Short-time-scale (year) variations of petroleum fluids from the US Gulf Coast

Evolving short-term (less than 5 yr) compositional changes in hydrocarbon c harge from some Eugene Island Block 330 (EI-330) wells are demonstrated. St orage, analytical, and production artifacts are shown to be minimal. In som e wells, compositions remain constant from 1985 to 1993, whereas in others in the same reservoir, significant changes are observed. In some cases, tem poral variability is greater than spatial variability. Maximum temporal cha nge is strongest for specific compounds: toluene and C-6 to C-9 normal alka nes, but is also observed to a lesser extent for higher-molecular-weight co mponents (up to n-C-32). Principal coordinate analysis shows the highest de gree of overall temporal compositional change over an 8-yr period in the sh allowest wells where there is also evidence of biodegradation. Small tempor al compositional changes are also observed in two deeper wells that are bel ow the thermal window favorable for biodegradation. An exception is an unus ual oil, where a very large increase in toluene, as well as smaller changes in a number of n-alkanes, was observed in 1993. The delta C-13 compound-sp ecific isotopic signature of toluene, in addition to several other C-7-C-8 compounds in this oil, yields convincing evidence that it is related to the same family as other EI-330 oils and unlikely to be due to a drilling or l aboratory contaminant. Minor isotopic differences in other C-7 compounds (1 .5 parts per thousand) are consistent with extensive gas washing of this oi l. The short-term compositional changes in EI-330 oils are attributed to ga s washing, which causes overprinting of biodegraded oils with light n-alkan es in shallower GA and HB reservoirs where oils are currently being biodegr aded in situ. Patterns of smaller changes in heavier compounds in both shal lower and deeper wells are also consistent with this interpretation. Copyri ght (C) 2001 Elsevier Science Ltd.