Quantity of Organic Matter
Adequate quantities of organic material must be present for a geologic unit to be considered a potential source rock. A variety of geochemical parameters measure the petroluem potenital of a source rock based on the amount of organic material present. These parameters are listed in the table below. Geologists assess the quantity of organic material in sedimentary rocks by evaluating several analyses including total organic carbon (TOC), and Rock-Eval pyrolysis.
Total Organic Carbon (TOC)
- TOC is a measure of the organic richness of a rock, i.e. it is the quantity of organic carbon (both kerogen and bitumen) in a rock sample (Jarvie, 1991; Peters and Casa, 1994).
- Kerogen is the portion of organic matter preserved in sedimentary rocks that is insoluble in organic solvents. It derives from the breakdown and diagenesis of plant and animal matter disseminated in fine grained sediments. Kerogen includes both marine- and land-derived organic matter. The latter is identical to the components of coal.
- Bitumen is the portion of organic matter in sedimentary rocks which is soluble in organic solvents. Hunt (1996) broadly defined bitumen as any native substance of variable color, hardness, and volatility, composed principally of the elements carbon and hydrogen and sometimes associated with mineral matter, the nonmineral constituents being largely soluble in carbon disulfide. The term bitumen is also used to describe organic matter that may be thermally extracted from rocks, and used informally to mean tar, pitch, and asphalt.
- TOC is reported in weight percent (wt. %) carbon, (e.g., 1.0 wt. % carbon means that in 100 grams of rock sample there is one gram of organic carbon).
- TOC is useful as a qualitative measure of petroleum potential. The qualifiers of petroleum potential which we use in this report ( Fair, etc.) apply to rocks at a thermal maturity equal to vitrinite reflectance (R o ) of 0.6% (beginning of the oil window). It is important to consider that TOC decreases with increasing thermal maturity.
- A TOC of 0.5 wt. % is widely regarded as the minimum value for defining a petroleum source rock, but most geochemists consider rocks with less than 1.0 wt. % TOC as organically lean and unlikely sources for commercial hydrocarbon accumulations (Peters and Moldowan, 1993; Hunt, 1996). The worldwide average TOC of all shales is about 0.9 wt. % and the average TOC of source rock shales is 2.2 wt. % (Miles, 1989). The average TOC of calcareous shale source rocks and carbonate (mudstone) source rocks is 1.9 wt. % and 0.7 wt. %, respectively. The worldwide average TOC of all source rocks is 1.8 wt. %, a value much higher than the oft quoted minimum of 0.5 to 1.0 wt. %. We recommend using a TOC of 1.0 wt. % as a minimum value for defining potential source rocks.
Rock-Eval Pyrolysis Parameters
Rock-Eval pyrolysis is a powerful analytical tool that assesses quantity, type and thermal maturity of whole rock and kerogen samples. The S1 and S2 values measured by Rock-Eval Pyrolysis provide information about the quantity of organic or generative potential of the rock sample. S1 and S2 are reported in milligrams of hydrocarbon per gram of dry rock. Rock-Eval pyrolysis involves two heating step: 1) Volatization of hydrocarbons in the source rock; and 2) pyrolysis of kerogen and conversion to free hydrocarbons, A third pyrolysis step generates trapped CO 2 and is an indicator of the oxygen trapped in the kerogens.
S1 : the already generated oil in the rock. These are the hydrocarbons already present in the sample and they are distilled out of the sample at the initial heating of the sample to a temperature of 350 ° C. These values may be anomalously high from migration and contamination by drilling fluids and mud.
- S1 = 1.0 mg HC/g dry rock-- Minimum value for good source rocks. .
S2 : the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter. S2 is an indication of the quantity of hydrocarbons that the rock has the potential of producing should burial and maturation continue. This parameter normally decreases with burial depths >1 km
- S2 >= 5.0 mg HC/dry rock -- Minimum value for good source rocks.