Disintegration Mechanism and Hydrogeochemical Processes of Red-Bed Soft Rock Under Drying-Wetting Cycle

Red-bed soft rock in the drawdown area on bank slopes of landslide easily disintegrates upon exposure to water, and its properties experience comprehensive deterioration, which will cause bank slope instability. To better study disintegration mechanism of the red-bed soft rock, a series of laboratory tests were conducted in this paper to investigate the disintegration characteristics, durability and hydrogeochemical process of red-bed argillaceous siltstone under drying-wetting cyclic conditions. Experimental results showed that, with increasing number of drying-wetting cycles, red-bed argillaceous siltstone gradually disintegrated, from initial appearing the cracks on the surface of the samples to large particles gradually breaking up into small fragments. Significant changes in grain size distribution, and the durability index of the samples progressively decreased. Microstructural analysis showed that the size and distribution of pores and cracks in the sample surface significantly increased, such that the sample surface became disordered and complicated. Notable changed in concentrations of ions in the soaking solutions indicated continuous mineral dissolution and loss during the cyclic drying-wetting. Based on the results obtained from the experiment, it is concluded that the disintegration of samples undergoing drying-wetting cycles was the result of the synergistic action of water and temperature. To be specific, the dissolution of calcite, albite, gypsum, montmorillonite and kaolinite during the wetting procedure, which promotes the decrease in mineral content and increases in pores and cracks. The increases in temperature and the dehydration shrinkage of sample during the drying procedure accelerated the disintegration of the samples.

Evaluation of Lithology Variations in Layered Red Beds with Depth: An Example of the Yellow River Guxian Dam, NW China

Lithology variations, which are recognized as rock type differences, significantly affect the physical properties of rock masses in red beds. In this paper, we introduce a statistical method for quantitatively evaluating the degree of lithology variations in layered red beds with depth. The core of this method is to use borehole logs as random variables for statistical analysis to calculate the percentage of a specified lithology at a given elevation, which involves seven steps of attitude calculation, reference point selection, distance calculation, elevation modification, data discretization, data statistics, and curve plotting. The Yellow River Guxian Dam is chosen as a field case study. We classify rock types of feldspar sandstone, fine sandstone, and conglomerate into hard rocks and that rock types of calcareous siltstone, argillaceous siltstone, and mudstone into soft rocks. Borehole logs recorded during the geological investigation are used to plot the percentage curve of hard rocks. We find that the degree of lithology variations for each lithology group differs greatly, the general behaviors of lithology variations on two sides of the Guxian Dam riverbed are quite similar but still with some differences, and that some thick lithology groups can be finely divided into several subgroups. On the basis of the hard rock percentage curve, we introduce a lithology variation index to quantitatively characterize the degree of lithology variations, which can be used as an important index to supplement the traditional methods when performing rock mass classifications in red beds. We also plot the trilithology percentage curve of sandstone, calcareous siltstone, and argillaceous siltstone, which serves for the determination of physical parameters of the dam foundation rock mass, the identification of the potential shear sliding surface, and the search for an impervious grouting bottom. Moreover, we find that the crest and trough, which are local high and low points in the hard rock percentage curve, can be used to show some characteristics of shear zones. The locations of shear zones are well represented in the form of troughs and that the development of shear zones has a good linear relationship with the hard rock percentage of the corresponding crest. The method proposed in this paper can be promoted and applied in similar projects or studies.

Seismic prediction of lithofacies heterogeneity in paleogene hetaoyuan shale play, Biyang depression, China

Shale lithofacies identification and prediction are of great importance for the successful shale gas and oil exploration. Based on the well and seismic fine calibration, extraction, and optimization of seismic attributes, root mean square (RMS) amplitude analysis is used to predict the spatial–temporal distribution of various lithofacies in the fifth organic-matter-rich interval, and the prediction results are confirmed by the logging data and geological background. The results indicate that in the early expansion system tract, dolomitic shale and calcareous shale were widely developed and argillaceous shale, silty shale, and argillaceous siltstone only developed in the periphery of deep depression. With the lake level rising, argillaceous shale and calcareous shale were well-developed, and argillaceous shale interbedded with silty shale or argillaceous siltstone developed in deep or semi-deep lake. In the late expansion system tract, argillaceous shale was widely deposited in the deepest sag; calcareous shale presented in eastern sag with belt distribution.

