5. Numerical testing of SQp and SQs attributes on rock physics model

There are at least two main aspects recorded on seismic data: lithological effect and fluid effect. To understand the effect of both lithological and fluid changes, a rock physics model of soft sediment is used to test the response of SQp and SQs attributes. Initial parameters of model are derived from well log data. Gassmann's fluid substitution was applied on the model to get the new lithology and pore fill at different water saturations and porosities. In this test, the lithological effect is represented by the changes of porosity, while the fluid effect is represented with different water saturation conditions. Water saturation is set from 1 to 100%, where gas is used as a complement, and porosity changes are set from 7 to 25%, while the aspect ratio is assumed to be 0.1. The responses of SQp and SQs attributes at different water saturation and porosity changes are shown in the following figure.

#### Figure 5.

Responses of SQp and SQs attributes on water saturation changes (left column) and porosity changes (right column). (a) SQs attribute versus water saturation, (b) SQs attribute versus porosity, (c) SQp attribute versus water saturation, and (d) SQp attribute versus porosity. The initial state of the models is Vp = 2231.9 m/s, Vs = 1127.04 m/s, density = 2.11 g/cc, Vsh = 0.56, Sw = 0.49, and porosity = 25%.

The responses of SQs attribute decrease when water saturation increases (Figure 5a). In constant porosity, the SQs value of gas sand is higher than water sand. The porosity changes also affect the SQs attribute; increasing in porosity is followed by increasing in SQs (Figure 5b). It can be interpreted also that when porosity of rock increases, the number of fluid inside the rock also increases, which tends to increase the SQs value. This attribute is sensitive to fluid changes (saturation) which means that SQs can be correlated to fluid content conditions. For every different conditions, gas sand has higher SQs value compared to water sand.

Figure 5c and d shows the SQp responses due to water saturation and porosity changes, respectively. The responses of SQp attribute increase when water saturation increases. However, the increment is significant when the water saturation is close to fully water-saturated conditions. Water saturation is from 0 to about 80%; the increment of SQp is not significant. In the condition where the gas saturation is low (where gas saturation is about 5 or 95% of water), SQp value increases exponentially (Figure 5c). This phenomenon is the same as in Gassmann's fluid substitution case where only 5% gas can boost seismic velocity exponentially. On the other side, when porosity increases (where the fluid content is more), the SQp values decrease (Figure 5d). It tells us that the number of fluid does not so much affect the SQp. In this example the change of lithology is represented by the change of porosity. SQp is more affected by lithology rather than fluid content. Hence, the SQp attribute might be better as a lithology indicator, while the SQs attribute would be better as a fluid indicator. This hypothesis will be proven by testing the attributes using real data.
