Evaluation of reservoir environment by chemical properties of reservoir water‒A case study of Chang 6 reservoir in Ansai oilfield,Ordos Basin,China

The Ordos Basin is the largest continental multi-energy mineral basin in China,which is rich in coal,oil and gas,and uranium resources.The exploitation of mineral resources is closely related to reservoir water.The chemical properties of reservoir water are very important for reservoir evaluation and are significant indicators of the sealing of reservoir oil and gas resources.Therefore,the caprock of the Chang 6 reservoir in the Yanchang Formation was evaluated.The authors tested and analyzed the chemical characteristics of water samples selected from 30 wells in the Chang 6 reservoir of Ansai Oilfield in the Ordos Basin.The results show that the Chang 6 reservoir water in Ansai Oilfield is dominated by calcium-chloride water type with a sodium chloride coefficient of generally less than 0.5.The chloride magnesium coefficients are between 33.7 and 925.5,most of which are greater than 200.The desulfurization coefficients range from 0.21 to 13.4,with an average of 2.227.The carbonate balance coefficients are mainly concentrated below 0.01,with an average of 0.008.The calcium and magnesium coefficients are between 0.08 and 0.003,with an average of 0.01.Combined with the characteristics of the four-corner layout of the reservoir water,the above results show that the graphics are basically consistent.The study indicates that the Chang 6 reservoir in Ansai Oilfield in the Ordos Basin is a favorable block for oil and gas storage with good sealing properties,great preservation conditions of oil and gas,and high pore connectivity.

Integrated U-Pb,Lu-Hf and(U-Th)/He analysis of zircon from the Banxi Sb deposit and its implications for the low-temperature mineralization in South China

Low-temperature Sb(Au-Hg) deposits in South China account for more than 50% of the world’s Sb reserves,however,their genesis remains controversial.Here we report the first study that integrates U-Pb and Lu-Hf analysis by LA-(MC)-ICPMS and conventional(U-Th)/He analysis,all applied to single zircon crystals,in an attempt to constrain the origin and timing of world-class Sb(Au-Hg) deposits in Banxi(South China).Zircon separated from a quartz-stibnite ore and an altered country rock samples revealed similar U-Pb age spectra defining two major populations-Paleoproterozoic(~1900-2500 Ma) and Neoproterozoic(~770 Ma),which are characterized by variable εHf(t) values(-10.7 to 9.1 and-16.5 to 11.2,respectively) and Hf crustal model ages(TDMC)(2.48 to 3.24 Ga and 0.97 to 2.71 Ga,respectively).The U-Pb age and Hf isotopic features of the zircons are consistent with the Banxi Group in the region,indicating that the zircons involved in the low-temperature hydrothermal system were originally from the Banxi Group country rocks.Thirty-three mineralization-related zircon crystals yielded a mean(U-Th)/He age of 123.8±3.8 Ma,which is interpreted to represent the timing of the latest low-temperature mineralization stage of the Banxi Sb deposit.The combined U-Pb,Lu-Hf and(U-Th)/He data suggest that Precambrian basement rocks were the major contributors to the low-temperature mineralization,and that Early Cretaceous(130-120 Ma) could be the most important ore-forming epoch for the Sb deposits in South China.This study also demonstrates the analytical feasibility of integrated U-Pb-Lu-Hf-(U-Th)/He "triple-dating",all applied to single zircon crystals.This approach reveals the full evolution of zircon,from its origin of the magmatic source,through its crystallization and low-temperature cooling.Although this study demonstrates the usefulness of this integrated approach in dating low-temperature mineralization,it has great potential for zircon provenance and other studies that may benefit from the large amount of information that can be extracted from single zircon crystals.

Ore-forming environment of Pb−Zn mineralization related to granite porphyry at Huangshaping skarn deposit,Nanling Range,South China

Multiple metallogenic types(skarn-type and vein-type)related to hypabyssal granites are found at the Huangshaping polymetallic deposit in the Nanling Range,South China.To constrain the crystallization and mineralization processes of skarn formation,three generations of magnetite and pyrrhotite from the hydrous silicate stage,oxide stage,early quartz–sulfide stage,and late quartz–sulfide stage were distinguished.The geochemical compositions of magnetite and pyrrhotite were obtained by electron probe microanalyzer(EPMA)and in-situ ablation-inductively coupled plasma-mass spectrometry(LA‒ICP‒MS).The results show that there may be silicate inclusions in magnetite and interaction of wall rock occurred in the mineralization process.The geochemical trends recorded in pyrrhotite show the influence of limestone during the crystallization of pyrrhotite.The re-equilibration temperatures of Po I,Po II,and Po III are 420.46,380.45,and 341.81°C,respectively,which suggests a continuous evolution following the high-temperature W–Sn mineralized system.The content change of Ni and V reflects a gradual decrease of oxygen fugacity from Mag I to Mag III,while the sulfur fugacity calculated from pyrrhotite gradually decreases.This continuous skarn mineralization evolution process helps us to better understand the change of metallogenic environment in the retrograde stage of the Huangshaping deposit.