Origin and alteration of Lower Cretaceous Mannville Group oils from the Provost oil field, east central Alberta, Canada

The Lower Cretaceous oils in the Provost oil field of east central Alberta (Townships 35-40; Ranges 1-12W4) show a range of API gravity and oil quality. This study indicates that the main causes for the observed variation in oil quality are oil source rock character, thermal maturity of the source rocks and biodegradation. Analysis of the gross oil composition, gasoline range (C (sub 5) to C (sub 8) ) compounds and the C (sub 15+) saturate and aromatic fractions (biomarker analysis) of twenty oils from this area allow us to assess the relative importance of each of these factors. Two oil families were identified. Family D consists of only one oil sample, and is a light, thermally mature, non-degraded, low sulphur crude oil. Lower pristane/phytane, C (sub 23) /C (sub 24) tricyclic terpane, C (sub 35) /C (sub 34) homohopane and C (sub 21) /C (sub 29) sterane and higher Ts/Tm, C (sub 29) norhopane/C (sub 30) hopane and S/R biomarker ratios, as well as the absence of C (sub 28) -bisnorhopane and the Q compounds, serve to distinguish this oil from the other oils at Provost. The source for the Family D oil is the Upper Devonian Duvernay Formation (i.e. it is similar to Devonian Leduc oils). This oil is inferred to have migrated up-dip from mature Duvernay sources via porous zones in the Upper Devonian carbonates, and accessed Lower Mannville porous and permeable strata through breaches in the Devonian Ireton Formation at the northern end of the Bashaw complex. The oil then migrated eastward into the Dina reservoir in the Provost field. Family EQ oils are non-degraded to moderately biodegraded, with API gravities typically <30 degrees , and moderate to high sulphur contents (1.43 to 4.37%). The Family EQ oils are less mature than the Family D oils, and represent a mixture of oils from the Upper Devonian/Mississippian Exshaw Formation (E) and the Lower Cretaceous Ostracode Zone (Q). The most biodegraded Family EQ oils occur in the eastern part of the study area, and in stratigraphically younger reservoirs (McLaren, Lloydminster sandstones). The least biodegraded oils are present in older reservoirs (Dina, Cummings) in the western and southern portions of the field. The Family EQ oils have migrated some 150 to 200 km from mature sources to the west and southwest of the Provost field. Based on biomarker distributions, early mature oils in the eastern portions of the field were predominantly sourced by the Exshaw Formation, with an increase in contribution from the Ostracode Zone with increasing thermal maturity of the migrating oils.