Joffre oil field is long (about 38 km or 23.6 mi) and narrow (3-4 km or 1.8-2.5 mi) and trends northwest-southeast. Production is from the Viking Formation, which consists of cross-bedded glauconitic sandstones up to 8 m (26 ft) thick encased in bioturbated marine mudstones and siltstones. Although an overall coarsening-upward facies sequence appears to exist, the sandstones are separated from the underlying mudstones by two erosion surfaces. The lower surface dips northeastward, with up to 9 m (29.5 ft) of erosional relief. This surface truncates facies and log markers and is overlain by a thin veneer of pebbles. The upper erosion surface appears to be flat southwest of the field but steps downward about 11 m (36 ft) along the length of the southern margin of the field. he step is interpreted as an incised shoreface cut during a sea level lowstand. During the lowstand, sand and granules were supplied to the shoreface by incised rivers and were transported along the shore by tidal and/or wave-generated currents. An ensuing rise of sea level covered the sands with marine muds and planed off any evidence of subaerial exposure southwest of the field, leaving only a transgressive lag. This interpretation overcomes some of the problems of offshore bar interpretations, namely, how the sediment was transported across the shelf and molded into ridges with internal coarsening-upward sequences. Apparently, (1) the linear nature of the field is due to its formation along the shoreface, (2) sediment was not transported across the shelf, and (3) relative sea level fl ctuations and consequent shifts in the position of the shoreface control the location of the reservoir sand body.