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USGS2018

Spatial distribution and ecological risk assessment of heavy metals in coastal surface sediments in the Hebei Province offshore area, Bohai Sea, China

Seven hundred and nine surface sediment samples, along with deeper sediment samples, were collected from Hebei Province along the coastal section of the Bohai Sea, China, and analyzed for grain size, concentrations of organic carbon (Corg) and heavy metals (Cu, Pb, Zn, Cr, Cd, As, and Hg). Results indicated that the average concentrations in the sediments were 16.1 mg/kg (Cu), 19.4 mg/kg (Pb), 50 mg/kg (Zn), 48.8 mg/kg (Cr), 0.1 mg/kg (Cd), 8.4 mg/kg (As), and 20.3 μg/kg (Hg). These concentrations generally met the China Marine Sediment Quality criteria. However, both pollution assessments indicated moderate to strong Cd and Hg contamination in the study area. The potential ecological risk index suggested that the combined ecological risk of the seven studied metals may be low, but that 24.5% of the sites, where sediments were more finer and higher in Corg concentration, had high ecological risk in Hg and Cd pollution.

Bohai Sea, Hebei Province↗
USGS2018

Integrating forest inventory data and MODIS data to map species-level biomass in Chinese boreal forests

Timely and accurate knowledge of species-level biomass is essential for forest managers to sustain forest resources and respond to various forest disturbance regimes. In this study, maps of species-level biomass in Chinese boreal forests were generated by integrating Moderate Resolution Imaging Spectroradiometer (MODIS) images with forest inventory data using k nearest neighbor ( k NN) methods and evaluated at different scales. The performance of 630 k NN models based on different distance metrics, k values, and temporal MODIS predictor variables were compared. Random Forest (RF) showed the best performance among the six distance metrics: RF, Euclidean distance, Mahalanobis distance, most similar neighbor in canonical correlation space, most similar neighbor computed using projection pursuit, and gradient nearest neighbor. No appreciable improvement was observed using multi-month MODIS data compared with using single-month MODIS data. At the pixel scale, species-level biomass for larch and white birch had relatively good accuracy (root mean square deviation < 62.1%), while the other species had poorer accuracy. The accuracy of most species except for willow and spruce was improved up to the ecoregion scale. The maps of species-level biomass captured the effects of disturbances including fire and harvest and can provide useful information for broad-scale forest monitoring over time.

Canadian Journal of Forest Research↗
USGS2018

Paleozoic shale gas resources in the Sichuan Basin, China

The Sichuan Basin, China, is commonly considered to contain the world’s most abundant shale gas resources. Although its Paleozoic marine shales share many basic characteristics with successful United States gas shales, numerous geologic uncertainties exist, and Sichuan Basin shale gas production is nascent. Gas retention was likely compromised by the age of the shale reservoirs, multiple uplifts and orogenies, and migration pathways along unconformities. High thermal maturities raise questions about gas storage potential in lower Paleozoic shales. Given these uncertainties, a new look at Sichuan Basin shale gas resources is advantageous. As part of a systematic effort to quantitatively assess continuous oil and gas resources in priority basins worldwide, the US Geological Survey (USGS) completed an assessment of Paleozoic shale gas in the Sichuan Basin in 2015. Three organic-rich marine Paleozoic shale intervals meet the USGS geologic criteria for quantitative assessment of shale gas resources: the lower Cambrian Qiongzhusi Formation, the uppermost Ordovician Wufeng through lowermost Silurian Longmaxi Formations (currently producing shale gas), and the upper Permian Longtan and Dalong Formations. This study defined geologically based assessment units and calculated probabilistic distributions of technically recoverable shale gas resources using the USGS well productivity–based method. For six assessment units evaluated in 2015, the USGS estimated a mean value of 23.9 tcf (677 billion cubic meters) of undiscovered, technically recoverable shale gas. This result is considerably lower than volumes calculated in previous shale gas assessments of the Sichuan Basin, highlighting a need for caution in this geologically challenging setting.

