Evolution of diagenetic fluid of ultra-deep Cretaceous Bashijiqike Formation in Kuqa depression

Abstract

Diagenetic fluid types of the Cretaceous Bashijiqike formation are restored based on the analysis of petrographic, electron microprobe composition, inclusions homogenization temperature, salinity and vapor composition and laser carbon and oxygen isotope of diagenetic mineral, and regional geological background. Diagenetic fluid evolution sequence is analyzed on this basis. The crystalline dolomite cement has a low concerntration of Sr, high concerntration of Mn and higher carbon isotope, showing that the crystalline dolomite is affected by meteoric fresh water, associated with the tectonic uplift of late Cretaceous. Similar δ13CPDB, negative transfer of δ18OPDB and the differentiation of the concerntration of Fe and Mn indicate that the diagenetic fluid of the vein dolomite cement is homologous with the diagenetic fluid of the crystalline dolomite cement, temperature and depth are the dominant factors of differential precipitation between these two carbonate cements. Anhydrite cements have high concerntration of Na, extremely low concerntration of Fe and Mn contents. Based on these data, anhydrite cements can be thought to be related to the alkaline fluid overlying gypsum-salt layer produced by dehydration. The barite vein has abnormally high concerntration of Sr, ultra-high homogenization temperature and high-density gas hydrocarbon inclusions, which is speculated to be the forward fluid by intrusion of late natural gas. Coexistence of methane inclusions with CO2 gas proves existence of acid water during the accumulation of natural gas in the late stages. Therefore, the alkaline environment and associated diagenesis between the meteoric fresh water in epidiagentic stage and carbonic acid in the late diagenesis have dominated the process of diagenesis and reservoir, the secondary porosity and fracture zone formed by gas accumulation is a favorable play for the exploration of ultra-deep reservoirs.