Maturity and the Hydrocarbon Windows
Learning objectives
- Distinguish richness (TOC) from maturity (what the kerogen became)
- Place a rock in the immature, oil, condensate, or gas window by Ro
- Name the hydrocarbon each window generates
- See that maturity scales the Passey TOC for a fixed Delta-logR
How Much, and What It Became
Richness and maturity are two different questions. The TOC says how much organic matter the rock holds; maturity says what heat and time have turned it into. A rich but immature shale has generated nothing yet; a rich, over-mature one has already given up everything it had. The standard maturity measure is vitrinite reflectance, Ro, in percent.
The Windows
As a source rock is buried and heated it passes through windows. Below about 0.55 percent Ro it is immature. From roughly 0.55 to 0.9 it is in the oil window. From 0.9 to 1.3 it makes condensate and wet gas; from 1.3 to 2.0, dry gas; above 2.0 it is over-mature, spent. The same rock, deeper and hotter, is a different play.
Why the Passey TOC Needs It
Maturity is also what scales the Passey separation into a TOC. A given Delta-logR reads a high TOC in an immature rock and a much lower one in the gas window, because mature kerogen has already converted much of its carbon into the hydrocarbons it expelled. Without a maturity estimate, the separation is only a relative richness, not a number.
References
- Tissot, B. P. and Welte, D. H. (1984). Petroleum Formation and Occurrence, 2nd ed. Springer.
- Passey, Q. R. et al. (1990). A practical model for organic richness from porosity and resistivity logs. AAPG Bulletin, 74(12), 1777-1794.