Organic geochemistry and sequence biostratigraphy of the Upper Devonian-Lower Mississippian Exshaw and lower Banff formations, southern Alberta.

摘要

The integration of sequence stratigraphic, biostratigraphic, and organic geochemical techniques at both subsurface and outcrop locations provide new insights into the Upper Devonian-Lower Mississippian petroleum system. This system is represented by the upper Palliser, Exshaw, and lower Banff formations, and includes both reservoir and hydrocarbon source-rock units. Many oil families in southern Alberta have been linked to their source; an exception is oil Family M. Biomarker data for these oils indicate a source of Mississippian age capable of producing relatively large quantities of oil. One probable source for these oils is the lower Banff Formation, predominantly a Type II kerogen with total organic carbon (TOC) values from immature samples that commonly exceed10 wt.%.; At outcrop, the lower Banff Formation comprises overmature shales that vary in thickness from three to four metres in the eastern Front Range exposures (e.g. Inverted Ridge), to greater than 20 metres in westernmost exposures (e.g. near Fortress Mountain), and possesses TOC values up to 18.2 wt.%. In the subsurface, the shale members of the Exshaw and Banff formations vary dramatically in thickness from being not present to thicker than 7 metres. These changes in thickness may explain the existence of two distinct mid-late Paleozoic oil families.; Sequence stratigraphic techniques and conodont biostratigraphic data were combined to aid in the correlation of surfaces within this Upper Devonian to Lower Carboniferous interval. These data indicate that locally, the Exshaw Formation is missing; the absence of these strata is attributed to the development of a stratigraphic high after the deposition of the Big Valley Formation and prior to the deposition of the lower Exshaw Formation. In this area, oils generated from the lower Banff Formation could have migrated without mixing with Exshaw Formation sourced oils providing a possible explanation for the existence of Family M oils in Cretaceous reservoirs.