Geochemistry of oils and condensates from the lower Eagle Ford formation, south Texas. Part 4: Diamondoids

Abstract

A rich variety of diamondoid indices exists that are assumed to be diagnostic of source facies, thermal maturity, the extent of oil cracking, mixed oils, and reservoir alteration processes. These parameters were developed either from empirical relationships observed in source rocks and oils produced from conventional reservoirs or laboratory experiments. Diamondoid parameters are now being routinely applied to unconventional oils; however, their accuracy and reliability have not been rigorously tested in these petroleum systems.Here, we examine diamondoid compositions in a suite of lower Eagle Ford oils and condensates with well-constrained source facies and thermal maturity established by other geochemical measurements and basin modeling. Diamondoid concentrations are low in samples from ∼0.8 to 1.35 %Ro, and then increase significantly at higher maturity due to the cracking of less stable hydrocarbons. Most published maturity-dependent diamondoid indices are not diagnostic when maturity is ≲ 1.1 %Ro and are influenced by source facies. Based on our data, their predictive accuracy is no better than ±0.2 %Ro at higher maturity. Published source indices are not diagnostic of a specific kerogen type or lithofacies but may be used to differentiate sources in complex petroleum systems. Quantitative diamondoid analysis can be used to calculate the degree of oil-to-gas conversion; however, the natural variance in the baseline concentration of 3-+4-methydiamantanes limits its accuracy at maturities ≲1.35 %Ro. Three lower Eagle Ford oils associated with Karnes Trough faults exhibit anomalous diamondoid distributions behavior that these reservoirs received secondary charges from a source other than lower Eagle Ford (likely Austin Chalk). Evidence for a secondary charge was not apparent from the study of the saturated and aromatic biomarkers. In summary, established diamondoid maturity and source indices based on compound ratios should be used with caution to characterize unconventional oils/condensates while quantitative diamondoid analysis can be used with greater confidence and can provide insights that are not apparent from other geochemical parameters provided that highly accurate determination of diamondoid concentrations can be obtained. In the case of the Eagle Ford oils and extracts, analyses must be accurate to less than 1 ppm.