Analysis of Well Logs from the Nechako Basin

Abstract

Physical rock properties from wireline logs acquired in several wells that intersect volcanic and sedimentary rocks in the Nechako Basin have been compiled. Different rock types can be classified based on their distinct geophysical rock property characteristics. Porosity, resistivity, density, compressional velocities and acoustic impedance of volcanic rocks are distinctly different from those of sedimentary rocks suggesting that they can be successfully imaged by geophysical techniques such as seismic, magnetotelluric and gravity. Empirical relationships between porosity, resistivity, density and compressional velocities were established. These relationships provide a means of comparing models from datasets acquired from surface seismic, magnetotellurics and gravity measurements.

New geophysical models for subsurface velocity structure in the Nechako–Chilcotin plateau from 2.5-D waveform tomography

“…However, the region that shows overestimation of velocities by FWI is bounded at the top by lithological changes according to well-log interpretations from other workers. Figure 7 reproduces interpretations from two studies (Mwenifumbo and Mwenifumbo 2010;Riddell 2011) including sonic log velocities and overlays them on segments extracted from Figs. 4 and 5 for direct comparison.…”

“…2). These are underlain by Jurassic volcanic rocks, possibly of the Stikine or (less likely) Cache Creek terranes (see Mwenifumbo and Mwenifumbo 2010, and Riddell 2011; well-log and seismic interpretation). Jurassic rocks are interpreted in well a-4-L at ϳ2400 m depth (elevation z ≈ -1.5 km), but extrapolation from the waveform tomography velocity model suggests that these units may be found 500-1000 m deeper to the north (see Fig.…”

“…Exploration well a-4-L is located in the Nazko River valley alongside Line 05 (see Fig. 1), and has been interpreted by Mwenifumbo and Mwenifumbo (2010) and Riddell (2011). Figure 6 shows a direct comparison between the TT and FWI velocity models ( Fig.…”

“…The positions of four exploration wells are indicated in Fig. 1; the well logs are interpreted by Mwenifumbo and Mwenifumbo (2010) (both a-4-L and b-16-J) and Riddell (2011) (all four wells); their interpretations provide information about the rock units and lithology at depth. Surficial geology information is available from the BC-wide dataset of Massey et al (2005), and detailed mapping along several of the profiles in the region has been carried out (Bordet et al 2011).…”

“…Bordet 2014); (2) potentially indistinguishable from the underlying Cretaceous sediments (e.g., Andrews et al 2011b;Kushnir et al 2012); and (3) interbedded with volcanic and volcaniclastic rocks that may exhibit widely varying seismic responses. Interpretation of well sonic logs and additional work in rock physics yield information about the high-frequency seismic response of different rock types that can be correlated to changes in lithology (Mwenifumbo and Mwenifumbo 2010;Riddell 2011;Kushnir et al 2012). However, geophysical investigations that make use of direct and refracted P-wave signals Hayward and Calvert 2011;Clowes 2012, 2013) have indicated that the in-situ response to low-frequency seismic waves cannot be reliably compared to the high-frequency sonic logs.…”