The majority of previous studies of induction logging in fractured formations have typically focused on the variation in spatial resistivity distribution around the wellbore. However, the effect of cumulative charge on the boundaries of an inclined fracture with a distinct morphology on induction logging has rarely been discussed. In this work, the 3-D finite element method (3-D FEM) is adopted to investigate the dual-induction logging response of inclined fractures in formations of different resistivities considering the volume contribution and charge contribution. First, induction logging theory is studied, and a 3-D fractured formation model considering cumulative charges is established. Second, the induction logging responses under various fracture resistivities, apertures, dip angles, and formation resistivities are simulated and analyzed. Subsequently, the corresponding logging response characteristics are studied, and the different response laws of inclined fractures in high- and low-resistivity formations are systematically summarized. Third, a 1:5 miniaturized dual-induction logging instrument was designed and manufactured. Furthermore, two devices were built to measure the induction logging response of the inclined fracture (simulated by nanoscale silver film or metal wire mesh) in low- and high-resistivity formations. The results of the physical experiments are consistent with the numerical results, validating the accuracy of the numerical simulation. Both the results reveal that due to the existence of cumulative charge layers on the upper and lower inclined fracture boundaries, the induction logging response rules of the conductive fractures in high-resistivity formations differ from those in low-resistivity formations, and hence, they should be treated differently.

When the fracture normal forms an angle relative to the borehole axis, the induction logging response to the fractured formation consists of two contributions: one is the volume contribution caused by the eddy current, and the other is the charge contribution caused by the cumulative charges on the upper and lower boundaries of the fracture; both contributions are nonlinear. Researching the distribution of surface charges and eddy current in inclined fractured formations is of great significance for logging analysts and geologists to fully understand induction logging laws in such setting. However, there is insufficient research on this aspect. In this work, first, an electromagnetic field calculation method under the condition of existing cumulative charges at the interfaces was developed. Second, an induction logging numerical simulation model of inclined fractures based on the three-dimensional (3D) finite element method (FEM) was established, and the distribution laws of cumulative charges and eddy current in fractures under different dip angles were summarised. Third, we used the annular conductive region in the fracture as the object of our research, and analysed the influence law of different conductive regions distribution on induction logging. Finally, we carried out physical experiments to verify the numerical results and achieved good agreement between the two results. Hence, it proves the validity of the methods and conclusions.

The types of organic carbon in shore and shallow lacustrine shales are more diverse than those in marine formations. It is of great importance to establish log interpretation models of the separate types of organic carbon content for hydrocarbon generation potential estimation. First, we summarize the main rock types and their logging characteristics in the shore and shallow lacustrine shale reservoirs and analyze the types of organic carbon in shore and shallow lacustrine shales using core pyrolysis analysis and kerogen maceral microscopic identification. Second, we establish the calculation method of the total organic carbon (TOC) content in the lacustrine shale formation by regression analysis. Third, we divide the organic matter in shore and shallow lacustrine shales into two parts: perhydrous organic carbon (perhydrous macerals) with a higher hydrogen index, higher H/C atomic ratio, and stronger hydrocarbon generation potential and subhydrous organic carbon (subhydrous macerals) with a lower hydrogen index, lower H/C atomic ratio, and poorer hydrocarbon generation potential. Furthermore, we introduce the concept of the mixed skeleton, combined with the formation component triangular chart, and establish calculation methods for perhydrous and subhydrous organic carbon contents. Finally, we apply this method to the Mesozoic shore and lacustrine shale in the western Sichuan Basin, China. The application indicates that the kerogen types of the Mesozoic shore and shallow lacustrine formations in the western Sichuan Basin are mainly type III and type IIB kerogen with a low content of perhydrous macerals. The perhydrous organic carbon content calculated by the logging evaluation model is positively correlated with the perhydrous maceral content obtained by kerogen maceral microscopic identification and the oil generation potential S2 obtained by core pyrolysis analysis. Logging analysts should use the perhydrous organic carbon content to estimate the hydrocarbon generation potential of shore and shallow lacustrine shale formations.

Shale adsorption capacity is affected by many factors including temperature, pressure, geochemical characteristics of organic matter, clay, and water saturation. The traditional calculation model of adsorbed gas content only considers the influence of temperature, pressure, and organic geochemical characteristics. The influence of clay and water saturation on adsorption capacity is seldom considered. Isotherm adsorption experiments were conducted on synthetic specimens and natural specimens with varying clay types, clay contents, and water saturations. Then, the influences of clay and water saturation on the adsorption capacity were systematically studied. The experimental results found that the order of clay adsorption capacities was smectite > kaolinite > chlorite > illite. The multicomponent superposition rule was applicable in evaluating shale-adsorbed gas content. The total adsorption capacity was equal to the accumulation of the adsorption capacities of all types of clay and organic matter. Moisture will significantly reduce the adsorption capacity of shale. The adsorption capacities of synthetic specimens and natural specimens after being fully saturated with water were 9%–14% and 42%–61% of those in dry states, respectively. Then, a new shale-adsorbed gas evaluation model was established based on the Langmuir equation considering clay and water saturation. The calculation error of this new model was approximately 11%, which provides a new method for evaluating the adsorbed gas content of shale.