Abstract

Thick Spathian deposits of the Lower Triassic Montney Formation are preserved in northeastern British Columbia and west-central Alberta, where they hold massive amounts of unconventional resources. Understanding the internal architecture of these marine deposits at basin-scale can provide a framework to better predict the distribution of source-rocks, reservoirs and seals within this petroleum system and to investigate their control on hydrocarbon generation and migration pathways. Ultimately, this high resolution stratigraphic framework can be used to investigate the impact of geological heterogeneities on well performance at the regional scale.

In northeastern British Columbia, the Spathian deposits consist mainly of offshore and offshore transition sediments forming a wedge prograding from northeast to southwest. This wedge is punctuated by major marine flooding surfaces bounding parasequence sets that can be correlated regionally owing to their characteristic gamma ray logs signature and to the high density of well and core control. The regional correlation of these parasequence sets, based on over 1450 wells, reveals well-defined clinoform morphologies characterized by topset, foreset and bottomset geometries along a proximal-distal depositional profile. The facies analysis and the characteristic dimensions of these morphologies are consistent with deposition in a predominantly siliciclastic shoreface to shelf setting and marks a significant contrast to the ramp setting of hybrid clastic-carbonate lithologies which prevailed during the Griesbachian to Smithian. The stratigraphic architecture is analogous to “subaqueous shelf-prism clinoforms” that have been described on numerous present-day and ancient continental shelves. Subaqueous shelf-prism clinoforms typically display a sigmoidal shape in the dip direction and along-shore-elongated depositional thick in plan-view. This geometry results from the interaction of clastic sediment input with shelf hydrodynamic processes such as storm generated waves and sediment gravity flows, as well as nearshore and offshore bottom currents. Consequently, the topset, foreset and bottomset of these clinoforms are characterized by different depositional facies that can be predicted and mapped at basin-scale, over hundreds of kilometres.

In the Spathian depositional system of Western Canada, clinoform bottomset facies are mainly a product of suspension deposition, hemipelagic sedimentation and mineral precipitation. These facies form the main source-rock intervals within the Montney Formation, due to anoxic conditions and lower sedimentation rates resulting in better preservation of organic matter. Clinoform foresets result from traction transport processes of coarser siliciclastics and higher sedimentation rates, forming thick, mostly organic-lean intervals with better reservoir quality. Foreset deposits form the thickest part of the Spathian parasequence sets and are the main targets of horizontal drilling and multistage fracturing. Clinoform topsets mainly consist of shoreface to offshore transition deposits and are poorly preserved due to the erosion under the top Montney unconformity. The distribution of the depositional thick in map view and along a strike-oriented regional cross-section suggest that these deposits were influenced by major structural elements at basin scale. The regional flooding surfaces bounding the parasequence sets might form extensive permeability barriers that potentially control up-dip migration of hydrocarbons within the Montney petroleum system.