Seismic Attribute Analysis of the Mississippian Chert at the Wellington Field, south-central Kansas

Abstract

Mississippian chert reservoirs, important hydrocarbon resources in North America, are highly heterogeneous, typically below seismic resolution and, therefore, present a challenging task for predicting reservoir properties from seismic data. In this study, I conducted a seismic attribute analysis of the Mississippian chert reservoir at the Wellington Field, south-central Kansas using well and 3D PSTM seismic data. The microporous cherty dolomite reservoir exhibits a characteristic vertical gradational porosity reduction and associated increase in acoustic velocity, known as a ramp-transition velocity function. I investigated possible relationships of the Mississippian reservoir thickness and porosity with post-stack seismic attributes, including inverted acoustic impedance. The analysis of well-log and seismic data revealed that fault #1 divides the Wellington Field diagonally from the southwestern corner to the northeastern corner. The reservoir in the southeastern part of the field is characterized by a vertical gradational porosity decrease (from 25-30 to 4-6%), variable thickness (6-20 m), lower seismic amplitude and frequency content, locally developed double reflector, and high correlation between seismic amplitude and reservoir thickness conformable with the theoretical amplitude response of a ramp-transition velocity function. Amplitude envelope was used to predict the reservoir thickness in this part of the field. The Mississippian reservoir in the northwestern part of the field has more heterogeneous porosity distribution within the reservoir interval, thins in the north-north-west direction, while no clear relationship was found between reservoir thickness and instantaneous seismic attributes. The model-based inversion and porosity model predicted from inverted impedance supported the well-log and seismic attribute interpretation. The reliability of the predicted porosity model is tested by cross-validation. Resolution limits were determined using wedge modeling as 1/16λ for the amplitude envelope attribute and 1/8λ for the model-based inversion within the Mississippian reservoir characterized by a vertical gradational porosity reduction. The seismic response of a ramp-transition velocity function, well established in theory, but poorly studied using field seismic data, could benefit the characterization of similar chert as well as clastic and carbonate reservoirs characterized by downward porosity reduction as shown in this study. In addition, it might improve an understanding of depositional and diagenetic histories of such reservoirs.