Applied Geochemistry (v.24, #1)

Mineralogy and environmental stability of slags from the Tsumeb smelter, Namibia by Vojtěch Ettler; Zdenek Johan; Bohdan Kříbek; Ondřej Šebek; Martin Mihaljevič (1-15).
Three types of smelting slags originating from historically different smelting technologies in the Tsumeb area (Namibia) were studied: (i) slags from processing of carbonate/oxide ore in a Cu–Pb smelter (1907–1948), (ii) slags from Cu and Pb smelting of sulphide ores (1963–1970) and (iii) granulated Cu smelting slags (1980–2000). Bulk chemical analyses of slags were combined with detailed mineralogical investigation using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM/EDS) and electron microprobe (EPMA). The slags are significantly enriched in metals and metalloids: Pb (0.97–18.4 wt.%), Cu (0.49–12.2 wt.%), Zn (2.82–12.09 wt.%), Cd (12–6940 mg/kg), As (930–75,870 mg/kg) and Sb (67–2175 mg/kg). Slags from the oldest technology are composed of primary Ca- and Pb-bearing feldspars, spinels, complex Cu–Fe and Cu–Cr oxides, delafossite–mcconnellite phases and Ca–Pb arsenates. The presence of arsenates indicates that these slags underwent long-term alteration. More recent slags are composed of high-temperature phases: Ca–Fe alumosilicates (olivine, melilite), Pb- and Zn-rich glass, spinel oxides and small sulphide/metallic inclusions embedded in glass. XRD and SEM/EDS were used to study secondary alteration products developed on the surface of slags exposed for decades to weathering on the dumps. Highly soluble complex Cu–Pb–(Ca) arsenates (bayldonite, lammerite, olivenite, lavendulan) associated with litharge and hydrocerussite were detected. To determine the mineralogical and geochemical parameters governing the release of inorganic contaminants from slags, two standardized short-term batch leaching tests (European norm EN 12457 and USEPA TCLP), coupled with speciation-solubility modelling using PHREEQC-2 were performed. Arsenic in the leachate exceeded the EU regulatory limit for hazardous waste materials (2.5 mg/L). The toxicity limits defined by USEPA for the TCLP test were exceeded for Cd, Pb and As. The PHREEQC-2 calculation predicted that complex arsenates are the most important solubility controls for metals and metalloids. Furthermore, these phases can readily dissolve during the rainy season (October to March) and flush significant amounts of As, Pb and Cu into the environment in the vicinity of slag dumps.

Hydrogeological and geochemical data, in conjunction with the results of an electrical imaging tomographic survey, were examined to determine the main factors and mechanisms controlling the groundwater chemistry and salinity of the unconfined aquifer of Bou-Areg, on the Mediterranean coast of NE Morocco. In addition, statistical and geochemical interpretation methods were used to identify the distribution of the salinity. Multivariate statistical analysis (cluster and principal component factors) revealed the main sources of contamination. Groups A, B, and C in the cluster analysis and Factors 1–3 (Factor 1: CE, Cl, K+, SO 4 2 - , and Mg2+; Factor 2: Ca2+, HCO 3 - , and pH; Factor 3: NO 3 - ) represent the ‘signature’ of seawater intrusion in the coastal zone, the influence of marly-gypsum outcrops in the upstream zone, and anthropogenic sources, respectively. The ionic delta, the ionic ratio, the saturation index, and Stuyfzand’s method were applied to evaluate geochemical processes. The results obtained indicate, on the one hand, the phenomenon of salinization in both the coastal and the upstream zones, and on the other, the dilution of groundwater by recharge. Cation exchange is shown to modify the concentration of ions in groundwater. Locally, with respect to salinization processes in the coastal zone, the results of electrical imaging tomography show that salinity increases both with depth and laterally inland from the coastline, due to seawater intrusion.

