Efficient identification of favorable exploration lithofacies with commercial value is of utmost importance in lacustrine shale petroleum exploration. Shales deposited under different salinity conditions exhibit considerably different organic matter (OM) accumulation and mineral compositions. However, the systematic summarization of favorable exploration lithofacies and their formation mechanisms in lacustrine shale systems formed under different salinity conditions is still lacking. Herein, lacustrine shale of the Lower Jurassic Qianfoya Formation (J 1 q) from the Langzhong Area of the Sichuan Basin is studied to systematically reveal its OM accumulation mechanisms and the factors controlling its physical properties. By combining our data with that published in the literature, the favorable exploration lithofacies and their formation mechanisms in lacustrine shales deposited under various different salinities (i.e., freshwater, saline, and hypersaline) are discussed. Our findings indicate that the J 1 q shale comprises four lithofacies, namely, organic-lean mixed shale (LM), organic-lean argillaceous shale (LCM), organic-rich mixed shale (RM), and organic-rich argillaceous shale (RCM). The pore systems of RCM and RM are dominated by OM-related pores, whereas those of LCM and LM are dominated by clay mineral-related pores. The J 1 q shale was deposited in an oxic environment with a semiarid to semihumid climate, moderate weathering, and freshwater to brackish conditions. The OM content is primarily influenced by the relatively reductive environment. The physical properties of RCM and RM are primarily determined by the total organic carbon (TOC) content, whereas those of LCM and LM are determined by the clay mineral content. Additionally, a novel model for describing the change in the TOC content, pore system characteristics, and fracability of shale with an increase in salinity is proposed. The model shows that with an increase in salinity, the TOC content first decreases, then increases, and finally decreases. Simultaneously, there is an improvement in the proportion of framework-grain related pores within the pore system. RCM, RM, and tight sandstone and limestone interlayers within freshwater lacustrine shale can be regarded as primary sweet spots. In the saline and hypersaline lacustrine shale systems, the main exploration targets are RM and organic-rich calcareous shale. These observations provide theoretical guidance for locating sweet spots in lacustrine shale systems deposited under different salinity conditions.
The complex and heterogeneous pore structure and oil content of lacustrine shales introduce significant challenges in the exploration of shale petroleum systems. To further expand our geological understanding of lacustrine shales and provide guidance for hydrocarbon exploration, in this study, based on programmed and improved Rock-Eval pyrolysis, total organic content (TOC) measurements, scanning electron microscopy (SEM) observations, and N2 adsorption (NA) experiments, a systematic evaluation of the mineral composition, geochemical features, pore structure, and oil contents of the third member of the Paleogene Shahejie Formation (Es3) shale in the typical lacustrine hydrocarbon enrichment sag (Nanpu Sag, Bohai Bay Basin) is conducted. The fractal theory is selected to quantify the pore system’s heterogeneity; also, the relationships between the pore structure parameters and the oil contents and mineral composition are revealed. Our results reveal the geochemical features of the Es3 shale are excellent, and thus, they can be classified as a set of high-quality source rocks (average TOC of 4.2%, mainly type II kerogen, and in the mature stage). Based on the SEM and NA analyses, the intercrystalline pores dominate the pore system. Moreover, the mesopores contribute the most specific surface area (SA) and pore volume (PV) to the samples (average contribution rates of 74.7 and 75.0%, respectively). The fractal dimensions have relatively strong positive correlations with PV and SA, which means more heterogeneous and complex pore structures provide more storage space and adsorption sites for shale oil accumulation. The clay mineral content is the most important factor which controls the pore structure parameter, and high clay content indicates high SA and PV. According to proposed numerical movable and adsorption hydrocarbon calculation models, the TOC content is the dominant factor controlling the movable and adsorption oil contents, but other factors’ impact also cannot be ignored. Our study not only provides guidance for shale oil exploration in the study area but also broadens our understanding of the oil contents of lacustrine shale.
Shale fracturing evaluation is of great significance to the development of shale oil and gas resources, but the commonly used shale evaluation methods (e.g., the method using the brittleness index based on mineral composition or elastic parameters) have certain limitations. Fractures and beddings affecting fracturing are not considered in these methods. Therefore, it is necessary to develop a new method to evaluate fracturing more comprehensively. The samples used in this research were taken from four typical continental shale basins of China, namely the Bohai Bay Basin, the Ordos Basin, the Songliao Basin, and the Junggar Basin. From a microscopic point of view, a three-parameter evaluation method involving multi-dimensional factors has been developed based on the nanoindentation method. Then, the fracturing coefficient K2 is obtained by combining the ratio β of the fracture indentation to the total indentation and the uneven coefficient m. After that, the fracability coefficient K3 is the ratio of the elastic modulus parallel to bedding to that perpendicular to bedding. Finally, the correlation between fracability coefficients K1, K2, and K3 is used to evaluate the overall fracturing performance of shale. The results of this evaluation method are in good agreement with the actual fracturing performance. It can be concluded that this method is highly reliable and practical and well worthy of promoted applications.
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