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The use of water detection (WD) indices to infer daily discharge (Qd) has a great potential to enrich needed hydrological data for understanding fluvial processes driving the morphological changes of braided rivers. However, no consensus has been reached on which one stands out for use in mid-sized braided rivers. In this study, we compared the physical characteristics of three most commonly used WD indices, the Normalized Difference Water Index (NDWI), Modified Normalized Difference Water Index (MNDWI), and Normalized Difference Moisture Index (NDMI), for two mid-sized braided reach segments in the Qinghai-Tibet Plateau, China, that have different morphological structures. Relying on the Google Earth Engine web interface, we calculated the total mean water width (WWt) based on the detected surface-water areas (As) and braiding index (BI), as well as the mean values (m) of these indices over about four decades at the braided corridor scale (cs) (mNDWIcs, mMNDWIcs, and mNDMIcs). We then examined different responses of these indices to water and non-water features and their best threshold values for characterizing channel structures. Our analyses demonstrated that (1) NDWI and MNDWI perform well for detecting braided channel structures with the threshold of zero; (2) WWt is generally better correlated to Qd in a linear style than WD indices do, particularly when calculated from MNDWI; and (3) among WD indices calculated at the braided corridor scale, mMNDWIcs shows a better relationship with Qd than mNDMIcs does. Finally, we provided mechanisms that may explain these differences in terms of photometric discrepancies in calculating WWt and WD indices and the impact of image resolution on their calculations. [ABSTRACT FROM AUTHOR]

Primary porosity is an important parameter in studies of reservoir and porosity evolution. However, the applicable conditions for the few existing mathematical models of primary porosity have yet to be clarified. A mathematical model based on the packing texture was proposed by the authors in a previous study; however, the proposed model was only applicable to well-sorted and well-rounded modern beach sands. In this study, the model was modified and applied to poorly-sorted and poorly-rounded braided river sands. The applicable conditions for the proposed models and the previous empirical formula were investigated based on 41 modern braided river sand samples from the Kuqa River in the Tarim Basin and 40 modern beach sand samples from eastern China. Modern sands can be divided into three types based on the particle size distribution. Modified mathematical formulas were derived for each type of sands. The constant parameters in the mathematical model, such as the filling coefficients, embedment coefficients, and packing void ratios were obtained through experiments and through linear fitting. Based on the mathematical model, the errors between the measured and the calculated primary porosities ranged from −3% to 3% for the modern braided sands and from −2% to 2% for the modern beach sands. Using the empirical formula, the primary porosity errors in the modern braided river sands were large, and the formula could not be appropriately applied except to well-sorted and well-rounded modern beach sands. Compared with the modified mathematical model, the original mathematical model could not be applied well to braided river sands owing to the additional errors caused by e f1. The modified mathematical model based on the packing texture was more credible when applied to poorly-sorted and angular braided river sands; further, it was applicable to all types of unconsolidated sands. • A modified model based on packing texture is applied to braided river sands. • Braided river sands are divided into three types by grain size distribution. • The effect of grain shape on primary porosity error is avoided by the new models. • The modified model is found to be more credible over a wider application range. [ABSTRACT FROM AUTHOR]

