Characteristics and controlling factors of the unconformity-related carbonate reservoir: a case study from Sinian Shuiquan Formation, Tarim Basin, northwestern China

Abstract

Deep-buried marine carbonate reservoirs in the Tarim Basin present challenges for exploration. A regional unconformity in dolomite at the top surface of the Shuiquan Formation makes the carbonate rocks ideal reservoirs. This study determines the unconformity belt distribution, lithology, reservoir space types and the controlling factors of the Shuiquan Formation, to establish a paleokarst development model and predict the favourable exploration targets. By flattening the seismic-event section and tracking truncation points on the seismic profiles, the unconformity area and denudation trends of the Shuiquan Formation are identified. The denudation area appears on a NEE-trending plane as a band with a width of 10–20 km and a thickness that expands from the depression to the slope and uplift. The Sinian Shuiquan Formation developed in a carbonate platform–tidal flat environment. Outcrop, core, well logging, and thin-section observations reveal that the lithology of the Shuiquan Formation consists of crystalline dolomite and granular dolomite, with pore types that include intercrystalline dissolution pores, intercrystalline pores and structural fractures. Unconformity, karstification and tectonic activity with atmospheric freshwater leaching, storage space and communication channels, respectively, are considered crucial factors influencing carbonate reservoir formation in the study area. A prediction model for the carbonate reservoirs in the Tadong area shows that the strong karst belt on the slope is the ideal area for forming a carbonate reservoir.

KEY POINTS

1. A regional unconformity at the top surface of the dolomite of the Shuiquan Formation makes the carbonate rocks favourable reservoirs.

2. Sedimentation, unconformity, karstification and tectonic activity play an important role in controlling reservoir formation and evolution.

3. The high-angle unconformity slope belt with long-term exposure and atmospheric freshwater leaching is the ideal area for large-scale epigenetic karst reservoir reconstruction and formation.

Keywords:

• unconformity surface
• carbonate rock
• karstification
• Shuiquan Formation
• Sinian
• eastern Tarim Basin

Acknowledgements

This work is based on the research project of the Research Institute of Exploration and Development of Daqing Oilfield Limited Company of PetroChina. We are grateful to the Research Institute of Exploration and Development of Daqing Oilfield Limited Company of PetroChina for the data and cooperative research for this study.

Disclosure statement

The authors declared that they have no conflicts of interest in this work.

Additional information

Funding

This work was funded by the Key Project of the National Natural Science Foundation of China (Grant No: 42330810) and the National Natural Science Foundation of China (Grant No: U19B6003-01-01).