Genesis of Dolostone of the Feixianguan Formation, Lower Triassic in the NE Sichuan Basin: Evidences from Rock Structure and Strontium Content and Isotopic Composition
川东北普光气田下三叠统飞仙关组白云岩成因——来自岩石结构与Sr同位素和Sr含量的证据

On the basis of analysis of rock structure and strontium isotopes composition and content,in this study the genesis of dolostone reservoir in the Feixianguan Formation,Lower Triassic of the NE Sichuan Basin is discussed. Results indicate that the dolostone reservoir is the product of replaced dolomitization during the deep burial diagenesis period. This conclusion comes from following evidences: (1) rock structure and characteristics of strontium isotope and strontium content of the deep burial dolostone are quite different from that of penecontemporaneous dolostone. (2) carbonate rocks and minerals of the Feixianguan Formation have the composition characteristics of Triassic seawater strontium isotope, and their 87Sr/86Sr ratios range from 0.706588 to 0.708187,covering the range (0.7076~0.7078) of strontium isotope in early Triassic and the average value 0.707656 being consistent with the average value of 0.707743 for global early Triassic. (3) 87Sr/86Sr ratios of burial dolostone range from 0.707122 to 707419,being slightly under the changing range of strontium isotope of the global early Triassic seawater,but within the range of 0.707330~0.707383 for early Triassic seawater of northeast Sichuan Basin suggested in earlier studies,and the average value 0.707421 is closer to the earlier reported value 0.707350 than global early Triassic seawater,which shows that dolostone diagenetic fluids were very similar to the Triassic seawater. (4) Based on comprehensive study of rock structure,geochemistry characteristics of strontium isotope and strontium content,it can be well proved that the dolostone diagenetic fluids come from sea source water rich in strontium and salinity sealed up and kept in the stratum.