Characteristics and formation mechanisms of dolomites in the Middle Permian Maokou Formation in southeastern Sichuan Basin

doi:10.13745/j.esf.sf.2023.2.19

Abstract

Abstract:

Breakthroughs in oil and gas exploration have been made recently in dolomite reservoirs of the Middle Permian Maokou Formation in southeastern Sichuan Basin. However, reservoir heterogeneity in the Maokou Formation is high, which makes it difficult to identify high-quality reservoirs without a clear understanding of the dolomitization mechanisms. Based on sedimentary facies of the region and hydrothermal events associated with the Emeishan Large Igneous Province, dolomites in the Maokou Formation are characterized and their formation mechanisms are summarized through lithological and geochemical analyses. In the Maokou Formation fine-medium-crystalline dolomite is the main type, followed by micritic dolomite and dolomite cements. In the Qijiang area stratified, fine-medium-crystalline dolomite is mainly found. The euhedral to subhedral dolomite crystals appear cloudy at the center and bright around the edges, with obvious graining lines; under cathodoluminescence (CL) microscope they appear brown-red at the interior and dark-red toward the exterior. The δ¹⁸O_V-PDB, δ¹³C_V-PDB and ⁸⁷Sr/⁸⁶Sr ratio values of fine-medium-crystalline dolomite indicate it was formed by seepage-reflux dolomitization at medium salinity during shallow burial and later underwent recrystallization and seawater-metasomatism. The Tailai area mainly occur porphyritic- micritic and fine-medium-crystalline dolomites, along with dolomite cements found in rock fractures. Dolomites are compact with no obvious grain structures, while dolomite cements are mainly saddle dolomite with wavy extinction, with multiphase ring-and-ribbon patterns visible under CL microscope. Isotopic data indicate dolomites in this area were formed during shallow burial under seawater environment, and late-stage thermal fluids flowing over clastic rocks directly contributed to the precipitation of dolomite cements and calcite. In general, the key reservoir-controlling factors of the Maokou Formation in southeastern Sichuan Basin are epigenetic dissolution, dolomitization during shallow burial, metasomatism and karstification; and seepage-reflux dolomitization during shallow burial is the main formation mechanism of dolomites.

Key words: Sichuan Basin, Middle Permian, Maokou Formation, dolomite, genetic mechanism

CLC Number:

TE121.3

HAN Yueqing, ZHANG Juntao, PAN Lei, LI Huili, LIU Guangxiang, LI Rangbin, WU Chongyang, HAO Yunqing, HE Zhiliang, HUANG Zhenkai. Characteristics and formation mechanisms of dolomites in the Middle Permian Maokou Formation in southeastern Sichuan Basin[J]. Earth Science Frontiers, 2023, 30(6): 45-56.

Figures/Tables 9

Fig.1 Facies map of and simplified stratigraphic column for Mao-3 section, southeastern Sichuan Basin

Fig.2 Identifying features of dolomites from the Maokou Formation in southeastern Sichuan Basin

Fig.3 Lithological characteristics of micritic dolomite from the study area

Fig.4 Lithological characteristics of fine-medium-crystalline dolomite from the study area

Fig.5 Lithological characteristics of dolomite cements from the study area

Fig.6 Cross-plot of Carbon and oxygen isotope values for carbonate rocks from the study area

Fig.7 Comparison of 87Sr/86Sr ratios of carbonatites from the study area

Fig.8 REE patterns of carbonatites from the study area

Fig.9 Diagenetic features and diagenetic process of the Maokou Formation in southeastern Sichuan Basin

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