--> Rock Physics Modeling as a Tool to Improve Interpretation from Tight Gas Sands Reservoirs, Dillon, Lucia D.; Schwedersky, Guenther; Nunes, Cassiane; Justen, Julio; Abreu, Elita, #90100 (2009)

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Rock Physics Modeling as a Tool to Improve Interpretation from Tight Gas Sands Reservoirs

Dillon, Lucia D.1
 Schwedersky, Guenther2
 Nunes, Cassiane2
 Justen, Julio2
 Abreu, Elita1

1EXPLORATION, PETROBRAS, Rio de Janeiro, Brazil.
2RESEARCH CENTER, PETROBRAS, Rio de Janeiro, Brazil.

Lucia Dillon*, Guenther Schwedersky**, Cassiane Nunes**, Julio Justen** and Elita de Abreu*
* PETROBRAS EXPLORATION
** PETROBRAS RESEARCH CENTER

Most of the current Brazilian producing fields are related to high porosity Tertiary turbidite sands, classical bright spots and type III AVO anomalies. However, the new exploratory frontiers are moving more and more to deeper plays. For those plays, type I and II AVO start to be much more common as a DHI response, than we could imagine in the recent past. On the order hand, type I anomalies are dim outs, and when compared to type III AVO anomalies, are much more difficult and subtle to recognize. For this reason, aiming the minimization of the exploration risk in some important Brazilian offshore exploration areas, we have used rock physics to model and constrain our elastic attributes interpretation by integrating rock, log and elastic seismic attributes analysis. As an example, we will present a complete case study for a Cretaceous tight gas sandstones.

Well log AVO modeling (1D) confirmed the same type of anomalies (I and II) that we observe on AVO sections derived from the angle stacks at the reservoir level. In the same direction, 3D Simultaneous Elastic Inversion also showed good elastic attributes anomalies for DHI. Lateral samples were taken from the reservoir and lab measurements (dry) for P and S velocities and densities were performed. Rock Physics modeling showed us that for those low porosity sandstones, the dry mineral modulus input are very important for Gassmann fluid substitution simulations. The presence of higher contents of calcite, for instance, will increase those moduli values and, for this low range of porosities, the consequence is a higher seismic sensibility to fluids. For instance, the contrast of Poisson Ratio, due to the change of water to gas saturation, can increase from 2% to 8%, just assuming a 20% of calcite (80% quartz) instead of 100% quartz for the mineral composition. So, for our case study, even being a very low porosity sandstone reservoir, the high contents of heavier minerals made this reservoir much more sensible to fluid than we could imagine a priori. In fact, enough to be “seen” by the DHI elastic attributes as observed.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil