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This paper describes the history, stratigraphy, structure and some of the development problems of the Gachsaran oilfield, which is situated near the boundary of the provinces of Khuzestan and Fars in southern Iran (Persia), and is probably one of the world's largest oilfields. The field was discovered in 1928, but until 1956 was, on account of the relatively heavy and sulphurous character of its crude, developed only to a limited degree. Shifts in the marketing, demand and refining situation, however, have now led to extensive development. Gachsaran is at present the most southeasterly of the line of fields in the Khuzestan foothills of the Zagros Range. As to structure, stratigraphy and producing characteristics, the field is of the classic Khuzestan type, and has individual well Potentials of the order of 20,000 to 40,000 bpd from the Asmari Limestone (Oligocene-Miocene). The latter is folded into a very simple pattern of large antidines and synclines, but is overlain by a complicated and disconformable overburden of Miocene evaporites and clastics (the Lower Fars formation); the principal development problems are consa quently the forecasting of the attitude of the Asmari below the disconformable cover, and the difficulties entailed in drilling through the unstable, mobile and highly-pressured Lower Fars overburden. The field as so far provisionally delineated is 130 square miles in area to the oil-water eontact, and has au Asmari Limestone oil column of 7150 ft. and two small separate gas-caps. The present production of the field is 60,000 bpd of 32"API oil through a 12-inch pipeline to the Bandar Ma'shur crude terminal and the Abadan refinery. The first stage of the present extensive development calls for increase in production capacity to 450,000 bpd, to be exported mainly through a new 26/28/30" pipeline, one hundred miles long, terminating on the island of Kharg in the Persian Gulf.
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