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Geochemistry of the fracture-cave calcite of the Ordovician Yingshan Formation and its paleokarst environment in the Tazhong area, Tarim Basin, China

  • Yong Dan
  • Liangbiao LinEmail author
  • Bin Liang
  • Qingyu Zhang
  • Jianwen Cao
  • Jingrui Li
Original Article
  • 29 Downloads

Abstract

The fracture-cave reservoir of the Yingshan Formation is an important oil and gas producing zone in the Tarim Basin. Karstification occurs at the top surface of the carbonate rocks of the Yingshan Formation on the northern slope of the Tazhong area due to a short exposure, so abundant fracture-cave spaces are formed. On the basis of the data of nearly 49 samples obtained from 21 wells, the carbon and oxygen isotopes of the fracture-cave calcite were analyzed. The carbon and oxygen stable isotopic characteristics recorded during paleokarstification were discussed to provide geochemical evidence for predicting the distribution of the fracture-cave reservoirs, as well as to understand the genetic mechanism of karst fracture-caves and the fluid properties and paleo-hydrological conditions of paleokarstification. The fracture-cave calcite can be divided into three types based on its carbon and oxygen isotope values, which indicate three types of paleokarst environments with different fluid properties. Type I calcite has similar carbon and oxygen isotope values to the bedrock of the Yingshan Formation, with δ13C values of − 2.0 to 1.1‰ and δ18O values of − 5.76 to − 8.16‰. These values indicate that the calcite was precipitated in the eogenetic karst environment of the Yingshan Formation. Type II calcite has a δ13C value of − 2.13 to 1.0‰ and a δ18O value of − 12.41 to − 8.0‰. It has a similar δ13C value, but a significantly negative oxygen isotope value compared with values of the bedrock of the Yingshan Formation, indicating a buried karst environment. Type III calcite has an extremely negative δ18O value (δ18O value < − 14.50‰), indicating hypothermal karst genesis. Its formation is related to magma or deep hydrothermal fluids. No calcite with atmospheric freshwater genesis and significantly negative δ13C and δ18O values was found in the samples, indicating that the formation of fracture-caves is weakly affected by atmospheric freshwater. The main reservoir space of the Yingshan Formation is karst caves. The calcite in the karst cave fillings belongs to type I, indicating an eogenetic karstification environment. Therefore, the formation of the fracture-cave reservoirs of the Yingshan Formation in the Tazhong area is related to eogenetic karstification, which occurred during exposure and mainly contributed to the formation of the reservoir spaces.

Keywords

Carbon and oxygen isotopes Paleokarst environment Paleokarst reservoir Eogenetic karstification Tarim basin Yingshan Formation 

Notes

Acknowledgements

This paper was supported by the Geological Survey Program of China Geological Survey (No. DD20190723), the Nonprofit Industry Research Program of Chinese Academy of Geological Sciences (No. YYWF201723) and the National Key R&D Program of China (No. 2018YFC0604301). Senior Engineer Zhang Zheng-Hong of the Research Institute of Exploration and Development at Tarim Oilfield Company of PetroChina have been of great assistance during the core observations and sample collection processes. Mr. Deng Zhen-ping (Manager) and Senior Engineer Yang Hui of the Karst Geology Resources and Environment Test Center of the Ministry of Land have been of great support during the sample testing and analysis processes.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Institute of Karst GeologyChinese Academy of Geological Sciences/Karst Dynamics Laboratory, MNR & GZARGuilinChina
  2. 2.State Key Laboratory of Oil and Gas Reservoir Geology and ExploitationChengdu University of TechnologyChengduChina
  3. 3.Institute of Sedimentary GeologyChengdu University of TechnologyChengduChina

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