Origin of CO_2 in natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin

The natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin contains high H2S whereas relatively low CO2 concentrations and the CO2 display high δ13C values (ranging from -5.81‰ to 3.3‰ (PDB)). This seems to contradict the conventional wisdom that TSR should be a primary source of CO2 in natural gas from the Feixianguan Formation. In contrast, many authigenic calcite samples from these sites display very low δ13C values (ranging from -18.4‰ to -10.3‰ (PDB)). This suggests that the carbon from TSR source dominated the formation of calcite whereas the carbon from inorganic source came into CO2 in natural gas. In order to assess the origin of CO2 from these H2S-rich sites, we have calculated the relative contributions of organic and inorganic carbon sources to the CO2 and authigenic calcite. The organic carbon source possibly originated from TSR, whereas the inorganic one might be generated from marine carbonates dissolution. This calculation is based on the carbon isotopic compositions of CO2 and authigenic calcite as well as an isotopic mass balance. The results show that the contribution of organic carbon source to the CO2 is only 2%, whereas that to authigenic calcite is as high as 43% on average. Such results combined with thermodynamic evidence indicate that the isotopically light CO2 produced by TSR process may contribute to authigenic calcite precipitation during burial diagenesis. Distinguishable from Ordovician reservoir of Tarim Basin, Feixianguan reservoir of Northeast Sichuan Basin experienced rapid tectonic uplift due to Yanshanian movement after TSR occurred. Such tectonic event could induce temperature decrease and further promote carbonates dissolution. During these processes, secondary porosity has developed in Feixianguan carbonate reservoirs. Therefore, much attention should be paid to the structural highs in search of high quality carbonate reservoirs.     

下扬子区海相中、古生界地质结构分区及其油气勘探选区意义

基于对下扬子区海相中、古生界构造及沉积沉降特征的研究, 以影响和反映构造稳定及晚期生烃的基底性质、构造变形特征、印支面埋深、沉积沉降中心迁移特征等为依据, 将下扬子区划分为5个一级、17个二级地质结构区。以构造稳定、有效保存及晚期生烃、晚期成藏等为评价标准, 综合评价南黄海中部晚期隆起稳定区(Ⅰ₂)、江都-东台-南黄海南部晚期沉降深冲断区(Ⅰ₃)、泰兴-海安晚期沉降深冲断区(Ⅱ₁)、南通-南黄海南部晚期沉降浅推覆区(Ⅱ₂)及太湖晚期沉降浅推覆区(Ⅱ₄)为下古生界油气勘探有利区; 南黄海北部晚期沉降深冲断区(Ⅰ₁)、江都-东台-南黄海南部晚期沉降深冲断区(Ⅰ₃)、滨海-大丰晚期沉降深下古卷入区(Ⅰ₄)和泰兴-海安晚期沉降深冲断区(Ⅱ₁)为上古生界油气勘探有利区; 南黄海北部晚期沉降深冲断区(Ⅰ₁)、滨海-大丰晚期沉降深下古卷入区(Ⅰ₄)、金湖-高邮晚期沉降深下古卷入区(Ⅰ₅)及泰兴-海安晚期沉降深冲断区(Ⅱ₁)为古潜山油气勘探有利区。研究对于该区海相中古生界勘探选区具有重要意义。      

东海盆地西湖凹陷构造样式及其演化

西湖凹陷是经历多期构造作用的白垩—第三纪盆地。多期裂陷和反转构造作用的叠加导致了相对复杂的构造格架样式和盆地地质结构。凹陷总体上具有东西分带,南北分块的构造格局。裂陷期的同沉积断裂组成了多种复合的构造样式：西部斜坡带主要发育断阶、地堑和地垒构造;中央洼陷带则以复合“Y”字型、羽状和似花状断裂组合为主;东缘断阶带主要发育“Y”字型、梳状和羽状组合。坳陷反转期,早期的同沉积断裂发生了不同程度的逆冲反转,形成了多种复合的构造样式,例如反冲叠瓦扇构造、后断叠瓦式逆冲构造等叠合构造样式。从盆地的构造样式出发,探讨了盆地的形成演化和构造- 沉积充填过程对有利圈闭带发育分布的控制作用,对富生烃凹陷油气预测具有重要指导意义。     

全面禁采以来锡西澄南地区地面沉降动态特征分析

锡西澄南地区在苏锡常地区地面沉降研究中具有一定代表性,是地面沉降的典型地区之一。本文在已有的长期地下水水位、地面沉降监测资料的基础上,结合区域地质条件就全面禁采以来锡西澄南地区的地面沉降特征的诸多方面作进一步的探索分析。分析结果显示:目前锡西澄南地区出现南北沉降差异;差异特征与相应区域的地下水补给条件密切相关;北部Ⅱ承压层沉降为预防重点。     

管幕预筑隧道地表沉降分析

沈阳地铁2号线某暗挖车站是国内首个采用管幕预筑法施工的地下工程,对该工法施工引起的地面沉降进行了监测,并建立了修正的Peck模型对沉降进行预测,结果表明：(1)管幕预筑隧道施工引起的地表沉降预测值与实测值基本吻合;(2)管幕预筑浅埋大断面隧道施工地表沉降表现为整体下沉特点,沉降值与地下结构基础的沉降值基本相等,控制管幕预筑隧道结构的墙脚下沉可以有效控制地表的沉降;(3)管幕预筑法隧道施工预先施作的地下永久结构对隧道围岩的约束作用明显,砂土地层中地层损失率为0.000 5%～0.002 0%,沉降槽宽度系数为0.5;(4)管幕预筑法建造地下空间,引起的地表沉降较常规施工方法小得多,环境效益和社会效益明显,该施工方法适用于修筑中心城区软土地层中地下交通枢纽工程及下穿交通干线的隧道工程,值得进一步研究和推广     

