Petrography, fluid inclusion and isotope studies in Ordovician carbonate reservoirs in the Shunnan area, Tarim basin, NW China: Implications for the nature and timing of silicification

The Shunnan (SN) area, located in the center of the Tarim basin, NW China, is a gas field discovered in 2013, where the gas is hosted from deeply buried Ordovician carbonate reservoirs with burial depth > 6000 m and temperature> 190 degrees C. The most important reservoir rocks in the SN area are silicified limestones, which are characterized by multiple generations/types of authigenic quartz (Qz1-Qz2) and coarse calcite cement (CC1-CC3), in addition to other diagenetic phases. Qz1 is a replacement quartz postdating burial stylolites in both limestone and strongly silicified limestone, and Qz2 are equant and bladed quartz cements developed in fractures or vugs in strongly silicified limestone, also postdating burial stylolite. CC1 is a coarse calcite cement found in the vugs, which postdates medium crystalline dolomite and predates saddle dolomite. CC2 (including CC2a, CC2b and CC2c) is the calcite postdating Qz1 and burial stylolites. CC2a is found in fractures in limestone or slightly silicified limestone. CC2b, CC2c and CO are only identified in strongly silicified limestone. CC2b fills intercrystalline pores of Qz1, and CC2c fills fractures, predating Qz2. CC3 is precipitated in remaining space left by Qz2c in fractures or vugs. Sr isotopes were analyzed in CC2a and CC2c. CC2a has Sr-87/Sr-86 ratios of 0.70890-0.70917. CC2c is characterized with Sr-87/Sr-86 ratios of 0.70949-0.70972. Fluid inclusions were studied in all the quartz and coarse calcite cements. Fluid inclusions in CC2a are characterized by T-h values of 118-131 degrees C and salinities of 22.9-25.2 wt% NaCI + CaCl2. Fluid inclusions from Qz2a, Qz2b, CC2b and CC2c have T-h values of 143-166 degrees C and salinities of 14.7-23.7 wt% NaCI + CaCl2. Fluid inclusions in Qz2c are characterized by T-h values of 125-132 degrees C and salinities of 24.8-26.8 wt% NaCI + CaCl2, and those in CO by T-h values of 86-101 degrees C and salinities of 22.9-252 wt% NaCI + CaCl2. The T-h drop, from Qz2a, Qz2b and CC2c to Qz2c and CO, cannot be explained by normal burial diagenesis, suggesting a hydrothermal event associated with the main phase of silicification. The relatively low temperature recorded by CC3 implies that the hydrothermal event possibly took place in the Devonian rather than Permian as previously thought. (C) 2017 Elsevier B.V. All rights reserved.