Diagenetic differences caused by the charging of natural gases with various compositions - a case study on the lower Zhuhai Formation clastic reservoirs in the WC-A sag, the Pearl River Mouth Basin. (March 2017)

• Record Type: Journal Article
• Authors: Duan, Wei; Luo, Chengfei; Lou, Zhanghua; Liu, Jianzhang; Jin, Aimin; Zhu, Rong
• Is Part Of: Marine and petroleum geology. Volume 81(2017)
• Page Start: 149
• Page End: 168
• Publication Date: 2017-03
• Subjects: Pearl River Mouth Basin -- WC-A sag -- Diagenesis -- Natural gases with various compositions; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2017.01.004
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Only a limited number of comparative studies have explored the diagenetic differences of reservoirs caused by the charging of natural gases with different compositions during the migration and accumulation of oil and gas. In this study, we quantitatively assessed the diagenetic variations of the lower Zhuhai Formation reservoirs in the WC-A sag, the Pearl River Mouth Basin, as a result of natural gas charging with different compositions. The employed methods included electron microscopy, scanning electron microscopy, cathode luminescence, X-ray diffraction, chemical composition analysis of formation water, stable isotope analysis, and fluid inclusion determination. The results indicated that: (1) in the lower Zhuhai Formation reservoir that are near the large fractures, the late-charging of CO2 -rich thermal fluid promoted the dissolution of minerals, and changed the porosity, permeability, pH value of fluid, and the contents of ions such as K +, Na +, Mg 2+ of the reservoirs. These changes, in turn, indirectly affected the content, form, and distribution of clay minerals in the reservoirs. In addition, the secondary enlargement of quartz was enhanced. (2) The charging of CO2 -rich thermal fluid caused strong dissolution and produced high contents of cements. The dissolution mainly occurred in a half open system with strong fluid activity. Consequently, the significantly decreasing of permeability was hindered and middle porosity–middle permeability reservoirs were generated. The reservoirs that are far from the large fractures, however, were mainly early charged by the hydrocarbon-rich fluid. The dissolution was weak and the generated cements were limited. In addition, the dissolution products could not be discharged in such closed diagenetic systems. As a result, the permeability of the reservoir was declined sharply and resulted in low porosity–low permeability reservoirs. Highlights: The gases changed from hydrocarbons near fracture to CO2 away from the fracture. CO2 -rich fluids induced higher cementation than hydrocarbon-dominated fluids. The charging of CO2 -rich fluids enhanced the secondary enlargement of quartz. The charging of different gases caused variations of clays in the reservoirs. CO2 -rich fluids resulted in high porosity and permeability reservoirs.