12: The Discovery Process behind the Giant Johan Sverdrup Field
Published:January 01, 2017
The giant Johan Sverdrup field was discovered in 2010 by well 16/2-6 drilled on the Utsira high, in the central part of the Norwegian North Sea. This area was considered exhausted after more than 40 years of disappointing on-and-off exploration drilling. The discovery of the significant Edvard Grieg field by well 16/1-8 in 2007 converted the Johan Sverdrup prospect to a high probability prospect. The predrill hypothesis was that the two prospects could be part of one large deposit with a 40 to 50 m (131 to 164 ft) saturated oil leg beneath a gas cap and a common oil–water contact (OWC) shallower than 1950 m (6398 ft) mean sea level (MSL). The exploration wells unfolded two normal pressured discoveries with undersaturated nonbiodegraded oil on the flank of a saturated system with biodegraded oil and 6 bar overpressure.
The Edvard Grieg discovery well proved a 40 m (131 ft) oil column above 1939 m (6362 ft) MSL on the west side of the high. The reserves in the plan for development and operations (PDO) were estimated to be 186 million BOE. This number has later been adjusted upward. The predrill estimate was 250 BOE. The reservoir sand was potassium rich and was indicated as shale and water bearing on the wireline logs. Without coring, the reservoir could have been overlooked. The reservoir is proximal Jurassic–Triassic deposits consisting of aeolian, braided river, and alluvial facies. Reservoir quality is also documented in Valanginian bioclastic sandstone and weathered basement.
The Johan Sverdrup discovery well was located to obtain maximum sequence stratigraphic information above the potential OWC at 1939 m (6362 ft) MSL on the east side of the high. The well proved an oil–water contact at 1922.5 m (6307 ft) MSL and showed that the discovery extended into the neighboring license to the west. The main reservoir in Johan Sverdrup is locally derived shallow marine transgressive Volgian sand, overlying reservoir rocks from Zechstein carbonates, lower and upper Jurassic continental to shallow marine sandstones separated by several unconformities. The PDO for the unitized field was issued in February 2015 with a reserve range of 1.7 to 3.0 billion bbl. The main reserve uncertainties are related to recovery factor, oil saturation, time–depth conversion variations, reservoir thickness estimation, and sequence resolution. The oil is nonbiodegraded and heavily undersaturated, and has low varying gas–oil ratio (GOR) and OWC varying from 1922 to 1935 m (6306 to 6348 ft) MSL with a substantial residual oil zone below the current free water level (FWL). This reflects the glacial-induced isostasy effects on FWL during Pleistocene.
The Edvard Grieg and Johan Sverdrup fields are situated on the southern part of the Utsira high referred to as the Haugaland High. This high is situated where the northwest extension of the Paleozoic to recent Tornquist wrench zone meets the Caledonian front between Scotland and Norway. In early mid-Jurassic the Haugaland High was part of a regional thermal inversion doming in the central North Sea. Several significant erosional and transgressive events determined the distribution of high-quality condensed Jurassic reservoir sequences from early to late Jurassic.
Late Pliocene and Pleistocene subsidence, including tilting toward the southwest and uplift in the northeast, brought the Johan Sverdrup field into its current structural position. The late structural formation implies late and ongoing migration into the reservoirs. Regional mapping of the shallow Miocene sands in the Utsira Formation showed seismic hydrocarbon indicators of petroleum migration from east to west, sourced by vertical leakage from the Johan Sverdrup field at Jurassic level. The westward migration ends in a glacial tunnel valley that cut into the Miocene sand. Gas flares at sea bottom consisting of gas derived from biodegraded oil have been sampled. This gas leakage has resulted in the formation of patchy carbonate crusts, a process that has been ongoing since the last glaciation.
Coring and production testing have been instrumental for the unfolding of the Edvard Grieg and Johan Sverdrup fields. Improved wireline logging procedures have been used in formation evaluation. The acquisition of 3-D ocean bottom seismic (OBS) and broadband seismic surveys in 2009 and 2012 has also been a catalyst for the unfolding process.