Pre-stack Seismic Attribute Analysis of the Mississippian Chert and the Arbuckle Group at the Wellington Field, South-central Kansas

Abstract

At the Wellington Field, south-central Kansas, the Mississippian reservoir is a microporous cherty dolomite, and the deeper Cambrian-Ordovician Arbuckle Group is a thick succession of interbedded dolomudstones, pack-grainstones, vuggy brecciated zones, and thin dolomudstone and shale beds. The Mississippian chert reservoir and individual Arbuckle reservoir units are highly heterogeneous and typically below seismic resolution. In this study I used 3D pre-stack depth migrated seismic data to map the main structural and stratigraphic features at the Mississippian and the Arbuckle reservoirs. A post-Mississippian normal fault that is striking NE-SW and dipping SE divides Wellington field diagonally into two parts. It cuts through the Mississippian and the Arbuckle Group down to the basement. The normal fault created accommodation space above the Mississippian chert reservoir in the southeastern part of the Wellington Field. The accommodation space allowed for depositing a layer that is thick enough to be resolved resulting in a localized double reflector in the seismic data. Furthermore, I conducted a pre-stack seismic attribute analysis of the Mississippian chert reservoir and the Arbuckle Group to extend previous work done using post-stack seismic data. The good porosity zones in both the Mississippian and the Arbuckle Group exhibit Class IV AVO response. This AVO classification was employed to identify the porous zones in the Wellington Field 3D seismic volume using the AVO intercept-gradient crossplotting technique. Simultaneous AVO inversion of pre-stack data showed better results than the model-based inversion of post-stack data for both the Mississippian reservoir and the Arbuckle Group. The inverted P-impedance by simultaneous AVO Inversion showed better correlation with the real P-impedance from well logs, and lower RMS inversion error. Also, Simultaneous AVO Inversion resolved low impedance zones that were not resolved by post-stack model-based inversion. Thickness resolution limit of simultaneous AVO inversion within the Mississippian chert reservoir was determined using wedge modeling as 10 m, which corresponds to 1/8 of a wavelength. In the Arbuckle, the low impedance zones in the inverted P-impedance volume show good contrast with the surrounding higher impedance zones, which makes it easy to define and trace the low impedance zones around the Wellington Field. In addition to the P-impedance, simultaneous AVO Inversion provided estimates of S-impedance and density, unlike the post-stack model-based inversion that inverts for P-impedance only. Inverted S-impedance was of good quality, but inverted density had the lowest recovery quality because density recovery depends mainly on the far offset data amplitude that can be easily distorted by noises. For porosity prediction at Wellington, multi-attribute linear regression analysis employed attributes from simultaneous AVO inversion results and attributes from post-stack seismic data to derive multi-attribute transforms that are used to predict porosity. A multi-attribute transform derived within the Mississippian chert reservoir only provided reliable porosity prediction within the Mississippian chert reservoir, but it did not provide meaningful porosity values outside the Mississippian reservoir. Another multi-attribute transform derived within a larger window, between the top of the Cherokee Group and the top of Reagan Sandstone, provided valid porosity values around the Mississippian chert reservoir that helped in determining the top and the base of the reservoirs. This multi-attribute transform also provided the best porosity prediction for the Arbuckle Group. Based on the estimated porosity volume and well data, the Mississippian reservoir thins to the northwest. The post-Mississippian normal fault is assumed to have lowered the southeastern part of the Wellington field area that remained underwater, while the northwestern part of the Wellington field was exposed resulting in the thinning of the Mississippian reservoir by erosion, and the deposition of thicker reworked Mississippian chert. The Arbuckle Group has five main low impedance and high porosity zones that are deeper in the eastern and southeastern parts of the Wellington field. The depth change of the five zones in the Wellington field is attributed to the post-Mississippian normal fault.