Log Correlation

Log correlation is a vital tool for understanding the relationship of the facies from different localities. The idea is to match up the same facies on different logs. It is important to note that the lines used to correlate logs are not time lines and are diachronous, i.e. they cross over time lines. Time lines can be drawn, but this is the last stage of correlation.

	Figure 1: A prograding sequence with the facies labelled

To explain how to correlate logs, it is probably best to do an example. This example looks at a prograding/retrograding sequence (figure 1) on a shoreline. There are three facies described; deep marine (DM), shallow marine (SM) and fluvial (F). Figure 3 shows the log data, with the symbols described in figure 2.

	Figure 2: The key used in the following logs.

	Figure 3: The two logs we are going to correlate.

The first step in correlation is to identify boundary horizons, using various pieces of information. Information that could be used includes palaeontological (fossil horizons), sediment cycles or a distinctive facies (e.g. coal layer). This has been done and the logs are shown in figure 4. Deep marine facies will consist of shales and fine grained rocks with abundant fossils. Shallow marine will be coarser sediments with cross bedding, ripple marks, laminations and fossils. A fluvial environment will show channelised erosion, roots and cross bedding. The main boundaries have been identified using sediment cycles (where one cycle is denoted by an inverted triangle) and a distinctive bed (the coal layer, shown in black). A cycle is the upward progradation from deep marine, shallow marine and then fluvial. The cycle does not have to start with deep marine, nor does it have to end with fluvial, but the cycle must proceed in the correct order. When the order reverses, e.g. fluvial to deep marine, that is a new cycle.

	Figure 4: First stage of correlation.

Once the boundaries have been identified, the facies can be correlated. If both logs contain the same facies, the correlation is straight forward. If one log contains a facies that does not appear in another log, the facies must grade out between the logs, see for example the first shallow marine facies in the left hand log. The correlation is shown in figure 5.

	Figure 5: Facies have now been correlated.

The last stage is to put in time lines. As we know this is a prograding sequence, the facies at the same time will be different at the two localities. The most proximal (nearest to the shore line) log will contain most shallow water facies and least deep marine facies, while the most distal (furthest from the shore line) log will contain the most deep water facies (figure 6). Bearing this in mind, the right hand log is the most distal log and the left log the most proximal. We can now draw on time lines. They are only an estimation, but will be in the same form as beds in a prograding sequence (figure 1).

	Figure 6: The final correlated logs

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