Characteristics and Controlling Factors of Methane Adsorption of the Longmaxi Shale in Northeastern Chongqing: Implications for Shale Gas Occurrence in Complex Structural Areas.

Accurately determining the adsorption capacity of Longmaxi shale in complex structural areas is crucial for evaluating the shale gas resources in northeastern Chongqing. However, studies on the pore characteristics and methane adsorption capacity of these Longmaxi shales are currently limited. In this paper, we collected core samples from the YDB-1 well in northeastern Chongqing and determined the pore structure and adsorption capacity of the Longmaxi shale using low-pressure gas adsorption and high-temperature, high-pressure methane adsorption experiments, respectively. The results indicate that the adsorption capacity of shale in complex structural areas is significantly positively correlated with the organic matter (OM) content, weakly positively correlated with the quartz content, and weakly negatively correlated with clay minerals. Meanwhile, gas-in-place content is simultaneously controlled by the pressure and temperature of the reservoir, and with increasing depth, the adsorbed gas rapidly increases to a maximum value (at 0.8 km) and then slowly decreases, whereas the free gas continuously increases. Compared with the shale in the stable structural areas, the Longmaxi shale in complex structural areas usually develops OM-hosted pores and intergranular pores of OM and minerals and contains more micropores due to tectonic compression, resulting in a relatively larger specific surface area and adsorption capacity. This is the reason shale in complex structural areas has high development potential. The final result can provide an important basis for the evaluation of the gas content and the optimization of dessert areas in the Lower Paleozoic shale gas in southern China.

Impact of Minerals and Sealing Systems on the Pore Characteristics of the Qiongzhusi Formation Shale in the Southern Sichuan Basin.

The characteristics, distribution, and preservation of pores are vital in controlling the storage and distribution of shale gas. The Qiongzhusi Formation shales taken from different members with similar tectonic and thermal evolutions were used to evaluate the response of pore characteristics to minerals and sealing systems using field-emission scanning electron microscopy and gas adsorption. Because of differences in mineral structure and arrangement, feldspar, organic matter (OM)-clay, OM-rutile, and OM-apatite aggregates facilitate multiple types of pores in the shale and influence the relative proportions of surface porosity for different types of pores owing to differences in mineral structure and arrangement. Rigid frameworks and pressure shadows formed by rigid minerals and OM-mineral aggregates preserved OM and pores to some extent. The sealing capacity of the floor controls the sealing system and hydrocarbon expulsion efficiency of the Qiongzhusi Formation in different members. During thermal evolution, the amount of hydrocarbons generated and expelled affected the stress equilibrium state between the pore pressure and external stress, influencing the compaction intensity of shales. The OM pore development characteristics were evolved with variation in the stress equilibrium state in different sealing systems. Once the stress equilibrium state was disrupted, the OM pores deformed, narrowed, or even closed under the influence of compaction owing to the loss of overpressure support. The pore characteristics of the Qiongzhusi Formation shales responded significantly to different sealing systems. A few OM pores are flat and slitlike in the open system, whereas numerous OM pores are round and elliptical in the semiopen system. Meanwhile, the average diameter of the OM pores in the open system was reduced by approximately 40.2% compared with that of the semiopen system. Furthermore, the pore volume and specific surface area of the mesopores for open system shales were reduced by 38.4% and 37.7%, respectively, compared to the semiopen system. These findings will improve the understanding of the distribution and preservation of pore in shale and help assess the sweet-spot members for the Qiongzhusi Formation shale gas.

Analysis of Developmental Characteristics and Dominant Factors of Pore-Fracture Systems in Lower Cambrian Marine Shale Reservoirs: A Case Study of the Niutitang Formation, Fenggang Block, Southern China.

Due to breakthroughs in the Lower Silurian Longmaxi Formation in the Sichuan Basin and multiple strata around the basin, the northern part of Guizhou adjacent to the Sichuan Basin has become a key area for shale gas exploration. Compared with the Longmaxi Formation, the Niutitang Formation displays greater TOC (total organic carbon) content, depositional thickness and distribution area, but the details remain undetermined. In the study area, the Lower Cambrian Niutitang Formation typically has high TOC content, maturity and brittle mineral content. The study area has experienced multiple periods of tectonic movement, which have great influence on the fracture and pore characteristics. The fractures are mainly structural fractures and have obvious zoning. The primary types of pores are intraparticle pores, organic matter pores, and interparticle pores. Further, macropores and mesopores less than 50 nm contribute most of the pore volume, while pores less than 2 nm contribute most of the specific surface area. Many factors affect the pore-fracture system, such as tectonism, TOC content, mineral composition, and sedimentary environment. Tectonic movements produce fractures based on the changing stress field, but the degree of fracture development does not agree well with the degree of pore development. The TOC content has good positive correlations with the development of fractures and micropores, especially for nanoporosity, while clay minerals show a negative correlation with the development of fractures but a strong positive correlation with the growth of micropores. Quartz displays a positive correlation with the development of fractures but no good correlation with pore development. Finally, the lithofacies, lithologies and mineral compositions under the control of sedimentary environments are internal factors that can impact the development of pore-fracture systems.

Study on the Shale Gas Reservoir-Forming Characteristics of the Taiyuan Formation in the Eastern Qinshui Basin, China.

Shales are widely developed in the strata of the Carboniferous-Permian coal measures in the Qinshui Basin, and these shales have great potential for shale gas exploration. In this paper, the shales of the Taiyuan Formation in the eastern Qinshui Basin are studied. The shales of the Taiyuan Formation in the study area are investigated through field investigation, organic geochemical testing, X-ray diffraction, scanning electron microscopy, high pressure mercury injection, low temperature liquid nitrogen adsorption and PetroMod simulation and through other tests to study the reservoir characteristics, such as organic geochemistry, mineralogy, petrology, pore permeability, and gas burial history. The results show that the shales of the Taiyuan Formation are well developed over the whole area with a thickness of more than 60 m. The average organic matter content is 2.95%, and the kerogen type is type III. The shale maturity (average value is 2.45%) corresponds to the stage of high maturity evolution, indicating that a large amount of shale gas has been generated in this area. A high content of quartz and clay minerals indicates a high fracturability. The nanopores in the shale reservoir are well developed at pore sizes between 2˜10 nm and greater than 1000 nm; however, the pores at the other pore sizes are poorly developed, resulting in weak pore connectivity in the reservoir. According to the results of the PetroMod simulation, the shale of the Taiyuan Formation has undergone two subsidence and two uplift processes. The Yanshanian magmatic intrusion is the key factor for the rapid increase in gas production. In addition, the geological structure of the area is relatively simple, and the burial history and caprock thickness are also the main controlling factors of gas generation and preservation. The shale-sandstone-shale combination and shale-coal-shale combination are the main models of shale gas preservation. This comprehensive study suggests that the shale gas of the Taiyuan Formation in the eastern Qinshui Bain has good potential for exploration and development.