海洋三维VC观测系统优化设计

观测系统设计在地震勘探中起着至关重要的作用,最佳观测系统不仅可以提高资料品质,还能够降低采集成本。通过对海洋垂直缆（vertical cable,VC）进行正演模拟我们发现,随着偏移距的增大,同相轴会出现交叉、合并现象,地层顺序也会发生变化。我们针对单VC和多VC的特点,分别设计了相应的观测系统。然后对所设计的观测系统进行评价与优化,并得出了一些有益的结论：采用增加激发点密度的方法对面元覆盖次数的改善效果要好于采用增加激发面积的方法对面元覆盖次数的改善效果;当目标层存在倾角时,通过在构造走向上增加激发线条数,同时在下倾方向上增加激发线长度可以补偿照明损失;当目标层为背斜或向斜时,通过增大最大环半径来增加背斜和向斜照明范围的方法奏效甚微;当激发面积和接收面积相等时,通过同时增大激发面积和接收面积来提高中心区域面元覆盖次数的方法行不通,而当激发面积大于接收面积时则该问题得以解决。     

天然气水合物开采过程甲烷气泄漏海床基高密度电阻率法监测效果模拟与分析

为实现天然气水合物开采过程沉积物内甲烷气泄漏过程有效监测,设计了一种海床基原位电学监测方法。为界定该方法对不同模式泄露甲烷气的探测能力,以南海神狐海域天然气水合物试采区为研究区,构建相应地质及电阻率模型,模拟利用设计电学系统对其进行监测,计算得到不同采集参数的电阻率剖面,并对其进行对比分析。研究结果表明：海水层会在探测剖面上某深度区间内形成层状低阻异常带,对该深度区间有效电信号形成压制。将该异常带顶界深度定义为系统有效探测深度后,发现该深度受电极距直接影响,10 m极距偶极装置有效探测深度约为50 m。电学探测剖面对有效探测深度内分布的层状和团状甲烷气聚集区、慢速甲烷气泄露区、沿断层泄露的甲烷气区具有良好反映能力,数据处理得到的相对电阻率剖面与电学探测剖面相比能更好地反映甲烷气聚集区边界。该监测方法能够实时监测含气区空间变化。     

Sedimentary control on the formation of a multi-superimposed gas system in the development of key layers in the sequence framework

Based on core observations, well logs and test results of siderite-bearing mudstone from the Benxi Formation to the Member 2 of the Shanxi Formations in the Linxing block, northeastern Ordos Basin, a logging identification model for siderite-bearing mudstone (key layer) was established. The porosity characteristics and sealing property were quantitatively evaluated by logging data. Sedimentary control on the formation of multi-superimposed gas-bearing system in the development of key layers in the sequence framework was also discussed. The results showed that the siderite-bearing mudstone has obvious logging response characteristics, e.g., high photoelectric absorption cross-section index (PE), high density (DEN), high amplitude natural gamma ray (GR), low acoustic (AC), low resistivity (M2RX) and low neutron porosity (CNCF). The quantitatively evaluated results of the porosity characteristics and sealing property for the key layer showed that the key layer has the characteristics of low porosity (with an average of 1.20 percent), low permeability (with an average of 2.29 × 10⁻⁸μm²), and high breakthrough pressure (with an average of 12.32 MPa) in the study area. This layer acts as an impermeable gas barrier in a multi-superimposed gas system. The results also indicated that the material composition of the multi-superimposed gas-bearing system can be established by the sequence stratigraphic framework. The sedimentary evolution results in a cyclic rhythm of material composition vertically. The spatial distribution of the corresponding transgressive event layer near the maximum flooding surface (MFS) in the sequence framework restricts the spatial distribution of the key layer with high breakthrough pressure and low porosity, which constitutes the gas-bearing system boundary. The siderite-bearing mudstone formed near the MFS in the second-order sequence and constitutes a stable comparison of the first-order gas-bearing system boundary, which has a wide range of regional distribution and stable thickness. The siderite-bearing mudstone formed near the MFS in the third-order sequence is often incompletely preserved due to the late (underwater) diversion channel erosion and cutting. This layer forms the coal-bearing reservoirs, which we termed as a second-order gas-bearing system in adjacent third-order sequences to form a uniform gas-bearing system.     

Structural integrity of Ellisolandia elongata reefs: A mechanical approach to compare tensile strengths in natural and controlled environments

Geniculate coralline algae are oases of biodiversity, providing nursery areas and shelter for the species that live amongst their fronds. The key to their success in the inter‐tidal is the ability to withstand hydrodynamic forces. Under culturing conditions most of the physical and ecological stressors such as intense hydrodynamic forces and grazing are extremely reduced, thus affecting species mechanical properties and their response to external threats. The aim of the present study was to investigate tensile mechanical properties of clusters of fronds of Ellisolandia elongata from natural (sheltered and exposed reef) and culturing conditions (after 1 month of culturing). The tensile test showed that the first failure stress (σ_I) was not significantly different between the natural and culturing conditions, indicating that the two reefs (sheltered and exposed) were characterized by the same distribution of pre‐existing, inherent structural flaws. Interestingly, the σ_max (maximum stress before rupture) was significantly different between the two conditions, with the culturing condition being more resistant to average load compared to the natural conditions. The maximum stress before rupture (σ_max) showed the influence of the environment in reducing the strength and elasticity of the fronds.     

