Characteristic foraminifera of the Pre-Cenozoic carbonate formations of Cat Ba island and northern Song Hong Basin, Vietnam

Abstract

Pre-Cenozoic carbonate formations of the northern Vietnam continental shelf have recently become important petroleum exploration targets, owing to the discovery of hydrocarbon accumulations in the northern Song Hong Basin. However, there is limited stratigraphic information about the carbonate basement in the area; further, the stratigraphic relationship between carbonate formations exposed in adjacent areas, such as outcrops on Cat Ba Island and the subsurface Song Hong Basin, is unclear. This study revealed foraminiferal assemblages that can be used to determine the age of carbonate basement rocks based on samples from outcrops on Cat Ba Island and wells in the northern Song Hong Basin. Foraminifera were identified and divided into eight biozones and one assemblage that are indicative of different ages. Carbonate formations on Cat Ba Island originated from the Famennian to Moscovian, and those in the northern Song Hong Basin originated from the early Tournaisian to early Wuchiapingian. Carbonates in the basement section of well SH1 and the Cat Co, Hung Son, Hien Hao, and Gia Luan sections of Cat Ba Island, originated in the early-mid Tournaisian. In well SH2, basement carbonates are coeval with the upper part of the Gia Luan section, indicating that it originated in the late Serpukhovian to the late Moscovian. This age range is significantly different from the Late Devonian to Early Carboniferous age determined by previous studies. The youngest basement carbonates were identified in well SH3 (early Capitanian to early Wuchiapingian). As these carbonate formations are contemporaneous, they were governed by the same tectonic conditions and may exhibit similar geological characteristics. Therefore, reservoir rock models of the carbonate basement in northern Song Hong Basin could be used to clarify geological carbonate models for Cat Ba Island.