Density extraction from P-wave AVO inversion: Tuscaloosa Trend example Jyoti Behura 1 , Nurul Kabir 2 , Richard Crider 2 , Petr J´ ılek 2 , and Ellen Lake 3 1 Center for Wave Phenomena, Colorado School of Mines, Golden, Colorado 80401, USA; 2 BP Exploration and Production Technology Group, Houston, Texas 77079, USA; 3 BP North American Gas Strategic Performance Unit, Houston, Texas 77079, USA. Summary Density extraction from AVO inversion is thought to be unstable and difficult. Recent research has, however, shown that it is possible to extract density reliably from P-wave reflection data if the density contrast is significant across the interface. This approach is tested over a gas reservoir where petrophysical evidence suggests that density is the primary driver of reflectivity. By adopting a linear as well as a non-linear approach, inversion is performed for density and P-wave velocity from a 3D prestack seismic dataset acquired over this gas reservoir. The non-linear inversion results agree well with the log and production data, thus proving that density can be extracted reliably in this case and used for reservoir characterization. We use the extracted density along with the P-wave velocity to distinguish the low-density porous gas sands from wet sands, tight sands, and shales. Introduction The Tuscaloosa Trend, located in South Louisiana, is a series of gas fields. All these sands are of Cretaceous age and were deposited in a near-shore marine depositional environment. Data from a 3D seismic survey over this area is used in this study. Lithology at the reservoir depths consists of gas-saturated sands along with wet sands, tight sands, and shales. Reduction of drilling risk and optimization of production requires delineating and separating gas sands from other lithologies. Amplitude-variation-with-offset (AVO), a useful tool in seismic exploration, is applied here to extract physical pa- rameters (P-wave velocity, S-wave velocity, and density) used for lithology discrimination. In isotropic media, P- wave AVO is a function of four quantities: g (background VP /VS ), P-wave velocity contrast, S-wave velocity con- trast, and density contrast across the interface. However, a common impression is that these individual parameters cannot be found uniquely using this tool i.e. all these parameters cannot be decoupled from one another using AVO without long-offset data. Estimation of density can be particularly difficult (Debski and Tarantola, 1995) and requires wide range of incidence angles to be stable as pointed out by Li (2005). Kabir et. al. (2006), how- ever, showed that density can be estimated reliably from near- and mid-offsets where density contrast is the dom- inant contributing factor in the AVO equation (Figure 1). Through a field data example, they could successfully relate the estimated density to gas saturation in the Ma- Fig. 1: Sensitivity curves for the density parameter generated by varying the density of the bottom layer between +20% and -20% (Kabir et. al. 2006). hogany gas field in offshore Trinidad. The same idea is expressed by Li (2005) who concludes that, “In general, high porosity reservoirs and reservoirs with better den- sity contrasts or anomalies are appropriate candidates for applying density inversion.” The prestack data in this Tuscaloosa Trend study, how- ever, spans from 0 ◦ to 30 ◦ . So the obvious question is whether density can be reliably extracted in this field us- ing only near- to mid-offset data. In other words, is den- sity the primary driver of reflectivity in this reservoir? Why density? To test the applicability of density inversion, well logs are analyzed for one of the Tuscaloosa fields. A cross-plot of the density and P-wave slowness logs (inverse of P-wave velocity) in one of the wells is shown in Figure 2a, where the data points have been scaled by the corresponding gamma ray recording at that depth. Note that the density of most of the sands is quite different from that of the shales. On the other hand, the P-wave velocities of sands and shales span over the same range. By knowing densities, sands can be separated from shales, which is not possible using P-wave velocities alone. So we conclude that density is the primary driver of reflectivity between gas sands and shales in this area. From Figure 2b, in conjunction with Figure 2a, also note that the low density and low P-wave velocity sands are the producing gas sands (with the low density resulting from high porosity). With increasing water saturation, the density and velocity of these porous sands increases.