The Relevance of Mineralogy to The Life Cycle of a Petroleum fieldP Joseph Hamilton1 and Ly Vinh Chi2

1. Digital Core, Canberra, Australia. 2. Fugro Robertson Inc., Houston, Texas, USA

ABSTRACT

Industry recognition of the importance of mineralogy is evident in increasing adoption of automated mineral analysis techniques into mud logging. Automated Mineral Analysis (AMA) techniques such as X Ray Diffraction (XRD), Scanning Electron Microscope/Energy Dispersive X Ray analysis (SEM/EDX) and hyperspectral imaging can be applied for both near real time mineral identification and quantification of core and cuttings and for subsequent laboratory based detailed investigation. At the rig site a combination of rapid and minimal sample preparation, rapid analysis, robust mineral identification and sufficient resolution to reveal poroperm character and rock texture would be ideal. No one method is yet able to deliver all these characteristics at the rig site and particularly in rocks that are fine grained and / or organic rich and / or with abundant chemically complex and poorly crystalline clays.

Nevertheless, knowledge of reservoir mineralogy at the rig site has the potential to provide information critical to reservoir quality assessment, well steering, addressing formation damage, improved log interpretation and documentation of interwell correlative mineralogies. In shale gas prospects, knowledge of mineralogy is critical to design of appropriate frac strategies, with quartz-rich clay-poor shales forming frac targets and claystones forming frac barriers. Volcanic horizons often provide laterally extensive marker horizons useful for interwell correlation, present low permeability barriers and baffles to fluid flow and cause increased rates of drill bit wear. Their occurrence in cuttings samples is rapidly recognised in automated SEM/EDX data. For example, co-variation in elevated abundances of plagioclase and pyroxene provide interwell marker horizons of basalt. Other minerals useful for recognising volcanic horizons include amphibole, albite and zircon. Even when abundances of diagnostic minerals are at trace levels the number of data points collected per sample is large enough to capture a statistically meaningful representation of that mineral.

The giant petroleum fields of the Gippsland basin, South Eastern Australia are nearing the end of production. They are currently being assessed for suitability for carbon dioxide geosequestration. The Lakes Entrance Formation regional seal and the Latrobe Group reservoirs and intra Latrobe Group seals have been assessed for the presence of minerals that may be reactive to carbon dioxide, forming solid carbonates and so trapping it and reducing porosity and permeability. The Gurnard Formation at the top of the reservoir section is of particular interest as it acts variously as a poor quality reservoir or seal. The presence of minerals such as glauconitic mica and albite in this formation are of interest as their potential reactivity could be the final containment mechanism for injected carbon dioxide that migrated to the top of the reservoir section.

INTRODUCTION

The purpose of this paper is to provide an overview of the topic of mineralogy in so far as it is a discipline of considerable relevance and value, particularly in well site application, for the petroleum industry.

The fundamental controls on the mineral composition of a siliciclastic sedimentary rock comprise provenance, depositional setting and diagenesis. Understanding these controls offers powerful constraints for prediction of reservoir quality and producibility, understanding reservoir architecture, modelling of mineralogical effects on log responses and targeting of injection sites for carbon dioxide geosequestration.

METHODS OF WELL SITE MINERAL ANALYSIS

X-Ray Diffraction When an X-ray beam interacts with a crystal lattice a spectrum of x-rays is generated from different incident angles with different planes of lattice symmetry. Each mineral has a distinct atomic structure and hence will diffract X-rays in a unique characteristic spectrum (Poppe et al., 2001). Analysis of bulk rock samples is undertaken on aliquots ground to <63 m and is rapid. Data can be interpreted quantitatively by reference to standard mineral spectra. Individual clay mineral spectra have many closely spaced peaks and specific clay mineral quantification for sandstones and siltstones requires further analyses of a separated <2 m fraction. The analyses are undertaken after different treatments (Figure 1) that include exposure to glycollation (to obtain diagnostic expansions of lattice spacings) and drying at different temperatures (to obtain diagnostic collapse of lattice spacings). The <2 m fraction does not necessarily capture all the clay minerals quantitatively as many diagenetic clay mineral grains are >2 m. Thus, ideally sized fractions > 2 m (e.g. 2-5 m, 5-10m and 10-20 m) should also be prepared and analysed. The clay mineral quantification also requires reconciliation with the bulk rock analysis by mass balance, which for small sample weights may introduce significant error.

