Paleo-environment and reservoir evolution of the Middle Ordovician Kelimoli Formation in the northern Tianhuan area, Ordos Basin, China: implications for high-quality reservoir development

ABSTRACT

The Middle Ordovician Kelimoli Formation carbonate reservoirs in the northern Tianhuan area, Ordos Basin, China are main exploration targets. Subsurface core samples, logging, drilling and production data from the area were used to characterise the carbonate reservoir and to construct a genetic model for exploration. The sedimentary facies identified include trough–continental rise, upper–lower slope, platform margin reef-shoal, open platform and evaporation-restricted platform. The slope and platform margin facies are potential zones for high-quality reservoirs. Porosity in the study area comprises intergrain, intercrystal, intragrain and intracrystal pores, fractures and vughs. The Sr/Ba (0.40–4.87) and V/(V + Ni) (0.64–0.97) ratios indicate deposition in a brackish water-dominated environment under reducing conditions, associated with sea-level fluctuations during the deposition of the Kelimoli Formation. The Sr/Ba, V/(V + Ni), ⁸⁷Sr/⁸⁶Sr ratios, δ¹⁸O values and crystal texture of dolomite samples suggest that meteoric water was involved in the diagenetic fluid in the near-surface depositional environment. Isotopes (δ¹³C and δ¹⁸O) and trace elements (Fe and Mn) allowed the identification of seepage-reflux and mixed-water dolomitisation. The crystal textures of the samples consist of micritic, bioclastic and reefal limestones, and dolomite with gypsum, which were easily dissolved during the early diagenetic stage. The epidiagenetic stage was the key period for the development of high-quality reservoirs because of large-scale bedding-parallel karstification from meteoric water. The reservoir zones, dominated by partially filled and unfilled vughs, and fractures, are favourable exploration targets in the northern Tianhuan area.

KEYWORDS:

• paleokarst fracture-vugh reservoir
• paleo-environment
• karstification
• dolomitisation
• Kelimoli Formation
• northern Tianhuan area
• Ordos Basin

Acknowledgements

We are grateful to PetroChina Company for permission to publish the manuscript and appreciate Senior Engineers Zhifeng Zuo and Jinhua Fu from the Research Institute of Exploration & Development of Changqing Oilfield Company, CNPC who provided many helpful comments and editorial suggestions on an earlier version of this manuscript. We are also grateful to two anonymous reviewers and the editors for their critical and constructive comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study has been supported by National Natural Science Foundation of China [grant number 41302087]; ‘Research on the characteristics and prospects of carbonates-evaporites hydrocarbon accumulation, CNPC’ [grant number 2016B-0401].