110 / 2022-02-19 17:32:48

Distinctions of shales and mudstones for Shale Oil E&P: the feature in nomenclature and classification

shales; mudstones; mud; clay; silt; lamination; fissile; lithofacies; Shale Oil E&P

With the deepening of global shale oil and gas exploration and development, shales and mudstones are becoming the important research objects of fine-grained sedimentology, but their sedimentary characteristics and relationships have been in contention. The phenomena mainly contributed to the different size definition of "mud" in fine sediments, and it resulted in confusion of concepts and usage. At present, the size of fine sediment mud (mud grade particles) is divided differently in sedimentology among the world. Scholars in Europe and US generally define mud as a particle size less than 62.5μm, including clay grade particles and silt grade particles. In China, since the 1950s, the particle size of mud corresponds to clay grade particles is less than 3.9μm. Meanwhile, due to the term clay contains the meaning of both grain size and mineral composition, in addition, many clay minerals contain silt-size particles; that leads to the boundary between CLAY-SIZE and SILT-SIZE particles implicitly extended. The fabric and compositions of consolidated mudstones are similar to that of shales, but they lack the "lamination" and "fissile" texture properties of shales. What’s more, "Lamination" highlights the stratigraphic vertical disparities created by sedimentary processes, whereas "fissile" refers to the mechanical weaknesses created by weathering during diagenetic processes. As a result, fine-grained sediments were recommended to be classified into lithofacies based on sedimentary processes, as well as mineralogy, fabric, structural properties and organic matter abundance. The production and usage of shale oil and gas required the combination of geology and engineering, in which recognizing the differences between shales and mudstones is critical for pay zone optimization.