三塘湖盆地二叠系干酪根热模拟气体产物的地球化学特征

针对三塘湖盆地的主力烃源岩二叠系芦草沟组泥质烃源岩开展封闭体系和半开放体系热模拟实验,前者将干酪根密封在金管中开展实验、后者直接对烃源岩样品进行压机半开放体系热解模拟,对实验中生成气体的组分和稳定碳同位素进行分析。实验结果显示,两种热解方法以无机气体生成为主,包括CO2、H2、H2S 和 N2等,其中 CO2含量最丰富,这与样品干酪根中丰富的氧含量一致。半开放体系模拟实验温度达到480℃时,气态烃的含量达40.64%,其中甲烷含量达到26.10%,封闭体系中随着模拟温度升高烃类气体产物的碳同位素呈现先变轻后变重的趋势,但同位素分馏程度不大, CH4气体的δ13C 值分布在-40.2‰~-46.6‰之间, C2H6分布在-34.3‰~-38.0‰之间, C3H8分布在-33.6‰~-36.0‰之间,而CO2气体δ13C值分布在-32.8‰~-28.8‰之间。三塘湖盆地针对气藏的勘探已经有所发现,但有关气藏的地球化学特征报道较少,该工作中的模拟实验结果可为三塘湖盆地深入的气藏地球化学勘探提供基础数据。     

塔中地区奥陶系喜马拉雅山期S干气充注的证据

塔中地区奥陶系天然气成因多样;Ⅰ号坡折带中东部奥陶系天然气以高干燥系数、 甲烷同位素值重为特征;与塔深1井寒武系原油裂解气接近;应主要来自寒武系原油裂解气成因。寒武系贫H₂S、 高成熟原油裂解气在喜马拉雅山期时;气侵奥陶系油气藏;得到了以下主要证据的支持: 1）天然气甲烷δ¹³C值大多比Chung et al.（1988）天然气模式甲烷δ¹³C值计算值高3‰以上;2）干燥系数与甲烷δ¹³C值大体上具有正相关关系;3）天然气干燥系数与H₂S含量大体上具有负相关关系。这些特征表明;存在贫H₂S、 相对富¹³C甲烷为主的干气与富H₂S、 相对贫¹³C甲烷的湿气混合作用。奥陶系中H₂S-δ³⁴S 值为14‰～20‰;远低于中深1井寒武系原地热化学硫酸盐还原作用（TSR）成因的H₂S（33‰）;支持了奥陶系中H₂S并不是来源于寒武系古油气藏。于是提出;来自寒武系贫H₂S的干气在喜马拉雅山期对良里塔格组和鹰山组油气藏发生了气洗;油气藏的气/油比值增大、 导致了原油蜡含量增高、 甲烷δ¹³C值发生正偏移。     

东秋里塔格构造带构造特征及油气勘探区带

东秋里塔格构造带是塔里木盆地库车坳陷的前缘构造带,整体表现为受新近系吉迪克组膏盐岩滑脱层控制的分层变形构造特征。盐上构造层主要发育断层相关褶皱,盐岩层以塑性流动变形为主,盐下构造层发育断层相关褶皱、冲起构造、逆冲断块等多种构造样式。依据盐下构造层变形特征,自西向东将东秋里塔格构造带划分为过渡构造段、叠瓦扇构造段、前端冲起构造段以及后端冲起构造段四段。过渡构造段内膏盐岩层发育层位由库姆格列木群转变为吉迪克组,盐下发育叠瓦扇构造;叠瓦扇构造段盐下发育典型的逆冲叠瓦构造样式;前端冲起构造段盐下发育断弯褶皱背斜,并在反冲断层的复合作用下发生冲起,主逆冲断层之下发育次级逆冲夹片;后端冲起构造段盐下发育断弯褶皱,并在背斜北翼发育一个独立的冲起构造。东秋里塔格构造带具备有利生储条件,构造分析表明过渡构造段、叠瓦扇构造段以及前端冲起构造段的盐下次级逆冲夹片具备勘探潜力,是有利的油气勘探区带。     

