ABSTRACT:

Unconventional tight shale reservoir systems are heterogeneous at all scales. This results from multiple sequences of deposition and accumulation of sediments in time, followed by locally varying and extensive post-depositional transformations. It has been said that the textural variability in shales at the thin-section scale rivals the variability of an entire outcrop in sandstones (J. Schieber). Given their colloidal size of organic and inorganic sediments, their large surface area to volume ratio, and their high chemical potential for undergoing geochemical transformations, the resulting distribution of material properties in tight shales is highly heterogeneous. Understanding scale-dependent heterogeneity in tight shales and other unconventional reservoirs is important for hydrocarbon production and recovery. It is also important for characterization, modeling, and for extending our observations, experience and understanding from one scale (e.g., core-scale) to another (e.g., log- or seismic-scale). The presence of scale-dependent heterogeneity also poses additional important questions regarding sampling for characterization, including the number of samples needed, the adequate scale for sampling and others. Addressing and solving these questions will lead to significant progress on tight shale exploration and efficient production. This paper describes continuous measurements along the length of the core that result in significant improvements to geologic core descriptions and heterogeneous rock characterization. Using multiple high-resolution measurements (e.g., of strength, thermal conductivity, CT atomic number, and XRF mineralogy) we define the principal rock classes, with similar characteristic properties, that define the heterogeneous system. The thickness and cyclic stacking patterns of these units provide quantitative information of the depositional system and its sequences. The method also differentiates transitional contacts from abrupt contacts, and provides additional information for developing a geologic model. Although the cyclic nature of tight shale sequences is often visually apparent, the variability in properties within these sequences is only accessible by the continuous measurements.

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