Abstract
The topic of dolomite formation has long presented a challenge to researchers. In this study, the origin of widely occurring oolitic dolomites from the Yudongzi outcrop in the lower Triassic Feixianguan formation in northwest Sichuan, China, was investigated through petrographic observations, and mineralogical and geochemical analysis. Analytical methods used include cathodoluminescence, X-ray diffraction, stable isotopes, and electronic microprobe characterization. The dolomites were categorized into three major genetic types according to their textural and structural characteristics, which reflect their various origins. The first genetic type of these dolomites, seepage reflux dolomitization, occurs in marly to microcrystalline dolomite during the penecontemporaneous stage, and displays negatively skewed δ18Ο (−2.83‰ Pee Dee Belemnite (PDB)), positively skewed δ13C (2.71‰ PDB), a low degree of order (0.48), and 87Sr/86Sr ratios of 0.707509–0.707634, indicating involvement of a Mg-rich brine fluid in an open evaporative environment. The second type, shallow burial dolomitization, is the most significant genetic type of dolomite reservoir in this area. This process produced dominantly silty to fine crystalline dolomite in a platform-margin oolitic beach facies with negatively skewed δ18Ο (−3.26‰ PDB), positively skewed δ13C (1.88‰ PDB), a high degree of order (0.70), and 87Sr/86Sr ratios of 0.707318–0.707661, which are related to seawater-derived fluids in a shallow burial environment. The third type is moderate to deep burial dolomitization, and is the main process responsible for zoned dolomite and dolomite with cloudy cores and clear rims (CCCR dolomite), which have the most strongly negatively skewed δ18Ο (−7.32‰ PDB), positively skewed δ13C (3.02‰ PDB), and 87Sr/86Sr ratios of 0.707217–0.707855, representing diagenetic alteration and fluid flow in a closed environment. These findings indicate that dolomite was likely affected by various degrees of burial and related marine-derived fluids, which will aid exploration efforts in high-quality hydrocarbon reservoirs in the Sichuan Basin.
Highlights
Dolomite is one of the most important reservoir rock types in the world: around one-third of oil and gas resources occur in carbonate reservoirs, of which half are composed of dolomite [1,2].Since the first description of dolomite by a French naturalist in 1791 [3], the origin of both the mineralMinerals 2017, 7, 120; doi:10.3390/min7070120 www.mdpi.com/journal/mineralsMinerals 2017, 7, 120 and rock form of dolomite has met with considerable controversy for over two hundred years
As studies on dolomite reservoirs have progressed and expanded, various formation mechanisms and dolomite deposition models have been proposed by sedimentologists, including seepage reflux [4,5,6,7], evaporation pumping [8,9], mixed water [10,11,12,13], and burial dolomitization [14,15,16,17]
Petrological, and geochemical methods, many oil and sedimentary scientists have conducted intensive research on the widely occurring dolomite reservoirs in the Sichuan Basin. These workers have proposed a number of dolomitization models, such as seepage reflux and mixed water dolomitization [32,33,34,35,36], burial dolomitization related to burial in a closed marine system [37,38,39,40], and tectonically controlled hydrothermal dolomitization characterized by saddle dolomites [41,42,43,44,45,46]
Summary
Dolomite is one of the most important reservoir rock types in the world: around one-third of oil and gas resources occur in carbonate reservoirs, of which half are composed of dolomite [1,2]. Petrological, and geochemical methods, many oil and sedimentary scientists have conducted intensive research on the widely occurring dolomite reservoirs in the Sichuan Basin These workers have proposed a number of dolomitization models, such as seepage reflux and mixed water dolomitization [32,33,34,35,36], burial dolomitization related to burial in a closed marine system [37,38,39,40], and tectonically controlled hydrothermal dolomitization characterized by saddle dolomites [41,42,43,44,45,46]. This study conducted a detailed petrographic characterization through mineralogical and geochemical analyses using cathodoluminescence (CL), stable isotopes, X-ray diffraction, and electron microprobe methods to investigate the oolitic dolomites from the Yudongzi outcrop in the Feixianguan Formation, in order to constrain the most likely origin of the dolomite, and promote further exploration efforts in high-quality reservoirs
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