.3, having more nSA and smaller pore throat sizes in the nPSD plot than the other sample (Figs. 4C–4D; Table 4). The increased nSA and smaller throat sizes are attributed to the lepispheres ~250 nm in diameter visible in isolated areas within E 1225.3, as shown in Figure 4D (arrow). In comparison, E 1224.6 contains opal-CT as infrequent coalescent mats with little evidence of distinct lepispheres (Fig. 4C). At reduced magnifications, the surface of this mat occurs as the wall of a moldic pore formed by the dissolution of a frustule that had served as the nucleation site for opal-CT (Fig. A2D, circle). Lepispheres are also observed in opal-A to opal-CT transition samples from Field B, as shown for B1421.3 (Fig. A2C, arrow). Despite containing less opal-CT (1 wt%; Table 3), this Field B sample has similar nSA and greater nPV values as well as second peaks at larger throat sizes than E 1225.3 (Fig. 4D; Fig. A1E; Table 4). The lepispheres in the Field B sample are larger (~400–700 nm) with more broadly spaced blades (Fig. A2C). Lepisphere size and morphology account for the larger throat sizes for Field B (e.g., B 1421.3) versus Field E (e.g., E 1225.3) opal-A to opal-CT transition group samples.

This relationship is similar for samples representing the opal-CT silica group. Opal-CT silica group sample E 1362.5 contains large (1–2 µm diameter), short-bladed lepispheres with “ball-of-yarn” morphology, as described by Rodgers et al. (2004); see Figure 4F (arrow). In comparison, A 1009.7 contains lepispheres ~0.75–1.25 μm in diameter with extremely fine, closely spaced, short blades (Fig. 4E, arrow). The ball-of-yarn morphology of E 1362.5 yields a greater nPV accessed through larger pore throat sizes (Fig. 4F) compared to the finely bladed morphology of A 1009.7 with greater nSA and smaller throat sizes (Fig. 4E; Table 4). The greater nPV for E 1362.5 could also be attributed to its greater opal-CT content, 70 wt%, compared to 58.5 wt% for A 1009.7 (Table 3). More examples are available in Figures A2, S11, and S12.

A complication for opal-CT group samples is that more than one opal-CT morphology may be present within a given sample. Three distinct sizes and morphologies are visible in the “cherty” lamina opal-CT example, E 1363.7 ch (Figs. A2E–A2G). In this sample, the largest and best-developed lepispheres are 2–5 µm in diameter with large, well-defined blades (Fig. A2E). These lepispheres occur in moldic pores formed by the dissolution of sponge spicules similar to those shown by Murata and Larson (1975). Penetrating blades are broadly spaced at the outer edge, yielding “house-of-cards” structures, as described by Flörke et al. (1976, 1991); see Figure A2E (arrow). In contrast, the matrix of E 1363.7 ch consists of two other textures. One texture consists of coalescent lepispheres with a dense cluster of interpenetrating blades (Fig. A2F, circle). The other texture contains spheroids ~0.75–1.25 µm in diameter with short, fine blades that are somewhat similar to the morphology observed in Field A opal-CT group samples (Fig. 4E; Fig. A2G, arrow). The relative abundance of various opal-CT morphologies accounts for the greater variability in the nanometer-scale pore structure for samples on the large nSA trend (e.g., Fig. 3C, blue right triangles). In this way, opal-CT morphology, blade structure as well as their spacing, and opal-CT abundance are manifested in the nanometer-scale pore measures.

The nPV range for the opal-CT to quartz transition group is similar to that of the opal-CT group samples; however, the opal-CT to quartz transition group contains less nSA than the opal-CT group samples, placing this silica group on the small nSA trend (Fig. 3A). As shown for C X540.4, coarser, 3–8 µm lepispheres with quartz crystals between the blades explain this reduction in nSA for opal-CT to quartz transition group samples compared to opal-CT group samples (Fig. 4G, arrow). For the nPSD distributions, the opal-CT to quartz transition samples have porosity accessed through pore throat diameters of 10–100 nm compared to the pore throat diameters of 4–10 nm of opal-CT group samples (Figs. 4E–4G; Figs. A1G–A1J).

