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.

Seismic stratigraphy of the Qiongdongnan deep sea channel system, northwest South China Sea

Based on more than 4000 km 2D seismic data and seismic stratigraphic analysis, we discussed the extent and formation mechanism of the Qiongdongnan deep sea channel. The Qiongdongnan deep sea channel is a large incised channel which extends from the east boundary of the Yinggehai Basin, through the whole Qiongdongnan and the Xisha trough, and terminates in the western part of the northwest subbasin of South China Sea. It is more than 570 km long and 4-8 km wide. The chaotic (or continuous) middle (or high) amplitude, middle (or high) continuity seismic facies of the channel reflect the different lithological distribution of the channel. The channel formed as a complex result of global sea level drop during early Pliocene, large scale of sediment supply to the Yinggehai Basin, inversion event of the Red River strike-slip fault, and tilted direction of the Qiongdongnan Basin. The large scale of sediment supply from Red River caused the shelf break of the Yinggehai Basin to move torwards the S and SE direction and developed large scale of prograding wedge from the Miocene, and the inversion of the Red River strike-slip fault induced the sediment slump which formed the Qiongdongnan deep sea channel.

Analysis on deep crustal structure along the onshore-offshore seismic profile across the Binhai (littoral) fault zone in northeastern South China Sea

The onshore-offshore deep seismic experiment was carried out for the first time and filled the blankness of the seismic surveys in the transition area between South China and northeastern South China Sea. The seismic data were analyzed and processed. The different seismic phases were identified and their travel time arrivals were modeled by ray-tracing to study the P-wave velocity crustal structure of this area. The crustal structure of this area is the continental crust. The crust thickness is gradually decreasing southward along the on-shore-offshore seismic line. The low-velocity layer (5.5 similar to 5.9 km (.) s(-1)) exists generally in the middle crust (about 10.0 similar to 18.0km)with about 2.5 similar to 4.0 km thickness, which is also thinning seaward. No obvious high-velocity layer appears in the lower crust. The Binhai (littoral) fault zone is a low velocity zone, which is located about 35km southeast to the Nan'ao station and corresponding to the gradient belt of gravity & magnetism anomalies. The depth of the fault zone is close to the Moho discontinuity. The littoral fault zone is a boundary between the normal continental crust of South China and the thinned continental crust of the sea area.

Seismic profiles across the middle Tan-Lu fault zone in Laizhou Bay, Bohai Sea, eastern China

The stratigraphic architecture, structure and Cenozoic tectonic evolution of the Tan-Lu fault zone in Laizhou Bay, eastern China, are analyzed based on interpretations of 31 new 2D seismic lines across Laizhou Bay. Cenozoic strata in the study area are divided into two layers separated by a prominent and widespread unconformity. The upper sedimentary layer is made up of Neogene and Quaternary fluvial and marine sediments, while the lower layer consists of Paleogene lacustrine and fluvial facies. In terms of tectonics, the sediments beneath the unconformity can be divided into four main structural units: the west depression, central uplift, east depression and Ludong uplift. The two branches of the middle Tan-Lu fault zone differ in their geometry and offset: the east branch fault is a steeply dipping S-shaped strike-slip fault that cuts acoustic basement at depths greater than 8 km, whereas the west branch fault is a relatively shallow normal fault. The Tan-Lu fault zone is the key fault in the study area, having controlled its Cenozoic evolution. Based on balanced cross-sections constructed along transverse seismic line 99.8 and longitudinal seismic line 699.0, the Cenozoic evolution of the middle Tan-Lu fault zone is divided into three stages: Paleocene-Eocene transtension, Oligocene-Early Miocene transpression and Middle Miocene to present-day stable subsidence. The reasons for the contrasting tectonic features of the two branch faults and the timing of the change from transtension to transpression are discussed. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.

