Methane-derived authigenic carbonates from the Ulleung basin sediments, East Sea of Korea

Authigenic carbonates were sampled in methane-enriched piston core sediments collected from gas venting sites on the western continental slope of the Ulleung Basin, East Sea of Korea. Multidisciplinary investigations on these carbonates, including the scanning electronic microscope (SEM) observations and mineralogical-geochemical compositions, were carried out to identify the carbon and oxygen sources and the forming mechanism of these carbonates. The authigenic carbonates from the study area correspond to semi-consolidated, compact concretions or nodules ranging from 2 to 9 cm in size. X-ray diffraction and electron microprobe analyses showed that most of the sampled carbonate concretions were composed of almost purely authigenic high-Mg calcite (10.7-14.3 mol% MgCO3). Characteristically, microbial structures such as filaments and rods, which were probably associated with the authigenic minerals, were abundantly observed within the carbonate matrix. The carbonates were strongly depleted in delta C-13 (-33.85 parts per thousand to -39.53 parts per thousand Peedee Belemnite (PDB)) and were enriched in delta O-18 (5.16-5.60 parts per thousand PDB), indicating that the primary source of carbon is mainly derived from the anaerobic oxidation of methane. Such methane probably originated from the destabilization of the underlying gas hydrates as strongly supporting from the enriched O-18 levels. Furthermore, the strongly depleted delta C-13 values (-60.7 parts per thousand to -61.6 parts per thousand PDB) of the sediment void gases demonstrate that the majority of the gas venting at the Ulleung Basin is microbial methane by CO2 reduction. This study provides another example for the formation mechanism of methane-derived authigenic carbonates associated with gas-hydrate decomposition in gas-seeping pockmark environments. (c) 2009 Elsevier Ltd. All rights reserved.

Tectonics of the southern tip of the Parece Vela Basin, Philippine Sea Plate

We report new geophysical and petrological data collected at the southern tip of the Parece Vela Basin in the Philippine Sea. The Parece Vela Basin, which was formed as a backarc basin behind proto Mariana arc-trench system from late Oligocene to middle Miocene, provides us a good opportunity to study the nature of successive backarc basin formations in the Philippine Sea and the relationship between are and backarc magmatisms. Regional bathymetric map derived from satellite altimetry shows that the southern tip of the basin, now located just west of the Yap arc-trench system, has unique morphological and tectonic features which include: 1) the absence of spreading center or its trace, 2) shallow average depth, and 3) enigmatic curved structures. Our newly collected high-resolution bathymetric data reveal that the spreading fabric similar to the central Parece Vela Basin exists to the north of 9 degrees 20'N. Thus it appears that the present-day Yap arc and backarc region represent the western half of the seafloor that was produced by the early E-W and the following NE-SW spreading in the northern and central Parece Vela Basin, and that the eastern counterpart now lies west of the West Mariana Ridge. Unlike the northern Parece Vela Basin, there appears to be no evidence for a systematic propagation of spreading center in the southern part. Instead two rift segments, one which extends from the central Parece Vela Basin and the other which lies within the western remnant arc (Kyushu-Palau Ridge), overlap at the southern tip of the basin, producing a complex seafloor that includes curvilinear deeps and deformed topographic highs. (c) 2007 Elsevier B.V. All rights reserved.

Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin, northern South China Sea

Our analysis of approximately 40,000 km of multichannel 2-D seismic data, reef oil-field seismic data, and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas (water depth >= 500 in) of the Qiongdongnan Basin (QDNB), northern South China Sea. High-resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid-Miocene Meishan Formation in the Beijiao and Ledong-Lingshui Depressions developed reef carbonates. The seismic features of the reef carbonates in these two areas include chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies, similar to the seismic characteristics of the LH11-1 reef reservoir in the Dongsha Uplift and Island Reef of the Salawati Basin, Indonesia, which house large oil fields. The impedance values of reefs in the Beijiao and Ledong-Lingshui Depressions are 8000-9000 g/cc x m/s. Impedance sections reveal that the impedance of the LH11-1 reef reservoir in the northern South China Sea is 800010000 g/cc x m/s, whereas that of pure limestone in BD23-1-1 is > 10000 g/cc x m/s. The mid-Miocene paleogeography of the Beijiao Depression was dominated by offshore and neritic environments, with only part of the southern Beijiao uplift emergent at that time. The input of terrigenous sediments was relatively minor in this area, meaning that terrigenous source areas were insignificant in terms of the Beijiao Depression: reef carbonates were probably widely distributed throughout the depression, as with the Ledong-Lingshui Depression. The combined geological and geophysical data indicate that shelf margin atolls were well developed in the Beijiao Depression, as in the Ledong-Lingshui Depression where small-scale patch or pinnacle reefs developed. These reef carbonates are promising reservoirs, representing important targets for deepwater hydrocarbon exploration. (C) 2008 Elsevier Ltd. All rights reserved.

Fine-grained Pleistocene deepwater turbidite channel system on the slope of Qiongdongnan Basin, northern South China Sea

Recently, as oil exploitation has become focused on deepwater slope areas. more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW-NE to W-E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel-levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting. (C) 2009 Elsevier Ltd. All rights reserved.

