Facies Models Walker Pdf 42
Saija Grzegorek
The concept of facies, that is a body of rock with specified characteristics, has been used ever since geologists, engineers and miners recognized that features found in particular rock units were useful in correlation and in predicting the occurrence of coal, oil, and mineral ores. The term was introduced by Gressly (1838) who used it to embrace the sum total of lithological and paleontological aspects of a stratigraphic unit. Since that time the term has been the subject of considerable debate, well summarized by Middleton (1973) and discussed extensively by Anderton (1985), Walker and James (1992), Reading (1987, 1996) and Miall (1999).
Photographs showing lithofacies A clast-supported gravelly and sandy facies (Gcm, Sm), B matrix-supported gravelly (Gmm), C matrix-supported graded bedding gravelly, D sandy facies (Sh, Sm, and Sp), E gravelly and mud facies (Gmm, Fm, and Fr)
FIGURE 5. Lithofacies of the Corcal section: (A) Limestone beds in the upper part of the section (scale object is 13 cm). (B) Thin section view of a massive calcimudstone (b/w; Sample C1 2,62). (C) Fenestral structures filled with calcite cement (b/w; Sample MP 2118). (D) Thin section view of an intraclastic grainstone with local ferruginous matrix (b/w; Sample MP 2119). (E) Thin section view of a marl (Sample MP 2115). (F) Specimens of C. werneri preserved along a surface in a shale bed (scale object is 2 cm). (G) Calcimudstone with elongated intraclasts and silicified anhydrite pseudomorphs, indicated by arrow (b/w; Sample MP 2123). (H) Recrystallized grainstone (Sample C3 24,60). (I) Grainstone with bioclasts of Cloudina (b/w; Sample MP 2128). (J) Recrystallized packstone (b/w; Sample MP 2132).
FIGURE 6. Macroscopic view of sedimentary facies in the lower part of the Laginha section. Sample identification and scale at the right-hand bottom corner of each picture. (A) Basal polymictic breccia with basement clasts, poorly sorted and with micritic matrix. (B) Muddy sandstone with quartz grains and lithoclasts. (C) Thin section photograph of a dolomitized desiccation breccia. Note the broken dark grains still positioned next to each other counterparts. (D) Intraclastic breccia with square, angular clasts. (E) Laminated carbonate with alternated finer and coarser layers. (F) Dolomitic marl displaying mica extraclasts. (G) Polymictic intraclastic breccia with homogeneous structure.
FIGURE 7. Macroscopic lithofacies of the Laginha section. (A) Polymictic breccia overlaid by muddy sandstone and then carbonate beds. (B) Sequence of dark gray fine grainstone, dark gray coarse lithic sandstone with carbonate matrix, and light gray dolomitic wackestone. (C) Cm thick layer of intraclastic breccia in the lower section. (D) Limestones with dm thick tabular bedding in the lower section. (E) Homogeneous, polymictic intraclastic breccia within dark gray limestones. (F) Limestones with dm thick tabular bedding in the upper part of the lower section, with chaotic bedding towards the top.
Introduction to carbonate petrology, classifications, photomicrographs, and exercises related to thin sections of carbonate, carbonate facies, carbonate sequences, hydrocabon plays, carbonate depositional settings and analogues.
While most carbonate grains are easy to recognize in hand specimen when seen in thin section they can be difficult to identify. The problem is, as Majewski(1969) remarked, that a "variety of shapes are produced by random cuts through a single geometric pattern, and shapes can be duplicated in such cuts by different designs; also, characteristic features may be obscured and others may become apparent" when the grains are exposed in a specific plane. Never the less the carbonate grains of this collection are separated from one another on the basis of their shape, size and internal structure.
Carbonate grains can be separated from one another on the basis of their shape, size and internal structure. Because the grains commonly collect near their site of origin, they can be used, in conjunction with other Rock characteristics including vertical and lateral facies relationship and sedimentary structures, to determine the depositional of the Rocks they occur in. Information about grain types and the manner in which they occur in Rocks can be communicated by means of limestone classifications.
Basic data requirements for facies analysis of subsurface rocks are listed in Table 1. Data associated with wells are most often used, but seismic data, particularly three-dimensional data, are becoming increasingly important in defining sandstone body geometries.[2] Conventional core is perhaps the most diagnostic for sedimentological interpretation of vertical sequences (see Core description). However, wireline tools such as dipmeters and formation imaging devices can provide electrical images suitable for sedimentological interpretation with the added ability to determine paleocurrent directions in appropriate cases.
One of the first steps in the facies analysis of a clastic reservoir is the description and interpretation of available conventional core.[4] An important result of core description is the subdivision of cores into lithofacies, defined as subdivisions of a sedimentary sequence based on lithology, grain size, physical and biogenic sedimentary structures, and stratification that bear a direct relationship to the depositional processes that produced them. Lithofacies and lithofacies associations (groups of related lithofacies) are the basic units for the interpretation of depositional environments.
Interpretation of the environment in which lithofacies were deposited from analysis of cored sequences involves relating the identified lithofacies to the physical and biological processes that produced them. This process-response relationship identifies the specific processes responsible for the sequence and, by inference, the depositional setting in which these processes occurred. The application of the process-response approach relies primarily on depositional models constructed through study of both modern and ancient analogs.
Depositional models are important for predicting the distribution of permeability and porosity within different reservoir types. These models are never exact matches to a reservoir; rather, they serve as guides to aid in the interpretation of any one reservoir.[3] Reservoir properties are generally observed to be correlative with lithofacies types to one degree or another (see Geological heterogeneities). This reflects the fundamental control on permeability and porosity by grain size, sorting, and spatial distribution of different lithofacies types. Even where rocks have experienced later physical and chemical diagenesis, permeability and porosity relationships are controlled, in large part, by the original sedimentary fabric of the rock.
Wireline logs to be used for facies analysis should, whenever possible, always be calibrated by core. This calibration involves (1) shifting core to log depths (see Preprocessing of logging data and Core-log transformations and porosity-permeability relationships) and (2) establishing a relationship between lithofacies associations and curve shape. Core gamma scans, obtained by passing the core through a device that measures the natural radioactivity of the rock, are particularly useful for shifting cores to logs. The calibration of wireline log shape by core is particularly important for firmly establishing the log response and the identity of vertical sequences on these logs.
Deltaic bodies are generally classified into three major categories or end-members on the basis of the dominant sediment transport process that influences their facies constituents and external geometries. These three end-members are as follows:
Barrier islands (Figure 3f) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are very well sorted, lack interstratified clay, and exhibit excellent reservoir properties where not cemented. Tidal inlet and flood tidal delta deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.
Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure 3i). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.
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