ORIGIN OF TAR MATS IN PETROLEUM RESERVOIRS .1. INTRODUCTION AND CASE-STUDIES

Three case studies of petroleum reservoirs containing tar mats (Oseberg field and Ula field, both from the North Sea, and a North American field) have been studied using a variety of organic geochemical techniques with special emphasis on the geochemistry of the asphaltenes. The results were evaluated in combination with the available geological data and with numerical simulations to evaluate possible mechanisms of tar mat formation (Part II; Wilhelms and Larter, 1994). In Part I the petroleum geochemistry of three sets of tar mats is described. The tar mat extracts were characterized by a sharp increase in the asphaltene content (20-60 wt.%) compared with the oil leg extracts (1-5 wt.%). The extract yields (kg EOM/t rock) in the tar mats (ca. 4-46 for the Oseberg field, 8-18 for the Ula field and 8-43 for the North American case study) are significantly higher than in the corresponding oil leg extracts, which have extract yields of ca. 1-14 kg/t rock. The tar mat extracts are geochemically related to the pooled oil as revealed by geochemical fingerprinting, including biomarker analysis. The tar mats are situated in high porosity and high horizontal permeability sandstone layers above permeability barriers. Several zones of low extract yield and immature petroleum geochemical characteristics were observed within the oil legs and between tar mats, which might represent minor amounts of petroleum generated locally from indigenous organic matter.