Posts Tagged ‘gas chromatography’

Biomarkers

Biomarkers are a group of compounds, primarily hydrocarbons, found in oils, rock extracts, Recent sediment extracts, and soil extracts. What distinguishes biomarkers from other compounds in oil is that biomarkers can reasonably be called “molecular fossils”. Biomarkers are structurally similar to, and are diagenetic alteration products of, specific natural products (compounds produced by living organisms). Typically, biomarkers retain all or most of the original carbon skeleton of the original natural product, and this structural similarity is what leads to the term “molecular fossils”.

Biomarkers have a variety of applications in petroleum exploration. For example:

  1. When samples of oil and candidate source rocks are available, biomarkers can be used to make oil-source rock correlations, or
  2. When samples of candidate source rocks are NOT available, the biomarker distribution in an oil can be used to infer characteristics of the source rock that generated the oil WITHOUT examining the source rock itself. Specifically, biomarkers in an oil can reveal (1) the relative amount of oil-prone vs. gas-prone organic matter in the source kerogen, (2) the age of the source rock, (3) the environment of deposition as marine, lacustrine, fluvio-deltaic or hypersaline, (4) the lithology of the source rock (carbonate vs. shale), and (5) the thermal maturity of the source rock during generation (e.g., Peters and Moldowan, 1993). Such data may be key inputs to effective basin modeling of a prospect or block.

 

Petroleum Biomarkers Indicative of Source Rock Organic Matter Input and Depositional Conditions (Table 1)

Below are a few examples of oil biomarker parameters that provide information about the depositional environment of the source rock and the origin of the organic matter in the source rock.

Source Information Biomarker Parameter Comments
Marine Source Rock 24-n-propylcholestanes Ubiquitous in oils derived from marine source rocks. (Moldowan et al., 1990)
C4246 Cyclopentylalkanes with odd/even carbon preference (Carlson et al. 1993; Hsieh and Philp, 2001)
Lacustrine Source Rock Botryococcane Presence = lacustrine source. Absence = meaningless. (e.g., Moldowan et al., 1980, Metzger and Laegeau 1999)
b-Carotane Presence = lacustrine source. Absence = meaningless. (Hall and Douglas, 1983; Jiang and Fowler, 1986)
Sterane/Hopanes Low in oils derived from lacustrine source rocks. (Moldowan et al., 1985)
C26/C25 tricyclic terpanes > 1 in many lacustrine-shale-sourced oils. (Zumberge, 1987)
Tetracyclic Polyprenoids High in oils from lacustrine sources. (Holba et al., 2000)
C4246 Cyclopentylalkanes with even/odd carbon preference or with no preference (Carlson et al. 1993; Hsieh and Philp, 2001)
Higher plant input to Source Rock Oleananes, Lupanes, Taraxeranes Biomarkers indicating flowering plant input to source. (e.g., Ekweozor and Udo, 1988)
Bicadinanes Derived from Dipterocarpaceae tree resins. (Cox et al., 1986)
Retene, Cadalene Biomarkers indicating conifer input to source. (Noble et al., 1985)
Tetracyclic diterpanes Biomarkers indicating conifer input to source. (Noble et al., 1985)
C29 steranes High relative to total C27-C29 steranes. (Huang and Meinschein, 1979; Moldowan et al., 1985)
Coal Source Rock Pristane/phytane Very high in coal-sourced oils; e.g., > 3.0 (Hughes et al., 1995)
C31 homohopanes High relative to total C31-C35 in some coal-sourced oils
Hypersaline Depositional Environment Gammacerane High relative to C31 hopanes in oils derived from sources deposited under hypersaline depositional conditions. High values indicate stratified water column during source deposition. (Sinninghe Damste et al., 1995)
Pristane/phytane Very low values (e.g., < 0.5) in oils derived from source rocks deposited under hypersaline conditions (due to contribution of phytane from halophilic bacteria). (ten Haven et al., 1987; 1988)
Anoxic Depositional Environment of Source Rock C35 homohopanes High relative to total hopanes in oils derived from source rocks deposited under anoxic conditions (Peters and Moldowan, 1991). Abundance of C35 homohopanes in oils (Relative to C31-C34 homohopanes) is correlated with source rock Hydrogen Index (Dahl et al., 1994).
Pristane/phytane 1.0 can indicate anoxic conditions, but the ratio is affected by many other factors.
Isorenieratane & related compounds (2,3,6 and 2,3,4 – Trimethylaryl isoprenoids), Chlorobacteria Presence in oil indicates anoxic photic zone during source rock deposition, since these compounds are biomarkers for green sulfur bacteria. (Summons and Powell, 1987; Grice et al., 1998; Koopmans et al., 1996)
V/(V+Ni) Porphyrins High = reducing conditions. (Lewan, 1984)
28,30-bisnorhopane High in certain reducing environments. (Schoell et al., 1992; Moldowan et al., 1984)
Carbonate Source Rock 30-norhopanes High in carbonate-sourced oils; e.g., C29/C30 hopanes ~ 1 (Fan Pu et al., 1987; ten Haven et al., 1988; Subroto et al., 1991)
Diasteranes/steranes Low in carbonate-sourced oils. (Rubinstein et al., 1975; Hughes, 1984)
Dibenzothiophene/phenanthrene > 1.0 in oils derived from high-sulfur carbonates. (Hughes et al., 1995)
2a-methylhopanes High in carbonate derived oils (Summons et al., 1999)
Age of Source Rock Deposition Oleanane Present in oils derived from Late Cretaceous or younger sources (Moldowan et al., 1994)
(24-norcholestanes)/(26-norcholestanes) High in many Tertiary sources. Low values are not age-diagnostic. (Holba et al., 1998A; 1998B)
Dinosteranes, triaromatic dinosteroids Absence always means Pre-Mesozoic, while presence USUALLY means Mesozoic or younger. (Moldowan et al., 1996)
C29 Monoaromatic Steroids High in oils derived from sources older than 350 mybp. (Moldowan et al., 1985)
C11-C19 Paraffins Odd-carbon-number predominance in oil from many Ordovician sources. (Douglas et al., 1991; Fowler, 1992)
(24-isopropylcholestanes)/(24-n-propylcholestanes) High in oils from pre-Ordovician sources. (McCaffrey et al., 1994B)

