Abstract
The detailed characteristics and formation mechanisms of organic-rich clasts (ORCs) in the Upper Paleozoic tight sandstone in the northeastern margin of the Ordos Basin were analyzed through 818-m-long drilling cores and logging data from 28 wells. In general, compared with soft-sediment clasts documented in other sedimentary environments, organic-rich clasts in coal-bearing tight sandstone have not been adequately investigated in the literature. ORCs are widely developed in various sedimentary environments of coal-bearing sandstone, including fluvial channels, crevasse splays, tidal channels, sand flats, and subaqueous debris flow deposits. In addition to being controlled by the water flow energy and transportation processes, the fragmentation degree and morphology of ORCs are also related to their content of higher plants organic matter. The change in water flow energy during transportation makes the ORCs show obvious mechanical depositional differentiation. Four main types of ORC can be recognized in the deposits: diamictic organic-rich clasts, floating organic-rich clasts, loaded lamellar organic-rich clasts, and thin interlayer organic-rich clasts. The relationship between energy variation and ORCs deposition continuity is rarely studied so far. Based on the different handling processes under the control of water flow energy changes, we propose two ORCs formation mechanisms: the long-term altering of continuous water flow and the short-term water flow acting triggered by sudden events.
Highlights
The soft-sediment clasts (SSCs), as aggregations of the fine sediments formed at the syngenetic sedimentary stage, are often dispersedly preserved in the water-transported sandy hosting sediments [1,2]
A total types of 28 exploration wells were as research objects in this study, from 2013 to Eleven of lithofacies related to selected organic-rich clasts are recognized withindrilled the Carboniferous2017 by the
In the process of transportation, organic-rich clasts (ORCs) undergo a series of dynamic processes including liquefying, represent carbonaceous fragments, carbonaceous mudstone clasts, shale clasts, dark mudstone clasts crushing, squeezing, squashing, winnowing, folding and pressing in the long-term and repeatedly, and plant fragments developed in coal-bearing sandstones
Summary
The soft-sediment clasts (SSCs), as aggregations of the fine sediments formed at the syngenetic sedimentary stage, are often dispersedly preserved in the water-transported sandy hosting sediments [1,2]. SSCs are widely developed in a variety of modern and ancient sedimentary settings, including glacial, alluvial, fluvial, estuarine, coastal, shoreline and deep-water environments [3,4,5,6,7,8,9,10,11,12]. Numerous targeted researches have been carried out on the morphology, classification, formation and transportation mechanism of different SSCs under particular settings [1,12]. The term “organic-rich clasts (ORCs)” is used for describing this outstanding feature
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