Sichuan Basin↗
USGS2018

Bed shear stress estimation under wave conditions using near-bottom measurements: Comparison of methods

Understanding the influence of waves on bed shear stress is critical for predicting morphodynamical behaviours in coastal areas. Near-bed flow was measured on the middle and lower intertidal mudflats along the Jiangsu coast, China, using a three-dimensional acoustic velocimeter that collected a 3.5-cm vertical profile at 1mm resolution and sample rate of 25 Hz. On the lower and middle tidal flats, velocities from ~2.5-6 cmab (cm above bed) and ~0-3 cmab were measured, respectively. Current-induced bed shear stresses were calculated from turbulent kinetic energy (TKE) at the 11th measurement layer (i.e., 5.1 cm below the probe) using wave-turbulence decomposition and from a logarithmic fit to the horizontal mean velocity profile (LP). A wave boundary layer extended from the bed up to 3 cmab when the significant wave height was 0.23 m; when it was present the near-bed mean velocity profile was non-logarithmic. Waves suppress the development of a vertical velocity gradient and lead to an overestimation of bed shear stress when calculated using the log profile assumption. The TKE method is more accurate than the LP method when waves are present and measurements are at least partially within the wave boundary layer. Accurate calculation of current-induced bed shear stress depends on probe height and wave conditions.

Jiangsu coast↗
USGS2017

Similarity of plant functional traits and aggregation pattern in a subtropical forest

The distribution of species and communities in relation to environmental heterogeneity is a central focus in ecology. Co-occurrence of species with similar functional traits is an indication that communities are determined in part by environmental filters. However, few studies have been designed to test how functional traits are selectively filtered by environmental conditions at local scales. Exploring the relationship between soil characteristics and plant traits is a step toward understanding the filtering hypothesis in determining plant distribution at local scale. Toward this end, we mapped all individual trees (diameter >1 cm) in a one-ha subtropical forest of China in 2007 and 2015. We measured topographic and detailed soil properties within the field site, as well as plant leaf functional traits and demographic rates of the seven most common tree species. A second one-ha study plot was established in 2015, to test and validate the general patterns that were drawn from first plot. We found that variation in species distribution at local scale can be explained by soil heterogeneity and plant functional traits. (From first plot). (1) Species dominant in habitats with high soil ammonium nitrogen and total phosphorus tended to have high specific leaf area (SLA) and relative growth rate (RGR). (2) Species dominant in low-fertility habitats tended to have high leaf dry matter content (LDMC), ratio of chlorophyll a and b (ratioab), and leaf thickness (LT). The hypothesis that functional traits are selected in part by environmental filters and determine plant distribution at local scale was confirmed by the data of the first plot and a second regional site showed similar species distribution patterns.

Zhejiang Province↗
USGS2017

Evaluation of diffuse and preferential flow pathways of infiltratedprecipitation and irrigation using oxygen and hydrogen isotopes

Subsurface-water flow pathways in three different land-use areas (non-irrigated grassland, poplar forest, and irrigated arable land) in the central North China Plain were investigated using oxygen ( 18 O) and hydrogen ( 2 H) isotopes in samples of precipitation, soils, and groundwater. Soil water in the top 10 cm was significantly affected by both evaporation and infiltration. Water at 10–40 cm depth in the grassland and arable land, and 10–60 cm in poplar forest, showed a relatively short residence time, as a substantial proportion of antecedent soil water was mixed with a 92-mm storm infiltration event, whereas below those depths (down to 150 cm), depleted δ 18 O spikes suggested that some storm water bypassed the shallow soil layers. Significant differences, in soil-water content and δ 18 O values, within a small area, suggested that the proportion of immobile soil water and water flowing in subsurface pathways varies depending on local vegetation cover, soil characteristics and irrigation applications. Soil-water δ 18 O values revealed that preferential flow and diffuse flow coexist. Preferential flow was active within the root zone, independent of antecedent soil-water content, in both poplar forest and arable land, whereas diffuse flow was observed in grassland. The depleted δ 18 O spikes at 20–50 cm depth in the arable land suggested the infiltration of irrigation water during the dry season. Temporal isotopic variations in precipitation were subdued in the shallow groundwater, suggesting more complete mixing of different input waters in the unsaturated zone before reaching the shallow groundwater.

