Geochemical characteristics and fluid origins of fracture- and cave-filling calcites of Ordovician in Yubei area, Tarim Basin

Abstract

The genesis of carbonate reservoirs of the Lower-Middle Ordovician strata has been an important topic for petroleum exploration in the Yubei area, Tarim Basin. To investigate the stages and properties of paleo-fluid and discuss the reservoir origin, the geochemical and cathodoluminescence features of the Ordovician fracture- and cave-filling calcites from wells in the Yubei area were analyzed combining with the regional tectonic background. Results show that the REE distribution patterns and occurrences of samples from different wells have the characteristics of seawater and fresh water, respectively. The average values of Sr isotopes gradually decrease from east to west. Moreover, the average value of 87Sr/86Sr in high-angle fracture calcites is higher than those of cave-filling and horizontal-fracture calcites, suggesting obvious variations of fluid properties in different regions and sample occurrences. Therefore, the diagenetic environment of the Lower-Middle Ordovician carbonates in the Yubei area may be open. The decreases of meteoric alteration from east to west can be attributed to the Middle Caledonian and Early Hercynian tectonic setting. Moreover, the progress of reservoir modification by fluids is mainly affected by fault activity intensity.

Keywords

• strontium isotope
• rare earth element
• calcite
• reservoir fluid
• carbonate
• ordovician
• yubei area
• tarim basin