Hydrocarbon Potential of the Paleozoic Carbonates and Mesozoic Siliclastics of the Liard Basin
By Bradley Parkes
The Liard Basin is located immediately to the east of the Cordilleran Thrust and Fold Belt and is a sub-basin extension of the Western Canada Sedimentary Basin. Within the Liard Basin the strata represented is aged Cambrian to Upper Cretaceous and is relatively unexplored (Morrow and Maclean 2004). Potential hydrocarbon resources exist in both conventional and unconventional targets. The western portion of the Liard Basin is known as the Liard Fold and Thrust Belt or Liard Plateau. Within the area several conventional hydrocarbon pools have been found in middle Devonian carbonates and more recently some unconventional gas reservoirs have been discovered in Mississippian shales (Walsh et al 2005). According to a June 14, 2012 article in the Globe and Mail, Apache Corp. suggests the Liard Basin may be more prospective than the nearby Horn River Basin (Vanderkippe 2012). The lack of well control in the area requires a methodology to determine the prospective areas from the non-prospective areas. One methodology that can help identify possible hydrocarbon targets is to define the area through seismic surveys. Using seismic interpretation, time structure maps and isochron maps one can determine the nature of the complex regional geology.
The dominant structural feature of the Liard Plateau is the Bovie Structure that separates the thick Paleozoic and Mesozoic sediments in the western portion of the basin from the thinner successions in the interior plain (Morrow et al 2005). The Bovie Lake Fault cuts from the Proterozoic through Mississippian strata creating a west dipping anticline and represents the eastern edge of the Cretaceous deposition that thickens to the north and west (Walsh et al 2005). This margin also marks the Liard Basin from the western edge of the Horn River Basin. The Bovie Lake Thrust Fault separates the Mississippian Banff shales from the more continuous upper Devonian carbonates of the Tetcho and Kotcho Formations. The westward normal Bovie Lake Fault and eastward Bovie Lake Thrust Fault, collectively known as the Bovie Lake Structure is believed to have developed in two stages. The first stage consisted of uplift and compression during the Cambrian to the Permian creating the Bovie Lake Fault. This fault was reactivated during the Laramide Orogeny by a horizontal convergence creating the Bovie Lake Thrust Fault (Maclean and Morrow 2004). Understanding this complex geology can aid with exploration, as gas fields such as the Maxhamish Field and Beaver River field are associated with the Bovie Structures (Morrow et al 2005).
The Horn River Basin is already a significant hydrocarbon producer, yet to date the Liard Basin has not become a significant producer. Gas has been recovered from the Cretaceous Chinkeh formation and Besa River formation (Dixon 1997), however little exploration has been conducted. Recently a well drilled by Apache Corp in 2009 lead to their classifying the Liard Basin as “best unconventional gas reservoir in North America” (Globe and Mail June 14, 2012). This newly realized potential has created new interest in the Liard Basin. The source rocks are considered to be radioactive shales with prospective reservoir units in both Paleozoic carbonates and Cretaceous siliclastic strata west of the Bovie Lake Fault (Dixon 1997). Formations of interest include the Devonian Dunedin, Mississippian Exshaw, Banff and Debolt, Triassic Grayling and Toad Formations and Cretaceous sandstone Chinkeh and Besa River shales. With the lack of well control in the area seismic surveys can identify prospective targets.
Seven seismic lines were shot in the Liard Basin region of north east BC and south east Yukon (see Figure A1 – Base map). These seismic lines helped distinguish regional stratigraphy. The Paleozoic limestones have a higher P-wave velocity than the Cretaceous siliclastic deposits and imaged both the younger decollement faults associated with the Laramide Orogeny (Figure A3 – Line 84-2) and older Bovie Lake Fault (see Figure A2 – Line 82-11, Figure A4 – Line 84-3).
Three time structure maps (see Figures A5, A6 and A7) and two isochron maps (Figure A8 and A9) were produced from the seven seismic lines. These maps depict where the thicker Cretaceous clastics can be distinguished from thinner Cretaceous clastics and where the lower Paleozoic carbonate strata dominant.
Interpretation and Results
The Bovie Lake Fault can be seen on seismic data and cuts the Proterozoic to the Mississippian strata of the basin (see Figure A2 – Line 82-11, Figure A4 – Line 84-3). The Cretaceous strata that uncomfortably overlies the Paleozoic limestones are deformed and folded by smaller decollement faults associated with the Laramide Orogeny (Figure A3 – Line 84-2). The location of the fault terminates the eastern edge of the Liard Basin from the western margin of the Horn River Basin (Dixon 1997). The Bovie Lake Thrust Fault also separates the siliclastic strata, starting with the Mississippian Banff Shales from lower Paleozoic limestone deposition.
The Bovie Lake Fault can be interpreted on seismic surveys. The interpretation of seismic data suggests the fault developed in a two stage process relating to the Laramide Orogeny and shallow decollement faulting.
The centre portion of the fault zone is interpreted to be an anticline striking north-south. Further to the north the Bovie Lake Fault diminishes in offset and is less affected by the decollement faulting. The Seismic Survey data shows the extensive nature of faulting in the area (Dixon 1997).
Siliclastic strata have lower velocity on seismic surveys than carbonate deposition. The time structure maps created show that to the west of the Bovie Lake Fault lower velocity deposition indicating areas of Cretaceous siliclastics with reservoir unit potential. The time structure maps also help identify the area of dolomitized carbonates, due to very high velocity, that represent an exploration fairway, as the area of dolomitization is believed to be associate with the intersection of the Bovie Structure and Trout Lake Fault (Morrow et al 2005).
The isochrons maps created from the seven seismic lines depict thicker deposition to the west and south west of the Bovie Lake Fault and an anticline hinge line representing thicker Cretaceous strata and better reservoir targets (See Figure A10 – Seismic Location Map).
With increasing interest in the conventional and unconventional hydrocarbon potential of Liard Basin and very little well control the use of seismic interpretation methods can help identify the areas of high prospectivity from the lower areas of interest. By imaging the Bovie Lake Fault and Bovie Lake Thrust Fault and creating time contour and isochron maps the edge of the basin can be determined, the lower velocity siliclastic deposits can be distinguished from the higher velocity carbonates and the areas of thicker Cretaceous deposition can be isolated from the areas of thinner deposition. By understanding these factors the different possible traps, structural for the Paleozoic carbonates and stratigraphic for the Mississippian and Cretaceous siliclastics can be determined. By identifying areas of high hydrocarbon potential without sufficient well control exploration cost and time can be reduced.
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