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A New Screening Tool for the Rapid Evaluation of Gas Sorption Capacity in Shales

Nicolaj Mahlstedt and Brian Horsfield
GFZ German Research Centre for Geosciences, Potsdam, Germany

Quantifying the gas stored in shale-gas reservoirs in “free” and “sorbed” states is critical for the assessment of Gas-In-Place (GIP) and the design of effective production strategies. Petrophysical methods deployed for these assessments are slow and costly. Here we report the development of a new geochemical logging method to evaluate the amount and composition of sorbed gas within source rocks quickly and inexpensively, thereby providing a practical tool for the rapid identification of “sweet spots” or heterogeneities within vertical profiles, e.g. high water saturation within certain horizons. Thermovaporisation GC-FID (Tvap) is the basis of the screening method which works on the principle that small amounts of C1-5 gases in powdered rock samples can be released and detected by temperature triggered desorption from organic and mineral matter surfaces. Tvap can be easily run alongside Rock- Eval pyrolysis in high resolution and is capable of defining the basic sorption characteristics of source rocks or gas shales, demonstrated by a very good correlation of the said data with standard excess sorbed methane versus pressure curves. It is clear that screening measurements alone cannot replace established procedures for determining correct and reproducible adsorption isotherms, but the technique offers important insights into heterogeneities in behaviour within shale sequences. Classical methods on the other hand can only be deployed for a small number of samples and a limited range of experimental temperatures, conditions, and gas species. Tvap thus additionally provides the opportunity to upscale determined Langmuir parameters for basin modeling.

Applying the aforementioned screening tool to an extensive sample set we linked gas retention behaviour to kerogen structure and maturity. One major outcome of paramount importance is that not the amount of “dead” carbon alone is crucial for the overall sorption capacity of a source rock but also the structure of the “live“organic matter. We found that aromaticity and sorption capacity of the organic matter are positively correlated. Keeping in mind that Type II kerogen in marine shales may be similar according to Rock-Eval parameter definition but can comprise very different structural OM units, a more aromatic and shortchain dominated kerogen exhibits higher sorption capacities than a less aromatic and shortchain dominated kerogen. Based on open-and closed-system pyrolysis GC-FID methods we demonstrate that Lower Paleozoic source rocks (Cambrian Shales - Alum Shale/Sweden; Australia) often exhibit, related to rare precursor structures directly inherited from biota, a much higher aromaticity than many Mesozoic source rocks (e.g. Posidonia Shale/Germany), which leads upon pyrolysis and throughout natural maturation to the formation of petroleums with unusually high gas-oil-ratios besides formation of large amounts of inert carbon. In any case, this causes a generally much higher sorption capacity of Lower Paleozoic shales compared to Mesozoic Shales at all maturity levels.

We illustrate all of the above findings using a backcloth of gas shales from Europe and North America.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013