For diagenetic quartz samples, the rough 1–3 µm spheres of quartz crystals are interpreted to be crude relics of precursor opal-CT lepispheres as shown for C X509.8 (Figs. 4G–4H, arrows). Quartz crystals that nucleated between the opal-CT blades, as shown for opal-CT to quartz transition samples, grew at the expense of opal-CT. The complete conversion of opal-CT to quartz results in a loss of nSA, nPV, and the second peak in the nPSD (Figs. 4G–4H). These values are similar to the least-mature opal-A group samples, as revealed by comparing Quarry 1 and C X509.8 in Figures 4A and 4H, respectively. This indicates that there are no second peaks in the nPSD if opal-CT is not present.

This implies that opal-A group samples with small second peaks contain the initial indications of opal-CT development, even though they contain no detectable opal-CT via XRD. In A 1596.8, embryonic lepispheres, as presented by Flörke et al. (1976) and Kastner et al. (1977), were observed. In most cases, they appear as bladed half spheres ~200–300 nm in diameter upon a silica substrate such as a sponge spicule and a diatom fragment (Fig. A2A, arrows). The space between the blades in these embryonic lepispheres and mat-like opal-CT occurrences are the source of the nanometer-scale porosity that contributes to subtle second nPSD peaks in more-mature opal-A group samples.

Composition-based silica groups can be determined using nitrogen sorption data alone due to the consistency in the nanometer-scale porosity measures for each silica group. To facilitate this, interpretative schematics were developed for nPV-nSA and nPSD plots based on known silica group memberships and then expanded to include interpreted silica group samples (Fig. 5; data for each sample are listed in Tables 4–5). Each schematic shows graphically the regions attributed to each silica group, with the data ranges listed in the corresponding key. Dashed lines are used to denote boundaries based on the study samples. The solid lines in the nPSD plot mark the ranges of the first peak and the second peak of each nSA trend.

All samples are correctly classified when using the schematics together rather than individually. Using only one schematic adds uncertainty for a few samples. For example, three opal-A group samples, A 1563.6, B 795.1, and D 1484.9, were misclassified using the nPV-nSA interpretive graph alone (Fig. 5A). In this region of the graph, with nPV values between 0.20 and 0.23 cm³/g, misclassification cannot be prevented, because samples from both opal-A and opal-CT to quartz transition groups coexist (Figs. 3A and 5A, striped area). The nPV-nSA interpretive graph must be used in conjunction with the nPSD interpretive graph (Fig. 5B) to identify accurately silica groups for samples that fall within this nPV range. This overlap is most likely due to the incomplete filling of the comparatively large volume of porosity accessed through pore throat diameters of 30–100 nm in the opal-CT to quartz transition group samples as shown in Figure 1F for C X539.5. If the nPV is adjusted for the opal-CT to quartz transition samples using mercury injection data, then the overlap in the nPV versus nSA schematic may disappear.

Likewise, single peak distributions for opal-A and diagenetic quartz are similar and should be identified using the nPV-nSA plot and not nPSD criteria alone. The interpretation for the initial peak, at 3.7–4 nm, in Figure 5B is based on the findings of Ross et al. (2016). Using experimental samples, this peak’s pore volume increased with prolonged exposure to high temperatures, whereas it decreased when dissolution dominated. More information on this topic is provided in the Supplemental Material. For the remaining nPSD samples, all except one sample are correctly classified using nPSD alone. For E 823.9, the height of the second peak is 0.0027 cm³/g/nm, which is significantly below the threshold for the opal-CT to quartz transition silica group (Table 5; Fig. 5B; Fig. A1J). This sample is discussed further in the Induced Alteration subsection.

In the schematics shown in Figure 5, opal-A group samples lie within their graphical boundaries despite representing different compositions, including relatively clean diatomites, clay-rich diatomaceous mudstones, and carbonate-rich diatomaceous samples. This consistency despite compositional differences indicates that the presence of other minerals does not interfere with evaluating silica alteration using nitrogen sorption data.