Yellow Sea, Bohai Sea↗
USGS2017

Nitrogen additions affect litter quality and soil biochemical properties in a peatland of Northeast China

Nitrogen (N) is a limiting nutrient in many peatland ecosystems. Enhanced N deposition, a major component of global climate change, affects ecosystem carbon (C) balance and alters soil C storage by changing plant and soil properties. However, the effects of enhanced N deposition on peatland ecosystems are poorly understood. We conducted a two-year N additions field experiment in a peatland dominated by Eriophorum vaginatum in the Da Xing’an Mountains, Northeast China. Four levels of N treatments were applied: (1) CK (no N added), (2) N1 (6 g N m −2 yr −1 ), (3) N2 (12 g N m −2 yr −1 ), and (4) N3 (24 g N m −2 yr −1 ). Plant and soil material was harvested at the end of the second growing season. N additions increased litter N and phosphorus (P) content, as well as β-glucosidase, invertase, and acid-phosphatase activity, but decreased litter C:N and C:P ratios. Litter carbon content remained unchanged. N additions increased available NH 4 + –N and NO 3 − –N as well as total Gram-positive (Gram+), Gram-negative (Gram−), and total bacterial phospholipid fatty acids (PLFA) in shallow soil (0–15 cm depth). An increase in these PLFAs was accompanied by a decrease in soil labile organic C (microbial biomass carbon and dissolved organic carbon), and appeared to accelerate decomposition and reduce the stability of the soil C pool. Invertase and urease activity in shallow soils and acid-phosphatase activity in deep soils (15–30 cm depth) was inhibited by N additions. Together, these findings suggest that an increase in N deposition in peatlands could accelerate litter decomposition and the loss of labile C, as well as alter microbial biomass and function.

Ecological Engineering↗
USGS2017

Lithospheric density structure beneath the Tarim basin and surroundings, northwestern China, from the joint inversion of gravity and topography

Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, v s , v p , and v p /v s , consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism.

Earth and Planetary Science Letters↗
USGS2017

Pulsed strain release on the Altyn Tagh fault, northwest China

Earthquake recurrence models assume that major surface-rupturing earthquakes are followed by periods of reduced rupture probability as stress rebuilds. Although purely periodic, time- or slip-predictable rupture models are known to be oversimplifications, a paucity of long records of fault slip clouds understanding of fault behavior and earthquake recurrence over multiple ruptures. Here, we report a 16 kyr history of fault slip—including a pulse of accelerated slip from 6.4 to 6.0 ka—determined using a Monte Carlo analysis of well-dated offset landforms along the central Altyn Tagh strike-slip fault (ATF) in northwest China. This pulse punctuates a median rate of 8.1 +1.2 / −0.9 mm/a and likely resulted from either a flurry of temporally clustered ∼Mw 7.5 ground-rupturing earthquakes or a single large >Mw 8.2 earthquake. The clustered earthquake scenario implies rapid re-rupture of a fault reach >195 km long and indicates decoupled rates of elastic strain energy accumulation versus dissipation, conceptualized as a crustal stress battery. If the pulse reflects a single event, slip-magnitude scaling implies that it ruptured much of the ATF with slip similar to, or exceeding, the largest documented historical ruptures. Both scenarios indicate fault rupture behavior that deviates from classic time- or slip-predictable models.

Altyn Tagh fault↗
USGS2017

Shallow microearthquakes near Chongqing, China triggered by the Rayleigh waves of the 2015 M7.8 Gorkha, Nepal earthquake

We present a case of remotely triggered seismicity in Southwest China by the 2015/04/25 M7.8 Gorkha, Nepal earthquake. A local magnitude ML3.8 event occurred near the Qijiang district south of Chongqing city approximately 12 min after the Gorkha mainshock. Within 30km of this ML3.8 event there are 62 earthquakes since 2009 and only 7 ML>3events, which corresponds to a likelihood of 0.3% for a ML>3on any given day by a random chance. This observation motivates us to investigate the relationship between the ML3.8 event and the Gorkha mainshock. The ML3.8 event is listed in the China Earthquake National Center (CENC) catalog and occurred at shallow depth (∼3km). By examining high-frequency waveforms, we identify a smaller local event (∼ML2.5) ∼15s before the ML3.8 event. Both events occurred during the first two cycles of the Rayleigh waves from the Gorkha mainshock. We perform seismic event detection based on envelope function and waveform matching by using the two events as templates. Both analyses found a statistically significant rate change during the mainshock, suggesting that they were indeed dynamically triggered by the Rayleigh waves. Both events occurred during the peak normal and dilatational stress changes (∼10–30 kPa), consistent with observations of dynamic triggering in other geothermal/volcanic regions. Although other recent events (i.e., the 2011 M9.1 Tohoku-Oki earthquake) produced similar peak ground velocities, the 2015 Gorkha mainshock was the only event that produced clear dynamic triggering in this region. The triggering site is close to hydraulic fracturing wells that began production in 2013–2014. Hence we suspect that fluid injections may increase the region’s susceptibility to remote dynamic triggering.