It is emphasised that XRD data are acquired for the crystalline portion of a sample so that amorphous material such as the mineraloid allophane is not reported. Further, poorly crystalline material such as many low temperature clays are difficult to identify and quantify and maybe under-represented in results. The method provides no textural information.

Figure 1. X-ray diffractograms for a separated clay fraction showing different changes for different clays in peak positions and intensities that accompany heating and glycollation, (after Zachos et al., 2004, Fig 18, Ch. 6).

Hyperspectral Imaging AnalysisHyperspectral imaging acquires and processes high resolution spectra from across selected contiguous bands of the electromagnetic spectrum (Smith, 2012). Different scanning instruments potentially suitable for rig site imaging of drill core and cuttings use different portions of the visible-infrared spectrum (between 0.4 m and 25 m). Data are presented as reflectance, the proportion of reflected energy compared with incident energy, as a function of wavelength, or wave number (Clark, 1999). Different minerals have different spectral reflectance curves because they selectively absorb different wavelengths of incident energy. The method is particularly useful for identification of clay minerals and their polytypes (Figure 2) and has the advantages of requiring minimal sample preparation and rapid data acquisition and processing times. Image resolution is limited by the line scan frequency and the diagnostic absorption wavelengths required for mineral identification (e.g. ~2-3 and 9-10 m for clays and ~ 9, 12-13 and 25 m for quartz; Clark 1999). For rig site operation it has the added advantage of providing identification of different oil types. Charsky and Herron (2012) recently demonstrated the use of calibrating mineral and kerogen standards for quantification of mineralogy and organic matter in cuttings with an infrared spectrometer.

Figure 2. Infrared spectra of clays illustrating the different diagnostic absorption peaks (after Nahin, 1955).

Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM/EDX)

In recent years ruggedized and portable scanning electron microscopes with energy dispersive X-ray detectors (SEM/EDS) have become commercially available for rig site deployment specifically aimed at achieving near real time analysis of cuttings. Figure 3 below illustrates the various reactions that result from electron beam incidence on a mineral surface. SEM/EDX instruments make use of the resultant X-ray spectrum and the intensity or brightness of backscattered electrons (BSE) for mineral identification at each analysis point. With the resultant x-ray spectrum processed to determine 1) the elements present at the analysis point and 2) the percentage amount of each detected element at the analysis point, while the BSE data provides a means to separate chemically similar minerals.

Figure 3. Illustration of the effects of electron beam incidence on a mineral surface.

Cuttings sample preparation and analysis is achieved in about one hour with typical analytical point spacings of 20-30 m. Laboratory analyses typically use analytical point spacings of 2-10 m and yield mineral maps of cuttings particles with sufficient resolution to provide good textural information (Figure 4). In rig site cuttings analysis, which is rapid and relatively low resolution, trace heavy minerals (e.g. zircon, rutile) are detected but thin coatings of minerals (e.g. hematite, chlorite) on detrital grains may not be detected.

Figure 4. Mineral maps of cuttings particles in which the combination of mineralogy and well resolved texture allows lithotype identification of each particle.

The ability of SEM based mineralogy tools to capture false colour mineral maps as well as high resolution and high magnification back scatter images (BSE) gives this technique a unique advantage over other rig-site instruments. Additionally, as this technique makes use of the chemical data to determine mineral type, the elemental composition of individual mineral grains, particles or whole samples is also available.

By way of a summary for this section the various methods of mineral analysis with potential well site application are summarised and their advantages and disadvantages compared in Table 1.

Table 1. Comparison of three rig site automated mineral analysis methods.METHOD SAMPLE PREP ADVANTAGES DISDAVANTAGES

XRD Select representative sample and grind to powder. Size separates required for clay mineral analyses as well as different treatments prior to analysis

For bulk rock analysis sample prep and analysis are rapid and space required in mud logging unit is minimal. Data processing does not require to be calibrated to formations of interest.

Clay mineral ID and quantification requires more sample preparation, more analysis time and more space in the mud logging unit. Data analysis requires expert knowledge. Data reported are for crystalline phases only. Amorphous and poorly crystalline phases are difficult to impossible to identify and quantify. Textural information is not generated.

SEM/EDX Select representative sample, epoxy mount, cut, polish, carbon coat.

