2014 Rocky Mountain Section AAPG Annual Meeting

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Controls on Porosity and Pore Type within the Eagle Ford Formation, South Texas


Scanning electron microscopy of Ar-ion milled samples from the Eagle Ford Formation, South Texas shows that compaction, cementation, bitumen generation, and generation of secondary porosity within organic matter (OM) are important factors in porosity evolution. Samples of varying compositions and maturities provide insight into these processes. Porosity in low-maturity samples is dominated volumetrically (values range from 0.1% to 11.4%) by relatively large, primary inter-particle pores (average sizes within samples ranging from 62.6nm to 143.1nm). Larger pores are typically associated with coccolith debris that is aggregated into pellets. Porosity and pore size correlate positively with calcite abundance and inversely with OM and clay volumes. Porosity loss is interpreted to occur through compaction of ductile kerogen and clays, and to a minor degree by cementation and infill of early bitumen. In high-maturity samples the observed OM includes a greater fraction of bitumen than in the low-maturity samples. Porosity in high-maturity samples is dominated volumetrically (values range from 0.2% to 3.6%) by smaller OM-hosted secondary pores (average size ranging from 11.1nm to 14.9nm). Mineral-associated pores are present, but are typically smaller (average size within samples ranging from 20.3nm to 40.7nm) and less abundant (values range from 0.0% to 1.9%) than at low maturity. Porosity correlates positively with volume measurements of OM and calcite, and correlates inversely with clay volume. Compared to low-maturity samples, porosity has been lost through cementation, bitumen filling, and, possibly, greater compaction in these high-maturity rocks, but additional porosity has been generated through organic maturation. At both low and high maturity, the evolution of the pore system can be explained by responses of the primary detrital assemblage to evolving physical and chemical conditions in the subsurface.