A031 Porosity and Shale Volume Estimation for the Ardmore Field Using Extended Elastic Impedance

Transcription

1 A31 Porosity and Shale Volume Estimation for the Ardmore Field Using Extended Elastic Impedance A.M. Francis* (Earthworks Environment & Resources Ltd) & G.J. Hicks (Earthworks Environment & Resources Ltd) SUMMARY Deterministic seismic inversion and extended elastic impedance (EEI) have been used to obtain quantitative estimates of porosity and Vshale over the Ardmore Field. The optimum EEI angles corresponding to porosity and Vshale were determined from the well logs, together with a lithology indicator. The prestack seismic data were then projected to the Chi angles corresponding to these three petrophysical indicators and deterministic inversions were performed to obtain three broadband EEI volumes. Three-parameter linear regressions were then performed to estimate quantitative porosity and Vshale volumes from the three EEI volumes. Regions of High porosity and low Vshale have been identified which may suggest possible future drilling locations. EAGE 68th Conference & Exhibition Vienna, Austria, June 26

2 Summary Deterministic seismic inversion and extended elastic impedance (EEI) have been used to obtain quantitative estimates of porosity and Vshale over the Ardmore Field. The optimum EEI angles corresponding to porosity and Vshale were determined from the well logs, together with a lithology indicator. The prestack seismic data were then projected to the χ angles corresponding to these three petrophysical indicators and deterministic inversions were performed to obtain three broadband EEI volumes. Three-parameter linear regressions were then performed to estimate quantitative porosity and Vshale volumes from the three EEI volumes. Regions of High porosity and low Vshale have been identified which may suggest possible future drilling locations. Ardmore Field Introduction The Ardmore Field is located on the south-western flank of the Central Graben in the North Sea. The main producing reservoir has been the Zechstein which has excellent characteristics due to the presence of both vuggy and fracture porosity. The Rotliegendes reservoir is composed of good quality massive Aeolian dune sandstones and is restricted to the central area of the field. Extended Elastic Impedance Analysis Extended elastic impedance (EEI) (Whitcombe et al., 22) provides a framework to work with pre-stack AVO but in terms of impedance instead of reflectivity. In the EEI analysis phase, EEI logs are generated for each well as a function of angle χ and correlated with the petrophysical logs. For each petrophysical log a plot is then made of the correlation coefficient as a function of angle. For Ardmore the resulting EEI angle correlation curves are shown in Figure 1. Curves are shown for gamma ray (GR), effective porosity (PHIE) and clay volume (VCL). The curves show the average response from 24 wells over the greater Ardmore area. The recorded seismic gathers for Ardmore have a χ angle range of to +14o, indicated by the shaded overlay in the figure below. PHIE is strongly negatively correlated at negative angles through acoustic impedance (zero angle) and peaks at a correlation of -.9 at an angle χ=+18o. The GR and VCL curves show no correlation with EEI logs representing the recorded seismic angle range but rapidly peak at a correlation of +.7 at an angle of +23o. Examples of the EEI logs are shown in Figure 2. In the left panel the PHIE log is compared to the EEI log at χ=+18o, showing the high correlation and predictive capability of the EEI log at this angle. The right panel shows the χ=+23o EEI log is a good predictor for the VCL log. 1.8 Correlation Coefficient.6 Vcl Maximum Angle Range of Seismic.4.2 GR PHIE VCL Porosity Maximum Angle (Chi) Figure 1: Correlation of petrophysical logs to EEI logs as a function of angle χ. EAGE 68th Conference & Exhibition Vienna, Austria, June 26