强震作用下松散海床地基的动力响应

海上和沿海区域砂性土地基在强震作用下需同时考虑液化、震陷以及流滑效应。选用杨朝晖叠套屈服面模型,提出了获取计算参数的简化方法,用OpenSees验证了该模型模拟液化的能力;开发了OpenSees与ANSYS的接口,对处于强震地区、地基土为松散砂性土的印尼某进水明渠堤坝进行了非线性有限元动力计算,判断了场地的液化情况,预测了堤坝及地基的震陷量和侧向流滑,计算结果对该类地基的加固处理具有一定的指导意义     

Influence of shear angle on hangingwall deformation during tectonic inversion

Tectonic inversion is a common phenomenon in island arc settings, especially in back‐arc basins. The reactivation of normal faults as thrusts, triggered by tectonic inversion, produces typical inversion fault‐related folds and thrusts in the hangingwall. These hangingwall inversion geometries are affected by two factors: the geometry of the underlying master fault and the angle of inclined simple shear relative to the regional dip of strata, in the case that the deformation is approximated by simple shear. This study employed numerical simulations to analyse the influence of the antithetic shear angle on the geometry of the hangingwall and displacement along the master fault. The simulation results reveal that a steeply inclined shear vector during extension produces a narrow, steep‐sided half‐graben, whereas a gently inclined shear produces a wide, open basin. After tectonic inversion, a tight anticline is formed under steeply inclined shear, whereas an open anticline is formed under gently inclined shear. Antithetic shear results in reduced total displacement along the master fault, and the greater the angle between the shear direction and the regional dip, the greater the displacement along the master fault. Because the deformation geometry of syn‐extension layers is affected by extension followed by contraction, a change in the shear angle during tectonic inversion produces a wide variety of deformation geometries. Comparison of the simulation results with the results of analogue modelling suggests that the shear angle decreases by 5° during the transition from extension to tectonic inversion and that such a change may be commonly observed in natural geological structures. These results highlight the benefits of numerical simulations, which can be used to readily examine a variety of constraining parameters and thereby lead to a better understanding of the mechanism of hangingwall deformation, avoiding erroneous estimates of the amount of fault displacement.     

Soft‐sediment deformation structures in Cretaceous non‐marine deposits of southeastern Gyeongsang Basin,Korea: Occurrences and origin

A variety of soft‐sediment deformation structures formed during or shortly after deposition occurs in the Cretaceous Seongpori and Dadaepo Formations of the southeastern Gyeongsang Basin exposed along coastal areas of southeastern Korean Peninsula for 0.5–2 km. These are mostly present in a fluvial plain facies, with interbedded lacustrine deposits. In this study, the features of different kinds of soft‐sediment deformation structures have been interpreted on the basis of sedimentology of structure‐bearing deposits, comparison with normal sedimentary structures, timing and mechanism of deformation, and triggering mechanisms. The soft‐sediment deformation structures can be classified into four morphological groups: (i) load structures (load casts, ball‐and‐pillow structures); (ii) soft‐sediment intrusive structures (dish‐and‐pillars, clastic dykes, sills); (iii) ductile disturbed structures (convolute folds, slump structures); and (iv) brittle deformation structures (syndepositional faulting, dislocated breccia). The most probable triggering mechanisms resulting in these structures were seismic shocks. These interpretations are based on the following field observations: (i) location of the study area within tectonically active fault zone reactivated several times during the Cretaceous; (ii) deformation structures confined to single stratigraphic levels; (iii) lateral continuity and occurrences of various soft‐sediment deformation structures in the deformed level over large areas; (iv) absence of depositional slope to indicate gravity sliding or slumping; and (v) similarity to the structures produced experimentally. The soft‐sediment deformation structures in the study areas are thus interpreted to have been generated by seismic shocks with an estimated magnitude of M > 5, representing an intermittent record of the active tectonic and sedimentary processes during the development and evolution of two formations from the late Early Cretaceous to the Late Cretaceous.     

东非裂谷东支South Lokichar盆地石油地质特征及成藏规律

东非裂谷东支由多个盆地组成,South Lokichar是唯一有油气发现的盆地,目前对于该盆地的石油地质特征及成藏规律仍认识不清,严重制约着东非裂谷东支下一步勘探.本文综合利用钻井、地震、岩芯、薄片以及分析化验等资料,对South Lokichar盆地的构造和地层发育特征、关键成藏条件和影响因素以及成藏规律进行详细分析...     

Genetic algorithm for fitting a mixed Bingham distribution to 3D orientations: a tool for the statistical and paleostress analyses of fracture orientations

The clustering of fracture orientations is important for tectonic studies and for geotechnical engineering. In this study, a real‐coded genetic algorithm was adopted to fitting a mixed Bingham distribution to orientation data by maximizing the log‐likelihood function of the distribution. The maximization is a difficult problem, because the function has multimodality and singularity. It was found that the algorithm was effective for this problem. Given the orientations of dilational fractures, the present method determines not only the stress axes and stress ratio of each of the fracture groups but also the maximum non‐dimensionalized fluid pressure at the time of their formation. In addition, the software calculates the 95 % error ellipses of the concentration axes. The present method found that the orientations of ore veins of the Akenobe Mine, SW Japan, should be partitioned into three clusters. It is shown that two of the groups had distinctive Zn and Sn contents, and that the ore fluids had overpressures only slightly greater than the minimum principal stress at the time of the deposition of Zn‐ and Sn‐rich veins.