Deltas as Petroleum Provinces: Past, Present, and Future

ABSTRACT Deltas have proven to be important worldwide petroleum provinces for two main reasons. Firstly, they are point sources from which sediment is introduced onto continental shelves and into marginal basins, where the interaction of fluvial and basinal processes frequently result in favorable associations of reservoir, source, and seal facies. Secondly, the progradation of deltas forms a geomorphic bulge which projects seaward and allows land and shallow offshore drilling rigs to reach deep targets which could only be reached by more expensive rigs in deeper water areas adjacent to the delta. Significant hydrocarbons have been discovered at least eighteen deltaic provinces worldwide. Almost 30 billion BO and over 100 TCF of gas have been produced from these deltaic provinces, which contain estimated ultimate reserves of 50+ billion BO and 350 TCF of gas. Fields with reserves greater than 500 MMBO occur in five deltaic basins and fields with more than 100 MMBO have been found in thirteen deltas. Future exploration efforts will focus upon deepwater delta-front environments and smaller deltas in more remote areas with hostile drilling environments, such as those in the circum arctic region. INTRODUCTION Ever since Herodotus described the land at the mouth of the Nile River as similar in shape to the Greek letter delta around 400 B.C. deltas have become one of the most studied of all depositional environments.1'2 Deltas form where fluvial systems enter a body of water and have been geologically defined as: "The low, nearly flat, alluvial tract of land at or near the mouth of a river, commonly forming a triangular or fan-shaped plain of considerable area, crossed by many distributaries of the main river, perhaps extending beyond the general trend of the coast, and resulting from the accumulation of sediment supplied by the river in such quantities that is not removed by tide, waves, and currents. Most deltas are partly subaerial and partly below water."z Deltas are significant components of many coastlines and their deposits make up a substantial volume of the stratigraphic record.1,3,4,5,6,7 Mfosdte7rn deltas are diverse in form and vary greatly in size and geomorphic setting. Deltaic morphology and three dimensional distribution of sediments is the result of a complex interplay between fluvial discharge, sediment supp" and the wave and tidal energy of the receiving basin. They occur at almost a" latitudes and climates, from the arctic the tropics.3