ABSTRACT: The Vajont landslide is an outstanding case history for the study and back analysis of complex instability mechanisms involved in sliding along planes of weakness in rocks. It moved a 250 m thick mass over a distance of about 500 meters horizontally, reaching velocities of 20 – 25 m/s, during less than 45 seconds. We use Discontinuous Deformation Analysis (DDA) to study velocity - distance evolution of that event by implementing a simple friction degradation algorithm. The slide moved across a basal plane that possessed an estimated friction angle of 12° only (µ = 0.231) due to previous sliding episodes. When the catastrophic landslide was triggered, friction had to be further degraded to explain the mapped runout distance and the estimated duration of the event. We find that further friction degradation of at least 25% must have taken place both across the basal plane (µ = 0.16) as well as in the rock joints consisting the sliding mass. This degradation resulted in peak velocity of 22 m/s, runout distance of 470 meters, and event duration of 37 seconds.

Abstract: We investigated the rock physics of immature organic-rich chalk from the Late Cretaceous Ghareb and Mishash formations in the Shefela basin, central Israel, and its variation upon pyrolysis-induced maturation. The study was carried out on core samples from a ~280 meters sequence of immature organic-rich chalk in the Zoharim well. We measured acoustic velocities using water-saturated core plugs in the bedding-normal direction, under simulated field pressures. Porosity and density data were obtained for oven-dried core plugs using nitrogen gas. Results measured for the immature chalk were compared with properties obtained at two different induced maturity levels: early-mature and over-mature. In the early-mature stage, where bitumen filled the newly generated and some of the pre-existing pore space, we observed an increase in S wave velocity by ~11%, and a slight decrease of P wave velocity, which was identical to the decrease in porosity and density. In the over-mature stage, where the pore space increased by ~29%, and the solid skeleton included kerogen residue and unaltered minerals, S wave velocity increased by ~19% and P wave velocity decreased by ~16%. We concluded that the kerogen experiences stiffening upon thermal maturation that results in faster shear waves. P wave velocity decreased upon maturation due to significant porosity enhancement. P wave - S wave velocities ratio (Vp/Vs) starts at ~2.2 in the immature stage, then reduces to ~2 in the early-mature stage, and further down to ~1.6 in the over-mature stage. Introduction Organic-rich sediments are widely known as source rocks due to their high potential within the organic matter to produce oil and gas. These rocks are typically fine-grained sediments that include organic matter in solid and fluid states. Kerogen is solid, and is the most abundant organic phase in thermally immature source rocks. The physical properties of source rocks are strongly influenced by kerogen properties such as density, chemical composition, elastic moduli and acoustic velocities. Thermal maturation causes significant changes in kerogen properties and thus in the entire rock properties. The dynamics of source rocks during thermal maturation is complex due to interaction of processes within the organic matter and field conditions (e.g. stress field, pore pressure, host rock properties, temperature). In addition to the importance for conventional oil systems, advanced technologies allows now to produce oil and gas from immature source rocks via in situ methods for inducing maturation. These aspects, among others, motivate the study of rock physics of organic-rich rocks and its variation upon maturation.

Abstract: Based on the assumption of Vertical Transverse Isotropy (VTI), we investigated the ultrasonic velocity and anisotropy of organic-rich chalks (ORC). These chalks, from the late Cretaceous Ghareb and Mishash formations in Israel, have a wide range of porosity (0 – 45 PU), high kerogen content (5 – 15 wt% TOC) and include sections with varying degrees of thermal maturation. Cores were extracted both from the Shefela basin of central Israel and two wells in the southern Golan Heights. We measured ultrasonic velocities of P and S waves in different directions using the ultrasonic transmission method, whilst adjusting effective (confining) pressure levels. The elastic constants were then calculated from the measured velocities and densities, as well as Thomsen anisotropy parameters. Compared to previous studies of organic-rich shale source rocks, which exhibit strong acoustic anisotropy, these organic-rich chalks display weak, yet still measureable transverse anisotropy. Our results present significant differences between the immature and early-maturation stage samples. In our samples, it had become evident that microcracks are present during the early-maturation stage, yet are absent throughout the immature stage. An additional distinction between the maturation stages reveals itself while examining the elastic constants, showing a much stiffer nature in the early-mature sample, which could also be attributed to compaction and lower porosity. Introduction Accelerating interest to develop unconventional oil and gas fields in the late Cretaceous Ghareb and Mishash formations in Israel motivates our laboratory work to measure and describe the seismic anisotropy of these rocks. These Late Cretaceous formations consist mainly of fine-grained carbonates with high concentrations of organic matter. The lithology of these rocks is mostly chalk, with occurrences of cherts, marls, porcelanites and phosphorites. These target layers show sedimentary layering and complex network of organic matter and minerals. Evidence of mechanical and petrophysical anisotropy had been recognized in previous studies of immature ORC from the Shefela basin [1], [2]. As far as we know, however, it is the first time that acoustic anisotropy of these formations is being measured. It is generally believed that at the field scale seismic anisotropy is influenced by three major factors: (1) interlayering of lithologies, or laminae, of contrasting elastic properties on scale much finer than the propagating seismic wavelength (2) preferred orientation of minerals and (3) stress-induced fractures and microcracks that show preferred alignment [3].