Salt Morphology and Hydrocarbon Trapping At El 188: Detailed Salt Sulface Mapping Yields New Opportunities In An Old Field

Abstract

Abstract Hydrocarbons are trapped within irregularities in the surface of a shallow piercement-type salt diapir at Eugene Island 188 field. Most of the oil is found in Pliocene age sands that terminate against a salt nose extending from the northeast flank of the dome. Substantial amounts of gas have also been found in late Miocene age sands that close against a deepseated salt nose extending due west from the dome. Ninety percent of the field reserves lie in Blocks 188 and 189 wherea rugose salt surface (caused mainly by faulting) provides excellent hydrocarbon trapping configurations. The southernhalf of the dome, up thrown to a major counter-regional fault system and characterized by a generally smooth salt surface, contains only 10% of the field reserves. In general, any Pliocene or Miocene sand closing against salt contains hydrocarbons. Five wells (four successful, one dry) drilled as a result of a 1992 field study added nearly 10 MMBOE to the field's ultimate reserves. Production rates were increased by 13 MMCFG and 2600 BO daily. Location and Development Eugene Island Block 188 field is located 110 miles southwest of New Orleans in Blocks 188, 189, 190, and 191 in 70 feetof water (Figure 1). The field was discovered in 1956, developed in the 1960's, redeveloped in 1983-84 and again in1992. A total of 125 wells have been drilled on Shell's acreage on the north flank of the salt dome. Ultimate recoverable reserves are estimated to be 60 MMBO and 150 BCFG from around the entire dome 90% of which will come from Blocks 188 and 189. Maximum daily production from Blocks 188 and 189 reached 19,500 BO and 21 MMCFG in 1971. Present daily production is 3900 BO and 45 MCFG (a new record for gas production) (Figure 2). In 1991-92 a field study was done to identify remaining opportunities in the filed. All pay and potential pay zoneswere remapped incorporating a new 3D seismic survey, gravity, production and well data. A series of sub regionalinterval isopachs over the dome and salt-withdrawal basin were also created to reconstruct the tectonic history. A very detailed picture of the salt surface was created using all available data. Five well locations were identified and subsequently drilled as a result of this field study. Exxon's 50% working interest was also purchased giving Shell a 100% working interest in the field. Paleogeography The Eugene Island 188 salt dome is part of a large counter regional fault/salt withdrawal mini basin complex (Figure 3). Major features of the complex are a large counter-regional expanding fault system, a salt withdrawal basin, and the shallow piercement salt dome with salt noses extending oul from it to the northeast and west. The north-dipping expanding fault system extends from western Ship Shoal area across Eugene Island to Vermilionarea, a distance of over 60 miles. The fault system appears to have been active throughout the late Miocene, Pliocene. and Pleistocene. Maximum throws across the fault are in thousands of feet.