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Facies maps show quantitatively the areal variation of either lithologic composition or other properties of a rock unit. They provide a three-dimensional picture of a stratigraphic unit and can form the framework for a paleogeographic and paleotectonic interpretation. The patterns shown on some maps have a relationship to oil- and gas-producing trends or mineralized areas.
Quantitative stratigraphic maps are classified in five categories according to the information they convey: (1) structure and topographic maps, (2) isopach maps, (3) facies maps, (4) vertical variability maps, and (5) maps based on either a statistical analysis or other kind of mathematical treatment of a contour map.
Compositional information can be presented in various ways by different types of facies maps. Ratiotype facies maps express the contrast between end members. Two sets of ratio contours describe a three-component system and subdivide the map into areas of different lithological composition. Facies maps based on a classifying function express the lithological composition in terms of classes defined by the relative amount of three components selected for end members. Class boundaries subdivide the map into areas of different lithological composition. Entropy function facies maps express the degree of intermixing of the end members and quantitatively define facies boundaries with one set of contours. The variation in composition can be shown by combining the entropy function with an overl y pattern to differentiate end members. Entropy-ratio facies maps provide both a contrast between the end members and information on the degree of intermixing of the end members. Facies departure maps express the degree of similarity to a pre-selected composition and are very effective for outlining trends related to a definite facies.
Two classes of maps show vertical variability. One class shows the degree of differentiation of a section into discrete units of different lithological types, and includes the following kinds of maps: average bed thickness, number of discrete beds of a given lithologic type, average interval entropy. The other class shows the relative position or vertical distribution of one lithological type within the section and includes the following kinds of maps: multi-partite, center of gravity, standard deviation or dispersion, and slice.
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