Middle Ground Shoal oil field, in upper Cook Inlet, Alaska, is located beneath water with an average depth of 100 ft (30 m). The Shell MGS State No. 1, drilled in 1963, was the first offshore oil completion in Alaska. The field produces oil from a gross interval of about 2,800 ft (850 m) in the Tertiary lower Tyonek Formation. The productive interval has been separated into seven pools; the A pool is produced separately, but production from the B, C, and D and the E, F, and G pools, respectively, is commingled. Three production platforms are in use, and the field contains 31 producing wells, 23 injection wells, 1 shut-in gas well, and 8 abandoned or suspended wells. As of January 1, 1974, the field had produced 78,662,670 bbl of oil, 37,270,730 Mcf of gas, and 9,162,874 b l of water. Because of declining reservoir pressures, pressure maintenance by water injection was started in 1969.

The structure at Middle Ground Shoal is a narrow anticlinal feature which strikes N10°E. Little or no paleostructural growth is thought to have occurred during deposition of the oil-bearing sandstone sequence. Channel fills and braided-stream deposits provide the reservoirs. The main productive interval in the field is the G pool, which is in the Hemlock Sandstone Member of the Tyonek.