From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, 10:00 AM, April 6, 2012
Quantitative Assessment of Organic Matter-Hosted Pores, Marcellus Formation (Devonian), Pennsylvania, USA
Dr. Kitty Milliken
Senior Research Scientist, BEG
Recognition of OM-hosted pore systems has sparked the notion that porosity evolution in shales may in part be controlled by organic matter maturation as function of thermal history. Samples from two wells of contrasting thermal maturity (late wet-gas, vitrinite %Ro ~ 1.0 to dry gas, vitrinite %Ro ~ 2.1) in the Marcellus Formation of Pennsylvania, however, show that total organic carbon content (TOC) is a stronger control on the character of OM-hosted porosity than is thermal maturity. Samples with TOC < 5.5 weight % display a positive correlation between TOC and porosity, but samples with TOC > 5.5 weight % display no additional increase in porosity. In a subset of samples (14) across a range of TOC (2.3 – 13.6 weight %) pore volume detectable by SEM is a small fraction of total porosity, ranging between 2-32 % of the helium porosity. Importantly, the SEM-visible porosity in OM decreases dramatically with increasing TOC, diminishing from 30 % of OM volume to < 1% of OM volume across the range of TOC. The morphology and size of OM-hosted pores also vary systematically with TOC.
Interpretation of this anti-correlation between OM content and SEM-visible pores is uncertain as it hinges in part on the interpretation of the OM as kerogen versus bitumen. Samples with the lowest OM-porosity (higher TOC) may represent gas expulsion (pore collapse) from either kerogen or bitumen that was more complete as a consequence of greater OM connectivity and framework compaction, whereas samples with higher OM-porosity (lower TOC) correspond to rigid mineral frameworks that inhibited compactional expulsion of methane-filled bubbles. Alternatively, higher TOC samples may contain OM (kerogen of low initial HI, relatively unreactive) that is less prone to development of SEM-detectable pores. In this interpretation, OM type, controlled by sequence stratigraphic position, plays the principal role in determining pore size distribution.