Geochemical evaluation of sedimentary rocks and hydrocarbon fluids in the Amistad offshore field, Progreso Basin Province, Southwest Ecuador

Abstract

This study contains a thorough isotopic and molecular characterization of a series of thirteen (13) gas samples and six (6) liquid hydrocarbons produced from late Miocene reservoirs in the offshore Progreso Basin (SW Ecuador), along with Rock-Eval and petrographic analyses of sixty-two (62) rock samples from the area. The principal aim of this research work is to investigate the geochemistry of hydrocarbon fluids and sedimentary rocks in the area of the Amistad field to determine the origin of the studied wet gases. Potential source rocks in the Amistad stratigraphic section are immature and have no oil-generating capability, except for the early Miocene Dos Bocas Formation. Studied low-boiling condensates have similar gas chromatographic fingerprints while their compound-specific isotopic analyses of lower diamondoids (CSIA-D) indicate an oil-source correlation between them and Dos Bocas extracts. Sampled gases are mainly methane of biogenic origin except for that from the Delfín B-17X well, which might represent the thermogenic end-member gas signature of the Amistad gases. Results suggest a complex hydrocarbon filling history with multiple charges of thermogenic gas and hydrocarbons generated from the distal deltaic Dos Bocas source rocks followed by biodegradation, leakage and partial displacement by biogenic methane due to the uplifting of the Amistad structure. The Amistad gases form three homogeneous groups located in the southern, central and northern parts of the Progreso reservoir, although the geological segmentation of it into compartments cannot be clearly identified.