Simulation of the Load-Unload Response Ratio and Critical Sensitivity in the Lattice Solid Model

The Load-Unload Response Ratio (LURR) method is an intermediate-term earthquake prediction approach that has shown considerable promise. It involves calculating the ratio of a specified energy release measure during loading and unloading where loading and unloading periods are determined from the earth tide induced perturbations in the Coulomb Failure Stress on optimally oriented faults. In the lead-up to large earthquakes, high LURR values are frequently observed a few months or years prior to the event. These signals may have a similar origin to the observed accelerating seismic moment release (AMR) prior to many large earthquakes or may be due to critical sensitivity of the crust when a large earthquake is imminent. As a first step towards studying the underlying physical mechanism for the LURR observations, numerical studies are conducted using the particle based lattice solid model (LSM) to determine whether LURR observations can be reproduced. The model is initialized as a heterogeneous 2-D block made up of random-sized particles bonded by elastic-brittle links. The system is subjected to uniaxial compression from rigid driving plates on the upper and lower edges of the model. Experiments are conducted using both strain and stress control to load the plates. A sinusoidal stress perturbation is added to the gradual compressional loading to simulate loading and unloading cycles and LURR is calculated. The results reproduce signals similar to those observed in earthquake prediction practice with a high LURR value followed by a sudden drop prior to macroscopic failure of the sample. The results suggest that LURR provides a good predictor for catastrophic failure in elastic-brittle systems and motivate further research to study the underlying physical mechanisms and statistical properties of high LURR values. The results provide encouragement for earthquake prediction research and the use of advanced simulation models to probe the physics of earthquakes.

美国西部地区加卸载响应比的时空扫描及其地震趋势研究

加卸载响应比理论是近年来提出的地震预报新方法,在中国大陆地区的实际地震预测中取得了较好的效果.为了研究加卸载响应比理论是否适用于不同地质条件的地震,是否具有普适性,选择了典型的美国圣安德烈斯断层带及其周围地区的地震作为研究对象,计算分析了美国西部地区加卸载响应比的变化情况,并对该地区未来的地震趋势进行了研究.

The degree of predictability of earthquakes in several regions of China: statistical analysis of historical data

The stress release model, a stochastic version of the elastic-rebound theory, is applied to the historical earthquake data from three strong earthquake-prone regions of China, including North China, Southwest China, and the Taiwan seismic regions. The results show that the seismicity along a plate boundary (Taiwan) is more active than in intraplate regions (North and Southwest China). The degree of predictability or regularity of seismic events in these seismic regions, based on both the Akaike information criterion (AIC) and fitted sensitivity parameters, follows the order Taiwan, Southwest China, and North China, which is further identified by numerical simulations. (c) 2004 Elsevier Ltd. All rights reserved.

Structures on sliding base subject to horizontal and vertical motions

In this paper particular investigation is directed towards the combined effects of horizontal and vertical motions of real earthquakes to structures resting on sliding base. A simplified method is presented to treat the nonlinear effects of time dependent frictional force of the sliding base as a function of the vertical reaction produced by the foundation. As an example, the El Centro 1940 earthquake record is used on a structural model to show the structural responses due to a sliding base with different frictional and stiffness characteristics. The study shows that vertical ground motion does affect both the superstructure response and the base sliding displacement. Nevertheless, the sliding base isolator is shown to be effective for the reduction of seismic response of a superstructure.

Experimental Research On Fatigue Property Of Steel Rubber Vibration Isolator For Offshore Jacket Platform In Cold Environment

Jacket platform is the most widely used offshore platform. Steel rubber vibration isolator and damping isolation system are often used to reduce or isolate the ice-induced and seismic-induced vibrations. The previous experimental and theoretical studies concern mostly with dynamic properties, vibration isolation schemes and vibration-reduction effectiveness analysis. In this paper, the experiments on steel rubber vibration isolator were carried out to investigate the compressive properties and fatigue properties in different low temperature conditions. The results may provide some guidelines for design of steel rubber vibration isolator for offshore platform in a cold environment, and for maintenance and replacement of steel rubber vibration isolator, and also for fatigue life assessment of the steel rubber vibration isolator. (C) 2009 Elsevier Ltd. All rights reserved.

