The deposition of the presalt section from Santos Basin began when Gondwana started to break up and South America and Africa were separating. Initial synrift carbonate deposits affected by relatively severe tectonic activity evolved to a lacustrine carbonate environment during the later stages of basin formation. Although the reservoirs are composed of carbonate rocks, the occurrence of faults and the intense colocation of igneous rocks served as a source of chemical elements uncommon in typical carbonate environments. Consequently, beyond the presence of different facies with complex textures and pore geometries, the presalt reservoir rocks present marked compositional and microstructural variability. Therefore, rock-physics modeling is used to understand and interpret the extensive laboratory measurements of P-wave velocities, S-wave velocities, and density that we have undertaken on the presalt carbonate cores from Santos Basin. We show that quartz and exotic clay minerals (such as stevensite and other magnesium-rich clay minerals), which have different values of elastic moduli and Poisson's ratio as compared to calcite and dolomite, may introduce noticeable “Poisson's reflectivity anomalies” on prestack seismic data. Moreover, although the authors concentrate their attention on composition, it will become clear that pore-space geometry also may influence seismic rock properties of presalt carbonate reservoirs.

In presented paper, we describe the technical and physical aspects of the application of a low-frequency (LF) apparatus based on a longitudinal type of forced oscillations. In particular, we investigate the influence of the strain gauge position on a tested sample on measurement results, we also examine the creep effect associated with the mineralogy of rocks, as well as the dispersion and attenuation in a liquid-saturated rock sample caused by the presence of the volume of pore fluid exterior to the sample (dead volume). The effect of the position of the strain gauges is investigated using a cylindrical acrylic sample and two pairs of strain gauges fixed in the middle and at one of the sample ends under a uniaxial pressure of 15 MPa. The obtained results demonstrate that elastic and anelastic parameters of the tested sample are independent from the location of the strain gauges. The impact of the creep phenomenon on elastic moduli was studied using three room-dry samples of Savonnieres limestone, Berea sandstone and Eagle Ford shale. The measurements were conducted for 120 h at a frequency of 2 Hz under a uniaxial pressure of 10 MPa and demonstrated that the LF moduli of all rocks were noticeably reduced with time. The effect of dead volume was investigated at seismic frequencies using limestone saturated with n-decane. It was found that the Young’s and bulk moduli exhibit strong dispersion at frequencies above 10 Hz if the dead volume is close to or greater than the pore volume of the sample. We also found that the characteristic frequency of dispersion corresponding to the attenuation peak is independent of the size of the dead volume and determined only by the physical parameters of the sample and pore fluid. We present also the results of the Young’s modulus and attenuation measurements conducted at seismic frequencies on vertical and horizontal shale samples saturated with water. It was shown that the relationship between the extensional attenuation and the Young’s modulus dispersion observed in the samples saturated at a relative humidity of 97.5% is consistent with the Kramers–Kronig relation.

Several analyses have been published in the specialized literature on the use and performance of distinct prestack seismic attributes as direct hydrocarbon indicators (DHI). The main idea is to use combinations of those attributes, in particular the P and S impedances (I P and I S ), to minimize the dry rock frame contribution, that cause the increase of the pore fluid discrimination capacity.The Batzle proposal (2001) refers to the calculation of K-G (where K and G represent the bulk modulus and the shear modulus, respectively). Several publications of Goodway et al. since 1997 have systematically presented the advantages of the parameter lambda*rho (Lamé parameter lambda λ multiplied by the density ρ as an excellent fluid indicator).More recently, a tutorial by Russell et al. (2003) presents the attribute I P 2 -CI S 2 with C being a function of local (V P /V S ) 2 where V P and V S are the P-and S-velocities for dry rock condition. As discussed by Russell et al., the Goodway (1997) and Hedlin (2000) proposals may be considered as particular cases of the I P 2 -CI S 2 attribute. In this study we examine the performance of several DHI attributes in the various scales involved in exploration geophysics, from rock samples to seismic.A total of 592 rock samples from Tertiary (188) and Cretaceous (404) siliciclastic reservoirs of Brazilian offshore fields were analyzed. The 3D seismic data set used in this study illustrates examples of siliciclastic oil-bearing Tertiary and Cretaceous intervals from offshore Brazil.The Cretaceous reservoirs are adjacent to igneous rocks with very high impedance properties, creating a greater challenge in the fluid identification process. Because of these high contrast zones, the use of fluid indicator attributes derived from conventional AVO weighted stacking methods is not appropriate. In order to prevent the pitfalls due to wavelet effects, the seismic attribute analysis was mainly based on the attributes generated from the inverted AVO data, particularly from the impedances (I P and I S ) derived from a simultaneous elastic inversion process. A 2D section containing three wells that crosses both the Tertiary and Cretaceous reservoirs was chosen to illustrate the attributes analysis on the seismic scale ( Figure 1). Rock samples, logs, and seismic data analysis.From the Gassmann equation and laboratory measurements of V P , V S , and ρ (dry, under real subsurface effective pressure), the elastic behavior of the samples was calculated according to two scenarios: 100% brine and 80% oil with 20% brine.Tables 1a and 1b summarize the results for each attribute analyzed, including I P -I S , λρ, Poisson's ratio, V P /V S , K-G, I P , fluid factor, and I P 2 -CI S 2 calculated for five distinct C values. We have also separated the attribute evaluations for two distinct sample groups, Cretaceous and Tertiary sandstones.These tables show the mean and standard deviation of the attribute value related to each scenario analyzed and also a fluid indicator coefficient which diagnoses the sen...

