Contrasting characteristics attributed to migration observed in petroleums reservoired in clastic and carbonate sequences in the Gulf of Mexico region

Abstract

Systematic differences among two subsets of petroleums selected geographically and by reservoir type were attributed to lithologically-characteristic migration phenomena. Most oils in offshore Louisiana Tertiary clastic reservoirs are deficient in light ends, evidently because of evaporative fractionation. Residual oils are enriched in cycloalkanes and aromatic hydrocarbons. Spuriously low evident maturities based upon concepts of paraffinicity are observed. Numerous oils are both fractionated and biodegraded. Mesozoic carbonate-sealed reservoirs at the margins of the basin generally contain pristine oils with intact light ends. However, many oils appear to be depleted in preferentially water soluble compounds: benzene, toluene, light cycloalkanes and phenanthrene, evidently due to aqueous fractionation during intraformational migration.

Experimental study of the effects of thermochemical sulfate reduction on low molecular weight hydrocarbons in confined systems and its geochemical implications

“…LMW hydrocarbon parameters are useful tools for correlating oils, evaluating source maturity and types, and identifying secondary transformations in reservoirs, such as biodegradation, thermal alteration, water washing and evaporation (Thompson, 1983(Thompson, , 1987(Thompson, , 1988(Thompson, , 1995Schaefer and Littke, 1988;Halpern, 1995;ten Haven, 1996;Mango, 1997;Whiticar and Snowdon, 1999). However, n-alkanes, isoalkanes, cycloalkanes and monoaromatics in LMW hydrocarbons behave differently during TSR as discussed above, which is different from the pattern during kerogen maturation.…”

“…The investigations of Thompson (1983Thompson ( , 1987Thompson ( , 1988Thompson ( , 1995 demonstrated that LMW hydrocarbons can be used to classify the types and thermal maturity of petroleum and evaluate secondary alteration in reservoirs, such as evaporative fractionation, water washing and biodegradation. Commonly used hydrocarbon parameters include heptane value, isoheptane value, n-heptane/methylcyclohexane and toluene/n-heptane (Thompson, 1988).…”

“…Lower toluene/n-heptane and higher isoheptane values for samples from the anhydrous and hydrous experiments leads to the incorrect conclusion that aromatics were removed by water washing in the reservoir (Fig. 8c) or were preferentially dissolved in water during secondary migration along carrier beds because toluene is more soluble in water than n-heptane, reducing toluene/n-heptane along migration pathways as indicated by Thompson (1995) (Fig. 8d).…”

“…Variations of molecular compositions in LMW hydrocarbons caused by TSR may complicate correlating oils, evaluating the maturity of kerogens, tracing hydrocarbon sources at high maturity and identifying secondary alterations, such as biodegradation, water washing and evaporative fractionation by using LMW hydrocarbon parameters and isotopes (Thompson, 1983(Thompson, , 1987(Thompson, , 1988(Thompson, , 1995Schaefer and Littke, 1988;Halpern, 1995;Rooney, 1995;ten Haven, 1996;Mango, 1997;Whiticar and Snowdon, 1999;Peters et al, 2005). For example, TSR affects the relative concentrations of methylhexanes and dimethylpentanes in crude oils.…”

Genetic features of gas-condensate reservoirs in the Kekeya Field, southwest depression in the Tarim Basin, China

“…The composition of the residual oil can be interpreted according to both the fractionation depth and the gas volume required to fractionate the oil [15] . In fact, gas washing and evaporative fractionation have been used to describe phase fractionation caused by multiple stage geological events or mixed processes with oils and gases [16][17][18][19][20] .…”