Lihua Liu

Partition Behavior of Polycyclic Aromatic Hydrocarbons Between Aged Coal Tar and Water

Coal tar aged in a large-scale, artificial aquifer experiment for five years was subsequently investigated for leaching behavior of polycyclic aromatic hydrocarbons (PAHs). After five years, the initially liquid coal tar had solidified and formed segregated particles with a grain size similar to that of the sandy aquifer material. The composition of the aged coal tar (ACT) with regard to PAHs was remarkably different from that of the original bulk coal tar (BCT), because most of the low-molecular-weight compounds had been depleted. Equilibrium aqueous-phase concentrations of 17 PAHs leaching from the aquifer material containing the ACT were measured from consecutive equilibration steps at increasing temperatures of between 25 and 100 degrees C using accelerated solvent extraction. The results showed 2- to 5,000-fold lower concentrations than those from BCT, indicating dramatic changes of dissolution behavior of PAHs from coal tar after the five-year aging period. Predictions based on Raoults law with the subcooled liquid solubilities substantially overestimated the equilibrium aqueous-phase concentrations of the PAHs from ACT, whereas the estimations were reasonable if the solid solubilities were employed instead. The enthalpies of phase transfer from ACT to water were determined based on the vant Hoff equation. The resulting values agreed with the dissolution enthalpies of pure solid rather than subcooled liquid PAHs.

Simulation of advective methane flux and AOM in Shenhu area, the northern South China Sea

Anaerobic oxidation of methane (AOM) occurring in the marine sediment is an important process for methane cycle and methane sequestration. In this work, a one-dimensional numerical model was developed to study the distribution of advective methane flux with the AOM process. The model has been applied to investigate the gas hydrates bearing sediments of Shenhu areas located in the northern South China Sea, where advective methane transport was detected. The modeling results suggest that methane flux will be consumed in the sediment column via dissolution, sorption, and AOM reaction. Only when the methane flux was one order of magnitude higher than current level, then a portion of methane will enter water column and possibly escape to the atmosphere. The numerical simulation also revealed that, due to the lower permeability of the silt–clay sediments, a much thicker sulfate-methane transition zone exists in the Shenhu area, where AOM is able to consume more.

Joint land-sea seismic survey and research on the deep structures of the Bohai Sea areas

This paper presents the survey and research work of two land-sea profiles in the Bohai Sea, China, carried out in 2010–2011, including the seismic sources on land and in the sea, the ocean bottom seismographs (OBS) and their recovery, the coupling of OBS and the environment noise in sea area, the data quality of OBSs, and the result of data analysis. We focused on the investigation of crustal structures revealed by the two NEEW-trending joint land-sea profiles. In combination with the Pn-velocity distribution and gravitymagnetic inversion results in the North China Craton, we propose that the undulation of the Moho interface in the Bohai and surrounding areas is not strong, and the lithospheric thinning is mainly caused by the thinning of its mantle part. The research result indicates that obvious lateral variations of Moho depth and seismic velocity appear nearby all the large-scale faults in Bohai Sea, and there is evidence of underplating and reforming of the lower crust by mantle material in the Bohai area. However, geophysical evidence does not appear to support the “mantle plume” or “delamination” model for the North China Craton destruction. The crustal structure of the Bohai Sea revealed “a relatively normal crust and obviously thinned mantle lid”, local velocity anomalies and instability phenomena in the crust. These features may represent a combined effect of North China-Yangtze collision at an early stage and the remote action of Pacific plate subduction at a late stage.

Coupled carbon and sulfur isotope behaviors and other geochemical perspectives into marine methane seepage

Methane seepage is the signal of the deep hydrocarbon reservoir. The determination of seepage is significant to the exploration of petroleum, gas and gas hydrate. The seepage habits microbial and macrofaunal life which is fueled by the hydrocarbons, the metabolic byproducts facilitate the precipitation of authigenic minerals. The study of methane seepage is also important to understand the oceanographic condition and local ecosystem. The seepage could be active or quiescent at different times. The geophysical surveys and the geochemical determinations reveal the existence of seepage. Among these methods, only geochemical determination could expose message of the dormant seepages. The active seepage demonstrates high porewater methane concentration with rapid SO42– depleted, low H2S and dissolved inorganic carbon (DIC), higher rates of sulfate reduction (SR) and anaerobic oxidation of methane (AOM). The quiescent seepage typically develops authigenic carbonates with specific biomarkers, with extremely depleted 13C in gas, DIC and carbonates and with enriched 34S sulfate and depleted 34S pyrite. The origin of methane, minerals precipitation, the scenario of seepage and the possible method of immigration could be determined by the integration of solutes concentration, mineral composition and isotopic fractionation of carbon, sulfur. Numerical models with the integrated results provide useful insight into the nature and intensity of methane seepage occurring in the sediment and paleooceanographic conditions. Unfortunately, the intensive investigation of a specific area with dormant seep is still limit. Most seepage and modeling studies are site-specific and little attempt has been made to extrapolate the results to larger scales. Further research is thus needed to foster our understanding of the methane seepage.

