Formation mechanism of dolomite reservoir controlled by fourth-order sequence in an evaporated marine environment – An example from the Lower Ordovician Tongzi Formation in the Sichuan Basin

Dongya Zhu,Zhiliang He,Fengcun Xing,Quanyou Liu,Rongqiang Zhang,Zihao Liu,Dianwei Zhang

doi:10.1177/0144598717736630

Abstract

The high-porosity dolomite reservoirs of the Lower Ordovician Tongzi Formation (Fm.) were widely developed in the Sichuan Basin of southern China. The characteristics and developing mechanisms of the high-porosity dolomite reservoirs under the control of fourth-order sequence boundaries are discussed. In the Tongzi stage of the Early Ordovician, the Sichuan Basin was in a restricted platform facies in an evaporated shallow seawater environment. From the western to eastern regions of the basin, the Tongzi Fm. was serially developed in a tidal flat-lagoon-high-energy shoal depositional system. The evaporated seawater consequently led to dolomitization by way of the refluxing model. The Tongzi Fm. dolomites were subdivided into four coarsening-upward fourth-order sequences. Many tiny dissolution pores were formed in the dolomite beneath the four fourth-order sequence boundaries due to syn-sedimentation meteoric water erosion. Exposure above the seawater due to the short-term fall of the relative sea level consequently led to contemporaneous meteoric erosion. The Tongzi Fm. dolomites in the belt surrounding the Central Paleo-uplift were further subaerially dissolved by meteoric water due to tectonic uplift in the Guangxi Movement since the end of the Silurian period. Therefore, dolomitization, syn-sedimentation meteoric erosion under the fourth-order sequence boundaries, and meteoric karst during the Guangxi tectonic uplift jointly controlled the development of the Tongzi Formation high-porosity dolomite reservoirs. In the eastern and southeastern Sichuan Basin, the favourable reservoirs are the high-energy shoal dolomites that were eroded by meteoric water under fourth-order sequence boundaries. Around the Central Paleo-uplift, the favourable reservoirs are the dolomites dissolved by subaerial meteoric karst during the Guangxi Movement.

Highlights

IntroductionAround the Central Paleo-uplift, the favourable reservoirs are the dolomites dissolved by subaerial meteoric karst during the Guangxi Movement
Dolomitization, synsedimentation meteoric erosion under the fourth-order sequence boundaries, and meteoric karst during the Guangxi tectonic uplift jointly controlled the development of the Tongzi Formation highporosity dolomite reservoirs
The occurrence of evaporative lagoons implies that the Tongzi Fm. sedimentary system was developed in a restricted shallow marine environment, which favoured the development of high energy shoal granular carbonates and propelled the dolomitization process

Summary

Introduction

Around the Central Paleo-uplift, the favourable reservoirs are the dolomites dissolved by subaerial meteoric karst during the Guangxi Movement. The restricted marine evaporative environment is most favourable for large-scale dolomitization. Large-scale sediments of dolomite and anhydrite, as well as widespread high-energy granular shoal dolomites were formed (Li et al, 2012; Men et al, 2010; Zou et al, 2014). During the post-sedimentation diagenetic process, karst reservoirs could be developed in carbonates due to tectonic uplift above the ground surface and subsequent meteoric erosion (Loucks, 1999; Loucks et al, 2004). The post-sedimentation tectonic unconformities and the karst reservoirs have been studied in detail (Konert et al, 2001; Loucks, 1999; Mazzullo and Chilingarian, 1996)

Results

Discussion

Conclusion