Using noble gases to trace subsurface fluid dynamics and helium accumulation in the Sichuan Basin, the Ordos Basin and the Qinshui Basin, China

Abstract

This PhD project mainly applies noble gases to investigate subsurface fluid behaviors in various conventional gas fields and coalbed methane (CBM) gas fields in China. Moreover, not only the controlling factors for the accumulation of hydrocarbon and helium in different subsurface environments but also the impact of hydraulic fracturing on subsurface fluid systems are studied in this thesis. In this project, 7 gas samples from the Weiyuan gas field in the Central Sichuan basin were investigated on noble gases. 4He concentrations ranging from 7.57 × 10-4 to 9.84 × 10-4 cm3STP/cm3. 3He/4He ratios range from 0.018 to 0.028Ra, suggesting a strong crustal signature. The fractionation of atmosphere-derived noble gases can be explained by the Two-Stage Oil-Modified Groundwater-Exsolution (OMG-E) fractionation model. According to the fractionation model, the ancient oil reserve in the Sinian reservoir is estimated as 8.11 × 108 t to 12.50 × 108 t at STP, and 70% ~ 77% of groundwater-exsolved gas was lost during uplift. 4He/40Ar* ratios in samples vary from 370 to 423, and 21Ne*/40Ar* ratios vary from 1.49 × 10-7 to 1.80 × 10-7. Helium present in reservoirs is mainly derived from both in-situ reservoir rocks and source rocks. The helium transport from source rocks could be achieved by diffusion processes in pore water, leading to the highly fractionated 4He/40Ar* ratios. 9 gas samples from the Longgang gas field in the Central Sichuan basin are studied in this project. 4He concentrations in samples range from 2.04 × 10-4 to 3.46 × 10-4 cm3STP/cm3. 3He/4He ratios vary from 0.014 to 0.020Ra, indicating a strong crustal signature. The samples show different noble gas fractionation patterns because reservoirs are discontinued and have different gas-water distributions. Noble gas data suggest that the majority of gas present in reservoirs is not released from groundwater. 4He/40Ar* ratios range from 1515 to 24000 and 21Ne*/40Ar* ratios vary from 2.1 × 10-7 to 5.4 × 10-6. The strong fractionation of crustal noble gases could be caused by diffusion and gas re-dissolution processes. Helium in the gas field could be mainly derived from the source rocks and migrated with hydrocarbon gases to the reservoirs by diffusion processes. 8 CBM gas samples from the Southwest Ordos basin are studied in this project. 4He concentrations in samples range from 2.83 × 10-6 to 7.23 × 10-4 cm3 STP/cm3. 3He/4He ratios range from 0.015Ra to 0.029Ra, displaying a strong crustal signature. Coherent fractionation of atmosphere-derived 20Ne/36Ar and crustal 4He/40Ar* are explained by the solubility-controlled Rayleigh fractionation models starting with the initial groundwater 4He/40Ar = 8.4 and 2.2, indicating the coal seam has experienced helium loss in different degrees, which could be due to diverse roof sealing. The helium accumulation ages of groundwater are calculated as 0.061 ~ 0.162 Ma. In addition, 13 CBM gas samples from the Southeast Qinshui basin are investigated in this project. All samples display air-influenced noble gas isotopic ratios and 4He/20Ne ratios, which could be attributed to mass-fractionated air-like components introduced by hydraulic fracturing. However, 20Ne/36Ar ratios (0.56 ~ 1.02) are obviously higher than the air value, which could be caused by the re-dissolution fractionation associated with hydraulic fracturing.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor) & Greg Holland (Supervisor)