Chongqing↗
USGS2017

Historical changes in organic matter input to the muddy sediments along the Zhejiang-Fujian Coast, China over the past 160 years

The burial of sedimentary organic matter (SOM) in the large river-influenced estuarine-coastal regions is affected by hydrodynamic sorting, diagenesis and human activities. Typically, the inner shelf region of the East China Sea is a major carbon sink of the Yangtze River-derived fine-grained sediments. Most of the previous work concentrated on the studies of surface sediments or used a single-proxy in this region. In this study, two cores from the Zhejiang-Fujian Coast were analyzed using bulk (TOC, TN and δ 13 C TOC ) and molecular biomarker ( n -alkane, brassicasterol, dinosterol and glycerol dialkyl glycerol tetraether lipids) techniques to clarify the sources, spatiotemporal distribution and fate of SOM in the Yangtze River Estuary and adjacent shelf. Results from this study indicated that the effects of diagenesis and diffusion on different sedimentary biomarkers resulted in overestimation of the relative contribution of terrestrial organic matter (%OM terr ), compared with those based on δ 13 C TOC . The amounts of terrestrial plant organic matter (OM plant ) and%OM terr in sediments decreased offshore. In contrast, the amounts of marine organic matter (OM marine ) increased offshore, but closer to the Yangtze River mouth, the amounts of soil organic matter (OM soil ) increased. Moreover, the amounts of TOC, OM plant and OM marine biomarkers increased, but OM soil and%OM terr decreased over time in recent decades. Our study suggests that spatial organic matter distribution patterns in marine shelf sediments were controlled primarily by hydrodynamic sorting and nutrient concentrations, and temporally diverse patterns were controlled predominantly by anthropogenic influence (e.g., dam construction and soil conservation, reclamation and agricultural plantations, anthropogenic nutrient input, dust storms, eutrophication, etc) and climate events (e.g., interdecadal climatic jump and heavy rain events) in the geological period.

Zhejiang-Fujian Coast↗
USGS2017

Depositional environment and organic matter accumulation of Upper Ordovician–Lower Silurian marine shale in the Upper Yangtze Platform, South China

The main controlling factors of organic matter accumulation in the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formations are complex and remain highly controversial. This study investigates the vertical variation of total organic carbon (TOC) content as well as major and trace element concentrations of four Ordovician–Silurian transition sections from the Upper Yangtze Platform of South China to reconstruct the paleoenvironment of these deposits and to improve our understanding of those factors that have influenced organic matter accumulation in these deposits. The residual TOC content of the Wufeng Formation averages 3.2% and ranges from 0.12 to 6.0%. The overlying lower Longmaxi Formation displays higher TOC content (avg. 4.4%), followed upsection by consistent and lower values that average 1.6% in the upper Longmaxi Formation. The concentration and covariation of redox-sensitive trace elements (Mo, U and V) suggest that organic-rich intervals of the Wufeng Formation accumulated under predominantly anoxic conditions. Organic-rich horizons of the lower Longmaxi Formation were deposited under strongly anoxic to euxinic conditions, whereas organic-poor intervals of the upper Longmaxi Formation accumulated under suboxic conditions. Positive correlations between redox proxies and TOC contents suggest that organic matter accumulation was predominantly controlled by preservation. Barium excess (Ba xs ) values indicate high paleoproductivity throughout the entire depositional sequence, with an increase in the lower Longmaxi Formation. Increased productivity may have been induced by enhanced P recycling, as evidenced by elevated C org /P tot ratios. Mo–U covariation and Mo/TOC values reveal that the Wufeng Formation was deposited under extremely restricted conditions, whereas the Longmaxi Formation accumulated under moderately restricted conditions. During the Late Ordovician, the extremely restricted nature of ocean circulation on the Upper Yangtze Platform in tandem with enhanced stratification of the water column promoted anoxic conditions favorable for the preservation of organic matter. During Early Silurian time, organic matter accumulation was principally controlled by changes in sea level, which affected terrigenous flux, redox conditions, and the degree of nutrition recycling.

