Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Shunli Zhang,Sibing Liu,Yi Wen,Zhengxiang Lv

doi:10.3390/min8070289

Shunli Zhang, Sibing Liu + Show 2 more

Open Access

https://doi.org/10.3390/min8070289

Copy DOI

Abstract

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ18O values generally ranging from −1.70‰ to −1.67‰ (with an average value of −1.69‰), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87Sr/86Sr ratios, δ18O values generally ranging from −6.10‰ to −2.50‰ (with an average value of −3.98‰), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ18O values generally ranging from −7.01‰ to −6.62‰ (with an average value of −6.79‰), relatively higher Mg/Ca values, and lower 87Sr/86Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.

Highlights

With the continuous innovation of science and technology, the degree of exploration in oil and gas with middle and shallow layers has been comprehensive
The most important pore types are the algal framework pores, intergranular dissolved pores, and moldic pores, which account for 70% of all pores
The results show that most dolomite samples have Z values of >130 (VPDB) and seawater (SMOW)

Summary

Introduction

With the continuous innovation of science and technology, the degree of exploration in oil and gas with middle and shallow layers has been comprehensive. Deep and ultra-deep reservoirs have gradually become important new fields for the development of oil and gas resources, while deep reservoirs often exhibit poor porosity due to having undergone intense compaction and destructive diagenesis. Some carbonate reservoirs that have undergone different stages of dissolution, dolomitization, structural fractures, abnormally high pressures, and early hydrocarbon charging can still maintain good reservoir space even at burial depths of more than 5000 m, such as the Ordovician dolomite reservoirs in the Mills Ranch Field (7663–8103 m), the Madden. Few studies have focused on the formation mechanism of deep-buried, high-quality carbonate reservoirs. The burial depth of the carbonate reservoirs of the Leikoupo Formation in the Western Sichuan

Methods

Results

Discussion

Conclusion