塔里木盆地石炭系烃源岩成熟演化研究

尽管塔里木盆地石炭系富含有机质且已发现其生成的原油,但关于石炭系烃源岩热演化研究却很薄弱。根据基础地质资料、古地温梯度和实测镜质体反射率数据,本文定量模拟了塔里木盆地6口典型井的石炭系烃源岩热演化史和这套烃源岩底界、顶界在二叠纪末期、三叠纪末期、白垩纪末期及现今成熟度的平面分布规律。塔北隆起南部、塔西南坳陷和阿瓦提凹陷在石炭-二叠纪作为沉降中心接受了巨厚的沉积物,造成这些地区石炭系烃源岩底界成熟度在二叠纪末期达到0.9%～1.3%（中-高成熟）,顶界为0.5%～0.9%（低-中成熟）,成为有利生油区。中生代时期,塔里木盆地中、东部长期处于沉降状态,石炭系烃源岩处于成熟演化阶段。白垩纪末期,塔中低凸起和满加尔凹陷的石炭系底界成熟度为0.7%～0.9%（中成熟）,顶界为0.5%～0.7%（低成熟）,而塘古孜巴斯坳陷底界成熟度为0.9%～1.1%（中-高成熟）,顶界为0.7%～0.9%（中成熟）。受羌塘地体、拉萨地体、印度板块分别与欧亚板块南缘碰撞远程效应的影响,塔西南坳陷和巴楚隆起在中生代一直处于隆升剥蚀状态,烃源岩热演化进入停滞状态。新生代以来,塔西南坳陷和阿瓦提凹陷演化为前陆盆地,再次接受了巨厚沉积物,致使石炭系烃源岩快速成熟演化,底-顶界成熟度现今已超过2.0%,进入干气阶段。满加尔凹陷石炭系烃源岩自石炭纪至今一直处于成熟演化阶段,现今达到最大,为0.8%～1.0%。另外,热史研究表明塔西南坳陷和阿瓦提凹陷石炭系烃源岩具有二次生烃潜力。本研究不仅丰富了塔里木盆地石炭系烃源岩热史研究成果,而且对油气勘探与开发具有重要的指导意义。     

安徽金寨三仙山地区梅山群碎屑岩成分及其对物源区构造属性和地层时代的意义

安徽金寨三仙山地区位于大别山北缘,该地梅山群主要为一套轻微变质的碎屑岩系。砂岩碎屑组分统计表明,砂岩类型主要为岩屑石英砂岩、岩屑石英杂砂岩,石英、长石、岩屑的平均含量为87.21%、1.67%、9.64%,杂基含量为15%,石英几乎全为单晶石英（95.79%）,长石以斜长石为主,岩屑主要为沉积岩屑（75.49%）,其次为变质岩屑（24.51%）。碎屑岩地球化学元素平均含量为：SiO₂（75.99%）,Al₂O₃（11.96%）,MgO（0.72%）,CaO（0.10%）,Fe₂O₃（4.02%）,K₂O（1.70%）,Na₂O（0.26%）。ΣREE=170.49×10^-6（74.49×10^-6～309.42×10^-6）,LREE/HREE=11.16（7.89～14.26）,轻稀土略有富集,δEu=0.72（0.59～0.90）,La/Yb=22.08（13.01～31.18）,（La/Yb）_N=14.89（8.77～21.02）,δCe=0.84（0.42～0.97）。碎屑岩地球化学特征指示三仙山地区梅山群母岩主要为古老沉积岩、长英质火山岩和古老变质基底,具有多重物源区。梅山群构造背景较复杂,主要为被动大陆边缘和活动大陆边缘,其次为大陆岛弧。三仙山地区梅山群碎屑岩的源岩成分、构造背景与商城-固始地区石炭系有很大差别,故其地层时代有待于进一步研究。     

