Background Shilajit is a commonly used Tibetan medicine, and its water extract is mainly used for various heat-related syndrome, especially that of stomach, liver and kidney. Shilajit is found to exudate from rocks of cliff at an altitude of 2000–4000 m as a water-soluble mixture of black paste and animal feces of Trodocterus spp. or Ochotona spp. Because it is difficult to reach the exudation points so as to explain the its formation process, the source of Shilajit still remains unclear and controversial, which severely impedes its safety and efficacy in clinical application. Methods In this work, a series of investigations as rock flakes identification, porosity determination, rock mineral analysis, scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS) have been carried out to clarify the source of Shilajit, including the storage condition and exudation process of its organic matter, and to investigate the geological structure of the exudation points as well as physical and chemical characteristics of the mother rocks. Results The Shilajit exudation points were mainly distributed on the steep cliffs, where there were cavities and sections that could not be eroded by rainwater. The fundamental structure of the exudation points was determined by the rock’s bedding planes, joints, fracture surfaces and faults, and developed into micro-topography later. The exudation points were distributed in the Triassic strata and scattered in the Early Mesozoic granitoids. The lithologic features were mainly slate, carbonaceous slate and sandy slate etc. The background rocks were characterized by intergranular pores, dissolved pore, joint and fracture development. Organic matter was widely distributed in these pores and fissures, which had condition for storage and exudation of organic matter. Conclusions Shilajit mainly distributed on sunny steep slopes and cliffs with a slope of 60° or above at altitude of 2000–4000 m. The lithology character of the Shilajit exudation area were mainly various metamorphic rocks of sedimentary rocks that were rich in organic carbon. The organic matter in Shilajit was found to flow out naturally from rocks along pore, structural plane and even accumulate on the surface of rock as a result of storage environment change caused by rock tectonic action.
In recent years, it has become more and more common to drill deep karst caves as a part of deep shale gas resource exploration and engineering construction in South China. However, the amount of research on the genesis and development mechanism of deep karst caves is relatively low. Based on drilling core karst morphology analysis and two-dimensional (2D) seismic and wide-field geophysical exploration methods, it is revealed that the deep karst in the Huanjiang area is mainly composed of net cracks, holes expanding along cracks and dolomite honeycomb pores, and that large karst caves are also developed, which are related to NW-trending faults. The deep karst is developed in the hanging wall of the fault, with a width of 500 m and a height of 1500 m, and a linear distribution along NW faults in the region. Based on Th-U dating, inclusion testing and rare earth elements from cave fillings, it is revealed that the development of deep karst space is related to two deep karst genetic types: The first is the hypogene hydrothermal karst, which developed in the Yanshanian period and is related to regional magmatism. The second is the groundwater deep circulation karst, mainly developed after the Himalayan period, which is related to the deep circulation of meteoric water. The genesis of deep karst space is the result of multi-stage karst superposition and is mainly controlled by faults. It is difficult to determine the specific time points of these two types of deep karst transformation, but according to regional tectonic evolution, we speculate that the Yunnan-Guizhou Plateau has been uplifted since the Himalayan period (>3.54 Ma), and the Carboniferous carbonate rocks and early faults in the Huanjiang area have been exposed, leading to the change and evolution of deep karst. Through comprehensive analysis, a fault-controlled hypogene hydrothermal karst pattern and a meteoric water deep karst pattern are established. The genetic pattern of deep karst provides theoretical support for predicting this kind of karst in southern China and for avoiding drilling deep karst caves as a part of resource exploitation.
The classification method of karst formations is widely used in engineering and environmental geology but is seldom used in petroleum geology. In this study, the classification method of karst formations is applied to the sealing study of shale gas roof and floor carbonate rocks, and the influence on shale gas accumulation and drilling is discussed. The Paleozoic black shale in southern China is primarily formed by marine and transitional faces, and the intergrowth between shale and carbonate rocks is a basic geological feature of the Paleozoic strata in southern China. Carbonate karst is an unavoidable problem in shale gas exploration in southern China. Around the black shale target layer, in the Upper Paleozoic trait region, the study starts from the development strength of karst strata, through geological profile survey, spring flow statistics, test, and other methods and means; the shale and the carbonate rock contacted with it are taken as a whole to explore the impact of karst strata on shale gas drilling. The Upper Paleozoic karst strata in the study area were divided into two kinds, four types, and six subtypes. It was determined that the limestone continuous karst strata of the Sidazhai Formation and the second member of the Nandan Formation are the sensitive layers of shale gas drilling, whereas the first number of Nandan and Wuzhishan formations are shale reservoir-forming packers. In addition, a method for evaluating the karst sensitivity of shale gas exploration is summarized. The karst avoidance, karst-sensitive, and karst-insensitive areas for shale gas exploration were divided. Combined with the surface and underground conditions and the basic geological conditions of shale gas not being significantly different, shale gas drilling should avoid the fold core and fault zones.
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