Carbon and Oxygen Isotopic Composition of Saline Lacustrine Dolomite Cements and Its Palaeoenvironmental Significance: A Case Study of Paleogene Shahejie Formation, Bohai Sea

Hailong Meng,Chenhao Xiong,Zhongmin Shen,Zhengxiang Lv

doi:10.3390/min9010013

Abstract

The dolomite reservoirs in the Paleogene Shahejie Formation in the Bozhong area of the Bohai Bay Basin contain a large amount of dolomite cement. Petrologic and mineralogic studies have shown that the dolomite cements can be divided into three types according to their occurrence: coating dolomite (CD), pore-lining dolomite (LD), and pore-filling dolomite (FD). The laser microsampling technique was used to analyze the C and O isotopes in the carbonate minerals. This method is an effective way to produce CO2 gas from a particular carbonate structure in a thin section, and it has a spatial resolution of 20–50 µm and an optimal precision of approximately ±0.22σ for δ13C and δ18O in carbonate standard materials. The carbon and oxygen isotopic compositions and the oxygen isotopic geothermometer results showed that the dolomitization fluid is mainly low temperature fluid, the lake basin environment is relatively closed, and the salinity index Z value is greater than 120, which indicates the invasion of seawater. CD and early-stage LD crystals were mainly very fine crystals with faint cathodoluminescence, which indicates the early formation of diagenesis. The high temperatures of late-stage LD and FD measured by oxygen isotope thermometers indicates that they formed at a deeper depth. The dolomite cements in the study area may have formed in two stages: seepage-reflux dolomitization during the penecontemporaneous period and burial dolomitization.

Highlights

In both carbonate and clastic reservoirs, carbonate cementation, such as calcite or dolomite cementation, is often a key factor that affects reservoir quality [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
Most dolomite reservoirs form in marine environments [20,21,22,23,24]; most are intercrystalline pore reservoirs that formed by dolomitization [25,26]; and some formed as karst caves or are related to hydrothermal solution and the thermochemical sulfate reduction (TSR) [27,28,29,30,31,32,33,34]
This study provides important guidance for reservoir prediction in the study area and provides an opportunity to better understand lacustrine dolomite reservoirs, which mostly form under deep burial conditions

Summary

Introduction

In both carbonate and clastic reservoirs, carbonate cementation, such as calcite or dolomite cementation, is often a key factor that affects reservoir quality [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The dolomite reservoir in E2 s is nearly 4000 m deep, its residual primary porosity is still greater than. The reservoir space of the dolomite reservoir in E2 s is largely composed of primary pores, residual primary pores, and moldic pores [19,35]. The main characteristic of the dolomite reservoir in E2 s is the development of a large amount of both syngenetic and authigenic dolomite cements [36].

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Results

Discussion

Conclusion