# Establishing a Geological Knowledge Base for Braided River Deltas Using Google Earth
## 3 Results and Discussion
### Construction of Geological Knowledge Base  
A geological knowledge base is critical for reservoir modeling and hydrocarbon exploration, providing statistical parameters and boundary conditions. For braided river deltas, knowledge base construction aids systematic sedimentary feature summarization and supports hydrocarbon exploration by offering quantitative foundations for sedimentary process simulation and reservoir prediction.  
1. Upper (Plain): The upper plain zone develops in proximal gentle slopes or plain environments, characterized by vertically stacked braided channels, controlled by stable hydrodynamic conditions and sustained sediment supply. Channel migration is dominated by vertical aggradation, forming thick, continuous, and well-connected sandbodies with stable thickness, commonly displaying a large-scale cross-bedding, scouring surfaces, and gravel lag deposits--features reflecting rapid accumulation under strong hydrodynamic forces. The sediment supply intensity is a core controlling factor, with the annual sediment input typically exceeding 10 million tons, primarily composed of medium-to-coarse sands of moderate maturity. The gravel content decreases with an increasing distance from the source. The channel width is significantly influenced by the sediment grain size: according to Google Earth measurements, typical widths range from 500 to 2000 m, with widths exceeding 3000 m in coarse-clastic supply areas. Individual channel sandbodies are 5-15 m thick, stacking vertically to form total thicknesses of 20-40 m. Logging curves display box-shaped or serrated box-shaped patterns, indicative of high-quality reservoir development.
2. Middle (Front): The middle front zone is characterized by the complex lateral migration of subaqueous distributary channels, exhibiting positive rhythmic changes in sandbody thickness from land to lake (decreasing sandbody thickness and increasing mud content). Channel morphology transitions from reticular to dendritic, developing microfacies such as mouth bars and distal sandbars, with sandbodies often separatedby interdistributary bay mudstones. Reservoir connectivity is directly influenced by channel spacing, sandbody thinning rate, and lateral migration frequency. Channel spacing is regulated by bifurcation angles: low-angle bifurcations (<45\({}^{\circ}\)) correspond to dense channel networks (100-300 m), while high-angle bifurcations (>60\({}^{\circ}\)) form isolated sandbodies (300-500 m). Sandbody thinning rates are < 1 m/km in gentle-slope areas (e.g., Lena River delta) and can exceed 3 m/km in steep-slope areas (e.g., Rufiji River delta), reflecting the gradient of the sediment supply energy decay. These parameters directly control reservoir connectivity and serve as key inputs for building geological models.
* Lower (Prodelta): The lower prodelta zone is located in the deep-water area of the lake basin, characterized by interbedded thin sheet sands and thick mudstones, with a weak provenance influence and dominance of fine-grained sediments. Sheet sands are distributed as lenticular or tabular strips, 1-5 m thick and covering 5-10 km\({}^{2}\) in the plan view, controlled by subtle paleotopographic undulations. Mudstones account for over 80% of the section, with individual beds 20-50 m thick, developing horizontal bedding and bioturbation structures. Carbon-oxygen isotopes indicate a lacustrine reducing environment, endowing an excellent sealing capacity.  
Key Parameter Analysis
* Wavelength and Bifurcation Spacing: Small-spacing areas like Poyang Lake (54% < 2000 m) form isolated lobes as stratigraphic traps.
* Amplitude and Sandbody Thickness: High-amplitude regions like Yukon-Kuskokwim delta correspond to thick sands but require caution for mudstone interlayers forming structural-lithologic traps.
* Angle and Palaeoslope: 30-60\({}^{\circ}\) angle concentrations (e.g., Lena River, 53%) indicate balanced depositional backgrounds favorable for structural-stratigraphic traps.