地震作用下大型升船机结构的时变动力可靠度

利用三维有限元方法对三峡升船机塔柱结构的动力学特性及随机地震响应进行了计算分析,结果表明塔柱结构柔度较大,其项部节点随机地震位移响应为中宽带过程.在此基础上,采用首次超越破坏机制,以塔柱结构顶部典型位置的位移限值为可靠度界限,对设计地震烈度下升船机塔柱结构的时变动力可靠度进行了计算分析,得到了塔柱结构设计基准期内的时变动力可靠度,并讨论了可靠度界限值的随机性对结构抗震时变可靠度计算结果的影响,建议升船机结构抗震可靠度计算模型采用Markov过程假定.该文可为升船机结构设计及安全运行提供必要的参考.

Gas-bearing fluid influx sub-system for gas hydrate geological system in Shenhu Area, Northern South China Sea

Based on the comprehensive interpretation and study of the Neogene fracture system and diapiric structure, it can be concluded that the diapiric structures, high-angle fractures and vertical fissure system are the main gas-bearing fluid influx sub-system for gas hydrate geological system in Shenhu Area, northern South China Sea. The Neogene fractures widely developed in the study area may be classed into two groups: NW (NNW)-trending and NE (NNE)-trending. The first group was active in the Late Miocene, while the second one was active since the Pliocene. The NE (NNE)-trending fractures were characterized by lower activity strength and larger scale, and cut through the sediment layers deposited since the Pliocene. Within the top sediment layers, the high-angle fracture and vertical fissure system was developed. The diapiric structures display various types such as a turtle-back-like arch, weak piercing, gas chimney, and fracture (or crack, fissure). On the seismic profile, some diapiric structures show the vertical chimney pathway whose top is narrow and the bottom is wide, where some ones extend horizontally into pocket or flower-shaped structures and formed the seismic reflection chaotic zones. Within the overlying sediment layers of the diapiric structure, the tree branch, flower-shaped high-angle fractures and vertical fissures were developed and became the pathway and migration system of the gas-bearing fluid influx. In the study area, the diapiric structures indicate a high temperature/over pressure system ever developed. Closely associated and abundant bright-spots show the methane-bearing fluid influx migrated vertically or horizontally through the diapiric structures, high-angle fractures and vertical fissures. In the place where the temperature and pressure conditions were favor for the formation of gas hydrate, the hydrate reservoir deposition sub-system was developed.

强地震荷载作用下含断层岩质边坡稳定性分析

基于岩石材料的弹塑性损伤理论, 引入两个独立的损伤变量分别控制岩石的拉压行为, 通过对地震菏载作用下边坡场地响应的有限元数值模拟, 总结含裂隙的岩质边坡发生局部损伤软化、破坏的仄域的扩展、演化过程和规律.结合对某大型水电坝坝址左岸的岩质边坡在强地震动载荷作用下(百年一遇、超越概率 2%)发生破坏的数值模拟, 揭示这类边坡滑坡的机理, 并根据模拟的结果, 对整治边坡破坏提出参考依据

地震作用下顺层岩质边坡动力响应的试验研究

基于相似理论建立顺层岩质边坡力学模型,并介绍爆炸模型试验的设计思路及试验方法.以导爆索为爆源,通过水下爆炸模型试验,研究近场地震作用下原型边坡的变形破坏模式以及支护结构对边坡稳定性的影响.试验结果表明,顺层岩质边坡的在近场地震作用下的破坏主要表现方式为节理岩体间的层间滑动,破坏模式为应力波导致的滑移-拉裂破坏;重力式挡墙的压应力峰值分布随着墙高的变化呈现先增大后减小的钟形分布;结合汶川地震现场考察及试验结果发现,支护结构对边坡稳定性起着重要作用