Sorption data can be used to better understand silica diagenesis, even at its earliest stages. This involves comparing samples with respect to the differences between nanometer-scale pore measures in conjunction with composition and SEM imaging. The pore structure development for each nSA trend is detailed next, followed by two subsections describing the different maturities of opal-A silica group samples and early indications as to which nSA trend their nanometer pore structure alteration will likely follow as they convert to more organized silica phases. The last two subsections demonstrate how this method can be used to identify unexpected or outlier samples for further scrutiny, including the detection of induced alteration.

The diagenetic evolution of the nanometer-scale pore structure can be observed for each nSA trend as demonstrated in Figures 6A and 6B. The small nSA trend starts with a single peak distribution (~3.8 nm) for an opal-A silica group example, A 752.1. As opal-A matures, a subtle second peak develops between 10 and 100 nm. This is demonstrated by D 1484.9 that has no detectable opal-CT (Fig. 6A; Table 3). This progresses until the opal-A has been fully converted, and larger, distinct peaks develop in place of the subtle second peaks observed for the opal-A silica group. This is represented by an opal-CT to quartz transition group sample, C X539.7, containing 68 wt% opal-CT (Fig. 6A; Table 3). Lepispheres of the opal-CT to quartz transition group are coarsely bladed and have quartz crystals protruding from them. The lepisphere-shaped clusters of quartz crystals observed in the diagenetic quartz samples (0 wt% opal-CT) from the same well indicate that these quartz crystals continue to grow as the opal-CT is depleted. As this occurs, the nanometer-scale pore structure is diminished by reducing both nSA and nPV as well as the loss of the second peak (Figs. 4G–4H).

For the large nSA trend, the initial state is a single-peak opal-A nPSD distribution (~3.8 nm; not shown) that develops a small second peak between 4 and 10 nm as opal-A matures (Fig. 6B). The developing second peak for A 1596.8 is attributed to the embryonic opal-CT lepispheres observed in SEM images, despite containing insufficient amounts of opal-CT to be detected via XRD (Fig. A2A, arrows; Table 3). As the amount of opal-CT becomes detectable via quantitative XRD, this peak continues to develop, as indicated by the opal-A to opal-CT transition group example, B 1421.3, containing 1 wt% opal-CT (Fig. 6B). This process continues until all of the opal-A has been converted, generating a substantial peak for the opal-CT group example, E 1362.5, consisting of 70 wt% opal-CT (Table 3). Of the opal-CT morphologies observed, large nSA samples contain well-developed lepispheres of opal-CT, as shown in Figures 4, A2, S11, and S12. For opal-A and opal-A to opal-CT transition group samples, these features are smaller, relatively rare, and easily overlooked.

The progressive development of a second nPSD peak is indicative of increased silica maturity within the opal-A group as it falls along a continuum from single-peak opal-A group samples to samples with two peaks containing measureable amounts of opal-CT (Figs. 6A and 6C). When present, the second peak in opal-A group samples is small, as indicated by peak heights in Tables 4 and 5 (see footnote † in tables). Behl (1999, p. 305) states, “Within each diagenetic zone, silica becomes increasingly well ordered with depth, temperature, or time, even though there may not be any lithologic change.” This is the case for the opal-A silica group, in which the relatively immature and near-surface samples, such as quarry samples, have a single-peak nPSD, and opal-A group samples with subtle second peaks heights are intermediate to samples with small yet measurable amounts of opal-CT (Figs. 6A–Fig. 6B). This phenomenon highlights subtle beginnings of opal-A maturation or opal-CT formation expressed in the nanometer-scale pore structure that would otherwise be undetected. For most of the opal-A group samples, the second peak develops and its height increases with depth within individual wells (e.g., Figs. A1C–A1D). Although general trends with depth are visible, exceptions do occur. For instance, A 752.1 and A 754.3 have somewhat different opal-A maturities based on sorption data despite originating from similar depths in the same well (Fig. 6C). Given the same burial history, other factors (such as mineralogy and particle size) most likely contributed to the maturity differences (as expressed by the second peak) between these two samples.

