Discovery of rare hydrothermal alterations of oligocene Dolomite reservoirs in the Yingxi area, Qaidam, West China

Abstract

Comprehensive research into core and micro-structure characteristics, petrology, trace elements, and carbon and oxygen isotope geochemical characteristics led to the initial discovery of a rare dolomite reservoirs reformed by hydrothermal fluid in the Yingxi area of Qaidam Basin; however, the degree of hydrothermal alteration is weak. The evidence mainly includes the following aspects: (1) the main rock reservoirs are dense, mainly developed ankerite and calcite with a micritic structure. Albite, pyrite, barite, celestite, and other hydrothermal minerals are occasionally found in fractures. (2) Trace amounts of FeO and MnO enriched with carbonate minerals with an average amount of 4.112 and 0.186%, respectively, are present. They were significantly higher than penecontemporary metasomatism dolomites. The amount of Ca2+ and Mg2+ also saw increases, indicating the hydrothermal fluid is an alkaline brine rich in Ca2+, Fe2+, Sr2+, Mn2+, and some Mg2+. (3) REE has a large range of changes and some of them have higher ΣREE values, which suffered from hydrothermal alteration. Clear Eu negative anomalies appeared on REE distribution model curves. It reflected the REE composition characteristics of the lake environment’s specific hydrothermal fluid. It formed in a deep lake with an alkaline brine environment. (4)The change to the range of δ18OPDB values is −6.8 to −3.6‰, with an average of −5.2‰. The values are between hydrothermal dolomite and penecontemporaneous metasomatism dolomite. This shows that the different types of dolomite have characteristics consistent with a penecontemporaneous metasomatic origin before hydrothermal fluid in a medium–low temperature condition reformed them. The degree of hydrothermal alteration is dependent upon the degree fractures developed as well as the distance between the sample and the hydrothermal channel. In short, the initial discovery of dolomite reservoirs reformed by hydrothermal fluid opens up new horizons for deep tight oil and gas exploration in Qaidam Basin’s Yingxi area in China. These results are substantial in the search for quality “desert” reservoirs present in deep stratum in the Yingxi area.