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

Three case studies of petroleum reservoirs containing tar mats (Oseber g field and Ula field, both from the North Sea, and a North American f ield) have been studied using a variety of organic geochemical techniq ues with special emphasis on the geochemistry of the asphaltenes. The results were evaluated in combination with the available geological da ta and with numerical simulations to evaluate possible mechanisms of t ar mat formation (Part II; Wilhelms and Larter, 1994). In Part I the p etroleum 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.%). Th e extract yields (kg EOM/t rock) in the tar mats (ca. 4-46 for the Ose berg field, 8-18 for the Ula field and 8-43 for the North American cas e study) are significantly higher than in the corresponding oil leg ex tracts, which have extract yields of ca. 1-14 kg/t rock. The tar mat e xtracts are geochemically related to the pooled oil as revealed by geo chemical fingerprinting, including biomarker analysis. The tar mats ar e situated in high porosity and high horizontal permeability sandstone layers above permeability barriers. Several zones of low extract yiel d and immature petroleum geochemical characteristics were observed wit hin the oil legs and between tar mats, which might represent minor amo unts of petroleum generated locally from indigenous organic matter.