Upper Yangtze Platform↗
USGS2017

A effective DNA vaccine against diverse genotype J infectious hematopoietic necrosis virus strains prevalent in China

Infectious hematopoietic necrosis virus (IHNV) is the most important pathogen threatening the aquaculture of salmonid fish in China. In this study, a DNA vaccine, designated pIHNch-G, was constructed with the glycoprotein (G) gene of a Chinese IHNV isolate SD-12 (also called Sn1203) of genotype J. The minimal dose of vaccine required, the expression of the Mx-1 gene in the muscle (vaccine delivery site) and anterior kidney, and the titers of the neutralizing antibodies produced were used to evaluate the vaccine efficacy. To assess the potential utility of the vaccine in controlling IHNV throughout China, the cross protective efficacy of the vaccine was determined by challenging fish with a broad range of IHNV strains from different geographic locations in China. A single 100 ng dose of the vaccine conferred almost full protection to rainbow trout fry (3 g) against waterborne or intraperitoneal injection challenge with IHNV strain SD-12 as early as 4 days post-vaccination (d.p.v.), and significant protection was still observed at 180 d.p.v. Intragenogroup challenges showed that the DNA vaccine provided similar protection to the fish against all the Chinese IHNV isolates tested, suggesting that the vaccine can be widely used in China. Mx-1 gene expression was significantly upregulated in the muscle tissue (vaccine delivery site) and anterior kidney in the vaccinated rainbow trout at both 4 and 7 d.p.v. Similar levels of neutralizing antibodies were determined with each of the Chinese IHNV strains at 60 and 180 d.p.v. This DNA vaccine should play an important role in the control of IHN in China.

Vaccine↗
USGS2017

Stochastic programming with a joint chance constraint model for reservoir refill operation considering flood risk

Reservoir refill operation modeling attempts to maximize a set of benefits while minimizing risks. The benefits and risks can be in opposition to each other, such as having enough water for hydropower generation while leaving enough room for flood protection. In addition to multiple objects, the uncertainty of streamflow can make decision making difficult. This paper develops a stochastic optimization model for reservoir refill operation with the objective of maximizing the expected synthesized energy production for a cascade system of hydropower stations while considering flood risk. Streamflow uncertainty is addressed by discretized streamflow scenarios and flood risk is controlled by a joint chance constraint restricting the occurrence probability. With the variability of flood risk level, two advancing refill scenarios for exploring operation benefit are presented. Scenario I loosens the current stagewise storage bounds conditions and allows advancing reservoir refills but keeps the flood risk level the same as the refill policies obtained under the current storage bounds. Scenario II keeps the current storage bounds unchanged but allows increases in flood risk level. The proposed methodology is applied to the Xiluodu cascade system of reservoirs in China and investigates the optimal refill policies obtained by both scenarios. Compared with the benchmark obtained under the current storage bounds and lowest flood risk level, the results show (1) the synthesized energy production can be improved by 2.13% without changing the flood risk level under Scenario I, and (2) the synthesized energy production can also be increased by 0.21% at the expense of increasing the flood risk level by 4.4% when Scenario II is employed. As Scenario I produces higher benefit and lower risk than Scenario II, it is recommended to loosen the current stagewise storage bounds but to keep the flood risk level unchanged during refill operations.

Xiluodu Cascade Reservoir↗
USGS2017

Organic petrology of peak oil maturity Triassic Yanchang Formation lacustrine mudrocks, Ordos Basin, China

An organic petrology evaluation and a determination of solid bitumen reflectance BR o "> BR o were completed for organic-rich Triassic Yanchang Formation mudrocks ( n = 15 "> n = 15 ) from the Ordos Basin, north-central China, as part of a larger investigation of “shale gas” resources. These data were integrated with information from Rock-Eval programmed pyrolysis to show that the samples are in the peak oil window of thermal maturity and that organic matter is dominated by solid bitumen with minor amounts of type III kerogen (vitrinite and inertinite) from vascular land plants. Describing a “kerogen type” for these rocks based strictly on parameters determined from programmed pyrolysis is misleading because the original organic matter has converted to hydrocarbons (present as solid bitumen), a large proportion of which may have been expelled into adjacent reservoir facies. However, based on the comparison with immature-early mature lacustrine mudrock (Garden Gulch Member of Green River Formation) and marine shale (Boquillas Formation), we suggest that the original organic matter in the organic-rich samples examined for our study may have been type I/II kerogen with hydrogen index values of &gt; 700 &#x2009;&#x2009; mg &#x2009; HC / g "> > 700 mg HC / g TOC.

