Systematic diagenetic changes in the grain-scale morphology and permeability of a quartz-cemented quartz arenite

David L. Boutt et al. — Mon, 09 Jan 2012 08:29:10 PST

The material properties of sedimentary rocks are controlled by a range of parameters, including grain size, sorting, and modification of the original sediment through the diagenetic processes of compaction and cementation. To isolate the effects of diagenesis and explore how they modify permeability, we quantified changes in grain and pore morphology accompanying progressive diagenesis of a simple system: a well-sorted, variably quartz-cemented quartz arenite of relatively uniform grain size. The most common type of authigenic cement in sandstones, quartz overgrowths, is responsible for significant porosity and permeability reduction. The distribution of overgrowths is controlled by available pore space and the crystallographic orientations of individual quartz grains. We show that progressive quartz cementation modifies the grain framework in consistent, predictable ways. Detailed microstructural characterization and multiple regression analyses demonstrate that both the number and length of grain contacts increase as the number of pores increases and the number of large well-connected pores decreases with progressive diagenesis. The aforementioned changes progressively alter pore shape and reduce pore-size variability and bulk permeability. These systematic variations in the pore network correlate with changes in permeability, such that we can use our data to calibrate the Kozeny-Carmen relation, demonstrating that it is possible to refine predictions of permeability based on knowledge of the sedimentary system. Jennie Cook completed her Ph.D. in 2010 at the University of Wisconsin-Madison on the structural and mechanical effects of diagenesis in sandstone. She presently works for BP as a geologist. Laurel Goodwin has been a professor at the University of Wisconsin–Madison since 2004, after working at New Mexico Tech. Her research interests include the mechanics of fracture and faulting and fluid-fault interactions in granular porous media. She received a B.A. degree from the University of Maine-Orono and an M.A. degree and Ph.D. from the University of California-Berkeley. David F. Boutt is an assistant professor of geosciences at the University of Massachusetts–Amherst. He received his Ph.D. from New Mexico Tech in 2004 with a specialization in hydrology. His research is focused on understanding the coupling between fluid flow and deformation in permeable media, with the goal of improving the characterization and conceptualization of hydrogeologic systems.

Quantification of Free Gas in the Kumano Forearc Basin detected from Borehole Physical Properties: IODP NanTroSEIZE drilling Site C0009

David L. Boutt et al. — Mon, 09 Jan 2012 08:28:08 PST

The Kumano fore-arc basin overlies the Nankai accretionary prism, formed by the subduction of the Philippine Sea Plate beneath the Eurasian plate offshore the Kii Peninsula, SW Honshu, Japan. Seismic surveys and boreholes within the framework of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project show evidence of gas hydrates and free gas within the basin. Here we use high-quality borehole sonic data from Integrated Oceanic Drilling Program (IODP) Site C0009 to quantify the free gas distribution in the landward part of the basin. The Brie theory is used to quantify gas content from sonic logs, which are calibrated from laboratory measurements on drill cores. First, we show that the sonic data are mainly sensitive to the fluid phase filling the intergranular pores (effective porosity), rather than to the total porosity that includes water bound to clay minerals. We then compare the effective porosity to lithodensity-derived porosity that acts as a proxy for total porosity. The combination of these two data sets also allows assessment of clay mineralogy of the sediments. Second, we compute free gas saturation and find a gas-rich interval that is restricted to a lithological unit characterized by a high abundance of wood fragments and lignite. This unit, at the base of the fore-arc basin, is a hydrocarbon source that should be taken into account in models explaining gas distribution and the formation of the bottom-simulating reflector within the Kumano fore-arc basin.