Facies are defined as the specific characteristics of rocks or strata that reflect the depositional environment, appearance, fossils, Sedimentary structures, and composition (Walker, 1992). Facies are also used in many contexts, which defines the purpose for which the term is used. For example, the environment in which rock is formed, i.e., fluvial facies, the process by which rock is formed, i.e., turbidite facies, etc. The change in facies distribution depends upon tectonic activity, changes in sea level, sedimentary processes, and sediment supply (Baker, 1999).
Walther’s law
The vertical and horizontal successions of rock units within the same depositional basin remain the same until there is a break or unconformity in rock records (Posamentier and Allen, 1999). This fundamental law of Stratigraphy helps visualize the vertical and lateral changes in rock units within a depositional setting and reconstruct the paleogeography along with temporal variations (Catuneanu, 2006).
This law is also helpful for studying facies dislocation during transgression and regression. Transgression is a condition in which the sea level rises and floods the shore so the shoreline moves towards land. At the same time, Regression is a condition in which the sea level falls and the line moves towards the basin or sea, exposing the areas previously invaded by sea (Bates and Jackson, 1987). The water depth changes with changes in sea level. Transgression results in rising water depth, while water depth decreases with Regression. This rise and fall of sea level results in facies dislocation so that coarser sediments lie directly over fine clays and fine sediments directly overlying coarser lithology without any gradation in grain size (Miall, 1997).
Cross Plots
The well-log cross plots are helpful for identifying lithologies, lithology variations on a regional scale, and Porosity variations. Cross-plot analysis can also easily identify The reservoir quality facies (Gray and Andersen, 2000). Different types of cross plots can be generated from well logs, and each cross plot has its own purpose and use. For example, GR with porosity cross plot indicates the porosity variations associated with facies variations. The cross plots of Sawan 1, Sawan 7, and Sawan 8 are generated by using Gr and effective porosity curves.
Facies Analysis Procedure
From the KINGDOM software main window menu bar, choose Tools > Cross plot > New to open the Select Data dialog box. A window will appear as shown in Figure. Data on each axis is plotted as X =Rho b (density log), Y =LLD (resistivity), and Z =Gr (gamma ray).
Sawan 1 Facies interpretation
Sand and Shale are two main facies identified in the facies analysis of sawan1. The data points in various colors are shown in the figure. The logs used for facies analysis are RHOB and LLD with GR as a reference log. The reservoir zone is focused on facies analysis ranging from 3240m to 3365m. The low resistivity, low density, and low GR values indicate Sand facie, and the data points corresponding to the sand are categorized in the same polygon. The shale-prone data points show high GR, high resistivity, and high-density values, which are kept in separate polygons. The red polygon sand facies exist dominantly in the reservoir zone and are also shown on log curves. The results of the facies analysis for Sawan 1 are shown in the figure.
Sawan 7 Facies interpretation
The zone selected for facies analysis in Sawan 7 ranges from 3265m to 3365, covering the reservoir interval. The figure shows data points representing various facies as colored dots. The data points associated with low resistivity, high Gr, and low density are kept in the same polygon. The data points with high GR, high resistivity, and density are kept in separate polygons, and the opposite of these characteristics are kept in separate polygons. The sand facie dominates the reservoir zone, as shown in red and indicated on log curves. The results are shown in the figure.
Sawan 8 Facies interpretation
The zone of interest for Sawan 8 lies at depths ranging from 3267m to 3350m. The data points are shown in the figure. The sand-dominated interval is indicated by low GR, resistivity, and low density, shown in red polygons, and the shale-prone zone is shown in violet polygons. The results are shown in the figure.
Cross Plot interpretation
The GR log is plotted on the X-axis, and PHIE is plotted on the Y-axis. The results are shown in figures for Sawan 1, Sawn 7, and Sawan 8. The figures show that Effective porosity decreases with an increase in GR values, which indicates a change in facies from Sand to shale, shown with an arrow.