For bulk rock analysis sample prep and analysis are rapid and space required in mud logging unit is minimal. Data processing does not require to be calibrated to formations of interest though is of benefit if available. Only technique to provide high resolution textural information, and also provide elemental data.

Limited time available for rig site analysis compared to lab analysis limits textural information. Method is inadequate for fine grained samples rich in organic matter and/or in samples containing mixed layer and / or smectitic clays

Hyperspectral imaging

None For bulk rock analysis, sample prep and analysis are rapid and space required in mud logging unit is minimal. Data processing does not require to be calibrated to formations of interest.

Textural information is limited by coarse resolution and step intervals between line scans (typically ~1mm). Best available method for clay mineral id.

APPLICATIONS OF MINERALOGY

Exploration Phase

The mineralogy of a sedimentary rock is a function of provenance, depositional setting and diagenesis. The influence of the former two factors may result in distinctive mineralogies across a field. This is of benefit for the development of exploration models, including the sequence stratigraphic framework, and understanding of reservoir architecture. Volcanic rocks are of particular value for such purposes. A triangular plot of detrital quartz, feldspar and lithic fragments (QFL, Figure 5), normalised to 100% is indicative of provenance (Dickinson et al., 1983; Pettijohn et al., 1987) with regard to tectonic setting and from which contributions of volcanic material may be identified. The value in volcaniclastic sediment for correlation lies in its potential for contributing specific minerals at specific times over a wide geographic area. A key horizon for correlation

across the compartmentalised Mengoepeh field in South Sumatra from rig site mineral analysis was realised by recognition of increased abundance of albite (Figure 6) accompanied by increased abundance of particles of dacite.

Figure 5. Relative abundances of detrital quartz, feldspar and lithic fragments on a triangular QFL plot (after Pettijohn et al., 1987).

Figure 6. Interwell correlation based on onset of dacitic volcaniclastic input as evidenced in sudden increase in albite abundance in cuttings.

Of common use for correlation are ash fall and other volcaniclastic deposits. Correlations thus established may be confirmed and further refined by radiometric age determination of minerals such as zircon (Schmitz and Davydov, 2012; Smyth et al., 2003, 2011). Deposition and subsequent alteration in marine waters give rise to bentonites through alteration to smectite in marine waters and to tonsteins through alteration to kaolinite in brackish / fresh water lacustrine environments. These are readily correlated in the field and from logs.

Some minerals are diagnostic of sequence stratigraphic context, useful information, for example, for modelling of seal-reservoir relationships and seismic data interpretation. Paralic and marginal marine environments are susceptible to relative sea level change with consequent changes in detrital mineral composition and in pore water chemistry giving rise to diagnostic diagenetic mineral assemblages (Morad et al., 2010). Khidir and Catuneanu (2009), for example noted increases in early diagenetic kaolinite abundances immediately below sequence boundaries and suggested the cause to be meteoric water flushing following uplift and erosion. This also causes dissolution of carbonates and detrital feldspars and creation of secondary porosity.

Note however that Amorosi (1995) and Hugget and Gale (1997) advise caution in the use of glaucony as a palaeo environment indicator given that mixtures of allochthonous and autochthonous grains are common because of reworking.

Electric log responses are affected to varying degrees by both mineral and fluid content of a drilled section. Correct log interpretation is critical to early inter well correlation and decision making during the exploration and development phases. The ray log is used to infer the relative volume in a logged section of productive sandstone and expressed as volume of shale (Vshale) Many clay minerals have high contents of radioactive elements, either intrinsically (e.g. illite) or as exchangeable cations (e.g. smectite). Kaolinite however has no ray emitting element content, resulting in an underestimate of Vshale if this is used in isolation. Some detrital sand sized minerals (e.g. zircon, apatite) are also capable of generating a high gamma ray signal reflecting the presence of “hot sands” but not “hot shale”. Table 2 provides examples of these effects of some key minerals on log response and further shows the relevance of on-site mineralogy to refine wireline data.

