Sedimentary architecture of a sandy braided river with seasonal hydrodynamic variations: insights from the Permian Lower Shihezi Formation, Ordos Basin, China
. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266000, China . Research Institute of Exploration and Development, Sinopec North China Petroleum Bureau, Zhengzhou 450000, China
A sandy, braided river is a typical type of river that exists in ancient and modern alluvial plains and is inherent with significant seasonal water discharge variations. The variations play an important role in the depositional process and the formation of the sedimentary architecture of braided rivers. In this paper, a braided river outcrop along the Yellow River in Fugu is used to describe the effects of seasonal hydrodynamic variations on braided river sedimentary architecture. The results show that the braided channel network exhibits two different patterns during flood period and normal period. During flood periods, the main braided channels surrounding channel bars and the secondary braided channels distributed on the top of the channel bars coexist, forming a highly braided channel network. Migration of the main braided channels control the formation of middle channel bars and side bars. The generation and evolution of the secondary braided channels reformed the upper part of preexisting channel bars and produced affiliated bars along their flow path. During the normal period, water levels decrease, causing the secondary river channels to be abandoned and forming abandoned channels, and only the main braided channels stay active. In the long term sedimentation process, strong water flow during the flood period continuously erodes pre-existing sediments and forms new sediments, while weak water flow during the normal period can only reform the main braided channels and their adjacent channel bar sediments. Based on differences in sedimentary processes and associated hydrodynamic conditions, braided river sediments are divided into two combinations. The strong hydrodynamic combination includes main braided channels, middle channel bar, and side bar, while the weak hydrodynamic combination includes secondary braided channels, abandoned channels, and affiliated bars. The proportion of strong hydrodynamic combinations is much larger than that of weak hydrodynamic combinations. Based on this, we construct a braided river sedimentary architecture model that is helpful for the fine characterization of subsurface oil and gas reservoirs.
Online First Date: 23 August 2024Issue Date: 29 September 2024
Cite this article:
Xiaohui LI,Yuliang SU,Guanglei REN, et al. Sedimentary architecture of a sandy braided river with seasonal hydrodynamic variations: insights from the Permian Lower Shihezi Formation, Ordos Basin, China[J]. Front. Earth Sci.,
2024, 18(3): 671-682.
Fig.1 (a) Sedimentary facies diagram of Lower Shihezi Formation in Ordos Basin. (b) Digital Outcrop Model and formation exposure of He8 member. The outcrop is about 1400 m, and the stratum slope is 4°. The outcrop was divided into 3 long profiles (A-B, C-J, and G-H). (c) Stratigraphic framework of He8 member. M. means member unit, L. means layer unit within a member unit. The text in (c) marked from left to the right as M, S, F, M, C, and G means mud, silt, fine sand, medium sand, coarse sand, and gravel sediments.
Fig.2 The whole view of the Tianshengqiao outcrop (a), the A-B profile of the outcrop (b) and the partial image of the A-B profile. The boundaries of the architectural elements at different levels and the sedimentary structures inside the architectural elements are clearly observed.
Fig.3 Typical lithofacies types of braided river deposits of He8 member. (a−b) Massive sandy conglomerate (Gm1 and Gm2); (c) parallel bedded sandstone (Sh); (d−e) trough cross-stratification sandstone (St1 and St2); (e) small trough cross-stratification sandstone (St2); (f−g) tabular cross-stratification sandstone (Sp1 and Sp2); (h) massive sandstone (Sm); (i) massive mudstone (Mm).
Lithofacies
Code
Sub-lithofacies
Code
Lithology
Interpretation
Massive sandy conglomerate
Gm
Massive sandy conglomerate, lag deposits
Gml
Fine- to medium-grained sandy conglomerate
Lag deposits at the bottom of channels. Gravel particle size is 0.2−1.5 cm, massive bedding. Formed under strong hydrodynamic conditions (Fig.3(a)).
Massive sandy conglomerate, non-lag deposits
Gm2
Fine- to medium-grained sandy conglomerate
Non-lag deposits within channel bar. Gravel particle size is 0.5−1.0 cm, massive bedding or large-scale trough cross-bedding. Formed under strong hydrodynamic conditions (Fig.3(b)).
Parallel bedded sandstone
Sh
Parallel bedded sandstone
Sh
Medium-to coarse-grained sandstone
Commonly observed in the middle and upper parts of large channels, middle channel bars and side bars. Formed under strong hydrodynamic conditions (Fig.3(c)).
Trough cross-bedded sandstone
St
Large trough cross-stratification sandstone
St1
Medium-to coarse-grained sandstone
Typical channel deposits, mainly found in the middle and lower parts of large-scale braided channel deposits. Formed under strong hydrodynamic conditions (Fig.3(d)).
Small trough cross-stratification sandstone
St2
Medium-to coarse-grained sandstone
Typical channel deposits, mainly found in the middle and lower parts of small-scale braided channel deposits. Formed under relatively weak hydrodynamic conditions than that of St1.
