The Chemical Remagnetization of Ediacaran Dolomite in the Taishan Paleo‐Reservoir, South China

• Published in: Journal of geophysical research. Solid earth 2018-08, Vol.123 (8), p.6161-6175
• Main Authors: Zhang, Yong, Jia, Dong, Muxworthy, Adrian R., Li, Yongxiang, Xia, Bin, Xie, Zengye, Hu, Jing, Zi, Jinping, Liu, Weiliang
• Format: Article
• Language: English
• Subjects: Barite; Cambrian; carbonate reservoir rocks; Carbonates; chemical process; Chemical reactions; Dolomite; Dolostone; Exploration; Geophysics; Haematite; Hematite; hydrocarbon migration; Hydrocarbons; Magnetic intensity; Magnetic studies; Magnetite; Migration; Mineral assemblages; Nanoparticles; Oil; Oil exploration; Ordovician; Organic carbon; Organic chemistry; Outcrops; Palaeomagnetism; Paleomagnetism; Pyrrhotite; remagnetization; Remagnetization dating; Remanent magnetization; Reservoirs; south China; Sulfur; Sulphur; Total organic carbon; Transition zone
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2018JB015547

To understand the origin of chemical remagnetization in carbonate reservoir rocks and associated chemical processes, we have conducted detailed petrologic, geochemical, and magnetic studies of the hydrocarbon‐impregnated Ediacaran dolomite in the Taishan Paleo‐reservoir, south China. Samples were collected across a continuous outcrop consisting of oil‐free dolomite, transition zone dolomite, and highly hydrocarbon‐impregnated reservoir dolomite. Paleomagnetic results show that dolomite from the reservoir and transition zone were remagnetized and carries a possible Cambrian‐Ordovician direction (Dec = 163.7°, Inc = 10.6°, and α95 = 1.9°), and the oil‐free dolomite shows unstable magnetizations. Hematite is the magnetic mineral in the oil‐free dolomite, whereas the hydrocarbon‐impregnated reservoir dolomite contains no hematite but mainly pyrrhotite nanoparticles. Transition‐zone dolomite contains both hematite and pyrrhotite. In addition, the natural remanent magnetization intensities decrease from the reservoir dolomite, through the transition zone, and into the oil‐free dolomite, and a similar trend was observed in the total organic carbon and bitumen contents. The results suggest that the magnetic mineral assemblage and magnetic intensity in dolomite appear to be closely related to the degree of hydrocarbon impregnation and that remagnetization of the Ediacaran dolomite was likely caused by hydrocarbon migration into the dolomite. Pyrrhotite nanoparticles, magnetite, and barite are suggested to be formed during hydrocarbon migration, by the reduction of detrital hematite in the dolomite and sulfur species in the hydrocarbons. The new findings into the origin of pervasive remagnetization have the potential to guide future hydrocarbon exploration within Ediacaran dolomite that is host to the largest single carbonate gas reservoir in China. Key Points An integrated paleomagnetic, rock magnetic, and petrographic study of the Ediacaran dolomite was conducted The hydrocarbon‐impregnated reservoir dolomite retains a CRM carried by authigenic pyrrhotite, which was related to hydrocarbon migration Authigenic pyrrhotite was formed by the reduction of detrital hematite in the oil‐free dolomite and sulfur species in the hydrocarbon