Geochemical Characteristics and Origin of Formation Water From the Upper Triassic Xujiahe Tight Sandstone in the Xiaoquan-Fenggu Structural Belt, Western Sichuan Depression, China

Formation water represents an important driving force and carrier for the migration and accumulation of oil and gas; thus, research on its origin is a hot spot in petroleum geology. The Upper Triassic Xujiahe Formation in the Xiaoquan-Fenggu Structural Belt in the western Sichuan Depression, China, has developed thick tight sandstone gas reservoirs. However, previous studies have provided different conclusions on the origin of the formation water in the Xujiahe tight sandstone reservoir. In this paper, the origin of the formation water in the Xujiahe Formation was determined based on the latest major and minor elemental concentration data, hydrogen and oxygen isotopes data of formation water, and carbon and oxygen isotope data of carbonate cements. The results show that the salinity of the formation water of the Xujiahe Formation in the study area is generally greater than 50 g/L. The water type is mainly the CaCl2 type, although a small proportion of NaHCO3 type water with high salinity is observed, which is related to hydrocarbon expulsion by overpressure. Moreover, the formation water in the sandstone of the Xujiahe Formation is obviously rich in Br, which is related to membrane infiltration, overpressured hydrocarbon expulsion of shale and diagenesis of organic matter. The composition of Cl− and Na+ ions in the formation water in the Xujiahe tight sandstone reservoir is consistent with the seawater evaporation curve, which deviates significantly from the freshwater evaporation curve. The hydrogen and oxygen isotopes of condensate water in the Xujiahe Formation tight sandstone are similar to those of atmospheric precipitation water, while the hydrogen and oxygen isotopes of the formation water in the Xujiahe Formation show that it is of seawater origin. Therefore, to use hydrogen and oxygen isotopes to determine the origin of formation water, condensate water must be accurately differentiated from formation water. Otherwise, if the condensate water is misjudged as formation water, then incorrect conclusions will be drawn, e.g., that the formation water of the Xujiahe Formation originated from fresh water. Affected by organic carbon, the carbon isotope Z value of the carbonate cements in the Xujiahe Formation is low (mainly distributed between 110 and 130). A Z value of less than 120 does not indicate that the ancient water bodies formed by cements were fresh water or mixed water bodies. However, Z values greater than 120 correspond to a formation temperature lower than 80 C, which indicates that carbonate cement was not affected by organic carbon; thus, the Z value can reflect the origin of ancient water bodies. The results of this study indicate that the formation water of the Xujiahe tight sandstone in the study area is of seawater origin. The determination of the origin of the formation water and seawater of the Xujiahe Formation provides strong evidence for the determination of the marine sedimentary environment of the Xujiahe Formation in the study area, and can provide scientific guidance for the search for high-quality reservoirs.

Hydrogeochemical characteristics and genesis of Seismic observation wells in Shandong Province

Hydrogeochemical characteristics, controlling factors, and recharge sources of the seismic observation wells in Shandong Province were investigated by analyzing cation and anion concentrations, hydrogen and oxygen isotopes in well water. A total of 17 water samples in seismic observation wells were collected on April 25-29, 2018. The results show that temperatures of seismic observation wells were in the range of 14.8 to 52.1°C, and the values of δD and δ 18 O ranged from -72.4‰ to -37.9‰ and from -9.4‰ to -4.3‰, respectively. Using C.A. ЩукаЛев’s classification method, the water samples of 17 seismic observation wells were classified into 7 types: Cl·SO 4 -Na·Ca, SO 4 -Na, Cl-Na, HCO 3 -Na·Ca, HCO 3 -Mg·Na·Ca, HCO 3 -Na and HCO 3 -Mg·Ca·Na. The results indicate that the hydrogeochemical characteristics of 17 seismic observation wells, with a certain spatial distribution pattern, are affected by several factors, such as the tectonic, topography, stratigraphy, hydrology and meteorology. The analyses of ratio coefficients, Schooller diagram, hydrogen and oxygen isotopes compositions, Giggenbach and Gibbs diagram suggest that the atmospheric precipitation is the main recharge source of 17 observation wells. The recharge sources of deep lateral runoff and sedimentation water, moreover, play a significant role in some seismic observation wells. Combined with the amount of precipitation, the distance from recharge areas, the closure degree of observation wells and the stage of water-rock reactions, the development directions of faulting and topography control the directions of groundwater recharge, runoff and discharge, which make hydrogeochemical characteristics represent complex spatial distribution rules.

Characteristics of Stable Hydrogen and Oxygen Isotopes in Precipitation over the Jiaolai Plain, and its Water Vapor Sources

Precipitation is the sole input of regional water resources in mountainous or hilly areas that are not traversed by large rivers. A prerequisite for using isotopic techniques to study the regional water cycle of a mountainous area is to examine the stable isotopic composition of its precipitation. The findings are of great significance for in-depth understanding of the water-cycle processes. In this study, each event of precipitation was sampled and used to investigate the characteristics of stable hydrogen and oxygen isotopes (δ2H and δ18O, respectively) in precipitation on the Jiaolai Plain and its surrounding areas. NCEP/NCAR data was used for the wind speed and direction, relative humidity, and precipitable amount in the study area during the sampling period. The water vapor sources of precipitation over the plain were revealed through a comparative analysis of seasonal variations in precipitation isotopes, between the Global Network of Isotopes in Precipitation (GNIP) stations located along different vapor transport paths. The results showed that the local meteoric water line (LMWL) was δ2H = 6.38 δ18O + 0.72, with a gradient less than 8. This indicated that the precipitation process was affected by non-equilibrium evaporation occurred when the drops fell below the cloud base. Significant temperature and amount effects existed in the δ18O of precipitation, although the altitude effect was not significant. The water vapor source of the precipitation was controlled predominantly by the East Asian Monsoon from June to September, with the main source being evaporation from the adjacent Pacific Ocean. The plain was controlled by Westerlies from October through May, with the predominant vapor source being local evaporation. Water vapor from the polar region had minimal impact. During the sampling period, water vapor brought by Typhoon Lekima produced heavy precipitation on the plain. There was a significant depletion of δ18O in the precipitation at that time, indicating the existence of the cloud–rain zonal effect. These findings can serve as the basis for studying surface water–groundwater–seawater transformation.

Using Isotopes (H, O, and Sr) and Major Ions to Identify Hydrogeochemical Characteristics of Groundwater in the Hongjiannao Lake Basin, Northwest China

Hongjiannao Lake is the largest desert freshwater lake in the Ordos Plateau, China, and the relict gull is an endangered species that uses the lake for its habitat, with the largest colonies being located there. Using hydrochemical parameters, stable hydrogen and oxygen isotopes, and strontium isotopes, we investigated the hydrogeochemical characteristics of groundwater. As a result, the major cations of the groundwater were found to be Ca2+ and Na+, the major anion was found to be HCO3−, and the hydrochemical facies were mainly found to be HCO3–Ca, HCO3–Na. and HCO3–Ca–Na. The hydrochemical formation of groundwater was controlled by both evaporation and water–rock interactions, and carbonate and sulfate minerals dissolved or precipitated in the groundwater. On the basis of isotope analysis, groundwater was affected by evaporation and δ18O enrichment, and the higher salinity of Hongjiannao Lake suffered from intensive evaporation. The higher 87Sr/86Sr ratio and lower concentrations of Sr2+ in the groundwater were derived from the dissolution of silicate minerals, whereas the opposite concentrations were due to the dissolution of carbonate and sulfate minerals. Based on this work, such results can be used to research groundwater recharge into the lake and to protect water quality.