Linking Stratigraphic Architecture and Petroleum System Elements of the Niobrara Formation to Oceanographic and Far-Field Tectonic Events

Abstract

The Niobrara Formation was deposited in the Western Interior Seaway (WIS), which filled an active foreland basin during the Upper Cretaceous. The WIS experienced important oceanographic variations and tectonic events during Niobrara time, and resolving their influence is critical to mapping petroleum system elements of the greater Denver-Julesburg (D-J) Basin area. To unravel stratigraphic complexity through time, sequence stratigraphic surfaces suited to the distal carbonate ramp, and in the context of biozone and isotope geochronology data, were correlated through basin-scale well control to create a series of age-constrained isochore maps. At a WIS scale, thickness patterns suggest that the basin’s flexural forebulge had migrated eastward to a position along trend with the future Rocky Mountains where it defined the western edge of the distal carbonate ramp that developed in the backbulge of the basin. At the D-J Basin scale, early Niobrara deposition (Upper Turonian - Lower Santonian) was dominated by patterns of differential sediment accumulation with compensational infilling in the form of systematic reversals of stratigraphic thicks and thins through time. This pattern was interrupted in the Lower Santonian by the development of sublinear basement uplifts along the trend of the emergent Transcontinental Arch. As a result, sediment accumulation became dominated by patterns of draping over the long-lived seafloor paleohighs. Absolute timing of architectural changes in the Niobrara suggests a link between Sevier thrusting episodes, a migrating flexural forebulge, and uplifts along reactivated basement shear zones in the distal foreland. The well-known transgressive-regressive cycles of the Niobrara in the D-J Basin appear to be broadly overprinted by two distinct influences: circulation-related bottom currents and deposits and the later interference of tectonic uplifts. This dynamic paleoseafloor morphology was a first-order control that shaped the depositional patterns of Niobrara source rock and reservoir rock intervals.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019