Membership in either the large or small nSA trend is indicated by the pore throat size of the developing second peak, specifically 10–100 nm for the small nSA trend and 4–10 nm for the large nSA trend (Fig. 5B). The initial stages of trend development are demonstrated by two opal-A samples from the same well separated by only 9 m (30 ft), A 1563.6 and A 1596.8 (Fig. 6D). These cores were retrieved at a natural geothermal gradient temperature of ~47 °C. No opal-CT was detected via quantitative and spectral XRD (Table 3; Figs. S10A–S10B [see ^{footnote 1}]). A 1596.8 contains ~4 wt% calcite (Table 3; peak between 29 and 30° 2θ in Fig. S10B), whereas A 1563.6 does not contain calcite based on the XRD spectrum (Fig. S10A). In Figure 6D, the nPSD of A 1596.8 reveals an incipient second peak at 8.3 nm within the 4–10 nm range indicative of large nSA trend membership, as found for opal-A to opal-CT transition and opal-CT groups (Fig. 6B). In contrast, A 1563.6 exhibits a small, broad second peak at 28 nm that is within the 10–100 nm range as observed for opal-CT to quartz transition group samples on the small nSA trend (Figs. 6A and 6D). Within the opal-A group, only A 1596.8 shows a peak developing between 4 and 10 nm (Figs. A1A–A1D). The ramped nPSD observed for B 794.3 might also indicate the initial stages of large nSA development (Fig. A1A). Without A 1596.8, the link between opal-A and opal-A to opal-CT transition group samples would not be adequately represented.

As described previously, embryonic lepispheres were observed in an opal-A sample, A 1596.8 (Fig. A2A, arrows). These embryonic lepispheres are the source of the 4–10 nm pore volumes measured for the sample. In comparison, smaller surface area morphologies of opal-CT are expected to occur in the opal-A sample A 1563.6. Although a few potential candidates were observed, these features are much smaller than the lepispheres found in A 1596.8. Charging issues impeded image collection at higher magnifications to verify these observations (not shown).

Analysis of the nanometer-scale pore structure readily identifies outlier samples that warrant further scrutiny that would otherwise be overlooked. These outliers are found by having nSA, nPV, and nPSD results outside of those found for similar samples in their silica group (Fig. 5). Outlier samples identify additional avenues of inquiry by triggering new questions. Outlier samples of note include (1) the three opal-A group samples plotting with opal-CT to quartz transition group samples in the nSA-nPV plot, (2) initial signs of nSA trend membership for opal-A group members (A 1563.6 and A 1596.8), (3) adjacent samples (such as E 1224.6 and E 1225.3 spaced 0.2 m [0.7 ft] apart in the same well), with different morphologies for opal-CT and the resulting effect on nanometer-scale pore structure, and (4) the visibly laminated sample with different nanometer-scale pore structures in which the first and second peaks overlap for E 1363.7 ch. Outliers resulting from induced alteration are presented in the following section.

Two outlier samples were altered in situ from steam-enhanced recovery operations. In addition, two other samples were subtly altered and are presented first. Both B 794.3 and B 795.1 are more extreme with respect to nPSD and nPV-nSA than other opal-A group samples from these shallow burial depths. For example, B 794.3 plots adjacent to an opal-A to opal-CT transition sample, whereas B 795.1 plots with opal-CT to quartz transition samples in the nPV-nSA graph (Figs. 3B and 3D; Table 4). Both samples have similar compositions, containing ~42 wt% opal-A and ~28 wt% clay (Table 3), and are from the same well. These relatively shallow samples from <244 m (<800 ft) were expected to be similar to shallow opal-A samples from Field D, as well as the near-surface quarry samples (Figs. A1A and A1D, note different vertical scales). Instead, the nPSD of B 795.1 is more similar to the nPSD for deep (>457 m or >1500 ft) opal-A group samples from Fields A and D (Figs. A1C–A1D), whereas B 794.3 exhibits a more ramped nPSD in the 4–10 nm range similar to A 1596.8. Further inquiries into the field and well information revealed that B 795.1 and B 794.3 were heated (up to 49 °C compared to the geothermal gradient temperature of 32 °C for this depth and location) by an overlying steamed sandstone interval.

