The Santiago Formation (Late Hettangian–Sinemurian), described in the area of Santiago in the Oriente Basin of eastern Ecuador, consists of three distinct sedimentary members. The Santiago River Member is composed of limestones and calcareous sandstones. The Yuquianza Member is a monotonous sequence of black shales. The Patuca Member consists of a sequence of sandstones, greywackes, and shales, intercalated with lava flows and dikes. The fine-grained sediments of the three members are characterized by a high content of particulate organic matter (POM). Palynofacies and rock-eval analyses indicate the predominance of kerogen types II and III, with HI values that indicate a moderate to low source potential. At the type locality, the organic matter is thermally mature and locally overmatures.