Influence of natural fractures on gas accumulation in the Upper Triassic tight gas sandstones in the northwestern Sichuan Basin,China

The Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin, China, is a typical tight gas sandstone reservoir that contains natural fractures and has an average porosity of 1.10% and air permeability less than 0.1 md because of compaction and cementation. According to outcrops, cores and image logs, three types of natural fractures, namely, tectonic, diagenetic and overpressure-related fractures, have developed in the tight gas sandstones. The tectonic fractures include small faults, intraformational shear fractures and horizontal shear fractures, whereas the diagenetic fractures mainly include bed-parallel fractures. According to thin sections, the microfractures also include tectonic, diagenetic and overpressure-related microfractures. The diagenetic microfractures consist of transgranular, intragranular and grain-boundary fractures. Among these fractures, intraformational shear fractures, horizontal shear fractures and small faults are predominant and significant for fluid movement. Based on the Monte Carlo method, these intraformational shear fractures and horizontal shear fractures improve the reservoir porosity and permeability, thus serving as an important storage space and primary fluid-flow channels in the tight sandstones. The small faults may provide seepage channels in adjacent layers by cutting through layers. In addition, these intragranular and grain-boundary fractures increase the connectivity of the tight gas sandstones by linking tiny pores. The tectonic microfractures improve the seepage capability of the tight gas sandstones to some extent. Low-dip angle fractures are more abundant in the T₃X³ member than in the T₃X² and T₃X⁴ members. The fracture intensities of the sandstones in the T₃X³ member are greater than those in the T₃X² and T₃X⁴ members. The fracture intensities do not always decrease with increasing bed thickness for the tight sandstones. When the bed thickness of the tight sandstones is less than 1.0 m, the fracture intensities increase with increasing bed thickness in the T₃X³ member. Fluid inclusion evidence and burial history analysis indicate that the tectonic fractures developed over three periods. The first period was at the end of the Triassic to the Early Jurassic. The tectonic fractures developed during oil generation but before the matrix's porosity and permeability reduced, which suggests that these tectonic fractures could provide seepage channels for oil migration and accumulation. The second period was at the end of the Cretaceous after the matrix's porosity and permeability reduced but during peak gas generation, which indicates that gas mainly migrated and accumulated in the tectonic fractures. The third period was at the end of the Eogene to the Early Neogene. The tectonic fractures could provide seepage channels for secondary gas migration and accumulation from the Upper Triassic Xujiahe Formation into the overlying Jurassic Formation.     

In-situ stress state in the Linxing region,eastern Ordos Basin,China: Implications for unconventional gas exploration and production

The Carboniferous and Permian sedimentary rocks (mainly the Shanxi and Taiyuan formations) in the Linxing region, eastern Ordos Basin, China, host a significant volume of unconventional gas resources (coalbed methane, shale gas and tight sandstone gas). Currently, the in-situ stress state is poorly understood but knowledge of this is extremely important for a range of applications, such as gas exploration and production, fracture stimulation and wellbore stability. The maximum horizontal stress (S_Hmax), minimum horizontal stress (S_hmin) and vertical stress (S_v) magnitudes, and the S_Hmax orientation in the Linxing region were systematically analyzed for the first time in the present study, which can provide a reference for subsequent numerical simulation and hydraulic fracturing design. Based on borehole breakouts and drilling-induced tensile fractures interpreted from borehole imaging logs, the S_Hmax orientation rotates from ∼NEE-SWW-trending in the southern part to ∼ NWW-SEE-trending in the northern part of the Linxing region. Both conventional logs and extended leak-off tests were used for stress magnitude determination. The results revealed three types of in-situ stress fields (S_v > S_Hmax > S_hmin, S_Hmax > S_v > S_hmin and S_Hmax > S_v ≈ S_hmin), and a dominant strike-slip stress regime (S_Hmax > S_v ≥ S_hmin) was found for the entire well section in the target Shanxi Formation and Taiyuan Formation in the Linxing region. In addition, differential stress increased with depth in the Linxing region, which indicates that wellbore instability might be a potentially significant problem when drilling wells that are vertical or ∼ N-S-trending.     

