Abstract

Regional studies, involving 185 cores and over 300 well logs, show that deposits in the Caroline-Garrington-Harmattan fields are typical of the Viking Formation. Three main depositional events occurred: 1. construction and progradation of a clastic wedge associated with a shoreline; 2. cut-and-fill of channels that dissect the clastic wedge in the offshore shelf; and 3. deposition of sheet-like sandstones and conglomerates in the offshore shelf. Some of these sheets have been reworked into 5 to 10 m thick ridge-and-swale deposits. The Caroline-Garrington fields are characterized by a 20 to 35 m thick coarsening-upward sequence of shale, sandstone and conglomerate/pebbly sandstone. Three facies associations comprise the composite sequence: 1. interbedded, bioturbated and loaded fine-grained sandstone and shale (20 to 25 m thick); 2. fine-grained sandstone (6 to 12 m thick) with low-angle inclined cross-stratification; and 3. chert-pebble conglomerate and pebbly to coarse-grained sandstone (0 to 4 m thick). All of the facies thin and become finer grained to the north-northeast, corresponding to an increasing paleowater depth and more distal setting. At Caroline, progradation of the clastic wedge controlled the facies distribution. At Garrington, marine reworking of the sediment was more important. Locally, conglomerate and coarse-grained sandstone occur as lag deposits on ridge crests and in swale troughs. Ridge-and-swale paleotopographies predominate in proximal shelf areas; more sheet-like deposits characterize distal shelf settings. The Garrington deposits are distal equivalents of the clastic wedge at Caroline. In particular, thin “grit” units located above the main Viking deposits at Garrington can be correlated with coarser grained units at Caroline. These “grit” units can constitute a distinct petroleum reservoir. At Harmattan, the location of the best reservoir rocks is controlled by quite different sedimentological features. Here, the major paleotopographic feature is an east-west oriented channel (15 km long × 1.8 km wide × 2 m deep), which cuts through the clastic wedge deposits. The fill consists of chert-pebble conglomerate that occurs as three fining-upward sequences. Maximum net pay is along the trough.