Table 2. Examples of mineral effects on log responses.Mineral Logs affected Effectskaolinite Not detected\ undercall Vshale

Fe bearing clays

NMRdensity

IncreaseDecrease t2 cutoff

Increase

smectites

NMRdensity

IncreaseIncrease Swirr

Decreaseillite Increase

K feldspar, micas, zircon, apatite increase

Production Phase

Petroleum producibility from a reservoir depends on the distribution of porosity, permeability and wettability, properties controlled by the nature of the mineral surfaces in contact with pore fluid. Particle size and morphology of clay minerals varies considerably and hence has a major effect on many petrophysical properties. Particle size affects surface area (Table 3), and this in turn affects the amount of microporosity, the degree of charge on clay particle surfaces and hence cation exchange capacity (CEC), resistivity and sensitivity to fresh water.

Table 3. Surface areas and CEC values for common clay minerals.

Mineral Specific surface (m2/g) CEC (meq/100g)

kaolinite 10-20 3-10

illite 80-100 20-30

montmorillonite 800 80-120

chlorite 80 20-30

The typical effects of clay mineral cement morphology on porosity and permeability in sandstones are illustrated in Figure 6. Booklets of stacked of kaolinite platy crystals occlude macropores but do retain much microporosity which may not be producible. Chlorite often has a grain coating morphology with large surface area often restricting pore throats and so reducing permeability. However, small amounts of such chlorite are instrumental in aiding the preservation of intergranular porosity to great depths (Ehrenberg, 1993). Fibrous illite has a major effect in reducing permeability by orders of magnitude compared to the effect of other clay mineral cements. The illite fibres, up to tens of microns long, bridge pores and pore throats such that fluid flow pathways have more tortuosity and hence experience much reduced porosity.

Figure 6. Poroperm relationships in illite, chlorite and kaolinite cemented sandstones with SEM photomicrographs illustrating the different morphologies that result in the observed petrophysical properties (adapted after North 1985). SEM images reproduced from the 'Images of Clay Archive' of the Mineralogical Society of Great Britain & Ireland and The Clay Minerals Society (www.minersoc.org/gallery.php?id=2").

Some minerals may contribute to the costly consequences of formation damage, defined by Bennion (1999) as “the impairment of the invisible, by the inevitable and uncontrollable, resulting in an indeterminate reduction of the unquantifiable”. Impairment of reservoir permeability may be brought about by physico-chemical interactions of minerals and fluids during production. Fines migration occurs when drag forces dislodge fine particles into pore throughs thereby reducing pore connectivity and hence permeability. Microporous pore fill aggregates of small kaolinite platy crystals, thin grain coats of diagenetic mixed layer clays and chlorite and skeletal remnants of partially dissolved detrital feldspar grains (Figure 7) are prone to this phenomenon.

Acidizing and oxidising fluids used for enhancement of producibility by grain dissolution will lead to precipitation of iron oxyhydroxides if iron rich chlorite is present. Smectite rich clays have capacity to absorb water and swell, this leading to swelling, disaggregation and fines migration to block pore throats on contact with fresh / low salinity water. Knowledge of mineralogy is thus key to preventing and / or managing formation damage (Table 4).

Figure 7. SEM photomicrographs illustrating the fine particle size of alkali feldspar residual from diagenetic dissolution and which may be susceptible to fines migration.

Table 4. Examples of formation damage problems and their management.Mineral Problem Avoid Use

smectite swelling Fresh water KCL / Oil based systems

chlorite iron oxyhydroxide precipitation in pore throats after acidization

Oxidizing systems, high pH

Acid systems and oxygen scavengers

kaolinite fines migration High flow rates, sudden rate changes

Low and consistent flow rates

illite fines migration High flow rates, sudden rate changes, fresh water systems

Low, consistent flow rates, KCL / Oil based systems

alkali feldspar

Fines migration of skeletal remnants from dissolved detrital grains.

High flow rates, sudden rate changes

Low, consistent flow rates

Shale Gas Exploration and Production

Rig-site mineral analysis has also starting playing a part in better assessment of shale gas prospects. Pfau and Oliver (2011) reported on a 10 week field test of a SEM based mineral analysis instrument (RoqSCAN) applied to cuttings from a vertical drill hole and a long-lateral horizontal well targeting the Woodford Shale in Crockett County, Texas. In total, 600 cuttings were collected, analysed and interpreted on site. During the vertical pilot well the mineralogical data were used to characterise the target zone for the lateral well, the associated kick-off point and the curve build. The pilot well’s mineralogical data showed peaks in apatite and pyrite abundances which are key mineral features of the targeted interval for fraccing and gas recovery.