Arsenite adsorption on goethite at elevated temperatures by Michael Kersten; Nataliya Vlasova (32-43).
Experimental closed-system ΔT acid–base titrations between 10 °C and 75 °C were used to constrain a temperature-dependent 1-pK basic Stern model of the goethite surface complexation reactions. Experimental data for the temperature dependence of pHPZC determined by the one-term Van’t Hoff extrapolation yield a value for goethite surface protonation enthalpy of −49.6 kJ mol−1 in good agreement with literature data. Batch titration data between 10 °C and 75 °C with arsenite concentrations between 10 μM and 100 μM yield adsorption curves, which increases with pH, peak at a pH of 9, and decrease at higher pH values. The slope of this bend becomes steeper with increasing temperature. A 1-pK charge distribution model in combination with a basic Stern layer option could be established for the pH-dependent arsenite adsorption. Formation of two inner-sphere bidentate surface complexes best matched the experimental data in agreement with published EXAFS spectroscopic information. The temperature behaviour of the thus derived intrinsic equilibrium constants can be well represented by the linear Van’t Hoff log K T int vs. 1/T plot. Adsorption of arsenite on the goethite surface is exothermic (negative Δr H 298 values) and therefore becomes weaker with increasing temperature. Application of the new constants with the aqueous speciation code VMINTEQ predicts that the As(III) concentration in presence of goethite sorbent decreases by 10 times once the hydrothermal solution is cooled from 99 °C to 1 °C. The model curve matches data from a natural thermal water spring system. The increase of adsorption efficiency for As along the temperature gradient may well serve as an additional process to prevent ecosystem contamination by As-rich water seepage from geothermal energy generation facilities.

The results are reported of a geochemical study of sediment cores and surface waters taken over an annual cycle from the compost-based constructed wetland at Quaking Houses, NE England. The wetland was built to treat acidic and metalliferous waters emanating from colliery spoil. The influent waters contain up to 10 mM SO 4 2 - , total Fe around 100 μM, and a mean pH of 6.2. The organic-rich sediments sustain a coupled redox cycle of Fe and S which occurs throughout the year but which is more intense in the summer months. Throughout the sediments, reduction of Fe(III) and SO 4 2 - apparently occur within the same macroscopic volume of sediment, along with oxidation of sulfide and Fe(III). In the top 2 cm of the sediment, pore water Fe concentrations reach a maximum of ∼1 mM in the presence of high concentrations of Fe oxides and the occurrence of SO4 reduction. Partial re-oxidation of sulfide is indicated by the presence of significant elemental S. Total Fe in the surface 10 cm has a mean value of 7–9% of sediment dry weight, of which sulfide–Fe represents 7–12% and reactive amorphous Fe 20–45%; around 90% of the solid phase sedimentary Fe occurs as oxides or oxyhydroxides. It is suggested that the downwards diffusion of dissolved Fe from the near-surface maximum is sustained by the precipitation of Fe oxides as a result of radial O2 loss from roots in the dense rhizosphere. Pore water pH is between 7.2 and 7.8 and alkalinity increases downwards, coupled to microbial SO4 and Fe reduction. Transport processes occurring at and across the sediment–water interface are sufficiently rapid in the ∼20 h residence time of the waters to: (a) remove 70–90% of influent Fe and 15–25% of influent SO4 into surface sediments and (b) increase both the pH and alkalinity of effluent waters. Coupling of the Fe and S cycles is fundamental to effective remediation in terms of both alkalinity generation and the retention of metals.

Particle size distributions and the mineralogy of inorganic colloids in waters draining the adit of an abandoned mine (Goesdorf, Luxembourg) were quantified by single particle counting based on light scattering (100 nm–2 μm) combined with transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction. This water system was chosen as a surrogate for groundwaters. The dependence of the colloid number concentration on colloid diameters can be described by a power-law distribution in all cases. Power-law slopes ranged from −3.30 to −4.44, depending on water ionic strength and flow conditions. The same main mineral types were found in the different samples: 2:1 phyllosilicates (illite and mica), chlorite, feldspars (albite and orthoclase), calcite and quartz; with a variable number of Fe oxide particles. The colloid mineralogical composition closely resembles the composition of the parent rock. Spatial variations in the structure and composition of the rock in contact with the waters, i.e. fissured rock versus shear joints, are reflected in the colloid composition. The properties of the study colloids, as well as the processes influencing them, can be considered as representative of the colloids present in groundwaters.