Coal seam gas distribution and hydrodynamics of the Sydney Basin,NSW, Australia

This paper reviews various coal seam gas (CSG) models that have been developed for the Sydney Basin, and provides an alternative interpretation for gas composition layering and deep-seated CO₂ origins. Open file CSG wells, supplemented by mine-scale information, were used to examine trends in gas content and composition at locations from the margin to the centre of the basin. Regionally available hydrochemistry data and interpretations of hydrodynamics were incorporated with conventional petroleum well data on porosity and permeability. The synthesised gas and groundwater model presented in this paper suggests that meteoric water flow under hydrostatic pressure transports methanogenic consortia into the subsurface and that water chemistry evolves during migration from calcium-rich freshwaters in inland recharge areas towards sodium-rich brackish water down-gradient and with depth. Groundwater chemistry changes result in the dissolution and precipitation of minerals as well as affecting the behaviour of dissolved gases such as CO₂. Mixing of carbonate-rich waters with waters of significantly different chemistries at depth causes the liberation of CO₂ gas from the solution that is adsorbed into the coal matrix in hydrodynamically closed terrains. In more open systems, excess CO₂ in the groundwater (carried as bicarbonate) may lead to precipitation of calcite in the host strata. As a result, areas in the central and eastern parts of the basin do not host spatially extensive CO₂ gas accumulations but experience more widespread calcite mineralisation, with gas compositions dominated by hydrocarbons, including wet gases. Basin boundary areas (commonly topographic and/or structural highs) in the northern, western and southern parts of the basin commonly contain CO₂-rich gases at depth. This deep-seated CO₂-rich gas is generally thought to derive from local to continental scale magmatic intrusions, but could also be the product of carbonate dissolution or acetate fermentation.     

A geophysical view of the Southeastern Brazilian margin at Santos Basin: Insights into rifting evolution

This study investigates the rifting structures of Santos Basin at the Southeastern Brazilian margin, based on an integrated geophysical approach. Our aim is to constrain the crustal basement topography of central and northern Santos basin, the presence of magmatism and the role of inherited structures in space and time through the rifting processes. We present a new high resolution aeromagnetic dataset, which in correlation with gravity anomalies enables us to interpret the tectonic trends and crustal basement structures. We calculated the magnetic basement depth for the central and northern Santos Basin using power spectrum analysis. The obtained depths range between 2 and 9 kms, and are comparable with results from previous works. From our integrated study, three margin domains could be identified, which display distinct rifting structures and are characterized by important lateral variation along the margin. The proximal domain displays trends and magnetic basement blocks NE–SW oriented, i.e., parallel to inherited onshore crustal basement with an inflexion to E–W oriented trends; the necking domain is characterized by oblique magnetic basement highs and lows (E–W and NW–SE) and a structural trend change. The trends and magnetic basement highs are bounded by NW–SE negative anomalies, interpreted as transfer zones. Oceanwards at the distal domain, the lineaments and transfer zones show a progressive structural inflexion to ENE and E–W, sub-parallel to adjacent South Atlantic Fracture Zones. The observed crustal basement architecture and segmentation suggest the reactivation of pre-rift structures at the proximal margin and the obliquity of rifting relative to them. From the proximal domain oceanwards the structural pattern may reflect the passage from a “continental type” domain, where lithospheric inheritance controls the deformation, to a distal margin where this influence diminishes and “new” structural trends are formed. We propose that northern Santos Basin show evidences of an intensely deformed zone, where rift evolved under oblique extension, similar to that observed at transform margin segments.     

