Marine Fine-Grained Sediment Tortuosity Derived from the Analysis of Three-Dimensional Reconstructions of Organo-Clay Fabric at the Nanometer Scale

Douglas, Jessica R.; Curry, Kenneth; Bennett, Richard; Head, Andrew

We created 3-D reconstructions of marine fine-grained sediment from serial sections and photographic mosaics obtained using a transmission electron microscope (TEM). These reconstructions show aggregations of clay domains, surrounding organic matter (OM), and "voids" where water or free gas were located prior to processing for TEM. We examined laboratory model samples with 1% OM and natural samples with high levels of OM making qualitative observations and quantitative measurements of porosity and tortuosity. Tortuosity of clay fabric is important as OM must initially be trapped in the depositional phase during early sediment diagenesis prior to becoming an OM-rich shale over geological time. Tortuosity has historically been measured at micrometer scales from 2-D electron micrographs and in some cases light optical (mm) scales by tracing a line across the X and Y planes of a micrograph around clay and organic particles and calculating the ratio of the length of that tortuous line to a straight line (shortest possible path) across the same interval. Our 3-D constructs show that structures identified as "pores" in 2-D images are actually parts of a complex matrix of channels ramifying the X, Y, and Z planes. We measured porosity and tortuosity by electronically subsampling our 3-D constructs into a series of 300 nm cubes. Porosity was measured for each cube and the orientation of each of the particles (segments of aggregates) in each cube was obtained. We used porosity and particle orientation to determine the shortest possible pathways to calculate tortuosity across a portion of the whole 3-D construct. Our technique allows us to calculate tortuosity while restricting porosity to a minimum boundary condition prior to determining the tortuous pathway. For example, using our 1% OM laboratory model sediment we measured tortuosity (ratio of measured shortest path to straight line across) through a series of 300 nm cubes with 50-100% porosity to be 1.08. Measuring tortuosity across the same sample, but restricting flow to cubes of 90-100% porosity yielded a tortuosity measure of 1.19. Tortuosity measurements can be restricted to any range of porosity that, because of the subsample cubes, corresponds to ranges of "pore" diameters none of which can exceed 300 nm. The technique currently uses data to estimate predominant particle orientation in the X, Y, or Z plane of the construct, but approximations of azimuth and inclination can be applied, thus refining the model.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013