The ca. 1730 Ma Wollogorang Formation is a mixed carbonate-siliciclastic unit in the southern McArthur Basin of northern Australia. It is an exploration target for base metals and may be a petroleum source unit and unconventional reservoir. Facies analysis indicates that the Wollogorang Formation records deposition in supratidal to offshore environments. Lateral consistency in facies, thickness, and stratigraphic architecture suggests deposition on an east-west trending (present coordinates) epeiric platform. According to recent tectonic models for northern Australia, this epeiric platform likely deepened to the present-day east. The Wollogorang Formation comprises one second-order and five nested third-order sequences. Despite the unavailability of biostratigraphy and biofacies, and the general poor time control in Precambrian basins, we are able to correlate third-order (i.e., our lowest rank) sequences over tens to hundreds of kilometers using high-resolution facies and petrophysical logs. As the distinction between stratigraphic sequences (related to shoreline shifts) and sedimentological bedsets (not related to shoreline shifts) becomes more difficult with decreasing hierarchical level, sedimentological, stratigraphic, paleontological, and mineralogical (including diagenetic) criteria were recently proposed to distinguish these cycles. Our study suggests that sedimentological and stratigraphic criteria are the most useful in Precambrian sequence stratigraphy. In contrast, paleontological criteria are irrelevant in Precambrian successions. Mineralogical (including diagenetic) criteria have not been tested in this study. In addition to further study these criteria, we suggest testing geochemical criteria, such as the enrichment of redox-sensitive trace elements and minor carbonate carbon isotope variation, to distinguish stratigraphic sequences from sedimentological bedsets.
The Beetaloo Sub-basin is a concealed, composite depocenter and a component of a group of intra-cratonic Paleoproterozoic to Mesoproterozoic sedimentary basins collectively described as the Greater McArthur Basin. The Sub-basin hosts unconventional and conventional petroleum resources, particularly in the uppermost Roper Group where stacked play opportunities include liquids rich shale, dry gas shale and hybrid/ tight gas plays (Côté et al. 2018; Altmann et al., 2020).
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.
Abstract Trace metal elements (TMEs) are commonly used to reconstruct the environmental conditions present during the deposition of organic-rich sediments. For example, TME concentrations controlled by changes in primary productivity and redox conditions are widely used in paleoenvironmental studies. Recently, these proxies have undergone a resurgence of interest and are commonly used in large-scale (10–1000 km) studies. However, applying these geochemical proxies at basin scale while ignoring variations in sedimentation rates (SR) may lead to misinterpretation of paleoenvironmental conditions. Here, we show how SR can affect the geochemical records and may lead to incorrect interpretations of TME evolution. Accounting for SR, we computed the authigenic fraction accumulation rates of key TMEs in the Upper Montney Formation and Doig Phosphate (Triassic, western Canada), and we correct the concentration of these elements in the Vaca-Muerta Formation (Jurassic–Cretaceous, Argentina). Our SR-corrected TME proxies require a different interpretation of paleoenvironmental conditions (e.g., primary productivity, basin restriction) compared to conventional TME results and highlight that elementary enrichments commonly interpreted as indicative of anoxic depositional environments may reflect low SR and the formation of condensed intervals. This work also introduces a new workflow to account for SR in paleoenvironmental studies at basin scale and over long time periods.
This study focuses on the Lower–Middle Triassic Montney, Sunset Prairie, Doig and Halfway formations from the foreland basin of the Canadian Cordillera (Alberta and British Columbia). Based on core and outcrop descriptions, the correlation of 400 wells, and on mineralogical analyses, this study interprets the basin-scale, 3D-stratigraphic architecture of these formations and discusses the controls on its evolution. Well correlation highlights four sequences (1–4) interpreted to occur in two second-order cycles (A and B). In this work, the Early Triassic Montney Formation and the early Middle Triassic Sunset Prairie Formation are interpreted to consist of three third-order sequences (1–3) related to the first second-order cycle (cycle A). The Middle Triassic Doig and Halfway formations are interpreted to consist of a fourth sequence (4) related to a second second-order cycle (cycle B). Integration of the stratigraphic surfaces with previously published biostratigraphic analyses emphasizes a major time gap of c. 2 myr between these two cycles, interpreted to record a major reorganization of the basin. Mineralogical analyses suggest that during cycle A, sediments were delivered from the east (Canadian Shield); whereas in cycle B, additional sources from the west (proto-Canadian Cordillera) occurred. This study shows that the stratigraphic architecture evolution was affected by the structural heritage of the basin and continental geodynamic evolution. This study provides a large-scale understanding on the controls of the stratigraphic architecture of the Lower and Middle Triassic strata, suggesting local and regional controls on the reservoir extension and unconventional play configuration within these strata.
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