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Facies Distribution of Alluvial Fan: Implication for Genetic Types and Heterogeneity of Conglomeratic Reservoir

Abstract

Integrated studies of modern and subsurface alluvial fan systems provide potentially useful tools for understanding the characteristics of sedimentary distribution and reservoir heterogeneity of fan deposits. This paper provide a comparison of sedimentation between modern analogue and subsurface reservoir of alluvial fan deposits. Aoyukou fan, a Quaternary alluvial fan system located in northeastern Datong Basin, Northern China, was an analogue for studying genetic types and facies distribution of alluvial deposits. The Aoyukou fan located at fault zone of mountain frontier is dominated by debris-flow conglomerates. Proximal-distal sedimentary variation and depositional model were dissected from eleven trenches by recognition of gravelly sediments textures, grain size distribution and facies assemblages. Sediments within main channel of proximal fan which deposited by high density debris flows are mainly consist massive, matrix-supported, multiple-size conglomerates. Braided distributary bars which are comprised of clastic-supported conglomerates show coarsening upward of grain size. In middle fan, conglomerates commonly display stratified, imbricated structures with medium degree of sorting. The finer sediments of distal fan are primarily consist of massive sands, muds and minor mud-supported float conglomerates, which indicates the interaction between alluvial fan and lacustrine system. Mahu Depression in Junggar Basin, Northwestern China, is a significant hydrocarbon accumulation area. The major reservoir within Mahu Depression is Baikouquan Formation, Lower Triassic, which was developed from massive, thick conglomerates of alluvial fan systems. Based on grain shape, arrangement, support type, fabric and sedimentary structures, nine gravelly lithofacies were recognized from core observation which include mud-supported float conglomerate (Gmf), sandy-supported float conglomerate (Gsf), gravelly-supported float conglomerate (Ggf), clastic-supported homogeneous-size conglomerate (Gch), clastic-supported multiple-size conglomerate (Gcm), imbricated conglomerate (Gi), graded conglomerate (Gg), trough cross-bedding conglomerate (Gt) and planar cross-bedding conglomerate(Gp). The profitable reservoirs facies of Gch, Gp and Gt with better physical properties were deposited by braided distributary bars and channels at middle fan, which was implied from the modern analogue study.