Seismic Attribute Analysis of the Mississippian Chert at the Wellington Field, South-Central Kansas

Ayrat Sirazhiev, George Tsofias, and Lynn Watney

Mississippian chert reservoirs are highly heterogeneous, typically below seismic resolution and, therefore, present a challenging task for seismic characterization. We used well and 3D PSTM seismic data to investigate the relationships of thickness and porosity of the Mississippian microporous cherty dolomite reservoir with post-stack seismic attributes at the Wellington Field, south-central Kansas. The analysis of well-log and seismic data revealed distinct characteristics across the NE-SW trending fault. The reservoir in the southeastern part of the field was characterized by a vertical gradational porosity reduction (from 25-30% to 4-6%), a corresponding increase in P-wave velocity (referred as a ramp-transition velocity function), variable thickness (6-20 m) and lower seismic amplitude and frequency content. High correlation between seismic amplitude and reservoir thickness, conformable with the theoretical amplitude response of a ramp-transition velocity function, was used to predict reservoir thickness in this part of the field. The reservoir in the northwestern part of the field exhibited more heterogeneous porosity distribution with no clear relationship between reservoir thickness and seismic attributes. Inverted acoustic impedance showed high correlation with porosity throughout the field. The porosity model predicted from inverted impedance supported the well-log and seismic amplitude interpretation. Wedge modeling determined resolution limits as 5 m (1/16λ) for predicting thickness from seismic amplitude and 10 m (1/8λ) for porosity prediction from model-based inverted impedance for the Mississippian reservoir characterized by vertical gradational porosity reduction. This study revisited the seismic response of a ramp-transition velocity function as a characterization tool for Mississippian chert reservoirs with gradational downward porosity reduction.