AF

100 m

AF

The mineralogical data obtained from the vertical pilot well was then used during lateral drilling to assist the drilling team to stay within the desired target frac zone for the majority of the 5,000 foot-long lateral (Figure 8). In addition the data obtained were also used to also provide rock property data including the brittleness of the rock which can be used to assist in the intelligent placement of fraccing stations to maximise production.

Figure 8. Anomalous pyrite and apatite spikes in target zone of the Woodford Shale (Pfau and Oliver, 2011).

Carbon Dixode Geosequestration

Part of the international effort to reduce the contribution of CO2 emissions to global warming uses the concept of geological storage (Baines and Worden, 2004), geosequestration, in depleted petroleum reservoirs. Generalised features of geological storage are illustrated in Figure 9 with reference to the possibility of using depleted giant oil and gas fields nearing depletion in the Gippsland Basin (Gibson-Poole et al., 2006, 2008), offshore southeast Australia (Figure 10).

Figure 9. Illustration of carbon geosequestration process in the Gippsland Basin (modified after Gibson-Poole et al., 2006; Watson and Gibson-Poole, 2005)

Figure 10. Gippsland Basin location.

CO2 is injected as a supercritical fluid into a depleted reservoir where it is stored as liquid in pore space. Reservoirs should be of good quality with intraformational seals and other horizontal permeability baffles to promote lateral migration allowing opportunities for trapping as a dissolved phase in pore water. The requirement also for a regional seal that is thick enough and wide enough to contain substantial column heights is met by the Lakes Entrance Formation that has contained hydrocarbons for ~10 Ma (Goldie Divco et al., 2010; O’Brien et al., 2008). The Gurnard Formation beneath the regional seal comprises immature glauconitic siltstones with poor quality either as seal or reservoir. Its mineralogy offers potential for mineral - CO2

reactions to form carbonates. Silicate minerals bearing calcium, magnesium, sodium and ferrous iron react with CO2 to form carbonate minerals and provide a process whereby the gas is trapped as part of a stable, solid phase. In seal lithologies and low quality reservoirs these reactions enhance seal integrity and capacity. This is because almost all carbonation reactions lead to a volume of solid products that is greater than the volume of solid reactants (Marini, 2007). The Gurnard Formation contains such potentially reactive minerals (e.g. albite, glauconitic mica).

Carbonate minerals are all potentially soluble in the presence of CO2, though conditions such as temperature, pressure, pH, formation water salinity, relative concentrations of bicarbonate and gas in solution in formation water and CO2 partial pressure will determine whether dissolution occurs subsequent to CO2 injection. In a CO2 storage and monitoring project in the Weyburn oil field, Saskatchewan, produced fluid showed increases in bicarbonate and potassium (Raistrick et al., 2008). The increase in bicarbonate after CO2 injection was ascribed mainly to dissolution and dissociation of injected CO2. The increase in potassium was ascribed to alteration of K feldspar to kaolinite.

3KAlSi3O8 (K-feldspar) + 3H2O + 3CO2 ⇒ Al4Si4O10(OH)4 (kaolinite) + 6SiO2 (quartz) + 2HCO3- + 2K+.

This is an important pH buffering reaction increasing CO2 aqueous storage as potassium bicarbonate brines. There may also be CO2 trapping in carbonates if sufficient cations are available in solution.

The kinetics of mineral reactions to form carbonates is largely unknown though natural analogues provide some insights. Wilkinson et al. (2009) suggested that the mineralogy of a UK gas reservoir with long term natural CO2 storage indicated that chemical trapping may be insignificant compared with physical mechanisms in some siliciclastic reservoirs. Only small volumes of new carbonate minerals had formed during an estimated ~50 Ma of exposure to CO2. Lu et al. (2009) examined the efficacy of a mudrock seals to prevent CO2 leakage in a natural analogue in the North Sea Miller oil field. The seal above a CO2-rich reservoir is estimated to have experienced penetration of CO2 with accompanying precipitation of new carbonate minerals for only 12 m vertical height in ~70-80 Ma since the CO2 was first trapped.

SUMMARY

The above review of mineral analysis methods and their application for well site operations demonstrates an exciting, yet under-utilised, potential throughout the life cycle of a petroleum field – from exploration through development and production to the use of exhausted fields for CO2 geosequestration.

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