Tabular cross-bedded sandstone
Sp
Large tabular cross-stratification sandstone
Sp1
Medium-to coarse-grained sandstone
Formed in inter-river areas with strong hydrodynamic conditions and open terrain. Mostly observed in the lower to middle parts of the channel bars and side bars and upper parts of large channel deposits (Fig.3(f)).
Small tabular cross-stratification sandstone
Sp2
Medium-to coarse-grained sandstone
Formed in inter-river areas with weaker hydrodynamic conditions than that of Sp1. Mostly observed in the middle to upper parts of the channel bars, side bars and affiliation bars and the upper parts of small channel deposits (Fig.3(g)).
Massive sandstone
Sm
Massive sandstone
Sm
Medium-to coarse-grained sandstone
Formed by the rapid unloading of sediments under strong hydrodynamic conditions, mostly found in the main channel, middle channel bar and side bar deposits (Fig.3(h)).
Massive mudstone
Mm
Massive mudstone
Mm
Mudstone
Formed on open floodplains with mainly muddy sediments (Fig.3(i)).
Tab.1 Summary of the characteristics of lithofacies observed on the Tianshengqiao outcrop
5th-level element
4th-level element
3rd-level element
Thickness/m
Relative hydrodynamic
Lithofacies association
Roughclassification
Detailedclassification
River bedform
Braided channel
Main channel (Chm)
Lateralaccretions
8.0−15.0
Very Strong
Gm1, St1, Sm, Sh, Sp1
Secondary channels (Chs)
Lateralaccretions
1.5−5.0
Weak
Gm1, St2, Sp2, Sh
Abandoned channel (Cha)
Centripetalaccretions
1.5−4.0
Weak
Gm1, St2, Sp2, Sh, Mm
Channel bar
Middle channel bar (Bm)
Downstreamaccretions
9.0−15.0
Very strong
Gm2, St1, Sp1, Sp2, Sm, Sh
Side bar (Bs)
Lateralaccretions
4.0−10.0
Strong
Gm2, St2, Sp1, Sp2, Sm
Affiliated bar (Ba)
Lateralaccretions
1.5−3.5
Weak
Sp2
Overbank deposition
Overbank deposition
Overbank Deposition (O)
Horizontalaccretions
0.2−3.5
Weak
Mm
Tab.2 Architecture elements of a braided river
Fig.4 Sedimentary architecture of the braided river on profile A-B of the Tianshengqiao outcrop, profile location shown in Fig.1(b). (a) Image and architectural boundaries of profile A–B, (b) sedimentary architecture interpretation of the A–B profile, (c) inferred sedimentary architecture interpretation of the A–B profile.
Fig.5 Sedimentary characteristics of the main channel (Chm). (a) Image of a Chm on the Tianshengqiao outcrop, (b) architecture and lithofacies distribution of the Chm, (c) horizontal architecture of the Chm in a braided river.
Fig.6 Sedimentary architecture of the braided river on profile G-H of the Tianshengqiao outcrop, profile location shown in Fig.1(b). (a) Image and architectural boundaries of profile G-H, (b) sedimentary architecture interpretation of the G-H profile.
Fig.7 Sedimentary characteristics of the secondary channel (Chs) and the abandoned channel (Cha). (a1) Image of a Chs on the Tianshengqiao outcrop, (a2) architecture and lithofacies distribution of the Chs, (a3) horizontal architecture of the Chs in a braided river, (b1) image of a Cha on the Tianshengqiao outcrop, (b2) architecture and lithofacies distribution of the Cha, (b3) horizontal architecture of the Cha in a braided river.
Fig.8 Sedimentary characteristics of the middle channel bar (Bm) and the side bar (Bs). (a) Architecture of Bm and Bs, (b) horizontal architecture of the Bm and Bs in a braided river.
Type
Channel width/m
Channel belt width/m
Max
Min
Avg
Max
Min
Avg
High energy channel
764.5
45.6
206.5
4929.6
301.8
1345.7
Low energy channel
26.7
2.5
8.5
177.6
17.3
57.1
Tab.3 Quantified geometric properties of the sandy braided river from He8 outcrops in the study area
Fig.9 Schematic diagram of the architecture element combinations. Combination under strong hydrodynamics in flood periods, occupying more than 90% of the braided river. It contains the following elements: Bm, Bs, CHm, and OF (OF is concomitant with strong hydrodynamics when flowing over the bank, not formed in it). Combination under weak hydrodynamics occurring at the tops of unit bars (Bm or Bs). It contains the following elements: CHs, CHa, and Ba.
Fig.10 The sedimentary architecture model and flow pattern of a sandy braided river in flood and normal season. (a) sedimentary architecture and flow pattern in flood season, (b) sedimentary architecture and flow pattern in flood season.
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