Origins and Geochemistry of Oolitic Dolomite of the Feixianguan Formation from the Yudongzi Outcrop, Northwest Sichuan Basin, China
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
... ites 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 δ 18 O (−2.83‰ Pee Dee Belemnite (PDB)), positively skewed δ 13 C (2.71‰ PDB), a low degree of order (0.48), and 87 Sr/ 86 Sr 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 δ 18 O (−3.26‰ PDB), positively skewed δ 13 C (1.88‰ PDB), a high degree of order (0.70), and 87 Sr/ 86 Sr 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 δ 18 O (−7.32‰ PDB), positively skewed δ 13 C (3.02‰ PDB), and 87 Sr/ 86 Sr 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. Minerals 2017, 7, 120 2 of 21 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     , evaporation pumping [8, 9] , mixed water     , and burial dolomitization     . These models have been widely used to explain the origin and stratigraphic distribution of ancient platform carbonates. However, for saddle dolomites which are characterized by coarse-crystalline white dolomite aggregates with curved crystal facies, commonly associated with higher temperatures and structures such as broken layers, thrust belts, and tectonized rocks beneath impermeable caprocks, hydrothermal fluids are likely to be involved in the dolomitization process. Current research has increasingly focused on structurally controlled hydrothermal dolomite (HTD) reservoirs [18-21], following on from earlier discoveries in the Ordovician Lima-Indiana field, and other areas of the northeastern United States  , as well as the Trenton-Black River in Michigan, and the northern Appalachian basins of the northeastern United States and eastern Canada  . Additionally, the global research community is vigorously pursuing the use of anaerobic and aerobic microbial culture experiments to explore nucleation and precipitation mechanisms of obtained material, much of which is very high-magnesium calcite, rather than dolomite [24, 25] . Several researchers have proposed microbial and organogenic models of this "dolomite" formation, which emphasize the importance of metabolic activity in microbial carbonate precipitation     . Over the last 30 years, oolitic dolomite reservoirs have come to represent a resource of increased economic value, and have become the subject of extensive research, such as the oolitic dolomites of the Mississippian Ste. Genevieve Group in the North Bridgeport oil fields of the Illinois Basin in the U.S.  , and the oolitic dolomites in the Puguang gas field in the lower Triassic Feixianguan formation of the Sichuan Basin in China [31, 32] . Using mineralogical, 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      , burial dolomitization related to burial in a closed marine system     , and tectonically controlled hydrothermal dolomitization characterized by saddle dolomites       . To a certain extent, these results represent the current state of knowledge of dolomitization mechanisms; however, many problems remain unresolved, such as the source and nature of the mineral-forming fluid, and the time scales and dynamics involved in dolomization and dissolution of the carbonate rock, among others. In the northwestern Sichuan Basin, previous studies have documented the various reservoir properties, diagenetic features, and internal structures of oolitic beach reservoirs in the lower Triassic Feixianguan formation in Erlangmiao, Jiangyou      , however, these works have not thoroughly investigated the origins of the dolomite. 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 therefore promote further exploration efforts in high-quality reservoirs.