Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 175

O fluid is based on an assumption that the fluid is water in which case it is equal to
89 %. Based on these assumptions, the formation oxygen content be calculated as
follows:

( ) ´

O formation =´´+´- 089 . fr 0 53 . 1 fr

fluid matrix

For a C/O ratio of 0.1 even as ˁ matrix varies from 2.4 g/cc to 3 g/cc the calculated total
carbon decreases by less than 0.1 wt% so the choice of ˁ matrix is very forgiving and
one is justified in leaving it at 2.65 for a first approximation.
The oxygen content organic material (gas, oil, kerogen, etc.) is zero. Thus the
formation oxygen content will also depend on the water saturation within the pore
space available.

Table 11.4 Oxygen content of
matrix materials (After Herron)

Mineral Wt % O

Siderite 41

Orthoclase 46

Anorthite 47

Calcite 48

Albite 49

Illite 51

Dolomite 52

Quartz 53

Kaolinite 56

Gypsum 56

Montmorillonite 59

Total Carbon content of the formation is thus derived from the C/O ratio read
from the log and an estimate of the formation oxygen content. However the carbon
present in the formation may be in the form of organic carbon but could also be
present in the form of carbonates such as calcite and dolomite. An independent
measurement is thus required to “back out” the effect of any carbonate present. For
this the same wireline inelastic gamma ray tool can be used to measure the calcium
content of the formation and from there calculate the TOC as:

TOC = CC

Total Carbonates

An example calculation:
Porosity is 10 %, therefore,

O formation =´´+ ´´ = 08901105309265 135 . . . . . .

And if the C/O ratio is 0.1 then,

C total =´ = 01 1350135 . . .