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Stratigraphic Evolution of an Ancient Channel-Lobe Transition Zone (CLTZ) in the Windermere Turbidite System and Implications for Reservoir Development

Abstract

Deep-water strata of the Neoproterozoic Kaza Group (basin floor) and Isaac Formation (slope) of the Windermere Supergroup in the southern Canadian Cordillera were deposited in a Neoproterozoic passive-margin basin. Two study areas separated by ~20 km (dip direction) expose a continuous, 290 m thick, stratigraphic interval that records the transition from basin floor to slope deposition. Although deep-water channel-lobe transitional zones (CLTZ) have been described from ancient and modern deep-marine settings, due to their intrinsic lithological heterogeneity and architectural complexity they remain underappreciated and poorly understood. In the two study areas, two distinctive stratal assemblages stack to form three sharply-bounded channel-lobe systems, which are interpreted to be either detached or attached. Detached systems are part of a well-developed CLTZ and characterized by common small (up to few m deep, several 100s m wide) and few large (up to 10s m deep, km wide) scours intercalated within thick succession of mudstone (i.e. thin-bedded, fine-grained turbidites). Nested and vertical-stacked distributary channel fills are observed also. Being dominated by fine-grained strata with some isolated sand-filled channels and scours, detached systems have limited reservoir potential. In contrast, attached systems, in which the CLTZ is poorly-developed or absent, are thicker (almost 2x) and comprise numerous small scours intercalated with proximal basin-floor elements, including sand-rich distributary channels and less common splays separated by mudstone-rich deposits, which collectively are then overlain by slope leveed channel deposits. Many of the decameter-thick, sandstone-rich stratal units in this system would have good reservoir potential with competent traps and seals. The stacking of detached and attached channel-lobe systems in the Kaza-Isaac succession indicates that the CLTZ evolved through time, which in large part is linked to major changes in sediment supply and sedimentation style caused by changes in relative sea level. Detached systems were formed by highly mobile flows that mostly bypassed the CLTZ and deposited further downflow. These conditions coincide with falling and later highstand of relative sea level. Conversely, attached systems reflect less mobile flows that preferentially deposited in the proximal parts of the system, and mark the lowstand to ensuing transgressive system tracts.