To characterize charge risk, these biomarker parameters can be used in a variety of innovative ways. For example, specific biomarker parameters can be calibrated against specific kerogen quality parameters in a given basin. Then, the biomarker ratios are measured in an oil sample from the basin, and the values are projected onto calibration curves to quantitatively predict characteristics of the source rock. This approach, pioneered by the founders of OilTracers, allows explorationists to assess whether an oil was generated primarily from an oil-prone or gas-prone organic facies (Dahl et al., 1994; McCaffrey et al., 1994). The information gained from oil biomarkers (source type, age, maturity, kerogen quality) when integrated into a basin model has substantial economic impact because it provides early estimates of oil quantity and GOR for exploration targets in the area of interest.

 

Using Biomarkers in Oil to Assess Source Thermal Maturity

The relative abundances of certain biomarkers in petroleum change as a function of source rock maturity. As a result, a variety of biomarker parameters have been identified that are very useful for characterizing the source rock maturity simply from analysis of the migrated oil (e.g., Peters and Moldowan, 1993).

Biomarker maturity parameters (e.g., parameters such as those in Table 2) make use of several processes that occur during source rock maturation:

  1. Cracking–large molecules break into smaller molecules
  2. Isomerization–changes in the 3-dimentional arrangements of atoms in molecules.
  3. Aromatization–formation of aromatic rings (loss of hydrogen from naphthenes)

Petroleum Biomarkers Indicative of Source Rock Maturity (Table 2)

Petroleum Fraction (Compound Class) Biomarker Parameter Measured in Petroleum Fraction Effect of Increasing Maturity Comments
Saturated Hydrocarbons C29 Steranes [20S/(20S+20R)] Increase Useful in early to mid oil window. Decreases at very high maturity levels.
C29 Steranes [abb/(abb+aaa)] Increase Useful in early to mid oil window.
Moretane/Hopane Decrease Useful in early oil window.
C31 Hopane [22S/(22S+22R)] Increase Useful in immature rocks to onset of early oil window.
Ts/(Ts+Tm) Increase Also influenced by source lithology.
Tricyclic Terpanes/Hopanes Increase Useful in late oil window; also increases at high levels of biodegradation.
Diasteranes/Steranes Increase Useful in late oil window; also affected by source lithology (low in carbonates, high in shales); also increases at high levels of biodegradation.
Aromatic Hydrocarbons Monoaromatic Steroids: (C21+C22)/ [C21+C22+C27+C28+C29] Increase Useful in early to late oil window; resistant to effects of biodegradation.
Triaromatic Steroids: (C20+C21)/ [C20+C21+C26+C27+C28] Increase Useful in early to late oil window; resistant to effects of biodegradation.
Triaromatic /(Monoaromatic + Triaromatic Steroids) Increase Useful in early to late oil window; resistant to effects of biodegradation.

Several considerations must be kept in mind when using petroleum biomarkers to assess source rock thermal maturity. For example:

  1. The exact relationship between a biomarker parameter and the source maturity is a function of heating rate, source lithofacies, and source organic facies (kerogen type). As a result, the exact maturity (i.e., vitrinite reflectance equivalent) associated with a given value for a biomarker parameter can change from basin to basin. Furthermore, the relationship between a biomarker maturity indicator and source rock maturity is generally non-linear.
  2. With increasing maturity, many biomarker maturity indicators reach terminal values; hence, a given biomarker parameter is applicable only over a specific maturity range.
  3. The concentrations of biomarkers in petroleum decrease with thermal maturity.