Geochemical gradients in soil O-horizon samples from southern Norway: Natural or anthropogenic? by C. Reimann; P. Englmaier; B. Flem; L. Gough; P. Lamothe; Ø. Nordgulen; D. Smith (62-76).
Forty soil O- and C-horizon samples were collected along a south-to-north transect extending inland for approximately 200 km from the southern tip of Norway. The elements As, Au, Bi, Cd, Cu, Ga, Ge, Hf, Hg, In, Mg, Mn, Mo, Na, Ni, Pb, Sb, Se, V, W, Zn and Zr all show a distinct decrease in concentration in soil O-horizons with increasing distance from the coast. The elements showing the strongest coastal enrichment, some by more than an order of magnitude compared to inland samples, are Au, Bi, As, Pb, Sb and Sn. Furthermore, the elements Cd (median O-/median C-horizon = 31), C, Sb, Ag, K, S, Ge (10), Hg, Pb, As, Bi, Sr (5), Se, Au, Ba, Na, Zn, P, Cu and Sn (2) are all strongly enriched in the O-horizon when compared to the underlying C-horizon. Lead isotope ratios, however, do not show any gradient with distance from the coast (declining Pb concentration). Along a 50 km topographically steep east–west transect in the centre of the survey area, far from the coast but crossing several vegetation zones, similar element enrichment patterns and concentration gradients can be observed in the O-horizon. Lead isotope ratios in the O-horizon correlate along both transects with pH and the C/N-ratio, both proxies for the quality of the organic material. Natural conditions in southern Norway, related to climate and vegetation, rather than long range atmospheric transport of air pollutants (LRT), cause the observed features.

Investigation of a 17 m vertical profile of a silt and clay aquitard at a natural gas well site in Alberta, Canada revealed a contaminant plume of gas condensate, along with high concentrations of acetate, propionate and butyrate. The pattern of the distribution of these short-chain fatty acids in groundwater and sediment samples suggested that they have been produced by microorganisms in a process associated with degradation of the condensate hydrocarbons. It is suggested that, in certain zones, under water-saturated and/or anaerobic conditions, these acids were actively consumed by SO4-reducing bacteria. Analyses of DNA extracts by denaturing gel gradient electrophoresis (DGGE) indicated that, compared to sediment samples collected from outside the condensate plume, contaminated samples tended to have fewer, but more strongly developed bands of DNA, which typically had closest affinities to known anaerobes, including species of Fe-reducing Geobacter, and SO4-reducing Desulfosporosinus.

Sorption isotherms, time-dependent adsorption and surface complexation modeling studies were used to investigate the post-depositional mobility of three of the platinum group-elements (Pd, Rh, and Pt) in semi-arid soil and sediment samples with varying surface properties. The acidity constants (Log  K a1 and Log  K a2), optimized from batch titration data, ranged from 4.69 to 5.34 for Log  K a1 and from −6.51 to −7.61 for Log  K a2, suggesting the occurrence of both protonation and deprotonation reactions on the solid surfaces. Partition coefficients and removal rates of the metals had a general trend of Pd > Pt > Rh. The sediment sample, with the highest clay content and exchangeable cation concentrations, also had the highest affinity for the metals. The times required for sediment to adsorb 63% of the metals were 2.63 h, 4.08 h and 10.64 h for Pd, Pt and Rh, respectively. The FITEQL program successfully optimized the conditional binding constants of the metals on the solids from batch adsorption data. The constants decreased in the order of Pd > Rh > Pt, which was consistent with the observed high affinity of the solids for Pd. The modeling results also showed that aqueous Pd was the least sensitive to pH followed by Rh and Pt. However, metal adsorption below the points of zero net proton charges (ca. pH 6.7) is attributable to the involvement of permanent negatively charged binding sites in the adsorption process. Notably, partition coefficients, removal rates and conditional binding constants all showed a high affinity of Pd for the solids. A similarity between the model outputs and the batch adsorption data indicates the suitability of the model for describing the mobility and retention of the three metals in semi-arid soils and sediments.

Subsurface waters circulating in an unpolluted soil of a planosolic horizon (Massif Central, France) were studied in order to determine their physico-chemical characteristics. Three water sampling sites were chosen along a toposequence. For each site, two piezometers were placed above and in the gravelly and concretion-rich horizon (Fe- and Mn- oxyhydroxides). Concentrations of major-, minor- (cations, anions, Fe, Mn, P and Si) and trace elements (Al, Ba, Cd, Co, Cr, Cu, Ni, Pb, Rb, Sr, Zn and U) were monitored on bulk and filtered water (0.45 μm) to study both the particulate and the dissolved components, from 2004 to 2006, during the soil saturation period (i.e., from November to May). Chemical characteristics of soil solutions provide evidence for various chemical water compositions and for temporal variations of water quality, revealing that the hydrodynamic and chemical reactivity in the solution is different for the three sites. Calculations of pe values indicate a range of redox state of the soil solutions. The pe ranges are different for each piezometer but correspond to anoxic solution. For all piezometers, distribution between the dissolved and the particulate fraction and correlations between the various elements in the soil solutions indicate that: (i) Al and Fe show similar behaviour, (ii) Al is mainly present as oxyhydroxides and (iii) some trace metals are mainly associated with particles which have a mixed nature. The impact of a concretion-rich horizon is noticed both on the nature of particles and on the speciation of trace metals and could be explained by the hydrodynamic and chemical reactivity of the circulating solution. Very few correlations exist between elements in the dissolved phase.

