DHI (Direct Hydrocarbon Indicators) attributes extracted from pre-stack seismic data: integrated analysis from rock to seismic

Abstract

In this study we examine the performance of several attributes of Direct Hydrocarbon Indicators (DHI) in the different scales involved in exploration geophysics, from rock to seismic. The compressional and shear wave velocities (Vp, Vs) of hundreds of rock samples from Tertiary and Cretaceous siliciclastic Brazilian offshore reservoirs were measured in the laboratory. Special attention was given to the proposal of Russell et al.(2003), which defined an attribute (Ip2 – C.Is2) based on the p and s impedances (Ip and Is) and a C factor, defined by local calibration. The rock scale observation showed that for younger, poorly consolidated reservoirs, the sensitivity to fluids is very good, even the simple acoustic attributes, such as the acoustic impedance, are sufficient for good fluid discrimination. However, for more consolidated reservoirs, common in older sequences, the combination of elastic attributes, associated with the correct local calibration becomes essential for the optimization of fluid detection. It can be observed that the indicator suggested by Russell et al. when used with the properly calibrated C value, may be much more efficient than the other attributes. However, on the seismic scale the Russell attribute does not show a performance similar to that forecast on the plug scale, due to the deterioration of the signal-to-noise ratio inherent to the operations involved in its calculation. In this context, we also show that the DHI attribute given by the difference Ip – Is, as proposed by Thedy et al. (2003), which is a special case (C = 1) of a generically Ip – C.Is attribute, is, in fact, a very good option for the seismic scale.