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 2-12W4) show a range of API gravity and oil qualit y. This study indicates that the main causes for the observed variation in oil quality are oil source rock character, thermal maturity of the source r ocks and biodegradation. Analysis of the gross oil composition, gasoline ra nge (C-5 to C-8) compounds and the C15+ saturate and aromatic fractions (bi omarker analysis) of twenty oils from this area allow us to assess the rela tive 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, nondegraded, low sulphur crude oil. Lowe r pristane/phytane, C-23/C-24 tricyclic terpane, C-35/C-34 homohopane and C -21/C-29 sterane and higher Ts/Tm, C-29 norhopane/C-30 hopane and S/R bioma rker ratios, as well as the absence of C-28-bis-norhopane and the Q compoun ds, serve to distinguish this oil from the other oils at Provost. The sourc e 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 car bonates, and accessed Lower Mannville porous and permeable strata through b reaches in the Devonian Ireton Formation at the northern end of the Bashaw complex. The oil then migrated eastward into the Dina reservoir in the Prov ost field. Family EQ oils are non-degraded to moderately biodegraded, with API graviti es 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 repre sent a mixture of oils from the Upper Devonian/Mississippian Exshaw Formati on (E) and the Lower Cretaceous Ostracode Zone (Q). The most biodegraded Fa mily EQ oils occur in the eastern part of the study area, and in stratigrap hically younger reservoirs (McLaren, Lloydminster sandstones). The least bi odegraded oils are present in older reservoirs (Dina, Cummings) in the west ern 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 distributio ns, early mature oils in the eastern portions of the field were predominant ly sourced by the Exshaw Formation, with an increase in contribution from t he Ostracode Zone with increasing thermal maturity of the migrating oils.