Methods for assessing natural background water quality of streams affected by historical mining are vigorously debated. An empirical method is proposed in which stream-specific estimation equations are generated from relationships between either pH or dissolved Cu concentration in stream water and the Fe/Cu concentration ratio in Fe-precipitates presently forming in the stream. The equations and Fe/Cu ratios for pre-mining deposits of alluvial ferricrete then were used to reconstruct estimated pre-mining longitudinal profiles for pH and dissolved Cu in three acidic streams in Montana, USA. Primary assumptions underlying the proposed method are that alluvial ferricretes and modern Fe-precipitates share a common origin, that the Cu content of Fe-precipitates remains constant during and after conversion to ferricrete, and that geochemical factors other than pH and dissolved Cu concentration play a lesser role in determining Fe/Cu ratios in Fe-precipitates. The method was evaluated by applying it in a fourth, naturally acidic stream unaffected by mining, where estimated pre-mining pH and Cu concentrations were similar to present-day values, and by demonstrating that inflows, particularly from unmined areas, had consistent effects on both the pre-mining and measured profiles of pH and Cu concentration. Using this method, it was estimated that mining has affected about 480 m of Daisy Creek, 1.8 km of Fisher Creek, and at least 1 km of Swift Gulch. Mean values of pH decreased by about 0.6 pH units to about 3.2 in Daisy Creek and by 1–1.5 pH units to about 3.5 in Fisher Creek. In Swift Gulch, mining appears to have decreased pH from about 5.5 to as low as 3.6. Dissolved Cu concentrations increased due to mining almost 40% in Daisy Creek to a mean of 11.7 mg/L and as much as 230% in Fisher Creek to 0.690 mg/L. Uncertainty in the fate of Cu during the conversion of Fe-precipitates to ferricrete translates to potential errors in pre-mining estimates of as much as 0.25 units for pH and 22% for dissolved Cu concentration. The method warrants further testing in other mined and unmined watersheds. Comparison of pre-mining water-quality estimates derived from the ferricrete and other methods in single watersheds would be particularly valuable. The method has potential for use in monitoring remedial efforts at mine sites with ferricrete deposits. A reasonable remediation objective might be realized when the downstream pattern of Fe/Cu ratios in modern streambed Fe-precipitates corresponds to the pattern in pre-mining alluvial ferricrete deposits along a stream valley.

Phosphorus speciation and availability in intertidal sediments of the Yangtze Estuary, China by L.J. Hou; M. Liu; Y. Yang; D.N. Ou; X. Lin; H. Chen; S.Y. Xu (120-128).
In order to better understand P cycling and bioavailability in the intertidal system of the Yangtze Estuary, both surface (0–5 cm) and core (30 cm long) sediments were collected and sequentially extracted to analyze the solid-phase reservoirs of sedimentary P: loosely sorbed P; Fe-bound P; authigenic P; detrital P; and organic P. The total sedimentary P in surface and core sediments ranged from 14.58–36.81 μmol g−1 and 17.11–24.55 μmol g−1, respectively, and was dominated by inorganic P. The average percentage of each fraction of P in surface sediments followed the sequence: detrital P (54.9%) > Fe-bound P (23.7%) > organic P (14.3%) > authigenic P (6.3%) > loosely sorbed P (0.8%), whereas in core sediments it followed the sequence: detrital P (61.7%) > Fe-bound P (17.0%) > authigenic P (13.1%) > organic P (7.5%) > loosely sorbed P (0.7%). Post-depositional reorganization of P was observed in both surface and core sediments, converting organic P and Fe-bound P to authigenic P. The accumulation rates and burial efficiencies of the total P in the intertidal area ranged from 118.70–904.98 μmol cm−2  a−1 and 80.29–88.11%, respectively. High burial efficiency of the total P is likely related to the high percentage of detrital P and the high sediment accumulation rate. In addition, the bioavailable P represented a significant proportion of the sedimentary P pool, which on average accounted for 37.4% and 25.1% of the total P in surface and core sediments, respectively. This result indicates that the tidal sediment is a potential internal source of P for this P-limiting estuarine ecosystem.

