Page 106 - The Petroleum System From Source to Trap

100 Peters and Cassa

Appendix C). TOC is not a clear indicator of petroleum cuttings are given in Schaefer et al. (1978), Reitsema et al.
potential. For example, graphite is essentially 100% (1981), and Whelan (1984). Other procedures are used for
carbon, but it will not generate petroleum. Some Tertiary sampling gases under pressure (Gas Processors Associa­
deltaic marine shales contain up to 5 wt. o/o TOC but tion, 1986). The more advanced aspects of gas geochem­
generate little if any petroleum because the organic istry are beyond the scope of this chapter, which deals
matter is gas prone or inert. The theory and pitfalls of primarily with rapid screening methods for evaluating
Rock-Eval pyrolysis interpretation are discussed by oils and source rocks. However, readers should be aware
Peters (1986) and are not repeated here. Key parameters that analysis of gases is likely to become increasingly
are defined in Chapter Appendix D. important as future exploration shifts from oil to gas.

Gas Analysis Organic Petrography
Residual gas (Ct-Cs) and heavier hydrocarbons in Thennal Alteration Index
drill cuttings and mud arriving at the shaker table can be
liberated with a blender and analyzed by g as chromato g ­ Thermal alteration index (T AI) is a numerical scale
raphy (GC) at the well site as part of a process called based on thermally induced color changes in spores and
hydrocarbon mud lo gg in g . Some systems use a simple hot pollen. Strew-mount slides of kerogen are examined in
wire detector to make only two measurements, methane transmitted light, typically using a split-stage compar­
and ethane-plus hydrocarbons. Hydrocarbon mud log ison microscope. The analyst matches the color of the
gas curves are commonly available from wildcat wells specimen under one ocular with that of a standard under
and provide useful information on hydrocarbon shows the other ocular of the microscope. Several TAl scales
(e.g., see Figure 5.7). have been published (e.g., Staplin, 1969; Jones and
Alternately, gaseous hydrocarbons can be detected at Edison, 1978). An advantage of T AI is that the greatest
the well site or in the laboratory using an oil show color changes occur in the oil window. T AI measure­
analyzer (Espitalie et al., 1984) or by hydrogen stripping ments are imprecise because description of color is
GC (Schaefer, et al., 1978). In GC, an inert carrier gas subjective, palynomorph thickness and type affect
(mobile phase) passes through a column coated with a results, and many samples contain few palynomorphs.
nonvolatile, high molecular weight liquid (stationary Quantitative spore color measurements (Marshall, 1991)
phase). The temperature of the column is gradually offer the possibility of more precise assessment of
raised using a temperature-programmed oven. thermal maturity. Despite limitations, TAl commonly
Petroleum components are separated depending on their provides useful data, even when other maturity parame­
volatility and affinity for the mobile versus stationary ters fail.
phases as they pass through the column. A plot of
detector response versus time shows separated peaks Vitrinite Reflectance
representing single or multiple components and is called Vitrinite reflectance (Ro) increases during thermal matu­
a chromato g ram. ration due to complex, irreversible aromatization
H e adspace g as analysis is sometimes used as a screening reactions. Approximate Ro, TAl, and Tmax values have
tool because it assists in quantitative show detection been assigned for the beginning and end of oil genera­
(Tissot and Welte, 1984). For this method, cuttings are tion (Table 5.3). Ro versus depth plots generally show
frozen or canned with water and a bactericide. Agitation linear trends on semi-log paper. Dow (1977b) showed
and/ or heating releases some of the hydrocarbons from how these plots can be used to support the existence of
the cuttings into the headspace over the water, which can faults, intrusions, and changes in geothermal gradient
be sampled through a septum with a syringe and and how to estimate the thickness of a section lost at an
analyzed by GC (e.g., Bernard, 1978; Whelan, 1 9 84). unconformity. This information provides valuable cali­
Many choose not to use this technique because it is costly bration for reconstructing burial histories.
and time consuming and metal cans rust or leak in For vitrinite reflectance, kerogen isolated from sedi­
storage. Furthermore, this method is not particularly mentary rocks is embedded in epoxy on a slide or in an
useful for establishing maturity profiles because gas epoxy plug and polished to a flat, shiny surface (Bostick
readily migrates. Vitrinite reflectance and Rock-Eval and Alpern, 1977; Baskin, 1979). Measurements are made
pyrolysis are more reliable methods for establishing of the percentage of incident light (usually at a wave­
thermal maturity profiles than gas analysis. length of 546 nm) reflected from vitrinite particles
Light hydrocarbon gas distributions combined with (preferably telocollinite) under oil immersion (Stach et
isotopic compositions can be used to describe the origin al., 1982). The subscript "o" in Ro refers to oil immersion.
and level of thermal maturity of the gas (e.g., Rice and Some old papers refer to Ra and RWI reflectance in air
Claypool, 1981; James, 1983; Schoell, 1984). Reliable and water, respectively. Vitrinite becomes anisotropic at
sampling methods are important because sample high levels of maturity (above about 1 o/o Ro), resulting in
handling can alter these gas compositions. For example, the terms Rm in and Rmax for the minimum and
drill cuttings used for headspace gas analyses should be maximum reflectance values obtained upon rotation of
kept in gas-tight containers at deep freeze temperatures each particle. Most kerogen studies report random mean
to avoid evaporative loss of components. Examples of Ro rather than Rmin or Rmax because rotation of the
procedures for sampling gases in drilling muds and microscope stage is not required.