Model of Organic Carbon in a Sediment Sample

The formation of kerogen in a sediment results from alteration of organic matter as it is deposited and exposed to higher temperature as it is buried. Kerogen is, by definition, matter that is insoluble in organic solvents and acids. The bulk of sediment is an inorganic matrix.

If the total organic carbon (TOC) of a sediment sample is approximately 1.00% (by weight), there is 1 gram of organic matter per 100 grams of sediment. Is the entirety of the TOC available to generate hydrocarbons? Of course, the answer is no as the conversion of organic matter to hydrocarbons is also dependent upon the hydrogen balance. Therefore, only a portion of the TOC is available for generation of petroleum. A model of TOC illustrates this. Figure 1 illustrates approximately a 1% TOC sample. This TOC is present as the carbon in extractable organic matter (EOM) and kerogen. The carbon in kerogen is present in reactive and inert forms (Cooles et al, 1986). Reactive kerogen is composed of a labile (oil prone) fraction and a refractory (gas prone) fraction. The inert carbon has no potential to yield hydrocarbons as it is largely devoid of hydrogen. If a hydrogen donor was available, then inert carbon would have the possibility of generating hydrocarbons.

Sample Description
Fig 1

In pyrolysis jargon, the following relationships are known (Jarvie, 1991) (Figure 2):

Sample Description
Fig 2

EOM carbon = the carbon present in the free oil or S1 peak
Reactive carbon = the carbon present in kerogen or the S2 carbon
Inert carbon = the dead carbon present in kerogen or the S4 carbon

The EOM and reactive carbon can be calculated from S1 and S2 data as follows:

convertible organic carbon = 0.083 x (S1 + S2)

i.e., the oil and kerogen yields are about 83% carbon which must be normalized to weight percent by dividing by 10 (S1 and S2 are in o/oo)

The total organic carbon can be determined by measuring the inert carbon left in the rock after pyrolysis by oxidation. The resulting value is the dead carbon content (referred to as S4 in mg carbon/gram rock). Thus, the total organic carbon is:

TOC = 0.083 x (S1 + S2) + 0.10 x (S4)

The inert carbon can also be calculated by subtracting the EOM and reactive carbon from the TOC, i.e.,

Inert Carbon = 10 x TOC - 0.083 x (S1 + S2)

If a rock has 5% TOC is it a good oil prone petroleum source rock? That depends upon the quality of the organic matter largely in terms of its hydrogen content. A comparison of the organic carbon composition of three different source rock types is illustrated in Figure 3. All samples are of equal maturity approximately 10% converted (note EOM carbon contents are equal).

Pie Charts
Fig 3

The Type I kerogen illustrated has 60% of its 5% TOC, i.e., 3.00%, available as hydrogen rich labile carbon (see upper pie chart). A small percentage of the kerogen is available as hydrogen poor refractory carbon with 20% available as hydrogen depleted carbon.

On the other hand, the Type II and Type III kerogens have much lower amounts of oil prone labile carbon (40% and 5%, respectively). This means that 2.00% of the organic carbon is present as labile carbon in the Type II kerogen and only 0.25% in the Type III kerogen. Note the increasingly larger amounts of refractory and inert carbon going from the Type II to Type III gas prone kerogen.

What does a hydrogen index (S2 x 100 / TOC) of 270 mean? In classical geochemistry terms, we would describe an HI of 270 as mixed gas/oil prone organic matter (Jones, 1984). Unfortunately, from HI values alone we are not able to discern what the composition of the reactive carbon is from an HI of 270, i.e., what percent is kerogen present as oil prone labile carbon and what percent is available as gas prone refractory carbon. Of course, other geochemical techniques are used to further detail what type of organic matter is present, e.g., maceral composition and percentages.

Two of the best tools available that are widely under-utilized are artificial maturation techniques such as hydrous pyrolysis and MicroScale Sealed Vessel (MSSV) pyrolysis. MSSV pyrolysis is one of the easiest techniques available to evaluate the product yield and distribution in reactive kerogen (Horsfield et al, 1989). Only small amounts of sample are required (a few milligrams of rock or kerogen). The sample is loaded into 40 microliter glass tubes which are subsequently purged and sealed. The sealed tube is then heated at temperature ranging from 275 to 365° C typically for 24 to 72 hours. During this thermal exposure, products are generated from the reactive kerogen and contained in the sealed tube. The sealed tube is subsequently broken in a modified GC injector and the products formed – gas and oil – are fingerprinted. This is illustrated in Figure 4 which is a fingerprint of the products formed during laboratory maturation of a sample having an HI of 270. As can be seen the products formed result in a distribution of products ranging from methane to C25+ alkanes. By quantitating the results, the yield of gas and oil can be determined. Thus, the expected GOR of this sample at a given level of maturity or conversion can be determined.

GC Trace
Fig 4

References:

Cooles, G. P., A. S. MacKenzie, and T. M. Quigley, 1986, Calculation of petroleum masses generated and expelled from source rocks. In Advances in Organic Geochemisty, D. Leythaeuser and J. Rullkoetter, eds., Advances in Organic Geochemistry 1985, Org. Geochem. 10, pp. 235-245.

, 1991, Total Organic Carbon (TOC) Analysis. In Source and migration processes and evaluation techniques, R. K. Merrill, ed., AAPG Treatise of Petroleum Geology, Handbook of Petroleum Geology, pp. 113-118.

Jones, R. W., 1984, Comparison of Carbonate and Shale Source Rocks, in Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, J. Palacas, ed., AAPG Studies in Geology 18, pp. 163-180.

Horsfield, B., U. Disko, and F. Leistner, 1989, The micro-scale simulation of maturation: Outline of a new technique and its potential applications. Geologische Rundschau Vol. 78, No. 1, pp. 361-374.