3 Depth (ft) Depth (ft) PHIE 91 EEI(+23) Shale Content (fraction) 86 EEI(+18) Porosity (fraction) VCL 23.5 Figure 2: Porosity (left) and Vshale (right) logs with their corresponding EEI logs. Well Ties, Zero Phasing and EEI Projection After completing the EEI analysis, EEI logs corresponding to the mean angles in the supplied seismic angle stacks are used to establish initial well ties. After tying each angle stack, a wavelet is estimated and zero phase deconvolution applied to each angle stack using spiking deconvolution. This helps remove small timing errors across the angle stacks and assists in normalising their spectral content. The deconvolved angle stacks are then used to project new seismic volumes corresponding to the χ angles identified from the EEI impedance analysis procedure described in the previous section. In addition to the EEI angles used to predict PHIE (χ=+18o) and VCL (χ=+23o) an additional EEI angle of χ=-65o was also selected. This was chosen after modelling EEI logs as a function of angle (Figure 3) and was intended to provide an EEI angle with maximum capability for discriminating lithology. This angle approximately corresponds to a shear wave log. Optimal Lithology Descrimination Optimal Phi/Vcl Prediction Time (ms) Current Seismic Chalk Zech Rot SST Chi (Degrees) +9 Figure 3: EEI logs as a function of χ. Deterministic Inversion In order to provide a quantitative output for predicting reservoir properties, the EEI data require inverting to absolute impedance. Deterministic inversion is most effective when the reservoir interval has a strong reflectivity and where the reservoir layering is relatively thick with well defined units close to the seismic resolution limit. The Ardmore Field seismic is thus ideally suited for deterministic inversion. The inversion scheme used here is referred to as model-based inversion (Russell and Hampson, 1991). In model-based inversion an initial impedance model is modified iteratively to improve the fit to the seismic trace. Assuming a reasonable initial model, model-based inversion is able to remove the wavelet and hence remove tuning effects.

4 The initial model comprises interpolated impedance data from the wells guided by a stratigraphic framework defined by the picked seismic horizons. In this case study, the stratigraphic framework comprises the Top chalk, BCU and Base Zechstein interpreted seismic horizon picks. A total of 3 wells have been included in the model. Three models have been constructed, one for each of the χ=-65o, χ=+18o and χ=+23o EEI volumes. Each EEI volume is inverted using its own wavelet. Quantitative Vshale and Porosity Prediction In order to make quantitative predictions of Vshale or porosity, the EEI log data have been plotted as a 3-dimensional crossplot coloured by porosity or Vshale (Figure 4). In these 3D cross-plots low porosity values plot along the left hand side of the projection and contour progressively to the good porosity values in the lower right-hand corner. Conversely, low Vshale values (clean formations) plot along the upper right edge and the data become progressively shalier to the lower left corner. Using the three EEI logs, two separate linear regressions have been defined, one for porosity and one for Vshale, and the coefficients used to combine and transform the three deterministic EEI seismic volumes to porosity and Vshale volumes. Cross-sections through these two volumes are shown below. Figure 5 is a cross-line through the Vshale volume. The high Vshale (blue/purple) below the BCU seismic horizon corresponds to shalier intervals, most likely Kimmeridge clay. The apparently shaly intervals in the chalk are probably erroneous: the average Vshale predictor is not appropriate for the chalk interval. Also in the Vshale section the clean, blocky Rotliegendes sands are clearly seen (greens) below the Base Zechstein seismic pick. In the porosity section (Figure 6) the Rotliegendes is again clearly seen, good porosity being shown in green. The Jurassic and Zechstein intervals show more intermediate porosity, along with the Devonian. Figure 7 shows an average porosity slice for the interval 1-2 ms below the BCU seismic pick, corresponding to Jurassic and Zechstein in different parts of the field. The Ardmore Field fault pattern is overlaid. The good quality reservoir is shown in red. Note in particular how the higher porosity Jurassic sands appear to be draped on the northwest flank of the field and in the hanging wall of the main field bounding fault to the southeast. The latter may indicate a possible drilling target. The low porosity (blue colours) in the centre of the field indicates the presence of Kimmeridge formation. Figure 4: 3-D EEI crossplots coloured by porosity (left) or Vshale (right). References Russell, B. and Hampson, D., 1991, A comparison of post-stack seismic inversion, 61st Ann. Internat. Mtg: Soc. of Expl. Geophys., Whitcombe, D. N., Connolly, P. A., Reagan, R. L. and Redshaw, T. C., 22, Extended elastic impedance for fluid and lithology prediction. Geophysics 67 pp EAGE 68th Conference & Exhibition Vienna, Austria, June 26

5 .5 Top Chalk BCU Base Zech. Figure 5: Inverted EEI seismic volumes combined and transformed to Vshale volume. Inserted logs are Vshale..25 Top Chalk BCU Base Zech Figure 6: Inverted EEI seismic volumes combined and transformed to porosity volume. Inserted logs are porosity. Figure 7: Horizon slice through Porosity volume 1-2 ms below BCU..