A more extreme example of in situ alteration was observed for sample E 829.3. This sample was initially assumed to belong to the opal-A group due to its large total porosity, friable texture, shallow burial depth, and temperature profile for this well. E 829.3 plots near the nPV region containing samples from both opal-A and opal-CT to quartz transition groups in the nPV-nSA plot (Figs. 3A–3B, open square; Fig. 5A). E 829.3 stands out in that its nSA is much lower than other samples on this portion of the small nSA trend. In addition, the first peak is absent in the nPSD plot (Fig. A1J). The loss of the first peak is associated with extreme dissolution (Supplemental Material; Ross et al., 2016). E 829.3 came from an interval that was heated above the geothermal gradient temperature of 29.4 °C to 37.8 °C by long-term field-based steam operations. Further analysis revealed that it has an FTIR peak at 623 cm–1 and crystals protruding from lepispheres, similar to the opal-CT to quartz transition group samples (Table 5; Fig. A2H, arrow). The XRD spectrum contains the typical cristobalite and tridymite peaks (Fig. S10D). Semiquantitative mineral identification based on this spectrum found predominantly cristobalite (81 wt%) with zeolite (17 wt%) contributions. No quartz was detected. Upon examination at higher magnifications, the crystals protruding from the lepispheres are interpreted to be zeolite, based on their crystal habit (Fig. A2H, arrow). Despite the lack of dia- genetic quartz, E 829.3 was assigned to the opal-CT to quartz transition group samples based on FTIR, sorption, and SEM findings.

The last example, B 1498.7 I, appeared to be a typical opal-A group sample based on its nanometer-scale pore structure as designated by the green and white triangle in Figures 3A and 3B and the nPSD in Figure A1B. This sample came from a well drilled through an interval that was heated to 156 °C by distant steam operations. The company reported that it had converted to opal-CT based on wireline log data. In support, computed tomography images of whole core (not shown) reveal en echelon fractures perpendicular to bedding, indicating that this interval is more rigid than comparable samples from the same field. With only one peak, the nPSD for B 1498.7 I could represent either opal-A or diagenetic quartz. Diagenetic quartz was ruled out by the strong amorphous response in the XRD spectrum (Fig. S10C). SEM images display intact diatoms, giving it the appearance of a typical opal-A sample at low resolution (Fig. A2B). Dissolution and botryoidal silica with spheroids ranging from 200 to 750 nm in size are apparent at greater magnifications (Fig. A2B, arrow). This botryoidal silica is similar to that found in opal-A sinter (hydrothermal) deposits, except that it is less extensive (e.g., Herdianita et al., 2000; Lynne and Campbell, 2004; Lynne et al., 2005, 2007). This sample demonstrates that silica alteration other than the typical diagenetic sequence of opal-A to opal-CT to quartz occurs in intervals exposed to steam-based enhanced oil recovery methods.

This new tool based on nanometer-scale pore measurements provides an opportunity to investigate the earliest stages of silica alteration and previously undetectable variations in this process with academic, resource extraction, and manufacturing applications. Findings derived from such studies could improve our understanding of the processes and provide guidance on how to control induced alteration for optimal results. For example, how can porosity and permeability be preserved during induced silica alteration? Based on E 829.3, we now know that it can happen. The next step is determining how that process can be induced in other areas. This would involve conducting experiments and analyzing the products using this sorption method. Further development of the method also presents opportunities. The sorption methodology could be expanded to include different adsorbing gases such as carbon dioxide and other isotherm processing methods. This would allow for smaller pore sizes to be investigated. Nitrogen sorption could also be tested as a means to quantify opal-CT content if combined with mineralogy data determined using a consistent measurement method (e.g., quantitative XRD) with known accuracies.

Expanding the sample data set would better represent the diagenetic variability of the Monterey Formation and other siliceous formations. For example, many of the pore structures in opal-CT and quartz porcelanites as revealed in Kassa and Behl (2015) are not represented in this study. Hydrothermal deposits and gem opals could also be included in the sample set. In particular, samples with opal-CT compositions intermediate between opal-CT and opal-CT to quartz transition and diagenetic quartz silica groups would help to define the transitional changes in the nanometer-scale pore structure. Do the large and small nSA trends represent two separate diagenetic trends, or do samples on the large nSA trend switch to small nSA samples as opal-CT converts to diagenetic quartz (Fig. 3A)? Does the small nSA trend represent two diagenetic progressions: (1) from opal-A to more mature opal-A group samples as nPV increases and (2) from opal-CT to quartz transition to diagenetic quartz group samples as nPV decreases (Fig. 3B)? This method coupled with experiments such as those conducted by Kastner et al. (1977) could answer these questions, and further our understanding of the role played by mineralogy and other factors in the formation and morphology of opal-CT.

