Lithofacies characteristics and sedimentary model of a gravelly braided river-dominated fan: a case study of modern Poplar River alluvial fan (northwest Junggar Basin, China)

Abstract

Conglomerate reservoirs in alluvial fans commonly contain economic crude oil, but it is challenging to depict the distribution of conglomerate reservoirs or to summarise the sedimentary model in alluvial fans deposits owing to their complex lithofacies changes and variable hydrodynamic conditions. This paper focuses on the Poplar River alluvial fan in the northwest Junggar Basin, China, as a special case to analyse its sedimentary processes and to build a sedimentary model through detailed outcrop mapping and grainsize analysis. The Poplar River alluvial fan is a braided river-dominated fan with the characteristics of a large fan size (approximate 327.6 km2), gentle slope (∼<0.04–0.07°), coarse sediments (mainly cobbles or finer pebbles) and rare fine-mud sediments deposited from floods. In this alluvial fan, 15 lithofacies are identified that mainly formed in five hydrodynamic environments: (i) debris-flow deposits; (ii) high-flow traction current deposits; (iii) low-flow traction current deposits; (iv) still-water deposits; and (v) eolian deposits. Moreover, the sedimentary processes on the Poplar River alluvial fan can be divided into flood and interflood periods. During flood periods, sediment deposition plays a major role on the alluvial fan. From the proximal to distal parts, six lithofacies associations can be identified: feeder-channel and sheet-flow deposits in the proximal part, sheet flood and braided-stream deposits in the intermediate part, and channel deposits and wetland deposits in the distal fan areas. During the interflood periods, sediment reworking is the most important function, and three lithofacies associations are identified: main-channel deposits in the proximal fan, braided-trench deposits in the intermediate fan and wetland deposits in the distal area. The braided-stream deposits, braided-trench deposits and sheet-flood deposits have the best potential for high-quality aquifers or oil reservoirs based on their moderately well-sorted grains and high primary permeable intervals. KEY POINTS Fifteen lithofacies and eight lithofacies associations are identified in the Poplar River braided river-dominated alluvial fan. The aggradation process of the studied alluvial fan can be divided into flood and interflood periods. The flood-period braided-fluvial deposits, interflood main-channel and braided-trench channel deposits are the facies with best reservoir potential.