Hydrogeochemistry of formation water in relation to overpressures and fluid flow in the Qikou Depression of the Bohai bay basin, China

The Qikou Depression is the largest hydrocarbon bearing depression in the western part of the Bohai bay basin, dominated by fan delta and lacustrine strata with volcanic and volcaniclastic rocks. In this study, the formation pressures and hydrochemistry of the formation water in the Qikou depression are investigated. It is found that a significant overpressure occurs in the Dongying (Ed) Formation and the first member (Est), the second member (Es2), the third member (Es3) of the Shahejie Formation. The pressure coefficients commonly range from 1.2 to 1.6 with the highest pressure coefficient being 1.7. The analysis of hydrochemistry data shows that the whole depression is dominated by NaHCO3 water type. The concentration of total dissolved solid (TDS) ranges from 2.13 to 53.16 g/L and shows a distinct vertical variation of salinity and ion ratios. High salinity water (TDS> 10 g/L) occurs below a depth of 2500 m, which coincides with the presence of the overpressured system. However, the increasing trend of TDS is diminished below 3500 m because the generation of organic acids in Qikou Depression is inhibited in the presence of overpressure. The analysis of the relationship among different ions indicates that the present-day characteristics of the formation water result from the albitization of feldspar and the dissolution of sodium-rich silicate minerals and halite in the different hydrochemical and pressure systems. (C) 2009 Elsevier B.V. All rights reserved.

Polygonal faults and their implications for hydrocarbon reservoirs in the southern Qiongdongnan Basin, South China Sea

The Polygonal faults were identified in Qiongdongnan Basin, South China Sea, by using the technique of time coherent slice and horizon flattening of high-resolution 3D seismic data. These polygonal faults occur in three tiers of the upper Meishan Formation and the Huangliu Formation. The faults have lengths of 150-1500 m, spacings of 50-3000 m, throws of 10-40 m and dips of 50-90 degrees. Tectonic evolution in the Qiongdongnan Basin can be divided into a rifted stage and a post-rifted stage. Tectonic faults are widely distributed in the rifted sequences, but are not well developed in the post-rifted stage. Few faults in the post-rifted sequences might suggest the absence of a migration pathway for hydrocarbon or other fluids. However, the existence of polygonal faults in the post-rifted sequences can serve as the pathway and promote the hydrocarbon migration and accumulation in the Qiongdongnan Basin during the post-rifted stage. (C) 2010 Elsevier Ltd. All rights reserved.

Triterpane and sterane biomarkers in the YA13-1 condensates from Qiongdongnan Basin, South China Sea

Triterpanes and steranes in condensates from the YA13-1 gas field, Qiongdongnan Basin, were monitored. The YA13-1 condensates have unusual biomarker distributions dominated by terpanes and steranes derived from higher plants. Anomalously abundant 1 got-oleanane and remarkably abundant bicadinanes are present in the YA13-1 condensates, whereas the 17alpha-hopane contents are extremely low. Taraxastane and significantly abundant 17alpha-diahopanes occur in the condensates. In addition, a number of unknown C-29 and C-30 pentacyclic triterpanes including previously unreported compounds were detected in the condensates, some of which are significantly abundant. The unknown compounds may be terrestrial biomarkers. C-29 homologues are relatively predominant among the regular and rearranged steranes. The diasterane concentrations are markedly higher than those of regular steranes. The maturity of the YA13-1 condensates is relatively high, at the peak to late oil generation stage (corresponding to 0.85-1.10% Ro), based on sterane and terpane and including bicadinane maturity parameters (i.e. T/(T-1 + R) and 2T/R bicadinane ratios). The above maturity assessment result is different from that based on diamondoid maturity parameters (%Ro = 1.60-1.70) [Org. Geochem. 25 (1996) 179], which can be explained by a contribution of hydrocarbons from two sources at different depths. The YA13-1 condensates were probably generated from the Yacheng and Lingshui coal-bearing source rocks buried both in the Qiongdongnan Basin (3400-5000 m) and in the Yinggehai Basin (>5000 m). The possible contribution of lower maturity hydrocarbons from the Yacheng and Lingshui Formations (3400-4100 m) in the Qiongdongnan Basin to the YA13-1 gases and condensates should not be neglected. (C) 2003 Elsevier Science B.V. All rights reserved.

Polymorphisms in the ACE and ADRB2 genes and risks of aging-associated phenotypes: the case of myocardial infarction.

Multiple functions of the beta2-adrenergic receptor (ADRB2) and angiotensin-converting enzyme (ACE) genes warrant studies of their associations with aging-related phenotypes. We focus on multimarker analyses and analyses of the effects of compound genotypes of two polymorphisms in the ADRB2 gene, rs1042713 and rs1042714, and 11 polymorphisms of the ACE gene, on the risk of such an aging-associated phenotype as myocardial infarction (MI). We used the data from a genotyped sample of the Framingham Heart Study Offspring (FHSO) cohort (n = 1500) followed for about 36 years with six examinations. The ADRB2 rs1042714 (C-->G) polymorphism and two moderately correlated (r(2) = 0.77) ACE polymorphisms, rs4363 (A-->G) and rs12449782 (A-->G), were significantly associated with risks of MI in this aging cohort in multimarker models. Predominantly linked ACE genotypes exhibited opposite effects on MI risks, e.g., the AA (rs12449782) genotype had a detrimental effect, whereas the predominantly linked AA (rs4363) genotype exhibited a protective effect. This trade-off occurs as a result of the opposite effects of rare compound genotypes of the ACE polymorphisms with a single dose of the AG heterozygote. This genetic trade-off is further augmented by the selective modulating effect of the rs1042714 ADRB2 polymorphism. The associations were not altered by adjustment for common MI risk factors. The results suggest that effects of single specific genetic variants of the ADRB2 and ACE genes on MI can be readily altered by gene-gene or/and gene-environmental interactions, especially in large heterogeneous samples. Multimarker genetic analyses should benefit studies of complex aging-associated phenotypes.

Aspectos ecofisiológicos de dos forrajeras estivales de los pastizales de la Depresión del Salado (Paspalum dilatatum Poir. y Bothriochloa laguroides D. C.)

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