Spatial variation of angle-dependent impedance through a thin-layered oil reservoir

Abstract

ABSTRACT. The P-wave elastic impedance (EI) concept is currently used as a complementary petrophysical parameter for the characterization of oil and gas reservoirs. In this paper, we construct maps representing the spatial variation of isotropic and anisotropic EI at a specific depth through a turbiditic oil-producing reservoir.EI is thus used here not for calibration and inversion of seismic data at farther offsets as firstly introduced, but as an original application of a novel petrophysical concept.As long as bulk density, seismic velocities and incidence angle are common variables in the isotropic and anisotropic EI formulas, we used geophysical measurements at the surroundings of 39 wells drilled through the reservoir formation in order to generate isotropic and anisotropic EI maps. The reservoir inner structure consists of fine layering, allowing us to approximate the formation to a transverse isotropic medium with vertical axis of symmetry (VTI medium) for anisotropic EI map construction. We then applied the Backus-averaging technique for estimating VTI elastic stiffness logs from the well log measurements through the reservoir formation.For the construction of the maps, we implemented an interpolation process incorporating a robust inverse-distance square-weighted interpolator into a search radius scheme. In this scheme, only control points inside a distance specified a priori are considered in the interpolation process. The resulting EI maps for incidence angles of 0 • , 20 • and 40 • , at 3080 m depth, reveal zones of fine layering around certain wells. We observed that seismic anisotropy induced by fine layering is pronounced in these zones, coinciding with low oil-producing intervals. At the same selected depth of the reservoir formation, the remaining parts of the maps show a weak strength of VTI anisotropy.Keywords: geophysical well logs, elastic impedance, seismic anisotropy, Backus-averaging, spatial data interpolation, Namorado reservoir.RESUMO. O conceito de impedância elástica (EI) da onda compressionalé atualmente usado como um parâmetro petrofísico complementar para a caracterização de reservatórios de petróleo e gás. Neste artigo, construímos mapas que representam a variação espacial da EI isotrópica e anisotrópica a uma profundidade específica através de um reservatório turbidítico produtor de petróleo e gás. EIé então usada aqui não para calibração e inversão de dados sísmicos de grandes afastamentos como proposto originalmente, mas como uma aplicação original de um conceito petrofísico novo. Uma vez que densidade efetiva, velocidades sísmicas eângulo de incidência são variáveis comuns nas fórmulas isotrópica e anisotrópica da EI, usamos registros geofísicos nas vizinhanças de 39 poços perfurados através da formação que contém o reservatório a fim de gerar mapas isotrópicos e anisotrópicos de EI. A estrutura interna do reservatório consiste de camadas delgadas, o que nos permitiu aproximar a formação a um meio transversalmente isotrópico com eixo de simetria vertical (meio TIV) para const...

Dynamic Young Modulus Variation Through a Turbiditic Sedimentary Interval

“…As commented in Oliveira & Martins (2011), two possible limiting situations can occur in Eq. (11): r s = d i and d i = 0.…”

“…To complete the well log data set in this old exploration area, Augusto (2009) and Augusto & Martins (2009) used least-squares regression analysis for empirically estimating the missing P-wave sonic logs in a large number of wells. Oliveira & Martins (2011) then applied the methodology proposed in Lee (2006) for evaluating S-wave velocity logs from the resulting empirically-estimated P-wave velocity logs. In Figure 2, an interval of the lithology column of reference wells NA-01 (Fig.…”

“…We applied the same procedure as in Oliveira & Martins (2011) to represent the spatial variation of isotropic and VTI Young moduli through the Namorado reservoir. The construction of the Young modulus maps relied on the input data set containing the E, E ⊥ and E logs computed with Eqs.…”

“…In general the gridded area is irregularly populated by control points, possibly leading to a situation where the calculation of the weight is critical (i.e., the situation when the selected search radius surrounds no control points). In order to avoid capturing no control points we used here the same procedure as in Oliveira & Martins (2011), applying a variable search radius during the interpolation process. The section in the following shows Young modulus maps using a variable search radius from 2000 to 3000 m. As we selected m = 2 in Eq.…”

“…In this paper, we used bulk density and P-and S-wave sonic well log measurements from 36 wells of the data set prepared by Oliveira & Martins (2011). The data set holds petrophysical information from 2940 to 3300 m depth of the upper Macaé formation containing the Namorado reservoir in Campos basin.…”