Amplitude Variation With Offset (Avo) Attribute Analysis For Seismic Reservoir Characterization And Seismofacies Study Of Kuyolo Field, Niger Delta Basin

ABSTRACT

Amplitude variation with offset (AVO) attributes analysis, based on integration of geophysical

and petrophysical data has become an important and innovative tool for the detection of

hydrocarbons in many exploration provinces of the world, including the Niger Delta Basin. The

method has been demonstrated to be effective when seismic data processing is carefully done

with detailed petrophysical modeling from complete well log suites. However, routine

application of the method has from time to time yielded disappointing results largely due to poor

or non-availability of shear wave data. Considering the need for reduced risk and enhanced

productivity in today’s petroleum industry, a new and more efficient technique that ameliorates

the problem of poor or non-availability of shear wave data is required. AVO Seismofacies

methodology is one of such techniques and this approach was adopted for this study. It accounts

for expected Amplitude Variation with Angle (AVA) effects, uses information from angle

substacks, and effectively takes care of problems arising from the absence of data on shear

waves, missing well log sections, and difficult well-to-seismic ties. The objectives of the study

were to: (i) delineate (ii) map and (iii) characterize reservoirs in the Kuyolo field using the AVO

Seismofacies approach. This approach involves quality control analysis of data, seismic data

interpretation, shear wave data generation, reservoir fluid substitution via Gassmann technique,

crossplots of elastic parameters and finally AVO Seismofacies analysis. The result of the quality

control (QC) analysis shows that it was sufficient for the analysis of the Kuyolo field AVO

anomalies. The result of the seismic data interpretation showed excellent tie of the well and

seismic data with the R11 reservoir clearly delineated and mapped with high confidence. The

result of the comparison of the actual well log measurements with the modelled well logs

showed that the logs were relatively similar over all wells, with the trends and variations of

density and velocities correctly modelled, especially in clean reservoirs. The analysis of the

crossplots of modelled density (ρ) versus P-wave velocity (Vp) and Poisson’s Ratio (PR) versus

acoustic impedance (IP) generated after water substitution over all the wells, showed overall

good separation of water saturated sands and shales giving confidence that AVO Seismofacies

methodology could be used to characterize the reservoirs. Cross-correlation analysis showed that

substack pairs of Near – Mid and Mid – Far offsets were suitable for AVO analysis. The study

revealed that cross-correlation between Near and Far offset substack were too low to allow an

AVO analysis using this pairing of substacks. Substack pairs of Mid – Far were then selected to

be used for this study. Average lambda and slope in degrees values of 0.24 and 37 degrees (370)

extracted from the crossplots of filtered Mid and Far offset substacks at several locations and

depths where there was no hydrocarbon were used to compute the AVO Seismofacies cube. All

analyzed AVO anomalies gave fair responses fitting rather well on the AVO maps with the

structural, sedimentary and stratigraphic interpretation, with anomaly shut-off at their base that

could be interpreted as water/hydrocarbon contacts. Results of crossplot analysis of the

amplitude values extracted at the top of R11 reservoir from filtered Near and Far substacks

above the presumed Oil-Water-Contact (OWC) suggested Class II Reservoir sands. Thus the

analysis carried out in the study area enhanced the identification and delineation of the

hydrocarbon reservoir with high confidence. Additionally, the study has demonstrated that

crossplot analysis such as those developed and analyzed in this work has helped to characterize

the AVO Class of reservoir sands.