Average Normal Porosity

In this first approach, I assume that the seismic section without a BSR gives a good approximation to the normal background porosities without the effects of hydrate or gas saturation. Consequently, I use the baseline porosities, determined in this region by assuming 100% brine saturation, and average them into an averaged normal porosity trend.

The resulting normal porosity is overlain with the anomalous baseline porosities in Figure . The first panel shows that there are misfits between the normal and baseline porosities in regions assumed to be hydrate and gas free. Such misfits can result from lateral lithology variations, as well as from some uncertainties in the velocities which are directly translated into errors in porosity. The middle panel shows that the anomaly which is related to the presence of hydrate slightly deviates from the average normal trend. It might be hard, however, to separate this hydrate anomaly from those solely related to curve misfits (see left panel). The right panel displays a clear negative hydrate anomaly and a strong positive gas anomaly. Since the anomaly caused by the presence of gas is considerably larger than the one caused by the hydrate, it will result in a higher detectability of the areas of gas saturation.

 

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Figure 5 Average normal porosity (dashed line) overlying the anomalous baseline porosities (solid line).

 

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Figure 6 Residuals obtained by subtracting the average normal porosity from the anomalous baseline porosities.

The size of the anomalies, or deviations between the normal porosity and the anomalous baseline porosity, can be obtained by subtracting the two porosity trends. The resulting trace residuals are displayed in Figure . The gas-saturated section underneath the BSR yields a very strong positive residual, as expected from Figure . The residuals attributed to the presence of hydrate, however, appear to be of the same magnitude as some of the residuals in the region without BSR. These residuals are caused by local porosity variations that can represent lateral variations of lithology or variations introduced by velocity errors. A clear separation of some of the hydrate-related residuals from those caused by these other effects might be difficult, especially for the hydrate anomaly between 32 km and 42 km lateral distance.

 

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Figure 7 Histogram of the porosity residuals obtained using the average porosity trend. The solid line represents the residuals for the line between 0 and 27 km (without BSR). The dashed line represents the residuals for the line between 27 and 52 km (with hydrate and gas).

In order to examine the actual size of the residuals in more detail, I show a histogram of the porosity differences (Figure ). The solid line represents the residuals between a lateral distance of 0 and 27 km (region without BSR), while the dashed line represents the residuals in the region with BSR (27 km to 52 km). The comparison between both curves indicates the clear impact gas has on the residuals. It significantly increases the porosity anomalies and is therefore clearly identifiable. The negative residuals caused by the hydrate also appear to be separate from the residuals caused by local curve misfits. However, there is an overlap of the size of the small hydrate anomalies between 32 km and 42 km (as shown in the middle panel of Figure ) and the anomalies caused by the curve misfits. In this investigation, I chose to take anomalies less than -0.05 to be due to hydrate and anomalies larger than 0.08 to be gas-related. If I had taken a higher uncertainty for the hydrate anomalies, i.e., values higher than -0.05, I would not be able to resolve the hydrate-related anomalies between 32 km and 42 km lateral distance.