Genetic Mechanism of Dolomite in Middle Permian Maokou Formation:Case Study of Central and Eastern Sichuan Basin

doi:10.19657/j.geoscience.1000-8527.2020.02.13

Abstract

Abstract:

Dolomite reservoir is important in the Maokou Formation, and its genetic mechanism is a hot topic of research. Therefore, outcrop and well core observation, thin-section identification and geochemical analysis are conducted to unravel the dolomite genetic mechanism of the Maokou Formation (Fm.). The results show that the Maokou Fm. dolomite can be divided into five types: (1) layered powdered, (2) layered fine-medium crystalline, (3) scattered in limestone, (4) lentoid medium-coarse-grained and (5) saddle cement, each with different genetic origins. The layered powdered dolomite has similar geochemical characteristics to original limestone. Its original structure is well preserved whereas gas-liquid inclusions are not developed, and appears dark red in cathodoluminescence (CL) images, suggesting that the layered powdered dolomite was formed in an early burial environment. The layered fine-medium crystalline dolomite is generally developed under the layered powdered dolomite and remaining grain apparition, whilst the δ¹⁸O value (-7.31‰) is more negative than that of the limestone (-3.77‰). Its homogenization temperature (116.9 ℃) is higher than the normal burial temperature (100 ℃), and it appears as partially bright red in CL images, suggesting that the layered fine-medium crystal dolomite was formed by hydrothermal recrystallization of layered powdered dolomite. Genesis of the scattered dolomite in limestone was likely related to the mineral stabilization processes. Both the lentoid medium-coarse dolomite and the saddle dolomite cement, in which hydrothermal minerals (quartz, pyrobitumen, pyrite) are discovered, show a curved crystal edge and undulating extinction under the microscope. Both of them have markedly negative δ¹⁸O value (-10‰), high homogenization temperature (120 ℃) and positive Eu anomaly. It appears bright red in CL images, which indicates that these two kinds of dolomite are typically hydrothermal.

Key words: Maokou Formation, dolomite, geochemical characteristics, genetic mechanism

CLC Number:

TE122.2

LI Ting, ZHU Dancheng, LI Haiping, YANG Minglei, LI Tao, LI Pingping, ZOU Huayao. Genetic Mechanism of Dolomite in Middle Permian Maokou Formation:Case Study of Central and Eastern Sichuan Basin[J]. Geoscience, 2020, 34(02): 345-355.

Figures/Tables 11

Fig.1 Location (A) and stratigraphic division(B) of Maokou Formation of the study area

Fig.2 Macroscopic characteristics of dolomite in Moukou Formation

Fig.3 Microscopic characteristics of dolomite in Moukou Formation

Fig.4 Lithology of dolomite strata in Maokou Formation of Fangniuba profile,Fengdu county

Fig.5 Characteristics of δ13C and δ18O of dolomite samples in Maokou Formation

Fig.6 Characteristics of 87Sr/86Sr of dolomite samples in Maokou Formation

Fig.7 REE characteristics of dolomite in Maokou Formation

Fig.8 Fluid inclusion from different types of dolomite in Maokou Formation

Table 1 Homogenization temperature of inclusions from different types of dolomite in Maokou Formation

样品来源	岩石类型	包裹体类型	气液比/%	包裹体均一温度范围/℃	包裹体均一温度平均值/℃
丰都放牛坝剖面泰来6井	层状粉晶白云岩	液态包裹体	/	/	/
丰都放牛坝剖面华蓥二崖剖面	层状细-中晶白云岩	气液两相包裹体	5~10	108.3~134.4	116.9
华蓥二崖剖面	透镜状中粗晶白云岩	气液两相包裹体	5~10	92.7~185.6	114.8
丰都放牛坝剖面、泰来6井、华蓥二崖剖面	鞍形白云岩胶结物	气液两相包裹体	5~10	93.7~198.4	126.4

Fig.9 Cathodoluminescence characteristics of different types of dolomite in Maokou Formation

Fig.10 Comparison of dolomite of Maokou Formation in central and eastern Sichuan basin

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