Sources: Energy Information Administration, Petroleum Supply Annual 1990, Volume 1, DOE/EIA-0340(90)/1 (Washington, DC, May 1991), Table 39; Energy Information Administration, Petroleum Supply Annual 1996, Volume 1, DOE/EIA-0340(96)/1 (Washington, DC, June 1997), Table 40; Energy Information Administration, Petroleum Supply Annual 1997, Volume 1, DOE/EIA-0340(90)/1 (Washington, DC, June 1998), Table 37 and Table 38; and various news sources.