Aspects of petroleum basin evolution due to gas advection and evaporative fractionation

Criteria are proposed for the recognition of depletion by evaporative fractionation in reservoir fluid analyses (PVT data). The characteristics of original, as-generated, molar concentration profiles of marine oils are postulated: oils derived from clastic or carbonate sediments bearing, respectively, Type II or IIS kerogens. In unaltered marine oils two decreasing exponential concentration series occur, separated by a discontinuity between P5 (the 5 carbon pseudo-component) and P6, such that P6 exceeds P5, rendering the profile abruptly bimodal. A suite of 195 detailed PVT analyses (C1 to P30+) of reservoir fluid oils shows that concentrations in the P6+ fraction are frequently perturbed, the maximum migrating to P7, P8 or a higher carbon number, alteration attributable to additive gas advection, followed by evaporative fractionation. In the subject oils four variables are adduced which track these changes: C1/C2, P4/P8, E3 (a light gas–liquid/heavy end ratio) and E7 (a gasoline range/heavy end ratio). Increase in C1/C2 is accompanied by decreasing values of P4/P8 for which there is no a priori rationale other than the precepts of evaporative fractionation involving the advection of highly mature dry gas. ng a suite of 535 PVT analyses (detailed analyses plus 340 analyzed to P7+) of world wide provenance into seven subsets on the basis of C1/C2 ratios, a well defined, separate regression of gas/oil ratio on saturation pressure is revealed in each subset. Such correlations depend primarily upon the processes involved in evaporative fractionation. The curves embrace the full, commonly encountered, range of combinations of gas/oil ratio and saturation pressure, both of which correlate poorly with level of maturity expressed in a liquid fraction slope factor. These observations lead to the hypothesis that progressive evaporative fractionation is a widespread, if not universal, evolutionary process in petroleum basins.