Print this pageThe Added Value of Quantitative Seismic Integration in 3D Prediction of Petrophysical Properties and Facies. A Case History from Tunisian Ghadames Basin
An integrated seismic characterization study was carried out over 900sqkm exploration acreage, in the Tunisian Ghadames basin. The target is Silurian siliciclastic (Acacus,Tannezuft)consisting of interbedded shelfal clays and thin sandstone layers,characterized by low reflectivity.
A number of prospects had already been defined prior to this study:due to poor reflectivity at target level, structural setting was based on mimic the deeper Top Ordovician energetic reflector, while the stratigraphic information was derived from sparse well points. Seismic data quality issues (mainly very subtle reflections, multiples and quite low S/N ratio) discouraged a quantitative use of seismic for petrophysical and facies characterization.
Following a recent seismic reprocessing, data quality was positively re-assessed for seismic characterization purposes, providing a new opportunity to improve both structural and stratigraphic understanding of the area.
Applied workflow included: a)well-seismic tie and wavelet estimation; b)petro-acoustic analysis to evaluate Impedance vs petrophysical relationships; c)detailed interpretation at survey scale; d)a priori model building integrating logs, seismic and structural interpretation; e)inversion; f)multi-attribute calibration to petrophysical property.
The output is a Porosity cube generated for the whole 3D area and extends across 3 major fields and some discoveries. Statistical properties of predicted Porosity volume were extracted within each of the interpreted layers, to verify how it compared with the sedimentological interpretation.
Relationship between predicted vs actual Porosity Thickness was found reliable (R2 =0,73); residual errors were corrected via collocated cokriging. A probabilistic function relates Porosity Thickness and Sedimentological Facies distribution; this led to generation of “Facies Probability” maps. It is important to note that Porosity Thickness is strictly related to dominant facies distribution (shelfal lobes Sst., shelfal fines, interlobe-lobe fringe Sst, shales) in the examined sequences.
The post-study drilling of two exploration wells provided the value of the integration of seismic analysis into the purely well-based model, and a reliability evaluation of predicted porosity. Comparison of actual vs predicted logs clearly outlines considerable improvements, by reducing the prediction error and enhancing the ability to detect porous sand layers, that otherwise would not have been recognised.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil