Page 101 - The Petroleum System From Source to Trap

5. Applied Source Rock Geochemistry 95

Table 5.1. Geochemical Parameters Describing the Petroleum Potential (Quantity) of an Immature Source Rock
Organic Matter
Petroleum TOC Rock-Eval Pyrolysis Bitumenc Hydrocarbons
Potential (wt. %) S 1 a S 2b (wt. %) (ppm) (ppm)
Poor Q-0.5 o--o.5 0-2.5 o-o.o5 0-500 o-300
Fair 0.5-1 0.5-1 2.5-5 0.05--0.1 0 50o-1 0oo 30Q-600
1
Good 1 - 2 1 - 2 5- 0 0.1 o -0.20 1 0 0o-2000 60o-1 200
Very Good 2-4 2-4 1 o -20 0.2(}-(.40 2000-4000 1 2 0o-2400
Excellent >4 >4 >20 >0.40 >4000 >2400
amg HC/g dry rock cistilled by pyrolysis.
bmg HC/g dry rock cracked from kerogen by pyrolysis.
cEvapcration of the solvent used to extract bitumen from a source rock or oil from a reservoir rock causes loss of the volatile hydrocarbons below about n.C15. Thus, most extracts
are desaibed as "C15+ hydrocarbons. 'Ughter hydrocas can be at least partially retained by avoiding complete evapcration of the solvent (e.g., C1o.).

Table 5.2. Geochemical Parameters Describing Kerogen Type (Quality) and the Character of Expelled Productsa
HI Main Expelled Product
Kerogen Type (mg HC/g TOC) S2/S3 Atomic H/C at Peak Maturity
I >600 > 1 5 > 1 . 5 Oil
I I 30Q-600 1 o -1 5 1 . 2-1 .5 Oil
1
11/lllb 20o-300 5-- 0 1 . 0-1 .2 Mixed oil and gas
I l l so-2oo 1 - 5 0.7-1 .0 Gas
IV <50 <1 <0.7 None
aBase on a thermally immature source rock. Ranges are approximate.
bType lVIII designates kerogens with compositions between type II and Ill pathways (e.g., Figure 5.1) that show intermediate HI (see Rgures 5.4-6.11 ).

1
oil (�0.6-- . 35% Ro) or about 60°-150°C. Thermally post­ relationships are never proven because some level of
mature organic matter is in the wet and dry gas zones uncertainty always exists depending on the available
(gas window) and has been heated to such high tempera­ data. Nonetheless, effective source rocks satisfy three
tures (about 150°-2000 C, prior to greenschist metamor­ geochemical requirements that are more easily defined
phism) that it has been reduced to a hydrogen-poor (Tables 5.1-5.3):
residue capable of generating only small amounts of
hydrocarbon gases. • Quantity, or amount of organic matter (Table 5.1)
It is generally accepted that oil is unstable at higher • Quality, or type of organic matter (Table 5.2)
temperatures and progressively decomposes to gases • Thermal maturity, or extent of burial heating
and pyrobitumen, a thermally-altered, solidified bitumen (Table 5.3).
that is insoluble in organic solvents (e.g., Hunt, 1979;
Tissot and Welte, 1984). Mango (1991) shows evidence A potential source rock contains adequate quantities of
that hydrocarbons in oil are more thermally stable than organic matter to generate petroleum, but only becomes
their kerogenous precurs. He believes that oil and gas an effective source rock when it generates bacterial gas at
are generated by direct thermal decomposition of low temperatures or it reaches the proper level of
kerogen, but that hydrocarbons in oils show no evidence thermal maturity to generate petroleum. An active source
of decomposing to gas in the earth. This scenario does rock is generating and expelling petroleum at the critical
not exclude some oxidative decomposition of hydrocar­ moment, most commonly because it is within the oil
bons during thermochemical sulfate reduction (e.g., window (Dow, 1 9 77a). An inactive source rock has
Krouse et al., 1988). stopped generating petroleum, although it still shows
petroleum potential (Barker, 1979). For example, an
Source Rock T e rms inactive source rock might be uplifted to a position
where temperatures are insufficient to allow further
Sedimentary rocks that are, or may become, or have petroleum generation. A spent oil source rock has reached
been able to generate petroleum are source rocks (Tissot the postmature stage of maturity and is incapable of
and Welte, 1984). An effective source rock is generating or further oil generation, but may still be capable of gener­
has generated and expelled petroleum. This definition ating wet and dry gas.
excludes the requirement that the accumulations be Active source rocks include rocks or sediments that
"commercially significant," because (1) the terms signifi­ are generating petroleum without thermal maturation.
cant and commercial are difficult to quantify and change For example, a peat bog might produce microbially
depending on economic factors, and (2) oil-source rock generated gas (marsh gas consisting mostly of bacteria-