No abstract

No abstract

No abstract

With the exploration of tight oil and gas, shallow-water deltaic reservoirs have been attracting more and more attention. The sedimentary architecture of a shallow-water delta shows distinctive differences with that of a deep-slope delta. These differences may be associated with the mechanism and characteristics of the deposition in the area where the sediments unloaded. Based on modern sedimentary research of the Poyang Lake in China, this paper focuses on the processes of river flow entering a lake with a low dip angle. We conducted six sets of numerical simulations with different initial sedimentary flow velocities using Fluent software for analyzing the hydrodynamics and the sediment transportation in the shallow-water delta. We combined the simulation results with an analysis of the geomorphology of the Gangjiang Delta to reveal the deposition along the shoreline of the lacustrine shallow-water delta. The numerical simulation shows that the shallow-water delta is dominated by bed friction with an extensive hydrodynamical boundary layer. The bed shear stress, which varies with the changes in river flux, dominated the sediment transport and deposition at the shallow-water delta front, where the effluent flow mixes with lake water. The distributary channels show characteristics of repeatedly occurred erosion, scouring, filling, and reoccupation. We argue that the depositional characteristics are associated with the changes in bed shear stress controlled by variation of flow velocity. Mouth bars are less likely to grow to a reasonable scale because of the seasonal scouring of extreme floods. Moreover, the lake flow potentially reworks the mouth bars. Consequently, mouth bar deposits were difficult to preserve as hydrocarbon reservoirs in ancient shallow-water delta.

The reservoir architecture analysis of braided rivers, especially falling-silt seam forms, has played a key role in predicting remaining oil distributions. However, no studies have used architecture analyses that document braided river outcrops and researched the tapping of the few remaining oil distributions based on outcrops in the Songliao basin, northeast China. In this paper, the architecture characteristics and remaining oil distribution of braided river reservoirs are studied using a combination of an outcrop, modern deposition and subsurface well data. The new 8–13 m thick Lower Cretaceous Quantou Formation outcrop of the Songliao basin is a braided fluvial succession arranged in one large fining-upward cycle. Eight facies (Gt, St, Sm, Sh, Sp, Sw, Fl and Fm), four architecture elements (CH, DA, LV, and FF), and three orders of bounding surfaces (third-, fourth-, and fifth-order) are recognized. A new distribution pattern of falling-silt seams and a braided river architecture model are presented according to the analysis of the outcrop. In the mid-channel bar, the falling-silt seams thin from the mid-bar to the bar tail following the flow direction. Each falling-silt seam is oriented tangentially to the basal surface of the mid-channel bar, and the upper falling-silt seam extends farther than the lower one. In a Daqing Oilfield exploitation block in the Songliao basin, while channels and bars are the main reservoir units, they have different remaining oil distribution patterns. For bars, water injection wells located at the mid-bar, zonal injection technology, the drilling of horizontal wells, and proper well patterns are proposed. Fourth-order bounding surfaces, single braided channels, stacking patterns, and the lateral blocking of levees and floodplains are the key factors affecting the remaining oil distribution in channels.