Facies are defined as the specific characteristics of rocks or strata which reflect the depositional environment, appearance, fossils, Sedimentary structures and composition (Walker, 1992). Facies is also used in many contexts which defines for which purpose 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, sediment supply (Baker, 1999).
Walther’s law
The vertical and horizontal successions of rock units within same depositional basin remains same until there is a break or unconformity in rock records (Posamentier and Allen, 1999). It is a fundamental law of Stratigraphy which helps to visualize the vertical and lateral changes in rock units within a depositional setting and to 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 sea level rises and floods the shore so the shore line moves towards land while Regression is a condition in which sea level falls and shore line move towards 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 raising water depth while water depth decreases with Regression. This rise and fall of sea level results in facies dislocation in such a that there lie coarser sediments 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 regional scale, Porosity variations. The reservoir quality facies can also be easily identified by cross plot analysis (Gray and Andersen, 2000). There are different types of cross plots which 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 to facies variations. The cross plots of Sawan 1, Sawan 7, 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. Window will appear shown in Figure . Data on each axis is plotted as X =Rho b (density log), Y =LLD (resistivity), 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 colours are shown in figure . The logs used for facies analysis are RHOB and LLD with GR as a reference log. The reservoir is zone is focused for facies analysis ranging from 3240m to 3365m. The low resistivity, low density and low GR values indicates Sand facie and the data points corresponding to the sand are categorized in same polygon. The shale prone data points show high GR, high resistivity, high density values which are kept in separate polygon. The red polygon sand facies which is exist dominantly in reservoir zone, and also shown on log curves. The results of facies analysis for Sawan 1 is shown in figure .
Sawan 7 Facies interpretation
The zone selected for facies analysis in Sawan 7 ranges 3265m to3365 which covers reservoir interval. Figure shows data points representing various facies as coloured dots. The data points associated to low resistivity, high Gr, and low density are kept in same polygon. The data points with high GR, high resistivity and density are kept in separate polygon and opposite to these characteristics are kept in separate polygon. The sand facie dominates the reservoir zone shown in red colour, and indicated on log curves. The results are shown in figure .
Sawan 8 Facies interpretation
The zone of interest for Sawan 8 lies at th depth ranging from 3267m to 3350m. The data points are shown in figure . The sand dominated interval is indicated by low GR, low resistivity and low density are shown in red color polygon and shale prone zone is shown in violet coloured polygon. The results are shown in figure .
Cross Plot interpretation
GR log is plotted on X-axis and PHIE is plotted on Y-axis. The results are shown in figures for Sawan 1. Sawn 7, Sawan 8. It is evident from figures that Effective porosity decreases with increase in GR values which indicates change in facies from Sand to shale shown with arrow.
