Multi-Scale and Multi-Dimensional Imaging of Mineral Matrix Pore-Fracture Networks in Lower-Thermal-Maturity Lacustrine Shale: Examples from the Paleogene Shahejie Formation, Bohai Bay Basin

The pore-fracture network within shale significantlycontrols themicroscopic storage of hydrocarbons. The Paleogene lacustrine ShahejieFormation of Bohai Bay Basin represents an outstanding lower-thermal-maturityexample of a mineral matrix pore-fracture system. Recent studies haveshown that most pores in Shahejie shale detectable by high-resolutionelectron microscopy are associated with an inorganic mineral matrixinstead of organics. However, because of the lack of both qualitativeand quantitative data on nanometer to micrometer scales, such microstructureshave limited the ability to predict hydrocarbon storage feature. RepresentativeShahejie shale samples come from the Well F39X1 to investigate thepore-fracture network using a synergistic multi-scale multi-dimensionalworkflow by field emission scanning electron microscopy, multi-scaleCT, and FIBSEM. Investigations in both 2D and 3D and across cm-mm-& mu;m-nmlength scales indicate heterogeneity at every scale and dimensions.From 2D SEM images, clay flake-hosted interparticle pores, rigid grain-hostedinterparticle pores, intraparticle dissolution pores, grain-rim microfractures,intragranular microfractures, and matrix microfractures are identifiedwith varying numbers, sizes, and geometries. Large field-of-view 2Dscanning electron microscopy images were also used to quantitativelyand statistically study the abundance and size of pores and fractures.Using multi-scale CT techniques, pores and fractures from centimeterto nanometer scales were observed with preferential propagation alongthe mineral aggregate-rich laminations, resulting in an interconnectedpore-fracture network. Using FIBSEM, the mineral/organic particlesand their related pore phases were visualized, quantitatively computingthe pore size and abundance. The visual results suggest that the mineralmatrix pore-fracture network in lacustrine Shahejie Formation playsa key role as the liquid hydrocarbon storage space. The applicationof these techniques to the lower-thermal-maturity shale reservoirsprovides important insights into estimating and visualizing rock microstructuralheterogeneity and preliminarily establishing a liquid hydrocarbonmicroscopic storage model.