Fault activity history under absolute age constraints and its impact on hydrocarbon accumulation: A case study of the Tuoputai area in the Tarim Basin, China&#160;
Authors: Xie Honglei , Tian Jinqiang 
Published: March 2024

The fault system within petroliferous basins plays a pivotal role in governing the migration, accumulation, and preservation of oil and gas resources. Accurate determination of the timing of fault activities is critical for reconstructing the history of tectonic evolution and for the analysis of hydrocarbon systems. Conventionally, this determination is achieved through the analysis and interpretation of seismic data. However, this method's effectiveness is often constrained by the quality of the seismic data, typically yielding only an approximate timeframe for fault activity. This limitation poses significant challenges in precisely identifying the periods of fault activity, which is essential for the analysis of their impact on hydrocarbon accumulation. Calcite, a commonly developed vein mineral in response to fault activities, is noteworthy in this context. The use of LA-ICP-MS U-Pb dating for fault vein calcite has been established as an effective technique to reveal time information of fault activity. The Tuoputai area, situated within the Tarim Basin, is recognized as one of the exploration hotspots with a complex tectonic evolution history. In particular, the strike-slip faults connecting Lower Cambrian source rocks with Ordovician carbonate reservoirs are instrumental in hydrocarbon migration and accumulation. In this study, the TP39 fault zone in the Tuoputai area, recognized for its rich hydrocarbon content, was selected for detailed analysis. We initially analyzed seismic data to obtain the approximate period of fault activity, followed by LA-ICP-MS U-Pb dating of calcite veins from core samples, to precisely determine the fault activity time. Additionally, the study incorporated the analysis of oil fluid inclusions in the calcite veins to comprehensively analysis the impact of fault activities on hydrocarbon accumulation. The seismic data indicated that the TP39 fault zone has mainly experienced three activity periods, including the Caledonian (542-416 Ma) to Hercynian (416-251 Ma), Indosinian (251-199.6Ma) and Himalayan (65.5 Ma-present) periods. In situ U-Pb dating of calcite veins from Ordovician reservoir cores in two wells within the fracture zone yielded ages of 446.79&#177;2.67 Ma, 443.63&#177;1.31 Ma, and 278.23&#177;2.11 Ma, defining two significant active events in the TP39 fault zone during the Late Caledonian (~440 Ma) and Late Hercynian (~280 Ma), respectively. Furthermore, fluid inclusion analysis in calcite veins revealed a high prevalence of yellowish-green and bluish-green fluorescent oil inclusions, followed by blue fluorescent oil inclusions. The corresponding Th results from coexisting water inclusions indicated that the mainly phases of crude oil charging occurred during the Late Caledonian and Late Hercynian periods, followed by the Himalayan period, correlating well with the U-Pb dating results. This indicated that the fault activity during the Late Caledonian and Late Hercynian periods opened pathways for crude oil migration from the source rocks to the reservoirs. This study used the absolute dating method to constrain the activity time of the fault, demonstrating its impact on hydrocarbon accumulation, which is of great significance to basin structural research and oil and gas exploration.