Palaeohydraulic reconstruction and depositional model of the episodic flooding channels developed in the modern arid alluvial fan: Implications for the exploration target of the heterogeneous alluvial fan reservoirs

Highlights

• The episodic flooding channels are ribbon-shaped and characterized by the absence of floodplain and mudstone drapes.

• The episodic flooding channels predominately show a distributary pattern and may disappear on fan surface.

• The episodic flooding channels can be high-potential targets for hydrocarbon exploration within alluvial fan reservoirs.

• The evolution model of the episodic flooding channel provides guidelines for alluvial fan reservoir characterization.

Abstract

Alluvial fan reservoirs associated with episodic flood flows are generally heterogeneous, both laterally and vertically, owing to the variable depositional conditions and mechanisms. The complexity of sedimentary processes results in distinct challenges in terms of reservoir characterization and subsurface modelling. The ability to discern fan deposits in the subsurface directly impacts the assessment of the distribution, physical properties, and hydrocarbon recovery of reservoirs. Taking the Quaternary Poplar Fan as an example, this study aims to determine the characteristics, depositional model, and controlling factors of the episodic flooding channels developed in an alluvial fan through palaeohydraulic reconstruction. These research topics have rarely been studied and are poorly understood. The Poplar Fan is dominated by coarse conglomerates from apex to toe. In particular, lenticular gravelly and sandy flooding channel deposits can be identified throughout the fan and are easily distinguishable from the surrounding sheet-like conglomerates of the incised channel flood, sheetflood, and unconfined streamflood deposits in outcrops. Such flooding channels are characterized by the absence of fine-grained floodplain deposits or overbank deposits and the lack of in-channel mudstone drapes or plugs. This indicates the short-lived and episodic nature of the sedimentation processes. Individual channels are ribbon-shaped, with width/thickness ratios of less than 15:1, and show limited amalgamated characteristics. The estimated palaeoslopes of these flooding channels are all above the maximum gradient for modern typical rivers. In particular, the width, thickness, maximum particle size, calculated bankfull discharge, and stream power of these preserved channels all exhibit a decreasing downstream trend. Additionally, the channel width/thickness ratios increase initially and then decrease from the proximal to the distal fan. Accordingly, these episodic flooding channels predominately exhibit a distributary pattern in the plan view. Moreover, 70% of the estimated channel length is less than the maximum radial length of the fan, which suggests that these episodic flooding channels may gradually disappear downstream. Based on these outcrop data, a depositional model, together with an evolution pattern of the in-fan episodic flooding channels, was established. The variations in the water and sediment concentrations of the flood flow could be the major cause of the cyclic transformations between the episodic sheet-like and channelised flooding processes. Notably, compared with the sheet-like flood flow deposits, these episodic flooding channel deposits are much more organized in gravel arrangement, well-sorted, and more homogeneous, consequently having better physical properties and constituting a favourable exploration target. Moreover, the proposed cyclic evolution model of the episodic flooding channel can provide effective guidelines for stratigraphic correlation in characterizing heterogeneous alluvial fan reservoirs. The lithofacies and reconstructed palaeohydraulic parameters from the outcrops can also provide reasonable parameters and references for underground reservoir modelling and core analysis.