GEOCHEMICAL EVIDENCES FOR THE ORIGIN OF PALEO-WATER IN THE UPPER TRIASSIC SANDSTONE RESERVOIRS IN THE MIDDLE PART OF THE WESTERN SICHUAN DEPRESSION, CHINA

摘要

In order to clarify the origins of the paleowater of the Upper Triassic Xujiahe Formation in the middle part of the western Sichuan depression, we systematically analyzed the characteristics of the formation water compositions, the formation water hydrogen and oxygen isotopes, and the carbon and oxygen isotopes of the carbonate cements. The results showed that the composition of the formation water is in good agreement with the evaporation curve of seawater, however, it is far from the evaporation curve of freshwater. The hydrogen and oxygen isotopes of some waters in the Xujiahe Formation are characterized by atmospheric precipitation, and the analysis found that these waters are condensate water. The hydrogen and oxygen isotope characteristics of formation water showed that the formation water of the target layer was originated from seawater, and the oxygen isotope value of only part of the formation water is slightly lower, which is affected by CO_2-water exchange. The Z (paleosalinity) values of the samples affected by organic carbon were low, which were generally less than 120, and cannot indicate that the paleo-water was fresh water. Geochemical evidences of formation water components, hydrogen and oxygen isotopes, and carbon and oxygen isotopes from carbonate cements have fully confirmed that the paleo-water of the Xujiahe Formation in the study area is a seawater origin.