Ordos basin↗
USGS2017

Liquid hydrocarbon characterization of the lacustrine Yanchang Formation, Ordos Basin, China: Organic-matter source variation and thermal maturity

Source-rock samples from the Upper Triassic Yanchang Formation in the Ordos Basin of China were geochemically characterized to determine variations in depositional environments, organic-matter (OM) source, and thermal maturity. Total organic carbon (TOC) content varies from 4 wt% to 10 wt% in the Chang 7, Chang 8, and Chang 9 members — the three OM-rich shale intervals. The Chang 7 has the highest TOC and hydrogen index values, and it is considered the best source rock in the formation. Geochemical evidence indicates that the main sources of OM in the Yanchang Formation are freshwater lacustrine phytoplanktons, aquatic macrophytes, aquatic organisms, and land plants deposited under a weakly reducing to suboxic depositional environment. The elevated C 29 "> C 29 sterane concentration and depleted &#x3B4; C 13 "> δ 13 C values of OM in the middle of the Chang 7 may indicate the presence of freshwater cyanobacteria blooms that corresponds to a period of maximum lake expansion. The OM deposited in deeper parts of the lake is dominated by oil-prone type I or type II kerogen or a mixture of both. The OM deposited in shallower settings is characterized by increased terrestrial input with a mixture of types II and III kerogen. These source rocks are in the oil window, with maturity increasing with burial depth. The measured solid-bitumen reflectance and calculated vitrinite reflectance from the temperature at maximum release of hydrocarbons occurs during Rock-Eval pyrolysis ( T max "> T max ) and the methylphenanthrene index (MPI-1) chemical maturity parameters range from 0.8 to 1.05 % R o "> 1.05 % R o . Because the thermal labilities of OM are associated with the kerogen type, the required thermal stress for oil generation from types I and II mixed kerogen has a higher and narrower range of temperature for hydrocarbon generation than that of OM dominated by type II kerogen or types II and III mixed kerogen deposited in the prodelta and delta front.

Ordos basin↗
USGS2017

Controls on pore types and pore-size distribution in the Upper Triassic Yanchang Formation, Ordos Basin, China: Implications for pore-evolution models of lacustrine mudrocks

Our main objectives are to (1) learn if pore-evolution models developed from marine mudrocks can be directly applied to lacustrine mudrocks, (2) investigate what controls the different pore types and sizes of Chang 7 organic matter (OM)-rich argillaceous mudstones of the Upper Triassic Yanchang Formation, and (3) describe the texture, fabric, mineralogy, and thermal maturity variation in the Chang 7 mudstones. Lacustrine mudstones from nine cored wells along a depositional dip in the southeastern Ordos Basin, China, were investigated. Helium porosimetry, nitrogen adsorption, and field-emission scanning electron microscopy of Ar-ion milled samples were applied. Measured average total porosity of samples from a proximal to distal transect ( &#x3D5; = 5.0 % "> ϕ = 5.0 % ) is higher than those from the two adjacent cored wells ( &#x3D5; = 2.3 % "> ϕ = 2.3 % ). This difference in porosity partly caused by differences in the clay mineral content implies that in the fluvial-deltaic-lacustrine depositional environment, reservoir quality can vary significantly in a short distance. Owing to the uneven distribution of the sample set from proximal to distal area, we mainly evaluate variations in the proximal setting. Results from nitrogen-gas adsorption experiments show that there are four distinct patterns of pore-size distribution within the Chang 7 member of the Yanchang Formation with no particular correlation with mineralogical composition and thermal maturity. The pore network within Chang 7 mudstones is dominated by OM-hosted pores, with a lesser abundance of interparticle and intraparticle pores. The size distribution of mineral-hosted pores within these mudstones is found to be closely related to the rock texture (sorting and grain size) and fabric. Mudstones with well-sorted grains and a higher percentage of coarser grains have more abundant mineral pores. The sizes of OM-hosted pores in these compaction-dominated lacustrine mudstones were one to two orders of magnitude smaller than those in the marine mudstones that display abundant early cementation.

Ordos basin↗