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Sedimentary characteristics and genesis of a sandy, topset-dominated braided river delta from Huangqihai Lake were investigated using trenching and a ground penetrating radar survey. Ten lithofacies types were identified with assistance of grain size distribution to record a broad range of depositional processes within an overall coarsening upward sequence. Four distinct architectural elements, including channel fill, compound bar, sand sheet, and river mouth bar, built up this delta. No obvious foreset can be identified in this braided river delta. A gentle slope (0.2°) and shallow basin (<10 m deep), young age (~25 years), and low sediment supply caused by human disturbance and semi-arid climate, are believed to allow this type of topset-dominated delta to form. [ABSTRACT FROM AUTHOR]

Abstract: Although there are widespread indications of oil and gas in the Chang 6–4+5 layer of western Mahuang Mountian, well test proves that there is more water and less oil with the distribution and continuity of sand being not yet clear. To understand the distribution and evolution of sedimentary microfacies in the area, six kinds of sedimentary microfacies and ten kinds of sedimentary structures were identified, and ten patterns of contact between microfacies and five kinds of sedimentary sequence model were summarized through detailed observation and description of the core of rock and logging data. Each model represents different sedimentary facies. Reversed cycle is dominant and normal short-term cycle also exists in the vertical direction; Mouth bar and subaqueous distributary channel developed alternately in the lateral direction. The lower channel sand is “large in quantity but small in size with the layer narrow and thin”, whereas the upper sand is “few in quantity but large in size with a wide and thick layer”. The “single-factor analysis and multifactor comprehensive” research method was used to divide this area into three phase belts—delta inner front, delta outer front, and predelta. Subaqueous distributary channel, distributary mouth bar, and interdistributary deposits are prevailing in the delta inner front. Distal bar and sand sheet are dominant in the delta outer front, and predelta mud is dominant in the predelta. According to modern sedimentary facies model, the area is a constructive braided river-lake delta. Three stages appeared in its evolution: the initial development period (Chang 63), in which the predelta and the delta outer front are the main facies belts; the early-middle development period (Chang 62–Chang 61), in which the inner delta front and the delta outer front dominates and each micro-facies is a single phase and is deposited alternately; the middle-late development period (Chang 4 +52–Chang 4 +51), in which the delta inner front dominates and subaqueous distributary channel shows multiphase stacking pattern. [Copyright &y& Elsevier]

The article presents a study which aims to determine the Cretaceous shallow- water delta in the Songliao Basin, China. The study suggests that the shallow-water deltas were developed commonly in lake basins under a comparatively stable tectonic setting, shallow water depth, gentle slope having low subsidence rate, a frequently fluctuating lake level, open lake basin characteristics and sufficient sediment supply.

With the aid of modern sedimentology, seismic geomorphology, and the comprehensive analysis of geophysical data and core data from more than 10 wells, the Qingshankou Formation has been divided into 3 long-term cycles and further into 6 medium-term cycles in the Qijia area, Songliao Basin. The 3 long-term cycles, SQ1, SQ2 and SQ3 correspond to the members Qing I, Qing II and Qing III of the Qingshankou Formation, spanning 11.5 Ma during the Late Cretaceous. The time scale of the medium-term cycles, which are subunits of the long-term cycles, is approximately 0.5–1.5 Ma with thicknesses ranging from 45 m to 116 m. A shallow-water delta that consists of high-maturity fine-grained sandstone developed in the Qingshankou Formation. The scour surfaces reflect strong flow action, and the parallel bedding and large-scale cross bedding are well developed. Distributary channel sandstones are well developed in the shallow-water delta depositional system, in which a single channel can be several meters thick. The cumulative thickness of the distributary channels can reach tens of meters across a large area, extending tens of kilometers to the center of the lacustrine basin. The development of the subaqueous distributary channel of the shallow-water delta is closely related to base level (lake level) changes. In the early stage of a rising half-cycle and the late stage of a falling half-cycle, the fluviation is significant and the well-developed channel sandstone can extend towards the center of the lacustrine basin more than 50 km, with large cumulative thicknesses of compound sandbodies (17–28 m) and high sandstone-to-strata ratios (78–88%). In the late stage of a rising half-cycle and the early stage of a falling half-cycle, the lake level is high and the fluviation is weak, resulting in a distributary channel extent of less than 30 km, small cumulative thicknesses of compound sandbodies (5–12 m) and low sandstone-to-strata ratios (22–45%). The delta front can be divided into the inner front and the outer front. The subaqueous distributary channels in the inner front have good continuity and extend further, whereas the subaqueous distributary channels in the outer front have poor continuity, and sheet sand is well developed. Therefore, a shallow-water delta generally shows features including (1) sedimentary structures formed by strong currents, (2) stacked fining upward rhythm, (3) subaqueous distributary channels well developed, (4) delta front divided further into an inner and an outer front. There was a relatively good linear relationship between the thickness of the sandbody based on the well data and the maximum amplitude of the seismic trace next to the well. The reddish amplitude of the seismic attribution suggests thicker channel sandbodies (4–9 m), whereas the light amplitude of the seismic attribution indicates thinner channel sandbody (1–4 m). Of the two typical sedimentary models for shallow-water deltas, delta plains develop well in humid climates and delta fronts develop well in arid climates, both of which are found in the Qijia area, Songliao Basin. Stratal slices from seismic geomorphology are applied to display the branchlike sandstone dispersal patterns of the shallow-water delta and the broad distribution characteristics. This study also notes that the delta front distributary channel sandstone, which is near an oil source and faults, would be a favorable exploration target. [ABSTRACT FROM AUTHOR]