Kilometer-scale fault-related thermal anomalies in tight gas sandstones

Upper Carboniferous sandstones make one of the most important tight gas reservoirs in Central Europe. This study integrates a variety of geothermometers (chlorite thermometry, fluid inclusion microthermometry and vitrinite reflection measurements) to characterize a thermal anomaly in a reservoir outcrop analog (Piesberg quarry, Lower Saxony Basin), which is assumed responsible for high temperatures of circa 300 °C, deteriorating reservoir quality entirely. The tight gas siliciclastics were overprinted with temperatures approximately 90–120 °C higher compared to outcropping rocks of a similar stratigraphic position some 15 km to the west. The local temperature increase can be explained by circulating hydrothermal fluids along the fault damage zone of a large NNW-SSE striking fault with a displacement of up to 600 m in the east of the quarry, laterally heating up the entire exposed tight gas sandstones. The km-scale lateral extent of this fault-bound thermal anomaly is evidenced by vitrinite reflectance measurements of meta-anthracite coals (VR_rot ∼ 4.66) and the temperature-related diagenetic overprint. Data suggest that this thermal event and the associated highest coalification was reached prior to peak subsidence during Late Jurassic rifting (162 Ma) based on K-Ar dating of the <2 μm fraction of the tight gas sandstones. Associated stable isotope data from fluid inclusions, hosted in a first fracture filling quartz generation (T ∼ 250 °C) close to lithostatic fluid pressure (P ∼ 1000 bars), together with authigenic chlorite growth in mineralized extension fractures, demonstrate that coalification was not subject to significant changes during ongoing burial. This is further evidenced by the biaxial reflectance anisotropy of meta-anthracite coals. A second event of quartz vein formation occurred at lower temperatures (T ∼ 180 °C) and lower (hydrostatic) pressure conditions (P ∼ 400 bars) and can be related to basin inversion. This second quartz generation might be associated with a second event of illite growth and K-Ar ages of 96.5–106.7 Ma derived from the <0.2 μm fraction of the tight gas sandstones.This study demonstrates the exploration risk of fault-bound thermal anomalies by deteriorating entirely the reservoir quality of tight gas sandstones with respect to porosity and permeability due to the cementation with temperature-related authigenic cements. It documents that peak temperatures are not necessarily associated with peak subsidence. Consequently, these phenomena need to be considered in petroleum system models to avoid, for example, overestimates of burial depth and reservoir quality.     

The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain,Northwest China

Muri Basin in the Qilian Mountain is the only permafrost area in China where gas hydrate samples have been obtained through scientific drilling. Fracture-filling hydrate is the main type of gas hydrate found in the Qilian Mountain permafrost. Most of gas hydrate samples had been found in a thin-layer-like, flake and block group in a fracture of Jurassic mudstone and oil shale, although some pore-filling hydrate was found in porous sandstone. The mechanism for gas hydrate formation in the Qilian Mountain permafrost is as follows: gas generation from source rock was controlled by tectonic subsidence and uplift--gas migration and accumulation was controlled by fault and tight formation--gas hydrate formation and accumulation was controlled by permafrost. Some control factors for gas hydrate formation in the Qilian Mountain permafrost were analyzed and validated through numerical analysis and laboratory experiments. CSMGem was used to estimate the gas hydrate stability zone in the Qilian permafrost at a depth of 100–400 m. This method was used to analyze the gas composition of gas hydrate to determine the gas composition before gas hydrate formation. When the overlying formation of gas accumulation zone had a permeability of 0.05 × 10⁻¹⁵ m² and water saturation of more than 0.8, gas from deep source rocks was sealed up to form the gas accumulation zone. Fracture-filling hydrate was formed in the overlap area of gas hydrate stability zone and gas accumulation zone. The experimental results showed that the lithology of reservoir played a key role in controlling the occurrence and distribution of gas hydrate in the Qilian Mountain permafrost.     

海底管道出露悬空环境风险的定量评价研究

海底管道是海洋油气输送的主要方式,其运行状况直接关系到海上油气田的安全和海洋生态环境的质量。本文先以海底管道出露悬空作为故障树的顶事件,分析出海洋环境中各项基本事件,构建针对海洋环境的海底管道故障树模型;再以灰色模糊识别为理论基础,量化对海底管道安全有影响的海洋环境因素,并以层次分析法对每个风险因素进行分级赋权,确定评价指标范围和权重,进而构建海洋环境条件的海底管道风险评价体系。利用该体系对平湖油气田海底管道4个区段进行环境风险评价,避免了将整段管道一同评价导致的信息均等化,评价结果不能真实反映风险等级高的区段的情况;评价结果与管道2014~2016年现场检测数据对比,两者均显示P2分区中海底管道的风险等级较高,说明了该模型具有一定的合理性和实用性。     

自然延伸概念及相关地质要素在海域划界中的影响效力分析

The United Nations Convention on the Law of the Sea entitled the coastal States to naturally extend the continental shelf, which has caused more drastic disputes of maritime delimitation. This paper devotes to clarifying the significant concept of natural prolongation through an effect method combing the legal principles and technical analysis. Firstly, the classic samples with respect of the development of geological features are traced. Based on these samples, the classification with a model is proposed in order to affirm the concept''s significance under certain geomorphologic situations. Lastly, scientific analysis is used to present two potential prolongation situations and emphasize that all the technical analysis on maritime delimitation should be complied with international law and protect the common interest of all the mankind.