# Production Index (PI)

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Critical elements of the petroleum system Evaluating source rocks Carol A. Law Web page AAPG Store

Maturity of a homogeneous source section can be accessed from the production index (PI). PI is calculated from Rock-Eval data:

${\mbox{PI}}={\frac {{\mbox{S1}}}{({\mbox{S1}}+{\mbox{S2}})}}$

where S1 is the quantity of free hydrocarbons (gas + oil), in mg/g of rock and S2 is the quantity of thermally generated (cracked) hydrocarbons, in mg/g of rock [1].

## How to apply PI

PI increases continuously through the oil window up to a value of 0.50. The following cutoffs can generally be applied.

Production index Generation zone
< 0.10 Immature
0.10-0.30 Oil generation
> 0.30 Gas generation/oil cracking

## Caveat

PI data are misleading if the S1 peak includes nonindigenous hydrocarbons, such as drilling additives or migrated hydrocarbons. Expulsion characteristics of a source rock are not considered when looking at PI numbers. If the expulsion saturation threshold of the source rock is high, the PI data will be overestimated. If the expulsion saturation threshold is low, the data will be underestimated.

A trend is valid only if developed over a uniform source interval. If the depositional environment changes significantly, establish a different PI trend for each unique source rock type.