Maturation

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

The chemistry of organic matter contained within a sedimentary rock changes over time, reflecting its thermal and burial history. This change or maturation is measured and can be combined with quality and richness data to gauge the amount of hydrocarbon generated by the organic matter. The present-day maturity level is the product of a number of variables, such as tectonic setting, burial history, and thermal history. A number of variables such as paleolatitude, pressure, fluid flow, rock matrix chemistry, and pore fluid chemistry can affect the thermal history and thus a rock's rate of maturation.

Transformation rate

The rate at which hydrocarbons are generated from organic matter is called the transformation rate. Using source rock maturity, we can estimate the transformation rate. Different kerogen types (1) generate hydrocarbons at different maturity levels and (2) have different transformation rates. Maturity measurements can be made on several different components of a rock like vitrinite, kerogen, spores, apatite grains, and biomarkers; each has its own relationship to the kerogen transformation rate.

Caveat

Evaluating the maturation of a geologic section is based on a depth trend from samples in a well or basin. The level of maturity interpreted from most maturation indices is dependent on the type of organic matter or material being analyzed.

For example, Tmax cutoffs for hydrocarbon-generation zones are greater for type III than for type II kerogen. Therefore, a trend is only valid if based on analysis of samples from a homogeneous organic sequence or if differences in chemistry are accounted for in the interpretation process.

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