Abstract
Three large-scale episodes of volcanic activity occurred during the Tertiary in the Xihu Depression, located in the East China Sea. Intermediate-felsic magmas intruded along faults and the associated hydrothermal fluids resulted in the hydrothermal alteration of the clastic country rock. To better describe reservoir characteristics, reservoir samples were subjected to the following investigations: thin section examination, scanning electron microscope-energy dispersive spectrometer analysis (SEM–EDS), fluid inclusion homogenization temperature tests, vitrinite reflectance measurements, and X-ray diffraction. The results of this study provide evidence of the following hydrothermal alteration phenomena: brittle fracturing, clastic particle alteration, precipitation of unique hydrothermal minerals (celestite, zircon, apatite, barite, and cerous phosphate). The presence of abnormally high temperatures is indicated by fluid inclusion analysis, the precipitation of high-temperature authigenic minerals such as quartz, illite alteration, and anomalous vitrinite reflectance. Two aspects related to hydrothermal effects on reservoir properties have been investigated in this study: (1) Deep magmatic hydrothermal fluids carry large amounts of dissolved carbon dioxide and sulfur dioxide gas. These fluids percolate into the country rocks along fault zones, resulting in dissolution within the sandstone reservoirs and the development of significant secondary porosity. (2) Magma intrusions increase the temperature of the surrounding rocks and accelerate the thermal evolution of hydrocarbon source rocks. This results in the release of large amounts of organic acids and carbon dioxide, leading the dissolution of the aluminosilicate minerals and volcanic fragments in the reservoirs, and the generation of significant secondary porosity.
Highlights
The term “hydrothermal fluids” refers to all high-temperature aqueous fluids that contain many chemical materials in solution (e.g., H 2S, HCl, HF, SO2, CO, CO2, H2, N2, KCl, and NaCl), some of Edited by Jie HaoMagmatism occurs in many sedimentary basins around the world and has a significant impact on the generation, migration, and accumulation of oil and gas as well as the formation of hydrocarbon reservoirs due to the two following mechanisms (Ye et al 2005; Agusto et al 2013). (1) Thermal baking caused by the magmatic intrusion heats the surrounding rocks and results in mineral transformations
Girard and Nahon (1989) proposed the concept of “contact diagenesis,” the “abnormally high temperatures” from the magmatic intrusion result in changes in the authigenic mineral assemblage in the contact zone. (2)
In the Pearl River Mouth Basin in China, magmatism caused relatively strong deformation and contact metamorphism and provided a significant amount of heat, which resulted in abnormally high geothermal gradients accelerating the maturation of hydrocarbon source rocks and leading to the release of more organic acids, resulting in the development of secondary porosity (Nie et al 2005; Zhu et al 1994)
Summary
The term “hydrothermal fluids” refers to all high-temperature aqueous fluids (temperature range from 50 to 400 °C) that contain many chemical materials in solution (e.g., H 2S, HCl, HF, SO2, CO, CO2, H2, N2, KCl, and NaCl), some of Edited by Jie HaoMagmatism occurs in many sedimentary basins around the world and has a significant impact on the generation, migration, and accumulation of oil and gas as well as the formation of hydrocarbon reservoirs due to the two following mechanisms (Ye et al 2005; Agusto et al 2013). (1) Thermal baking caused by the magmatic intrusion heats the surrounding rocks and results in mineral transformations. Evidence for intrusive magmatic activity has been identified in different regions across the world during exploration for oil and gas, such as in the Michigan Basin in the United States (Wierzbicki et al 2006), the Songliao Basin and the Jiyang Depression in eastern China (Wang et al 1990), the offshore Bohai Bay Basin in Eastern China (Wang and Zhang 2001), the Dongpu Depression in eastern China (Zhu et al 1994) and the offshore Yinggehai Basin in Southern China (Wang et al 2010) In these basins, magmatic hydrothermal fluids have altered the reservoirs, and most of these altered reservoirs form a group of key hydrocarbon play pathways. The impact of magmatic activity and associated magmatic hydrothermal fluids on petroleum systems has drawn a significant amount of research interest in the field of oil and gas exploration and development (Shu et al 2003)
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