Neotectonics of the Bailongjiang and Hanan faults: New insights into late Cenozoic deformation along the eastern margin of the Tibetan Plateau

The way that far-field stresses and deformation propagated eastward in response to the growth and extrusion of the northeastern Tibetan Plateau remains a crucial scientific issue. This paper focuses on the Bailongjiang and Hanan faults, which are the easternmost part of the East Kunlun fault in northeast Tibet. Based on new field geological investigations, structural data, satellite imagery interpretation, and optically stimulated luminescence and 14C dating results, this paper presents the structural geometry and neotectonic activities of the two faults. The ∼200-km-long Bailongjiang fault, bounding the Bayan Har block in northeast Tibet, consists of two segments. Along the western segment, late Pleistocene lacustrine-facies deposits and Holocene activities were discovered in a great fault scarp. The left-slip rate of the fault is estimated to be ∼1.73–2.61 mm/yr, with an elapsed time of ∼2205 yr after a catastrophic paleoseismic event greater than M 7.2 ruptured the fault. The eastern segment splits into two branches and shows a positive flower structure where a pull-apart basin developed, filled with ∼200-m-thick mudstone and argillaceous siltstone, which record the mid-late Miocene deformation of the Bailongjiang fault. The Hanan fault features reverse faulting caused by NNW-SSE compression in the late Cenozoic. The two faults, together with the Maqên-Maqu-Tazang fault, confine the area of a strip block, the eastward extrusion of which was accommodated by thrusting due to the resistance of the stable Bikou massif since the late Cenozoic, which led to decreasing slip rates along the easternmost part of the Kunlun fault.

Coal Seam Roof: Lithology and Influence on the Enrichment of Coalbed Methane

To identify the lithology of coal seam roof and explore the influence of these roofs on the enrichment of coalbed methane, low-frequency rock petrophysics experiments, seismic analyses and gas-bearing trend analyses were performed. The results show that the sound wave propagation speed in rock at seismic frequencies was lower than that at ultrasound frequencies. Additionally, the P-wave velocities of gritstone, fine sandstone, argillaceous siltstone and mudstone were 1,651 m/s, 2,840 m/s, 3,191 m/s and 4,214 m/s, respectively. The surface properties of the coal seam roofs were extracted through 3D seismic wave impedance inversion. The theoretical P-wave impedance was calculated after the tested P-wave velocity was determined. By matching the theoretical P-wave impedance of the four types of rocks with that of the coal seam roofs, we identified the lithology of the roofs. By analyzing known borehole data, we found that the identified lithology was consistent with that revealed by the data. By comparing and analyzing the coal seam roof lithology and the gas-bearing trends in the study area, we discovered that the coal seam roof lithology was related to the enrichment of coalbed methane. In the study area, areas with high gas contents mainly coincided with roof zones composed of mudstone and argillaceous siltstone, and those with low gas contents were mainly associated with fine sandstone roof areas. Thus, highly compact areas of coal seam roof are favorable for the formation and preservation of coalbed methane.

Exploration of Mechanical Behaviors of Argillaceous Siltstone through Photoelastic Model Test and DEM Modelling

This paper aims at investigating mechanical behaviors of argillaceous siltstone through a photoelastic model test and DEM modelling. Both bonded and unbonded conditions were considered. The photoelastic model was fabricated in a controllable environment, and its scaled factor was 1 : 200 in length and 11.9 : 1 in Young’s modulus. The polariscope was designed to have an area-based LCD source and an automatic digital camera. A calibration test was carried out to obtain the relationship between the contact force and the average intensity gradient squared. A 2D DEM simulation is carried out for a 100 mm × 200 mm rectangular container filled with particles of 6 mm and 10 mm, in which the bonding effect is realized by adjusting the shear contact stiffness. The results show that both the photoelastic model and DEM modelling are able to capture the evolution of force chain network, effective contact number and stress concentration factor, and rheological behaviors when it is subjected to an increasing uniaxial load. The bonding effect plays an important role in the mechanical performance of argillaceous siltstone.