Stable isotope data, a dissolved gas tracer study, groundwater age dating, and geochemical modeling were used to identify and characterize the effects of introducing low-TDS recharge water in a shallow aerobic aquifer affected by a managed aquifer recharge project in California’s San Joaquin Valley. The data all consistently point to a substantial degree of mixing of recharge water from surface ponds with ambient groundwater in a number of nearby wells screened at depths above 60 m below ground surface. Groundwater age data indicate that the wells near the recharge ponds sample recently recharged water, as delineated by stable O and C isotope data as well as total dissolved solids, in addition to much older groundwater in various mixing proportions. Where the recharge water signature is present, the specific geochemical interactions between the recharge water and the aquifer material appear to include ion exchange reactions (comparative enrichment of affected groundwater with Na and K at the expense of Ca and Mg) and the desorption of oxyanion-forming trace elements (As, V, and Mo), possibly in response to the elevated pH of the recharge water.

Transport of 234U in the Opalinus Clay on centimetre to decimetre scales by Marek Pękala; Jan Dirk Kramers; Hans Niklaus Waber; Thomas Gimmi; Peter Alt-Epping (138-152).
The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements.Samples were digested using aqua regia followed by total HF dissolution, yielding two fractions. In all studied samples U was found to be concentrated in the HF digestion fraction. It has a high U/Th ratio and a study by SEM-EDS points to sub-μm up to several μm in size zircon grains as the main U-rich phase. This fraction consistently has 234U/238U activity ratios below unity. The minute zircon grains constitute the major reservoir of U in the rock and act as constant rate suppliers of 234U into the rock matrix and the pore water. The aqua regia leach fraction was found to be enriched in Th, and complementary to the HF fraction, having 234U/238U activity ratios above unity. It is believed that these U activity ratios reflect the surplus of 234U delivered from the zircon grains. Some cm-spaced samples show bulk rock 234U/238U activity ratios that are markedly out of equilibrium. In most of them a striking negative correlation between the total U content and the bulk rock 234U/238U activity ratios is observed. This is interpreted to indicate net 234U transfer from regions of higher supply of 234U towards those of lower supply which is, in most cases, equivalent to transfer from clayey towards carbonate/sandy portions of the rock. In contrast, the 25 cm averaged samples all have uniform bulk rock 234U/238U activity ratios in equilibrium, indicating U immobility in the last 1–1.5 Ma on this spatial scale. It is concluded that the small-scale lithological variations which govern U spatial distribution in the Opalinus Clay are the major factor determining 234U in situ supply rates, regulating its diffusive fluxes and controlling the observed bulk rock 234U/238U activity ratios. A simple box-model is presented to simulate the measured bulk rock 234U/238U activity ratios and to give an additional insight into the studied system.

Attrition efficiency in the decontamination of stormwater sediments by François Petavy; Veronique Ruban; Pierre Conil; Jean Yves Viau (153-161).
The purpose of this research is to propose a laboratory method based on attrition and sieving for the treatment of runoff water sediments in the aim of developing a pilot unit. The attrition process serves to remove fine particles and pollutants from the surface of coarse stormwater sediment particles. In all cases, the efficiency of pollutant removal is dependent upon various parameters, including cutoff threshold, residence time, solid density, temperature and impeller speed. This article presents the optimization of these various parameters along with method efficiency; for this work, several sediments have been tested. The results indicate that an attrition scrubber may be effectively used to remediate contaminated sediment and that reuse is definitely possible. A model of the method will also be proposed to study the behavior of fine particles and pollutants.

