THE ROLE OF LARGE SEDIMENT ROUTING SYSTEMS IN SAND MATURATION: IMPLICATIONS FOR HYDROCARBON PROSPECTIVITY

Reservoir quality variation along apparently identical margins is a major oil exploration risk. Reasons for such variation are poorly understood; most evidence is qualitative or anecdotal. In particular, large rivers are popularly equated with high-quality reservoir sands. However, it is clear that not all large river systems are associated with good reservoirs; equally, some small rivers appear to have produced high-quality reservoir sands, despite their relatively small catchment. One of the problems with previous work in this area is the tendency to concentrate either on processes within an individual sedimentary basin, or on the fluvial processes within an individual river system. However, sediment maturation is the sum of the tectonic, climatic and diagenetic processes, as well as the more commonly considered fluvial and authigenetic processes; to capture this total flux, it is better try to quantify the maturation within the whole sediment routing system. Large sediment routing systems are among the largest geological features on earth, commonly spanning ocean-continent boundaries and occasionally crossing plate boundaries.

The approach of this paper is to compare data from the sediment routing systems associated with the world's 25 largest rivers (Amazon, Amur, Chari, Danube, Ganges-Brahmaputra, Indus, Lena, Mackenzie, Mekong, Mississippi, Murray, Niger, Nile, Ob, Orange, Orinoco, Parana, St Lawrence, Shatt al Arab, Volga, Yangtze, Yellow, Yenisey, Zaire, Zambezi). We review all the identifiable fluxes, concentrating on processes that appear to influence sediment maturation as well as influencing their hydrocarbon potential: catchment lithology, topography, sediment erosion, transport energy, sediment storage, intra-catchment tectonism, climate, and age of the river system. Preliminary results suggest that the most important single factor in the development of mature sands is "residence", the likelihood of sediment being stored and modified during thoughput in the terrestrial part of the routing system.