Despite these limitations, biomarker indicators of source maturity can be extremely useful. For example, biomarker maturity parameters can be used to determine what the API gravity of a biodegraded oil was prior to biodegradation. This is accomplished by collecting a suite of non-degraded oils from the same petroleum system as the degraded oils. Using the non-degraded oils, the geochemist develops a correlation or “transform” between a biomarker maturity parameter and API gravity. The same biomarker parameter is then measured on a degraded oil, and the original gravity is determined using the transform developed from the non-degraded oil suite. Moldowan, et al. (1992) provide an excellent example of this approach in which they determine the original gravity of degraded Adriatic oils. For this application, the most effective biomarker parameters are those based on compounds that are highly resistant to biodegradation, such as [Triaromatic/(Monaromatic +Triaromatic steroids)].

Source Rock descriptions and source rock maturity information derived from oil biomarkers are often key input data for basin modeling of a prospect or block.

Biomarkers in Petroleum are analyzed by gas chromatography mass spectrometry (GC-MS) or gas chromatography – tandem mass spectrometry (GC-MS-MS). Analyses are typically performed on the saturated hydrocarbon fraction or the aromatic hydrocarbon fractions. The oil fractions are prepared by liquid chromatography.

 

source : Weatherford Laboratories Service

Secara umum, chromatography  merupakan suatu istilah yang menggambarkan teknik yang digunakan untuk memisahkan komponen-komponen dari suatu campuran/sample. Dalam gas chromatography (GC), gas (yang biasa disebut carrier gas) digunakan untuk membawa sample melewati lapisan (bed) material. Karena gas yang bergerak, maka disebut mobile phase (fasa bergerak), sebaliknya lapisan material yang diam disebut stationary phase (fasa diam). Ketika mobile phase membawa sample melewati stationary phase, sebagian komponen sample akan lebih cenderung menempel ke stationary phase dan bergerak lebih lama dari komponen lainnya, sehingga  masing-masing komponen akan keluar dari stationary phase pada saat yang berbeda. Dengan cara ini komponen-komponen sample dipisahkan.

Secara umum, peralatan GC terdiri dari: 1) Injection System; 2) Oven; 3) Control System; 4) Column; 5) Detector; dan 6) Data Acquisition System.

Injection system digunakan untuk memasukkan/menyemprot gas dan sample kedalam column.  Ada beberapa jenis injection system:
1) Packed column injector; umumnya digunakan dengan package column atau capillary column dengan diameter yang agak besar; injeksi dilakukan secara langsung (direct injection).
2) Split/Splitless capillary injector, digunakan dengan capillary column; sebagian gas/sample dibuang melalui split valve.
3) Temperature programmable cool on-column, digunakan dengan cool capillary column, injeksi dilakukan secara langsung.

Oven, digunakan untuk memanaskan column pada temperature tertentu sehingga mempermudah proses pemisahan komponen sample.

Column, berisi stationary phase dimana mobile phase akan lewat didalamnya sambil membawa sample. Secara umum terdapat 2 jenis column, yaitu:
1) Packed column, umumnya terbuat dari glass atau stainless steel coil dengan panjang 1 – 5 m dan diameter kira-kira 5 mm.
2) Capillary column, umumnya terbuat dari purified silicate glass dengan panjang 10-100 m dan diameter kira-kira 250 mm. Beberapa jenis stationary phase yang sering digunakan: a) Polysiloxanes untuk nonpolar analytes/sample.  b) Polyethylene glycol untuk polar analytes/sample. c) Inorganic atau polymer packing untuk sample bersifat small gaseous species.

Control system, berfungsi untuk: 1) Mengontrol pressure dan flow dari mobile phase yang masuk ke column. 2) Mengontrol temperature oven.

