Architectural Elements and Heterogeneity of Braided Fluvial Deposits, Middle Jurassic, Datong Basin, Northern China

Li, Shunli; Yu, Xinghe; Olariu, Cornel; and Steel, Ron J.
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The Yungang Formation of Middle Jurassic in the northwest of Datong Basin, northern China, is a set of typical sandy braided fluvial deposits. Outcrops of reddish coarse-grained sedimentary successions occur widely throughout the study area due to river and road cutting. Photographic mosaics, high resolution measurements and depictions of six outcrop sections nearby Yungang Rock Cave, northwest Datong, allowed the identification of architectural elements including channel (CH), unit bar (UB), compound bar (CB), downstream accretion (DA), laterally accretion (LA), hollow (HO) and overbank fills (OF). With developing architectural analysis and identifying different hierarchy of bounding surfaces, this study reconstructed stacking patterns of fluvial deposits, and tried to reveal the relationship between architectural elements and their reservoir characteristics based on petrophysic data in the braided channel-bar systems.

In micro-scale range, the porosities of different lithofacies are associated directly with the grain size. The coarser of grain size, the higher of porosity, and vice versa. The variations of permeability depend on particle arrangement, which coordinate to the paleo-currents, and the largest permeability parallel to the direction of paleo-flow in channels. At the channel-bar scale, isolated barriers like erosional surfaces and mud layers subdivided the multistory of channel deposits into different fluid-flow units. The porosity and permeability of channel elements decrease gradually from their center to the margins, lower and upper intervals. While the bar elements with less heterogeneity have better reservoir properties and decrease gradually down to the lower part interval due to the finer of grain size.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013