Application of extended elastic impedance for lithology and fluid discrimination in gharif, miqrat and amin formations
Author
Al-Shukaili, Faisal Khalifa
English abstract
Extended Elastic Impedance (EEI) method was first introduced by Whitecombe in 2007 as a lithology and fluid indicator. The EEI method rotates the conventional Acoustic Impedance (AI) with angles called Chi angle that ranges from -90 to +90. Each Chiangle can be correlated to different petrophysical parameters like Porosity, Vsh, Saturation and Gamma ray and also to elastic parameters such as Velocity ratio (Vp/Vs), Density, Acoustic impedance, Shear Modulus or Bulk modulus...etc. The EEI can be an alternative method to AVO inversion with much lower cost and time for fluid and lithology discrimination. The Objective of this study is to examine the EEI on a data from Oman for lithology and fluid discrimination; then to compare the results with the AVO inversion results that was run earlier to see the similarities and differences between the two approaches
The EEI method is applied to more than 30 wells from four different fields of the northem portion of Oman addressing two main issues:
• Lithology discrimination between sand and shale in the Gharif Formation. Gharif
sand and shale facies are known to have almost the same Acoustic Impedance (AI). which makes it difficult to discriminate using conventional deterministic
inversion. • Fluid discrimination in Amin and Miqrat Formations which are known to be very tight reservoirs.
Rock physics and EEI analysis at the well data are conducted to find the optimum parameters to discriminate between sand and shale facies at the level of Gharif Formation and between gas and brine saturated-sands in the level of Amin and Miqrat Formations. Neutron density crossover method turned out to be the optimum parameters for lithology discrimination within the Gharif Formation. Chi Angles of 87 and 16 were found to be the optimum angles that matches with Neutron and Density logs respectively. The two angles were compared to the real Neutron and Density logs and were found to be similar in both well and cross plots scale. The EEI operates in the domain of the intercept vs. gradient mainly by rotating the domain's coordinates by angle of Chi. So, rotating the extracted intercept and gradient by angles 87 and 16 enabled the study to have two relative volumes
of neutron and density. The interpretations of these two volumes needed thorough analysis to conclude whether the Gharif Formation channels could be seen by this approach or it is beyond the seismic resolution.
Velocity ratio (Vp/Vs) was found to be the most appropriate parameter for fluid discrimination in the tight rocks of Amin and Miqrat Formations. Chi angle of 30 was found to be the optimum angle that matches the Vp/Vs log. The extracted Intercept and Gradient were then rotated by the angle 30 and created a relative volume of Vp/Vs at Amin Formation level that was used to locate the brine and gas zones. The comparison between the EEI and AVO inversion results showed a high potential of similarities.
The study opened the doors for further studies for three possible business impacts opportunities at Amin and Shuaiba levels in the Kauther Survey. Low Vp/Vs anomalies - that have proven to be an indication of gas sands- were spotted at a three way dip closure associated with a fault plane trap system at Amin Formation that was never drilled before. The same anomalies were also noticed at a long features in the Amin Formation and in four way dip closure at Shuaiba Formation level. These low Vp/Vs anomalies were following the surface structural maps which rises the confidence of these prospects.
