Scaling and Hierarchy in Braided Rivers and their Deposits: Examples and Implications for Reservoir Modelling

Abstract

Globally, there are numerous significant hydrocarbon accumulations within braided fluvial reservoirs. Building successful subsurface models of braided fluvial reservoirs requires the inclusion of information from a wide variety of sources, including appropriate analogues (both modern and ancient) as well as specific field data. In addition to field data (primarily from boreholes), many published static modelling strategies for fluvial reservoirs use data collected from modern rivers or two-dimensional outcrops to infer three-dimensional reservoir architecture and geometry. However, there has been little attempt to evaluate the statistical significance of these various datasets or how they may be linked. The approach advocated in this paper utilizes statistical aspects of both modern rivers and ancient fluvial deposits such that the properties of analogue datasets allow their inclusion into reservoir modelling routines in an informed manner. This approach allows estimates of reservoir geometry at a variety of scales utilizing limited datasets such as are typically available in the subsurface. The scale invariant characteristics observed in fluvial systems and their deposits suggest that empirical predictions of key variables such as braid bar width and length can be made if additional data are known, or can be reasonably estimated (such as channel depth). In addition, confident observations of one level of reservoir hierarchy (e.g. bedsets) can allow the prediction of attributes of other levels of hierarchy (e.g. storeys) that may be more difficult to observe directly but are potentially significant for flow behaviour and reservoir performance.

Bar deposition in glacial outburst floods: scaling, post-flood reworking, and implications for the geomorphological and sedimentary record

“…Bar C was the only bar for which the position was controlled by mid-channel flow convergence (with 'channel width' defined by the extent of deeper flow between submerged unit bars), and even so, the extent of bar C was strongly controlled by the location of the adjacent bar B. The three bars have an average length-width ratio of 2.3, which is at the lower end of the range of dimensions reported for other rivers (Sambrook Smith et al, 2005;Kelly, 2006). This relatively short length may reflect the formation of the bars in only one flood.…”

“…This gives an average bar fragment length width ratio of 5.02, compared with the primary bar average of 2.3. Reworked bar fragments are therefore more than twice as long as the original bar on average, and at the upper limit of the range for length-width ratios of braid bars (Sambrook Smith et al, 2005;Kelly, 2006). No bar fragment was observed with a shorter lengthwidth ratio than the original bar.…”

“…Unit bars have length to width ratios of 5-5.5:1, whilst compound bars have length to width ratios of three to 3-3.5:1 (Lunt et al, 2004;Rubin et al, 2006). Using large datasets obtained from aerial photographs and field data, Sambrook Smith et al (2005) and Kelly (2006) obtain real-world average length to width ratios of 2.97 and 3.76 respectively, suggesting that real world 'braid bars' are mostly compound bars. The theory underlying the scaling of barforms in rivers is derived from work on pool-riffles and meandering rivers (Carling and Orr, 2000).…”

“…A partir de l'analyse de données de terrain et photo-interprétées, Sambrook Smith et al (2005) (Lunt et al, 2004). Lunt et al (2004) et Rubin et al (2006 (Sambrook Smith et al, 2005 ;Kelly, 2006 (Marren, 2002b(Marren, , 2005 …”

Geometry and compartmentalization of fluvial meander-belt reservoirs at the bar-form scale: Quantitative insight from outcrop, modern and subsurface analogues

“…Lynds & Hajek 2006;McKie 2011), it must be noted that the morphometry and scaling relationships of bars, channel fills and channel bodies produced by braided rivers are fundamentally different from what is described here (cf. Gibling 2006;Kelly 2006;Colombera et al 2013;Holzweber et al 2014). …”