Sedimentary architecture of shallow-water fan-delta front in a lacustrine basin: Sangyuan section of Lower Cretaceous Xiguayuan Formation, Luanping Basin, northeast China

The sedimentary architecture of fan deltas, which commonly constitute reservoirs, is a primary control on distribution and recovery efficiency of oil and gas. Deep-water fan deltas have been extensively discussed in the literature, whereas the sedimentary architecture of shallow-water fan-delta fronts, especially the characterization of mouth bar, quantification of architectural elements, and factors controlling the various architectural elements, remain poorly understood. Here, based on the integration of field survey and unmanned aerial vehicle (UAV) observation, the sedimentary architecture of Sangyuan section of the Lower Cretaceous Xiguayuan Formation in the Luanping Basin to is qualitatively and quantitatively characterized. Furthermore, factors controlling the architecture of the shallow-water fan-delta front are highlighted. Five facies associations, which differ in both lithofacies and dimensions, were interpreted in gravel-sand deposits of Sangyuan section, and indicate variations in flow conditions during deposition. The width/thickness ratio of facies associations generally increases rapidly as the flow energy and flow concentration decreases from debris flow to traction currents during the evolution of shallow-water fan delta. Both distributary channels and mouth bars dominate facies associations in the shallow-water fan-delta front, accounting for 53.42% and 36.88% of gravel-sand deposits respectively in Sangyuan section. Mouth bar consists of gravel accretions and sand accretions. Grain size of sediments influences the relative strength of the inertial force and bed friction, which determine the effluent behavior in a river-mouth system and the vertical grain-size trend of mouth bar accretions. Gravel accretions are characterized by normal grading, and these are interpreted as the products of inertia-dominated effluent. Sand accretions exhibit coarsening-upward trends in the river-mouth systems where friction-dominated outflow occurred. The overall vertical grain-size trend of mouth bar depends on the stacking pattern of accretions rather than the grading of one accretion.