Compositional and textural controls on the seismic properties of the Waipawa and Whangai Formation mudstones, East Coast Basin, New Zealand

Abstract

For many years, the Waipawa and Whangai formation mudstones have garnered considerable interest as petroleum source rocks. More recently, the Waipawa Formation has attracted attention as a potential unconventional reservoir and has even been likened to the Bakken and Barnett shales in North America. However, at present there is a paucity of information on the petrophysical and geomechanical properties of the Whangai and Waipawa formation mudstones. Understanding how the rock properties control the elastic behaviour of these mudstones is essential for accurate geophysical interpretation and reservoir characterization. Five preserved cores from Orui–1A and Te Mai–2 wells in the East Coast Basin have been analyzed in order to understand the compositional and textural controls on the propagation of elastic waves, the dynamic elastic moduli and their use in interpreting the geomechanical properties of the rocks. High density and directional P–wave velocity scans on large cores and subsampled plugs was undertaken using non-contacting laser-based ultrasonics. Additional P and S–wave data were acquired on core plugs using contacting ultrasonic transducers. Composition was determined with X–ray diffraction, X–ray fluorescence and Source Rock Analysis, and rock texture was characterized via thin section petrography. This study finds that composition is the dominant control on the wave velocities observed in these mudstones. Rocks which are higher in clay and kerogen composition have lower wave velocities, with kerogen exerting a stronger influence. Velocity anisotropy ranges from 2% to 15% within the samples with no apparent relationship between overall composition and the degree of anisotropy. High density elastic data acquired with a laser-ultrasonic system shows for the first time how sample heterogeneity (e.g. fractures or lateral changes in mineralogy/porosity) and layering affect the P-wave velocities. Anisotropy appears to be influenced by the alignment of minerals and variation in composition with respect to the orientation of the scans. The presence of fractures and veins have been found to enhance anisotropy. However, veins and open fractures have also been found to diminish the effects of laying induced anisotropy if the vein is approximately in the same plane as layering. Estimations of brittleness based on Young’s modulus and Poisson’s ratio indicate that the Whangai Formation may be well suited to hydraulic fracturing, with brittleness values as high as 63%. The Waipawa Formation is less brittle and maybe less well suited than the Whangai Formation.