One of the main benefits of this sorption-based method is that it responds to the earliest indications of silica alteration through changes in the nanometer-scale pore structure before opal-CT is detectable via XRD. Nanometer-scale pore structures may correspond to other measurements, such as the d(100) spacing of opal-CT and crystallinity index for quartz. Close examination of samples and/or carefully designed experiments could determine the causes of subtle differences in sorption measurements for adjacent samples identified in this study (e.g., B 794.3 + B 795.1 and A 1563.6 + A 1596.8; Tables 4–5; Fig. 6D; Fig. A1A). Potential suspects for these differences include particle size, mineralogy (particularly clays and carbonates), and organic matter content and its maturity.

As for the suitability of MIP as a proxy for gas sorption in the measurement of nanometer-scale pore structure, nitrogen sorption data processed using the BJH method (Barrett et al., 1951) are demonstrated here to produce results similar to MIP over the range where the incremental pore volume measurements from the respective techniques overlap in the pore throat size distribution (Fig. 1). High-pressure MIP measures pore throats as small as 3.9 nm in diameter at a pressure of 414 MPa (60,000 psi) and could potentially be used instead of BJH data, as long as interpretation of the first peak (3.7–3.9 nm) is not required. The surface area and pore volume from MIP data, however, would have to be recalculated to remove contributions from pores larger than those measured via the BJH method (>300 nm). Additional benefits of nitrogen sorption techniques over MIP measurements are that sorption is more reliable on structurally incompetent samples, and it avoids the generation of hazardous waste. For opal-CT to quartz transition samples (e.g.,C X539.5) with large pore volumes accessed through pore throats ranging from 30 to 100 nm, MIP data could be used to correct for the incomplete filling observed in sorption-derived nPSD (Fig. 1F).

Although this chapter focuses on natural silica diagenesis at standard geothermal gradient temperatures, silica alteration induced by thermal field operations, natural hydrothermal deposits, manufacturing, and laboratory experiments can be similarly examined by comparing the state of the nanometer-scale pore structure as measured by gas sorption. Diatomaceous reservoirs respond positively, from an oil recovery perspective, when heated (e.g., Kumar and Beatty, 1995; Kovscek et al., 1996a, 1996b; Murer et al., 2000). As a result, the potential conversion of silica from amorphous to increasingly crystalline phases at the field scale has been the subject of many studies and much speculation through the years (e.g., Strickland, 1985; Patel et al., 2005). At the field scale, this sorption-based method could be used to detect even subtle alteration induced by thermal recovery methods if both pre- and post-heating samples are available for analysis, as represented in samples used for laboratory experiments (Ross et al., 2016). Even when unaltered samples are not available, interpretations can be made as revealed in this study. Determining the impact on productivity requires laboratory experiments as most reservoir production is commingled. At the laboratory scale, uncoupling the physical processes associated with the heating and production of diatomaceous rocks is a nontrivial undertaking. There have been numerous observations, but the ability to detect subtle changes in the silica phase has been a challenge in that selective dissolution of susceptible minerals confounds direct comparisons of mineral weight percentages. In this regard, we find that gas sorption is particularly adaptable and useful. With this method, we can now determine the degree of opal-A maturity (i.e., single- to double-peak nPSD continuum for opal-A group samples) and detect the initial subtle shift toward opal-CT in laboratory and reservoir samples. These changes in the nanometer-scale pore structure can occur independent of changes in the total porosity and its micrometer-scale pore structures (Ross et al., 2016).

