No abstract

Tuning caused by closely spaced impedance boundaries affects seismic amplitudes. At zero‐offset the shape of the composite reflected signal approaches the time‐derivative of the original pulse as the layer thickness decreases. For layers thinner than half of the tuning thickness, the reflected amplitude is modified by a factor equal to twice the time‐thickness of the thin layer. Offset‐dependent tuning can be approximated by the time differences between primary reflections. For high‐velocity contrasts locally converted waves will also affect the total reflected seismic response. The contribution from intrabed multiples can, in most cases, be ignored. Correction for offset‐dependent tuning should be considered before conventional AVO analysis.

Tuning caused by closely spaced impedance boundaries affects seismic amplitudes. At zero-offset the shape of the composite reflected signal approaches the time-derivative of the original pulse as the layer thickness decreases. For layers thinner than half of the tuning thickness, the reflected amplitude is modified by a factor equal to twice the timethickness of the thin layer. Offset-dependent tuning can be approximated by the time differences between primary reflections. For high-velocity contrasts locally converted waves will also affect the total reflected seismic response. The contribution from intrabed multiples can, in most cases, be ignored. Correction for offset-dependent tuning should be considered before conventional AVO analysis.

Copyr!ghl 1996 Souety 01 Petroleum Englneets Tfus papr was prepared Ior presentatum at the EuroPan 3-D Resewo(r Modellmg Conference held tinSIavanger Noruay, 16-17 F.@ 19S6 Th!s paper was selected for presentatwn by the SPE Prcgram Commrttes following rwvlew o! Information conlamed m an abstrOct submlfted by the author(s) Conlenls of the paper as presented, have not been rWBWd by the Society of PekOleum Engineers and are sublect to correc!aon by the author(s) The mater,al, aspresented, doss not necesswfy rellect any pos{fton O( the .SOCtaty01 Petroleum Engineers or Its membas Papsrs plesenmd .91SPE mstxmgs ale subject to publlcellon rewew by Edllortal Comm!ltee of the Sccmty of Petroleum Engmeem Permlssm 10 copy IS restricted to an abslract of not more than 300 words. Illustrations may not be copIsd The ab$tracf should cwtam COOWNCUOUS acknowlsdgmenl of where and by wtmrn the parer was presented Write Llbrarlan, SPE, P O 6333636, Richardson, TX 75063 .3S36 USA, lax 01.214-952-9435 AbstractCalcite cemented sandstone layers are common heterogeneities in shallow marine reservoirs where they cause significant flow barriers. Field analogue studies indicate that the most laterally persistent cemented layers are associated with sequence stratigraphic bounding surfaces where the cementation forms a complex network of beds concentrated around both maximum flooding surfaces and sequence boundaries Although individual calcite cemented beds are thin, they arc associated with very high acoustic impedances and concentration of these thin, high impedance beds around bounding surfaces produces a significant seismic response which can be used to provide information on the spatial distribution of flow barriers. This information along with a sequence stratigraphic interpretation of the reservoir and geometrical data extracted from field analogues provides the input for an integrated reservoir description of calcite cemented barriers using a stochastic modelling approach.The stochastic model is a non-stationary' indicator field \vith an external trend in the indicator proportions, Vanograms are used to control the geometry of individual cemented beds. Sequence stratigraphy and seismic data are used to define spatial trends in the proportion of cemented beds; sequence stratigraphy provides the vertical constraints, and seismic data provide lateral constraints.The modelling approach has been tested using data from the TOGI area of the Troll gas field in the Norwegian North Sea. A controlled (verifiable) test has been carried out using synthetic seismic data based on a realistic, heterogeneous, 3D acoustic model of TOGI, These data have been inverted using a constrained sparse-spike seismic inversion technique. The test demonstrated that the prototype modelling procedure functioned satisfactorily. The integration of data from several sources contributed to the generation of realistic heterogeneity models constrained by a maximum amount of information.A test using real seismic data from TOGI has been initiated. The results of the sparse...

In exploration as well as in production of hydrocarbons, the overall goal is to extract information and estimate uncertainty about lithology and fluid in the subsurface. Different techniques are used to provide such predictions. Traditionally, standard prospect evaluation has been focusing towards identifying structural traps through seismic interpretation, which provides a framework for further and a more detailed geological analysis (basin analysis, modeling, structural reconstruction etc). Some of these activities are by no means un-important, but they are based on conceptual models and less on physical measurements. Analysis of seismic amplitude responses associated with a given target or prospect has usually been more of an add-on activity near the end of the prospect evaluation rather than an integral part of it. A change in the exploration focus in the past 10 years towards more subtle traps in complex depositional environments, i.e. deep-water clastic turbidite systems, has made the traditional prospect evaluation more uncertain. There are no doubts that improvements in the overall prospect risk assessment requires more quantitative techniques that uses seismic data, both geometry and amplitudes, to reveal knowledge about possible shape, lithology and fluid. The industry has been and is good at developing competent people that both develops and handles new and improved seismic technologies, but the pace of integrating it in a cross-disciplinary manner with all other pieces to shape the "big picture" puzzle is by far too slow and complicated. As long as the input to prospect risk exercises continue in a "bits and pieces" fashion, the result of the outcome will be no stronger than the weakest link.We are as part of the industry moving towards developing integrated prospect evaluation workflows that relies heavily on seismic data for predicting subsurface shapes and rock properties. The concept behind such an integrated approach is to establish a link between the geological and the seismic amplitude domains. Rock physics modeling make the connection between geology and seismic parameters, (P-, S-wave velocity and density). This enables us to investigate how various geological parameters such as, quarts cementation, sorting, shale content, porosity, layer thickness, softness of cap rock and fluid saturation affects seismic parameters. Seismic forward modeling connects variations in seismic parameters to seismic amplitudes and enables improved understanding of how geology and seismic amplitudes are linked in a geographical area.Next, our effort is focused towards customizing seismic data to secure optimal data quality prior to any attempt of extracting information about lithology and fluid. The aim