Source rock formations generally show a lower density, a lower sonic transit time, and a higher resistivity than other sediments of equal compaction and comparable mineralogy. This phenomenon can be used to identify source rocks on wireline logs provided the source rocks have a minimum thickness within the resolution of the sondes used and are sufficiently rich in organic matter. Classification rules have been established to assist in the recognition of source rocks on a combination of logs.
Because of the low density contrast between water and organic matter, the method becomes inaccurate at high water saturations (i.e., low compaction). Within limits, the amount of organic matter contained in a sediment can be estimated from log anomalies.
When source rocks become mature, free oil is present in addition to kerogen, and the resistivity increases by a factor of 10 or more.