Nanometer-scale pore structure as measured via nitrogen sorption provides a new investigative tool into natural silica maturation as well as changes induced in laboratory experiments and thermal reservoir production methods. This tool is useful for any application involving silica diagenesis and alteration, such as rocks derived from biosiliceous sediments, gem opals, hydrothermal deposits, industrial materials (e.g., sinters), and hydrocarbon reservoirs. This sorption-based method allowed for new insights into the natural diagenesis and induced alteration of Monterey Formation samples. Specific conclusions follow.

1. A new method based on nitrogen sorption was developed for evaluating silica phases or the degree of maturity of diatomaceous rocks and their diagenetic products. This method includes the preparation of mini cores, 50 °C outgassing, 70-point measurements for both absorption and desorption isotherms, BJH processing, and interpretation schematics. The application is based on nanometer-scale pore volume (nPV), surface area (nSA), and pore throat size distribution (nPSD) measurements. Clays, carbonates, and other minerals encountered in this study did not interfere with these measures and their subsequent interpretation.

2. Diatomites, related rock types, and alteration products can be classified based on the resulting BJH measures. Immature opal-A samples such as quarry samples present a single peak between 3.7 and 3.9 nm in their nPSD. As these samples mature, a second peak develops with a pore throat size range of either 4–10 nm or 10–100 nm. The second peak is distinct even if the amount of opal-CT is equal to or less than 1 wt%, thereby detecting the earliest signs of opal-CT formation. Second peaks reach their maximum pore volumes in opal-CT and opal-CT to quartz transition samples. The second peak is lost when opal-CT is fully converted to quartz.

3. Samples cluster by silica group in the nPV versus nSA plots. Two distinct trends are apparent: (a) a small nSA trend for samples with a single peak and second peak between 10 and 100 nm in the nPSD, and (b) a large nSA trend for samples with a second nPSD peak between 4 and 10 nm. Independent verification methods are recommended for samples that have been artificially heated.

4. The nanometer-scale pore structure as measured by nPSD, nPV, and nSA is controlled by the presence and morphology of opal-CT. Embryonic lepispheres were observed in a mature opal-A sample with no detectable opal-CT (XRD). In opal-A to opal-CT transition group samples, infrequent areas with isolated lepispheres and mats of coalescing lepispheres were observed. Opal-CT samples contain finely bladed opal-CT lepispheres as well as other morphologies. Opal-CT to quartz transition group samples have coarsely bladed lepispheres with quartz crystals protruding between the blades. For the diagenetic quartz samples, opal-CT lepispheres have been replaced with quartz crystals arranged in rough spheroids.

5. Nanometer-scale pore characterization reveals distinctions between opal-A samples that are not readily apparent by other means and the beginnings of opal-CT formation that are not detectable using other methods such as XRD. Two distinct diagenetic trends are obvious in opal-A to opal-CT transition, opal-CT, opal-CT to quartz transition, and diagenetic quartz group samples. Early indications of these nSA trends are also apparent in more mature opal-A samples that do not contain detectable amounts of opal-CT.

6. The sorption method provides a measurement of changes induced by thermal field operations and laboratory experiments. Hence, the method is useful as a monitoring tool. Dissolution and alteration (both full conversion and initial changes) are apparent. Examples include field samples heated above the natural geothermal gradient by distant steam operations to 38 °C, 49 °C, and 156 °C. Induced morphologies observed include embryonic lepispheres, lepispheres with protruding zeolite crystals, and botryoidal silica.

This work was supported by the Stanford University Petroleum Research Institute (SUPRI-A) Industrial Affiliates. Part of this work was conducted in the Stanford Nano Shared Facilities (SNSF), which is supported by the National Science Foundation as part of the National Nanotechnology Coordinated Infrastructure under award ECCS-2026822. Additional work was performed in the SUPRI-A, Benson, Stanford–U.S. Geological Survey Micro-Isotope Analytical Center, and Microchemical Analysis Facility laboratories of the School of Earth, Energy, and Environmental Sciences, Stanford University. We thank the reviewers and editors for contributing their time and expertise to improve this chapter. Portions of this chapter appeared in a paper presented at the Society of Petroleum Engineers Western Regional Meeting held in Anchorage, Alaska, 23 to 26 May 2016, and are used here with permission of the Society of Petroleum Engineers to republish via the Copyright Clearance Center.