Abstract
Bar top hollows (BTHs) are morphological elements recognized in modern braided rivers; however, information regarding their depositional features and types of filling units in ancient strata is unclear. This is an important reason behind why it is difficult to identify BTH units in subsurface reservoirs. A Middle Jurassic dryland sandy braided river outcrop in northwestern China is characterized in this study through the application of an unmanned aerial vehicle (UAV) surveying and mapping, and ground penetrating radar (GPR). A workflow of UAV data processing has been established, and a 3D digital outcrop model has been built. By observation and measurement of the outcrop model and GPR profiles, two types of BTH filled units were found: (a) sandstone-filled, and (b) mudstone-filled hollows. Both of these units were located between two adjacent bar units in an area that is limited to a compound bar, and were developed in the upper part of a braided bar depositional sequence. The ellipse-shaped, sandstone-filled unit measures 10 m × 27 m in map view and reaches a maximum thickness of 5 m. The transversal cross-section across the BTHs displays a concave upward basal surface, while the angle of the inclined structures infilling the BTHs decreases up-section. The GPR data show that, in the longitudinal profile, the basal surface is relatively flat, and low-angle, inclined layers can be observed in the lower- and middle part of the sandstone-filled BTHs. In contrast, no obvious depositional structures were observed in the mudstone-filled BTH in outcrop. The new understanding of BTH has a wide application, including the optimization of CO2 storage sites, fresh water aquifers exploration, and oil and gas reservoir characterization.
Highlights
Sandy braided rivers are often attractive as oil and gas reservoirs, as they have a high net-to-gross sand ratio, and have always been of interest to geographers, geomorphologists, and sedimentologists [1,2,3,4,5,6,7,8,9]
The present geomorphology was observed during the fieldwork to have a good correspondence with the lithology, depositional types, and forms of paleo-sedimentary units (Figure 5)
ground penetrating radar (GPR) survey reveals the reservoir bar top resulted in an uneven top of the hollow filled mudstone
Summary
Sandy braided rivers are often attractive as oil and gas reservoirs, as they have a high net-to-gross sand ratio, and have always been of interest to geographers, geomorphologists, and sedimentologists [1,2,3,4,5,6,7,8,9]. In addition to sandy architecture elements, braided river systems deposit fine-grained sediments [19]. Best (2006) proposed BTH, a new type of architecture element, in a study of the modern South Saskatchewan River [21]. The reservoir architecture of the Jurassic sandy braided river outcrops in northwestern China is investigated using rock outcrop observations, GPR detection, and UAV mapping. In the last ten years, GPR and UAV have been widely used in geological study and the mining industry as useful tools in the characterization of outcrops, modern deposition and in the mapping of mining areas [22,23,24,25,26]. A workflow of integrated outcrop study with UAV mapping, GPR detection, in lab test of rock samples, and field observation is introduced, and a new understanding of bar top hollows is proposed
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