Experimental estimation of physical properties of a series of rocks from a deep borehole of the Karoo Basin, South Africa

Abstract

The aim of this study is to estimate the physical properties (mainly density and seismic velocities) of the deep borehole cores from the Main Karoo Basin (South Africa). The samples were obtained from all the lithological units intersected by the KL1-65 borehole (~ 2034 m deep). The study also investigates the influence of applied uniaxial stress (up to 120 MPa) and mineralogical content on seismic wave propagation through the Karoo Basin rocks. The studied rock samples include sandstone, siltstone, shale, and tillite. The main objectives were to accurately distinguish the geological contacts, delineate the shale gas-bearing formations, and provide guidelines for future laboratory measurements and planning of reflection seismic surveys in the Karoo Basin. Thirty-eight samples were measured for density and seismic velocity using different techniques to obtain results that would represent true physical properties of the lithological units. Different techniques were used for density measurements, namely the instantaneous water immersion, wax immersion, and calliper methods using ADAM analytical balance scale with a sensitivity of 0.01 g. For ultrasonic measurements, two techniques were used: the Tektronix TDS 2021 and Proceq Pundit Pl 2000 ultrasonic pulser velocity tester consisting of two pairs of transducers with a centre frequency of 54 KHz. The water displacement and water displacement with wax techniques provided similar density results and the latter was chosen owing of its capability to preserve the sample’s integrity. Tektronix ultrasonic technique was more preferable over the Proceq technique for velocity measurements conducted under uniaxial stress. The average density for shale samples was 2.350 g/cm3, however the density values decreased with increased amount of carbonaceous materials within the sample. Moreover, the more carbonaceous shale from Collingham Formation measured the lower density values (e.g., 2.113 g/cm3), whilst carbonaceous shales from the Whitehill and Prince Albert Formation (main focus of this study) produced relatively higher average densities (e.g., 2.411 and 2.445 g/cm3). The P-wave velocities of sandstone (4500 –5600 m/s), shale (2100 –4800 m/s), and siltstone (1700 –4700 m/s) exhibited an increase in values as a function of depth. The S-wave velocities (Vs) mimicked the P-wave velocity (Vp) throughout the formations. The estimated Vp/Vs relation for sandstone, shale, and siltstone were linear, however the sandstone appears to be slanting horizontal while the relationship for shale and siltstone were directly proportional with ratio of approximately 1.52, and 1.73 respectively. The information from the Vp-Vs relationship was used to differentiate and identify several lithological units within the Karoo Basin. Mineralogical analysis indicates that the sandstone packages comprise quartz with relatively high seismic velocities. Shale samples, on the other hand, are mostly clayey with limited transmission of acoustic waves which result in relatively lower seismic velocity values within the package. Seismic velocities for siltstone fluctuate between clayey materials and sandstone rock types as they are a mixture of both. The estimated bulk densities and seismic velocities were used to compute synthetic seismograms to investigate the potential source of seismic reflectivity and their associated lithological boundaries. The strong reflections were observed at several lithological contacts including Waterford-Kookfontein, Kookfontein-Skoorsteenberg, Skoorsteenberg-Tierberg Formations, and the carbonaceous rich shale formations with the underlying or overlying rocks. The information from the borehole data and synthetic seismograms were used to constrain the seismic interpretation of the legacy 2D reflection seismic data acquired in the area for exploration purposes. Generally, seismic sections exhibit strong reflections at major lithological boundaries and various fault zones cross-cutting the strata. Delineation of faults, in particular, has significant impact on the future exploration projects in the area. Summarily, these results show the importance of analysing more than one sets of data to provide a realistic geological information of the subsurface. Ultimately, the integrated multi-geoscientific data that were collected were used to generate a geological model to provide a better understanding of the existing geological structures in the Karoo Basin