Review of Traditional and New Maturity Indicators — Differences and Complementarities

Chatellier, Jean-Yves; Perez, Renee

A review of how maturity is assessed will address some of the strengths and limitations of the tools traditionally used. It will introduce some correction algorithms as well as new tools or techniques that can effectively complement the geochemist tool kit.

Following a partial review of vitrinite reflectance, Tmax issues will be addressed when dealing with high maturity samples, some corrections that can be simply applied by any geoscientist will be proposed with the scientific reasoning behind them.

If ethane and propane carbon isotopes have proved to be extremely good indicators of maturity levels, calibration is needed for each shale formation and each basin before the absolute carbon isotope values are used as an Ro (vitrinite) equivalent or as a liquid phase predictor (dry gas, wet gas, retrograde gas…). Issues found using methane carbon isotope need to be kept in mind especially in absence of ethane and propane isotope analyses for the same samples.

Butane and to a lesser extent, pentane ratios are extremely good proxies for maturity. However, the usefulness of these ratios (iC4/nC4 and iC5/nC5) varies depending on their source (e.g. isojars, isotubes or chromatography).

When dealing with hydrocarbon ratios (e.g. C3+/C1+) local permeability enhancement associated with secondary porosity (diagenesis or fracture related) may obliterate and overwhelm the maturity indicator. Such a case will be demonstrated when hydrocarbon ratio changes are tied to features seen on image logs.

Understanding maturity can be efficiently enhanced by using quartz cement modeling as a complementary tool in case of hybrid shale (e.g. Lorraine shale). Thus, whereas all of the previously described tools deal with a maximum temperature reached during burial, quartz cement precipitation is time dependent and the fluid filling the pores has to be water at 80°C or higher.

Discrepancy can naturally exist between the maturity of the organic matter (or shale) and the maturity of the fluid in the pore system. Upward migration as seen in the Bakken Formation is expressed by hydrocarbon more mature than predicted by the geochemistry of the surrounding shale. On the other hand, early migration and entrapment will be expressed by lower maturity hydrocarbon than indicated by any geochemical analysis of the organic matter; i.e. retrograde gas in a reservoir where the organic matter indicates wet gas or dry gas domain.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013