Variations of Pb in a mine-impacted tropical river, Taxco, Mexico: Use of geochemical, isotopic and statistical tools by F. Arcega-Cabrera; M.A. Armienta; L.W. Daesslé; S.E. Castillo-Blum; O. Talavera; A. Dótor (162-171).
The potential environmental threat from Pb in Mexican rivers impacted by historic mining activities was studied using geochemical, isotopic and statistical methods. Lead geochemical fractionation and factor analysis of fractionated and total Pb indicate that anthropogenic sources have contributed significantly to Pb concentrations, while natural sources have contributed only small amounts. The analyses also indicate that two main processes are controlling the total Pb variation throughout the year in both rivers: erosion with discharge processes, and proportional dilution related to differences in grain-size distribution processes. Bio-available Pb in riverbed sediments was greater than 50% in 80% of the sampling stations indicating a high potential environmental risk, according to the risk assessment criteria (RAC). Nevertheless, based on the environmental chemistry of Pb and on multivariate statistical analysis, these criteria did not apply in this particular case. Significant differences (p  < 0.05) in total Pb concentrations (from 50 to 5820 mg kg−1) and in the geochemical fractionation were observed as a function of seasonality and location along the river flow path. In the Cacalotenango and Taxco rivers, the highest concentrations of total Pb were found at stations close to tailings during the rainy and post-rainy seasons. The geochemistry of Pb was mainly controlled, during the dry and post-rainy seasons by the organic matter and carbonate content, and in the rainy season by hydrological conditions (e.g., the increase in river flux), hydrological basin erosion, and the suspended solids concentration. Isotopic analyses of the 210Pb/214Pb ratio showed three processes in the Cacalotenango and Taxco rivers. First, the accumulation of atmospheric excess 210Pb, favoured during calmer hydrodynamic conditions in the river basin commonly during dry periods, is recorded by a 210Pb/214Pb ratio of >1. In the case of the Cacalotenango river, 210Pb did show preferential accumulation in sediments. Second, a 210Pb/214Pb ratio of <1 in some samples might be indicating (a) the presence of eroded material from weathered tailings with similar 210Pb depletion (probably by secular disequilibrium caused by weathering or mining processes, which was observed in both rivers), or (b) preferential transport of Pb and sediments during high energy events (e.g., flow increase, as is the case of Taxco river). Third, no significant changes in the 210Pb/214Pb ratio might be reflecting a situation where mining material is not entering the system, or where hydrodynamic changes throughout the year of equal magnitude allow the system to reach a new equilibrium for the 210Pb/214Pb ratio. Finally, based on these results it is recommended that inhabitants of the studied area avoid using water from the Cacalotenango river in the rainy and post-rainy seasons, and to take precautions for its use in the dry season, such as allowing suspended material to settle before use, and that they should avoid use of Taxco river water at all times.

Biomarker 17α(H)-diahopane: A geochemical tool to study the petroleum system of a Tertiary lacustrine basin, Northern South China Sea by Meijun Li; Tieguan Wang; Ju Liu; Meizhu Zhang; Hong Lu; Qinglin Ma; Lihui Gao (172-183).
Relatively abundant C30 17α(H)-diahopane has been detected in oils and rock extracts in the Fushan Depression, a Tertiary lacustrine basin of the Northern South China Sea. C30 17α(H)-diahopane/(C30 17α(H)-diahopane + C30 17α(H)-hopane) ratios (DI) increase with increased burial depth of the source rocks. DI values of most oils from the upper unit ( E 2 l 3 1 ) of the third member of the Eocene Liushagang Formation (E2 l 3) in the Huachang oil and gas field show a positive correlation with other molecular maturity indicators, such as the DNR (dimethylnaphthalene ratio), MDR (methyldibenzothiophene ratio), 4,6-/1,4-DMDBT (4,6-/1,4-dimethyldibenzothiophene), MPR (methylphenanthrene ratios), or the MPI-1 (methylphenanthrene index). The inflection of the DI-depth curve occurring at about 3200 m, corresponds to 0.55 (R o%). The oils of the Huachang oil and gas field have higher DI values than the extracts of in situ source rocks, indicating the oils might be sourced from deeply-buried source rocks. Two petroleum systems can be distinguished in the Huachang field in terms of the relative concentration of diahopane and other maturity parameters. The oils in the upper petroleum system, consisting of the reservoirs in E 2 l 3 1 , are mainly sourced from the E2 l source rocks in the Bailian sag, and are characterized by relatively higher maturity, higher oil and gas yields, and lower contents of diahopane and oleananes. The oils in the lower system, consisting of middle ( E 2 l 3 2 ) and lower units ( E 2 l 3 3 ) of E2 l 3, have higher contents of diahopane, higher contents of CO2 gas, and lower maturity and oil and gas yields. The occurrence and geochemical characteristics of the petroleum system are mainly controlled by the “Double Fault System” in the Huachang uplift. The C30 17α(H)-diahopane is an effective biomarker for oil–source rock correlation, oil–oil correlation, maturity assessment and petroleum system study in some terrestrial lacustrine basins.