Tight gas is an important unconventional resource with continuous consumption demand, and co-production from multi-layers are becoming an inevitable choice for areas with low reserve abundance. The Ordos basin contain tight gas production from generally the whole Permian, recording sandstones deposited from transitional to terrestrial environments. In order to reveal the reservoir sensitivities of different layers, petrophysical and sensitivity experiments were conducted with samples from members of different sedimentary environment, i.e., Tai 2 Member (P 1 t 2 , coastal environment), Shan 1 Member (P 1 s 1 , deltaic environment), He 8 Member (P 2 x 8 , deltaic-fluvial environment), He 4 Member (P 2 s 4 , fluvial environment), and Qian 5 Member (P 3 s 5 , braided fluvial environment) of Permian. The results showed: (1) The sandstones of P 1 t 2 , P 1 s 1 , P 2 x 8 , P 2 s 4 , and P 3 s 5 are dominated by feldspathic litharenite, litharenite, lithic arkose, litharenite, and lithic arkose, respectively; (2) From P 1 t 2 to P 3 s 5 , the content of illite decreases, those of chlorite and illite/smectite (I/S) increase, and kaolinite firstly increases and then decreases (the trend changed in P 2 s 4); (3) The permeability of all the analyzed sandstones ranges between 0.2 and 0.6 mD, the average porosity between 5.7% and 8.2%, with mercury withdrawal efficiency between 32.8% and 42.1% and average median radius between 0.10 and 0.20 μm; (4) All the sandstones have medium–weak flow rate sensitivities, with medium–weak (P 1 t 2), medium (P 1 s 1), medium (P 2 x 8), medium (P 2 s 4), and medium–weak (P 3 s 5) water sensitivities and weak (P 1 t 2), medium–weak (P 1 s 1), medium–weak (P 2 x 8), medium–weak (P 2 s 4), and medium (P 3 s 5) alkali sensitivities. The flow rate sensitivities are mainly influenced by pore structure and the quartz content, while water sensitivities are affected by pore structure, I/S and illite contents, and the alkali sensitivities are correlated with pore structure and kaolinite, I/S, and feldspar contents. The results can be beneficial for tight gas production engineering choices and strategies for improving gas production. • Petrophysics of a whole tight sandstones cross section were investigated. • Sandstones of different sedimentary environment show various diagenesis. • Flow rate/water/alkali sensitivities were revealed. • Sandstone sensitivities show correlation with pore structures and mineral contents. [ABSTRACT FROM AUTHOR]

Large and complex braided channels (>1 km wide) in the Source Region of the Yangtze River (SRYR) on the Qinghai-Tibet Plateau, China, are characterized by a unique alpine environment with very high elevations and glacier sources. Yet, little is known about the morphological complexity and changes of large braided channels owing to the scarcity of hydrological and topographical data. In this study, we provided the first insight into these issues using compiled long-term (several decades) climatic and suspended sediment data, and Landsat images of braided channels in two nested headwater watersheds (i.e., Tuotuo and Tongtian river watersheds) in the SRYR. Analysis of annual and monthly mean temperature, precipitation, and water discharges illustrated the nature of the hydrological regime and their response to glacier melting in the two watersheds. Examination of annual mean suspended sediment loads and concentrations (C) led to two sediment rating curves and generally similar C values between the two watersheds. Among a suite of morphological indices for braided rivers, braiding intensity may be quantified using four different ones. After comparing the indices, we showed that they had similar ability of characterizing braiding patterns and adopted branch count index (BI T3) for further analysis. We found that BI T3 was linearly correlated with active valley width (W R) during both flood and dry seasons for the 17 selected braided reaches distributed within the two nested watersheds. In addition, braiding density, defined as the ratio of BI T3 to W R , remained approximately unchanged irrespective of elevations and locations of these reaches, indicating clearly that morphological structures of braided channels are spatially uniform in the SRYR. Along the main channels of the two nested watersheds over the entire study period, BI T3 always reached the maximum as R W , defined as the ratio of water body to exposed active riverbed area, was around 0.43. Furthermore, R W was always positively correlated with the associated water discharge (Q). Variable degrees of braiding adjustment in response to changes of hydrological regime were represented by hysteresis loops between BI T3 and Q. These results revealed relatively homogeneous morphologic properties of braided channels across the two nested watersheds and provided an important benchmark for future morphodynamic research on braided rivers in the SRYR. • Changes of morphological structures of braided rivers are better captured using braiding intensity and active valley width • Braided rivers in the source region of Yangtze River (SYR) demonstrate morphological similarity across space • Variable morphological responses to discharge changes only occur in smaller watersheds • Increased glacier melting has had limited impact on sediment transport and morphological structures of braided rivers in SRYR [ABSTRACT FROM AUTHOR]