Detector, berfungsi mendeteksi adanya komponen yang keluar dari column. Ada beberapa jenis detector, yaitu:
1) Atomic-Emission Detector (AED); cara kerjanya adalah: campuran sample-gas yang keluar dari column diberi tambahan energy dengan menggunakan microwave sehingga atom-atomnya bereksitasi; sinar eksitasi ini kemudian diuraikan oleh diffraction grating dan diukur oleh photodiode array; kehadiran komponen dalam sample dapat ditentukan dari adanya panjang gelombang eksitasi komponen tersebut yang diukur oleh photodiode array.
2) Atomic-Emission Spectroscopy (AES) atau Optical Emission Spectroscopy (OES); cara kerjanya: campuran sample-gas yang keluar dari column diberi tambahan energy sehingga atom-atomnya bereksitasi; sumber energy tambahan ini (excitation source) terdiri dari beberapa jenis yaitu direct-current-plasma (DCP), flame, inductively-coupled plasma (ICP) dan laser-induced breakdown (LIBS); sinar eksitasi dari berbagai atom ini kemudian diukur secara simultan oleh polychromator dan multiple detector; polychromator disini berfungsi sebagai wavelength selector.
3) Chemiluminescense Spectroscopy; cara kerjanya sama seperti pada AES yaitu mengukur sinar eksitasi dari sample yang diberi tambahan energy; perbedaan dari AES adalah eksitasi molekul sample bukan atom sample; selain itu, energy tambahan yang diberikan bukan berasal dari sumber energy luar seperti lampu atau laser tetapi dihasilkan dari reaksi kimia antara sample dan reagent; sinar eksitasi molekul sample ini kemudian diukur dengan photomultiplier detector (PTM).
4) Electron Capture Detector (ECD); menggunakan radioactive beta emitter (electron) untuk mengionisasi sebagian gas (carrier gas) dan menghasilkan arus antara biased pair of electron; ketika molekul organik yang mengandung electronegative functional groups seperti halogen, phosphorous dan nitro groups dilewati detector, mereka akan menangkap sebagian electron sehingga mengurangi arus yang diukur antara electrode.
5) Flame Ionization Detector (FID); terdiri dari hydrogen/air flame dan collector plate; sample yang keluar dari column dilewatkan ke flame yang akan menguraikan molekul organik dan menghasilkan ion-ion; ion-ion tersebut dihimpun pada biased electrode (collector plate) dan menghasilkan sinyal elektrik.
6) Flame Photometric Detector (FPD); digunakan untuk mendeteksi kandungan sulfur atau phosphorous pada sample. Peralatan ini menggunakan reaksi chemiluminescent sample dalam hydrogen/air flame; sinar eksitasi sebagai hasil reaksi ini kemudian diukur oleh PMT.
7) Mass Spectrometry (MS); mengukur perbedaan mass-to-charge ratio (m/e) dari ionisasi atom atau molekul untuk menentukan kuantitasi atom atau molekul tersebut.
8.) Nitrogen Phosphorus Detector (NPD); prinsip kerjanya hampir sama dengan FID, perbedaan utamanya adalah hydrogen/air flame pada FID diganti oleh heated rubidium silicate bead pada NPD; sample dari column dilewatkan ke hot bead; garam rubidium yang panas akan memancarkan ion ketika sample yang mengandung nitrogen dan phosphorous melewatinya; sama dengan pada FID, ion-ion tersebut dihimpun pada collector dan menghasilkan arus listrik.
9) Photoionization Detector (PID); digunakan untuk mendeteksi aromatic hydrocarbon atau organo-heteroatom pada sample; sample yang keluar dari column diberi sinar ultraviolet yang cukup sehingga terjadi eksitasi yang melepaskan electron (ionisasi); ion/electron ini kemudian dikumpulkan pada electroda sehingga menghasilkan arus listrik.
10) Thermal Conductivity Detector (TCD); TCD terdiri dari electrically-heated wire atau thermistor; temperature sensing element bergantung pada thermal conductivity dari gas yang mengalir disekitarnya; perubahan thermal conductivity seperti ketika adanya molekul organik dalam sample yang dibawah carrier gas, menyebabkan kenaikan temperature pada sensing element yang diukur sebagai perubahan resistansi. 11) Photodiode Array Detector (PAD); merupakan linear array discrete photodiode pada sebuah IC; pada spectroscopy, PAD ditempatkan pada image plane dari spectroscopy sehingga memungkinkan deteksi panjang gelombang pada rentang yang luas bisa dilakukan secara simultan.

Data Aquisition, berfungsi sebagai: 1) Control automatic calibration; 2) Gas analysis; dan 3) Graphics & Reporting. Data aquisition merupakan perangkat gabungan dari Software dan Hardware (PC, Interface & Communication).

Technical Specification; Beberapa parameter yang menjadi ukuran spesifikasi teknis GC, antara lain:
1) Analytes; menyatakan komponen-komponen yang akan dianalisa/dideteksi.
2) Quantification limit (detectability); menyatakan kemampuan deteksi terkecil, dinyatakan dalam persen.
3) Measurement range; menyatakan kemampuan rentang pengukuran GC.
4) Communication Port ; digital port untuk komunikasi dengan PC atau perangkat digital lainnya.
5) Electrical Power Supply (voltage, phase, frequency, power).

 

Penulis : Ir Yosep Asro Wain
(Mantan Instrument Project Engineer, Kilang Putri Tujuh, UP II Dumai, PT Pertamina)

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