--> Shale resource systems: Geochemical assessment of reservoir rock

Southwest Section AAPG Annual Convention

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Shale resource systems: Geochemical assessment of reservoir rock


Shale resource systems are highly variable and one of the key differences is between organic-rich source rock that also serve as a reservoir and juxtaposed, organic-lean reservoir rocks, i.e., a hybrid system. These quite different reservoirs have substantially different in situ petroleum compositions that affect production rate and ultimate recovery. While it may be assumed that what is produced is what is in the reservoir, there are changes due to production fractionation and expulsion fractionation in terms of hybrid reservoirs. These differences include petroleum composition, quality, and intrinsic GOR also depending on storage type, e.g., matrix versus organoporosity in large or small pores.

During petroleum generation the first products are largely non-hydrocarbon constituents with minor amounts of hydrocarbons. These non-hydrocarbons are commonly referred to as ‘bitumen’ but they are essentially petroleum that is highly concentrated in polar compounds referred to as resins and asphaltenes. These polar compounds have are high polarity and low viscosity so their percentages in petroleum is very important factor in production results. With increasing thermal maturity additional bitumen is formed, but contemporaneously, previously formed bitumen is decomposed to hydrocarbons as well as more refractory bitumen. At this point the polars are solubilized and viscosity decreases. This process continues until petroleum is decomposed to dry gas and pyrobitumen above about 2.0% vitrinite reflectance.

The components and properties of the reservoir rock and retained or expelled petroleum is readily assessed by simple geochemical techniques that enable prediction of producibility including petroleum characterization in terms of API gravity and intrinsic GOR values.