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Preservational Complexity and Completeness in Channel Point Bars and the Heterogeneity of Heterogeneity in Their Reservoir Models

Abstract

Point bars tend to generate sandy lobate reservoir units that fill channels laterally and serve as primary development targets in both conventional and unconventional fluvial plays. Initial models for point bar growth build upon the presumption of periodic shingling of the convex inner channel bend with sheet-form sand layers that cover much of the wetted bend surface. Episodic and repetitive sheet addition causes the channel to migrate in expansional or translational vectors and produces sandy bodies partitioned with regularly spaced, gently dipping, bar-extensive, and sometimes draped accretion surfaces that record the channel form and resemble large cross sets. This results in reasonably predictable and easily modeled reservoir architectures. While field evidence argues that this fundamental modern process and rock product do occur in some approximation, an accumulation of additional field evidence argues that this process is not alone. At least three other processes also produce point-bar forms, and each of these processes preserves contrasting internal reservoir architecture. These processes are fragmentary bar accretion, counter point bar accretion, and mid-channel bar accretion. Fragmentary bar accretion results from high-frequency deposition of small unit bars over only limited areas of the wetted bar surface, commonly followed by dissection and erosional reshaping of the bar surface and local draping. This results in a bar deposit formed of highly fragmented reservoir units lacking through-going accretion sets and prone to unpredictable heterogeneity. Counter-point-bar accretion occurs by forced decoupling of the cut-bank flow shear and accretion along the cut-bank face. This produces concave accretion surfaces in strata typically much muddier and more heterogeneous than classic convex-accretion bars. Lastly, a lobe sandy body mimicking a true point bar may form in otherwise braided systems by preferential accretion of mid-channel bars to the inside bend of a braided river that meanders. These tend to form sets of amalgamated sandy mid-channel bars into point-bar shapes that have mounded accretion surfaces at various orientations. These surfaces may move reservoir fluid flow in erratic direction. Each of these forms are common, and each includes long internal hiatal surfaces that result in total bar accretion rates that are much slower than rates of short-term bar growth.