The Cretaceous (Cenomanian) continental record of the Laje do Coringa flagstone (Alcântara Formation), northeastern South America

The fossil taxa of the Cenomanian continental flora and fauna of São Luís Basin are observed primarily in the bone bed of the Laje do Coringa, Alcântara Formation. Many of the disarticulated fish and tetrapod skeletal and dental elements are remarkably similar to the chronocorrelate fauna of Northern Africa. In this study, we present a summary of the continental flora and fauna of the Laje do Coringa bone-bed. The record emphasizes the existence of a trans-oceanic typical fauna, at least until the early Cenomanian, which may be interpreted as minor evolutionary changes after a major vicariant event or as a result of a land bridge across the equatorial Atlantic Ocean, thereby allowing interchanges between South America and Africa. The paleoenvironmental conditions in the northern Maranhão State coast during that time were inferred as forested humid areas surrounded by an arid to semi-arid landscape.     

Distribution of trace metals in stream sediments along the Trans-Amazonian Federal Highway,Pará State,Brazil

A total of 64 sediment samples were collected along a stretch of about 988 km of the Trans-Amazonian Federal Highway, between Marabá and Itaituba, Pará State, in order to characterize the distribution of metals and trace elements. Due to the lithological and geological diversity along this stretch of the Amazon, the study region was divided in three distinct tracks. Statistical data analysis (Spearman correlation and Principal Factor Analysis) shows strong signature and predominance of regional rocks chemistry, such as mafic-ultramafic-bearing elements (Ni and Cr) and hydrothermalism-bearing elements such as Cu and Pb. Enrichment factors were calculated for three different normalizer agents: 1) Fe and Mn, representing the Fe and Mn (hydr)oxides, 2) Al representing clay minerals, and 3) organic matter (OM). The Fe and Mn (hydr)oxides showed to be the most proficient metal carrier among the geological matrices, likely due to the larger lithological diversity. On the other hand, OM positively correlated to trace element distribution.     

Low-pressure evolution of arc magmas in thickened crust: The San Pedro–Linzor volcanic chain,Central Andes,Northern Chile

Magmatism at Andean Central Volcanic Zone (CVZ), or Central Andes, is strongly influenced by differentiation and assimilation at high pressures that occurred at lower levels of the thick continental crust. This is typically shown by high light to heavy rare earth element ratios (LREE/HREE) of the erupted lavas at this volcanic zone. Increase of these ratios with time is interpreted as a change to magma evolution in the presence of garnet during evolution of Central Andes. Such geochemical signals could be introduced into the magmas be high-pressure fractionation with garnet on the liquidus and/or assimilation from crustal rocks with a garnet-bearing residue. However, lavas erupted at San Pedro–Linzor volcanic chain show no evidence of garnet fractionation in their trace element patterns. This volcanic chain is located in the active volcanic arc, between 22°00^′S and 22°30^′S, over a continental crust ∼70 km thick. Sampled lavas show Sr/Y and Sm/Yb ratios <40 and <4.0, respectively, which is significantly lower than for most other lavas of recent volcanoes in the Central Andes. In addition, ⁸⁷Sr/⁸⁶Sr ratios from San Pedro–Linzor lava flows vary between 0.7063 and 0.7094. This is at the upper range, and even higher than those observed at other recent Central Andean volcanic rocks (<0.708). The area in which the San Pedro–Linzor volcanic chain is located is constituted by a felsic, Proterozoic upper crust, and a thin mafic lower crustal section (<25 km). Also, the NW–SE orientation of the volcanic chain is distinctive with respect to the N–S orientation of Central Andean volcanic front in northern Chile. We relate our geochemical observations to shallow crustal evolution of primitive magmas involving a high degree of assimilation of upper continental crust. We emphasize that low pressure AFC- (Assimilation Fractional Crystallization) type evolution of the San Pedro–Linzor volcanic chain reflects storage, fractionation, and contamination of mantle-derived magmas at the upper felsic crust (<40 km depth). The ascent of mantle-derived magmas to mid-crustal levels is related with the extensional regime that has existed in this zone of arc-front offset since Late-Miocene age, and the relatively thin portion of mafic lower crust observed below the volcanic chain.