Research paperCharacteristic foraminifera of the Pre-Cenozoic carbonate formations of Cat Ba island and northern Song Hong Basin, Vietnam
Introduction
PetroVietnam recently discovered a hydrocarbon flow in the pre-Cenozoic carbonate basement rock of the northern Song Hong Basin, northeastern Vietnam, which has since become a potential target for oil and gas exploration and has attracted the interest of petroleum geologists (Tan et al., 2017). As the Song Hong Basin is filled with Cenozoic sediments, its petroleum systems have been considered within the framework of Cenozoic stratigraphy; this finding has suggested a new approach for managing petroleum systems in the basin. The northern Song Hong Basin has been extensively investigated and detailed studies have been conducted on: litho-sedimentological characteristics of the carbonate basement rocks (Tam et al., 2014); characteristics of the Mesozoic carbonate reservoirs (Tam et al., 2015); geological age determination and construction of geological prediction models for the pre-Cenozoic carbonate basement (Huy et al., 2014); and other projects (Hoang et al., 2014; Vo and Dam, 2016). However, geological data for this area are still limited, and PetroVietnam has indirectly conducted many studies using field models that have focussed on outcrops in areas adjacent to the northern Song Hong Basin (Vo and Dam, 2016; Thanh, 2007; Huyen et al., 2007). One of the problems is the relationship between the ages of the carbonate formations in the basement rock of the northern Song Hong Basin and adjacent areas. Many field studies have been conducted to evaluate the petrology, stratigraphy, and tectonic characteristics of Cat Ba, Co To, and Bach Long Vi Islands and their adjacent areas, including Hai Phong, Ha Long Bay, and Quang Ninh (Hoang and Duc, 2014; Huy et al., 2014; Long et al., 2014; Phuong et al., 2009; Phuong and Truong, 2007, 2005; Truong, 2003; Truong and Khoa, 1994). These studies showed that the ages of the carbonate formations in Cat Ba Island are similar to those of several wells in the northern Song Hong Basin, according to the characteristics of foraminiferal assemblages. This study will evaluate the foraminiferal fossils that be used to determine stratigraphic relationships between wells in the northern Hong Song Basin and outcrops on Cat Ba Island. This will aid stratigraphic correlations and the comparison of reservoir models in field work focusing on the wells.
Section snippets
General geology
The study area is located in northeastern Vietnam (Fig. 1) on the South China plate. In the Late Palaeozoic, the South China Plate collided with the Indochina plate and formed a broad northwest-southeast trending mobile belt. The belt includes the Ailaoshan, Song Ma, and Dian-Qiong sutures that represent the complex boundary zone between the Indochina and South China plates in northern Vietnam, and the southeastern part of the South China plate (Metcalfe, 2011). The Song Ma suture zone is
Study area
This study analysed the foraminiferal assemblages in 109 samples collected from a geological field trip to Cat Ba Island and 233 samples collected from wells in the northern Song Hong Basin. Four sections were surveyed along the length of Cat Ba Island, including the Cat Co, Hung Son, Hien Hao and Gia Luan sections (Fig. 1). The Cat Co section is a ~2.5 km section located in the southern area of the island. It is bounded by Cat Co 3 Beach and Ben Beo Village and extends in a south-north
Distribution of foraminifera in the carbonate formations of cat Ba island
Within the 211 thin sections of carbonate rocks from the 109 outcrop samples that were analysed, the ages of the foraminiferal assemblages ranged from the upper Famennian to the lower Moscovian, and their occurrences in each sample are shown in Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12 below.
Discussion
The characteristics, distribution, and age of foraminiferal assemblages from Cat Ba Island and the northern Song Hong Basin were obtained in this study, and correlations between the studied carbonate formations were determined.
The foraminiferal assemblages in this study were compared with the results of Devonian to Carboniferous foraminiferal studies from South China (Aretz et al., 2014, Aretz et al., 2012; Groves et al., 2012; Devuyst et al., 2003; Hance et al., 1993), Russia (Baranova et al.,
Conclusions
The findings of this study contribute significantly to our knowledge of known occurrences of late Paleozoic foraminifera in the northern continental shelf of Vietnam. In total, 74 genera, belonging to 27 families were identified, allowing for the identification of eight fossilized zones and one assemblage of benthic foraminifera in the carbonate basement. The foraminiferal fossilsshow stratigraphic distribution ranges from the latest epoch of the late Devonian to the earliest epoch of the late
Acknowledgments
The authors gratefully acknowledge the Vietnam Petroleum Institute for their financial support and for providing the samples used in this study. The authors would also like to thank Dr. Doan Nhat Truong for guiding our field trips to Cat Ba Island and adjacent areas.
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