In order to improve hydrocarbon production, it is often necessary to obtain more accurate rock, fluid and petrophysical information. For example, to obtain a reservoir porosity map using seismic data as reference, it is necessary to generate reliable correlations between seismic attributes and petrophysical properties like porosity and permeability. Again, to optimize drilling and/or hydraulic fracturing programs, it is also necessary to estimate better formation static mechanical behaviour from geophysical data. The main goal of this work is to establish for an offshore Brazilian field, relationships between compressional and shear wave velocities and petrophysical properties such as porosity and permeability.The large number of limestone samples (120) gave us a precise empirical relationship between Vs and Vp for limestone. In order to obtain a calibration reference, we also made, with the same samples, simultaneous measurements of dynamic and static elastic constants. Using all these laboratory relationships, it was possible to generate unmeasured pseudo-logs of in situ parameters, which include: shear wave velocity, static and dynamic elastic constants and permeability. The good experimental relationships obtained between k-~b and Vp-~b in this work together with available logs give us an additional method to estimate permeability which is impossible to obtain from in situ measurements.

ABSTRACT. There are numerous efforts to obtain information about saturation and pressure changes due to reservoir production from time-lapse seismic data. In spite of some good examples existing on the literature, there are a lot of approximations behind those studies. Generally, it is assumed that the seismic properties are functions of saturation and differential pressure P d (overburden minus fluid pressure). Actually, the stress that governs the seismic behavior is the effective stress P e f , which is not exactly equal to P d . In fact it is equal to the overburden stress minus n times the fluid pressure. Such coefficient (n) is known as the effective stress coefficient. Sometimes, depending on the circumstances, it might be identified as the Biot-Willis coefficient taking into consideration the bulk modulus of the porous media. In this paper, laboratory results in a tentative to quantify the n values for Brazilian limestones and tight sands will be presented. It is important to point out, that is the first effort to perform this sort of measurements on Brazilian rocks. The results have shown that the mentioned approximations can introduce errors on pressure estimations from time-lapse data even for high porosity rocks.Keywords: effective pressure, porepressure, seismic velocities, Biot-Willis coefficient, 4D seismic. RESUMO.Há vários esforços no sentido de obter informações sobre mudanças de saturação e pressão, devidasà produção dos reservatórios, a partir de dados sísmicos com lapso de tempo. A despeito de alguns bons exemplos existentes na literatura especializada, várias aproximações são feitas para a realização destes estudos. Geralmente,é assumido que as propriedades sísmicas são funções da saturação e da pressão diferencial P d (pressão de soterramento menos pressão de fluido). Na verdade, a tensão que determina o comportamento sísmicoé a tensão efetiva P e f , que nãoé exatamente igual a P d . De fato, estaé igualà tensão de soterramento menos n vezes a pressão de fluido. Tal coeficiente né conhecido como coeficiente de tensão efetiva. Eventualmente, a depender das circunstâncias, este pode ser identificado com o coeficiente de Biot-Willis considerando o módulo de compressão volumétrica do meio poroso. Neste trabalho serão apresentados resultados de medidas em laboratório procurando quantificar os valores de n para calcários e arenitos fechados brasileiros.É importante salientar que esteé o primeiro esforço de realização deste tipo de medidas em rochas brasileiras. Os resultados mostram que as aproximações mencionadas podem introduzir erros nas estimativas de pressão a partir de dados de monitoramento sísmico mesmo para rochas muito porosas.Palavras-chave: pressão efetiva, poro-pressão, velocidades sísmicas, coeficiente de Biot-Willis, sísmica 4D.