Abstract：

In multistory sandstones, lithofacies near the architectural bounding surface vary due to the hydrodynamic changes, which affect the heterogeneity inside the sand bodies. Especially for braided fluvial deposits, the hydrodynamic changes are more frequent, and the lithofacies near the bounding surfaces are distributed rather complicatedly and unstably. Lithofacies are identified and divided by investigating the classical sandy braided river outcrops in Datong, Shanxi Province, and eight modes of lithofacies assemblages are summarized, which correspond to eight types of architectural elements. The modes of lithologic complexes near architectural bounding surfaces in study outcrops are divided and measured quantitatively based on photographic mosaics and interpretation of the outcrop sections. The genetic mechanisms of lithologic variation of different orders and different types are analyzed, the architectural models of braided fluvial deposits are summarized, and the sandy braided river depositional process is remodeled. The fifth order architectural bounding surface is defined as the surface of a braided channel. Therefore, the lithology near the fifth order bounding surface is the floodplain mudstone which is relatively thick and develops stably between different periods of braided channels. The fourth order architectural bounding surface is considered as the surface of mid-channel bars or single channel-fills. So the lithologies near the fourth order surfaces can be divided into three types: covering mudstone on the flank of bars, erosional muddy gravels on the bottom of bars, and abandoned channel-fill mudstone. The third order architectural bounding surface is defined as the surface of a single accretion inside mid-channel bars. Thus, the lithologies near the third order surfaces can be divided into four types. The first type is muddy siltstone, also known as fall-silt seam, which deposits during the last phase of a single period of vertical accretion due to the weakened hydrodynamics. The second one is the thin-layer erosional muddy gravel, which generally locates at the bottom of an accretion. The third one is the muddy gully-fill deposit, and the fourth type is the diagenetic cementation belt. The lithologies near the third order architectural bounding surfaces develop unstably, due to its thin thickness and lateral discontinuities.

Key words：sandy braided river; architectural elements; architectural bounding surface; lithofacies assemblage; Jurassic outcrop in the Datong basin

Received： 2018-01-19

Corresponding Authors： houjg63@cup.edu.cn

Cite this article：REN Xiaoxu, HOU Jiagen, LIU Yuming, CHEN Depo, ZHANG Xiangyu. Architectural characterization and a distribution model of lithology near the boundary surfaces of different orders in a sandy braided river-a case study from the Jurassic sandy braided-river outcrops in the Datong Basin, Shanxi Province. Petroleum Science Bulletin, 2018, 03: 245-261.

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