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Ambiguity of GoogleEarth Images as Shallow Marine Reservoir Analogs

Abstract

With GoogleEarthâ„¢ and simular collections of Earth images widely available it is now standard practice to select images of coastal environments based on inferred depositional process as templates for expected subsurface facies patterns (e.g., MPS training images). Although remote imagery can be enlightening: 1) Relationships between environments observed on the surface are generally not the same as those preserved in the rock record, 2) Most coastal deposition occurs under water during major river floods and storms; thus images show just emergent tops of moribund depositional bodies, 3) It can be difficult to define the age of depositional bodies and Holocene climate changes have left many relict features that do not reflect modern processes, 4) We live in an early highstand world, following a major sea level rise, when wave-reworked strand plains and estuaries in partially filled valleys dominate coastlines; our reservoirs may have formed in very different settings. To illustrate difficulties defining modern reservoir analog internal character remotely, examples are discussed where Earth images proved to be misleading templates of subsurface facies patterns. Images of the Danube River Delta are used to support the asymmetric wave-dominated delta facies model that predicts sandy deposits updrift and more heterolithic deposits downdrift of a distributary channel. Assigning facies by location on a previously published time series of bathymetric maps and projecting the preserved facies horizontally though time between these surfaces led to an alternative prediction that resulting deposits are a symmetrical body with facies that become more wave-dominated down drift. The previous model focused on depositional environments of a thin surface veneer that does not reflect likely variations within the larger reservoir body. Similar considerations of barrier islands suggest that in many cases preserved deposits are not wave-dominated shoreline parallel sands (but rather tide-dominated tidal inlet deposits), the dimensions and shape of river-dominated mouth bar sands are poorly defined by their emergent parts, and the abundance of distributary channels may be controlled more by avulsion frequency than the number of coeval channels. These examples highlight the importance of considering the surface topography and shoreline trajectory when inferring reservoir facies relationships, rather than assuming patterns from static surficial images of depositional environments.