A review on the aging phenomena of organic components and their mass transfer through the NAPL interfacial phase

It occurs worldwide that the organic components of non-aqueous phase liquid (NAPL) enter the porous medium and become the source of contaminants in the subsurface. The transport of the organic components through NAPL interphase into the aqueous phase and the subsurface determines the extent of contamination, the persistence of residual NAPL phases and the techniques of remediation. During the transport process the NAPL interphase may experience “aging”, a physical and chemical change when NAPL is exposed to aqueous and or gaseous phases. This aging process alters vice versa the mass transfer behaviour of the organic contaminants in the porous medium.

Methane seepage intensities traced by sulfur isotopes of pyrite and gypsum in sediment from the Shenhu area, South China Sea

The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments. Anaerobic oxidation of methane (AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment, including pyrite and gypsum. Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor (cmbsf) in the entire core sediment of HS328 from the northern South China Sea. Most pyrites are tubes consisting of framboidal cores and outer crusts. Gypsum aggregates occur as rosettes and spheroids consisting of plates. Some of them grow over pyrite, indicating that gypsum precipitation postdates pyrite formation. The sulfur isotopic values (δ34S) of pyrite vary greatly (from–46.6‰ to–12.3‰ V-CDT) and increase with depth. Thus, the pyrite in the shallow sediments resulted from organoclastic sulfate reduction (OSR) and is influenced by AOM with depth. The relative high abundance and δ34S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone (SMTZ). The sulfur isotopic composition of gypsum (from–25‰ to–20.7‰) is much lower than that of the seawater sulfate, indicating the existence of a 34S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR. Pyrite oxidation is controlled by ambient electron acceptors such as MnO2, iron (III) and oxygen driven by the SMTZ location shift to great depths. The δ34S values of gypsum at greater depth are lower than those of the associated pyrite, revealing downward diffusion of 34S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate. These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral, which will be favor to the formation of gypsum. Overall, the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities, and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.

Theoretical simulation of the evolution of methane hydrates in the case of Northern South China Sea since the last glacial maximum

The distribution and inventory of gas hydrates in a region is determined by the sediment characteristics, methane supply and evolution of the reservoir. In recent decades, the geo-environmental constraints and sources of methane have been intensively investigated, and numerous experimental and numerical simulation tools have been developed to evaluate the inventory of hydrates. However, information regarding the evolution of hydrate reservoirs remains limited. This study developed a simulator to theoretically model the evolution of specific reservoirs since the last glacial maximum (LGM). The LGM was a recent cold epoch that occurred approximately 18,000xa0years ago. Since the LGM, the earth’s climate system has experienced a continuous increase of surface temperature and rising sea level. Given a sufficient supply of methane and a transport system, hydrates may form in marine sediments if sea level rises or melt if the temperature increases. A one-dimensional simulator that represents the sediment was developed and uses the current hydrate profiles as the initial conditions and reliable paleoenvironmental data obtained in sites located in the northern shelf of the South China Sea (SCS) as boundary conditions. Two types of hydrate profiles were reversely simulated till the LGM: (1) a Gaussian profile, which was observed in the Shenhu area and (2) a trapezoidal profile, which was observed in the Dongsha area, SCS. The evolution and past quantities of local hydrate reservoirs were estimated. The model results demonstrated that shallow (500–700xa0m below the seafloor, or mbsf) moderate-saturation (50xa0% pore volume, or v:v) hydrate deposits will form if they are subjected to recent climate changes. The inventory of hydrates in the NSCS increased by only 0–7xa0% over the past 18,000xa0years under various scenarios. A sensitivity analysis was performed to identify the most pertinent parameters that control the formation and dissociation of hydrates, including the grain size, temperature gradient and deposition depth. The distribution and depth of the reservoir were determined to be the most critical factors in the evolution of the studied hydrates.

Numerical modeling of early diagenetic processes in Haiyang 4 Area in the northern slope of the South China Sea

Early diagenesis affects the distribution of solutes and minerals in unconsolidated sediments. The investigation of diagenesis is critical to understanding the geochemical transformation and benthic fluxes of elements. During the cruise mission SO-177 in 2004, gravity coring samples were recovered in the Haiyang 4 Area of the northern slope of the South China Sea (SCS). The geochemical concentrations in interstitial water were determined onboard. The 1D C.CANDI reactive transport software was used to model the early diagenesis processes at four sites: 56-GC-3, 70-GC-9, 94-GC-11, and 118-GC-13. All of the simulations reproduced concentration profiles that matched the measurements with the implemented geochemical reactions. The degradation of organic carbon and anaerobic oxidation of methane (AOM) primarily determine the distribution of solutes in the working area. The degradation is active in the top 150xa0cm, and AOM is vigorous at depths below 200xa0cm. The local advective flux, sediment rate, and kinetic reaction constants of organic matter, methane and sulfate were calibrated based on the existing concentrations of pore water solutes. Geochemical reactions in this area occur in considerably deeper layers compared to depths cited in the literature. The model results provide evidence for the existence of deep hydrocarbon reservoirs that provide methane to the upper sediments.