Carbonate shedding and sedimentary cyclicities of a distally steepened carbonate ramp (Miocene, Great Bahama Bank)

Received: 10 May 1999 / Accepted: 16 October 1999     

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt,China

Natural gas, consisting primarily of methane(CH₄), has become a major source of clean energy in modern society in many parts of the globe. Recent experimental observations and discoveries of deep-sourced abiotic CH₄ in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs. However, most CH₄ flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations. Massive abiot...     

我国古生代海相碳酸盐岩成藏理论的新进展

近年来在中国古生界新发现大中型油气田40多个,其中大型油气田16个。在长期研究中国古生代海相沉积特征的基础上,总结归纳近几年来新发现的海相烃源岩,提出对碳酸盐岩有效烃源岩的新认识。从有效烃源岩、古岩溶储集体、油气成藏期次和成藏模式等四个方面概述油气成藏理论方面的新进展;根据油气成藏史研究,结合区域构造演化史、生烃史等,提出塔河油田奥陶系有四个成藏期:加里东中晚期至海西早期;海西晚期;燕山期-喜马拉雅早中期;喜马拉雅晚期。     

北部湾盆地海中凹陷油气成藏条件分析

海中凹陷位于北部湾盆地西南部,其勘探程度低,对油气成藏条件欠缺系统的认识和深入的研究。以含油气系统理论为指导,在简要介绍地层与构造特征的基础上,重点论述了烃源岩、储集层、圈闭及输导介质等油气系统的基本要素,认为海中凹陷发育多套烃源岩、多套储集层、多种类型圈闭和复合的输导介质,具备油气系统形成的基本条件,具有较为广阔的油气勘探前景。     

Origin of the Breno and Esino dolomites in the western Southern Alps (Italy): Implications for a volcanic influence

The Esino Limestone of the western Southern Alps represents a differentiated Ladinian-Lower Carnian (?) carbonate platform comprised of margin, slope and peritidal inner platform facies up to 1000 m thick. A major regional subaerial exposure event lead to coverage by another peritidal Lower Carnian carbonate platform (Breno Formation). Multiphase dolomitization affected the carbonate sediments. Petrographic examinations identified at least three main generations of dolomites (D1, D2, and D3) that occur as both replacement and fracture-filling cements. These phases have crystal-size ranges of 3–35 μm (dolomicrite D1), 40–600 μm (eu-to subhedral crystals D2), and 200 μm to 5 mm (cavity- and fracture-filling anhedral to subhedral saddle dolomite D3), respectively.The fabric retentive near-micritic grain size coupled with low mean Sr concentration (76 ± 37 ppm) and estimated δ¹⁸O of the parent dolomitizing fluids of D1 suggest formation in shallow burial setting at temperature ∼ 45–50 °C with possible contributions from volcanic-related fluids (basinal fluids circulated in volcaniclastics or related to volcanic activity), which is consistent with its abnormally high Fe (4438 ± 4393 ppm) and Mn (1219 ± 1418 ppm) contents. The larger crystal sizes, homogenization temperatures (D2, 108 ± 9 °C; D3, 111 ± 14 °C) of primary two-phase fluid inclusions, and calculated salinity estimates (D2, 23 ± 2 eq wt% NaCl; D3, 20 ± 4 eq wt% NaCl) of D2 and D3 suggest that they formed at later stages under mid-to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity, which is supported by higher estimated δ¹⁸O values of their parent dolomitizing fluids. This is also consistent with their high Fe (4462 ± 4888 ppm; and 1091 ± 1183 ppm, respectively) and Mn (556 ± 289 ppm and 1091 ± 1183 ppm) contents, and low Sr concentrations (53 ± 31 ppm and 57 ± 24 ppm, respectively).The similarity in shale-normalized (SN) REE patterns and Ce (Ce/Ce*)_SN and La (Pr/Pr*)_SN anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor. Positive Eu anomalies, coupled with fluid-inclusion gas ratios (N₂/Ar, CO₂/CH₄, Ar/He), high F⁻ concentration, high F/Cl and high Cl/Br molar ratios suggest an origin from diagenetic fluids circulated through volcanic rocks, which is consistent with the co-occurrence of volcaniclastic lenses in the investigated sequence.     

Serpulids living deep: calcareous tubeworms beyond the abyss

Although the carbonate compensation depth (CCD) for calcite, generally located in the depth range 4000–5000 m, is often proposed as a physiological barrier to deep-ocean colonization, many organisms with calcareous exoskeletons are found in the deepest oceanic trenches. Serpulid polychaetes inhabiting unprotected calcareous tubes are unlikely deep-sea inhabitants, yet, they are found at all oceanic depths from intertidal to hadal. Here we review and revise the published and unpublished records of Serpulidae from below 5000 m depth. We also describe tube ultrastructure and mineralogical content of available deep-sea serpulid tubes to obtain insights into their biomineralisation. Species belonging to the genera Bathyditrupa, Bathyvermilia, Hyalopomatus, Pileolaria (spirorbin) and Protis were found at depths from 5020 to 9735 m. However, only specimens of Protis sp. were truly hadal (>6000 m) being found at 6200–9700 m. Hadal specimens of Protis have irregularly oriented prismatic tube microstructure similar to that found in more shallow-water representatives of the genus. Initial EDX analysis suggested a mostly calcitic composition (i.e., the most stable CaCO₃ polymorph) on the basis of high Mg levels. Surprisingly, however, tubes of Bathyditrupa hovei and a species of Protis analysed using the more reliable method of laser Raman spectroscopy were found to be composed of aragonite. The compensation depth for this less stable CaCO₃ polymorph in the oceans is usually 2000–3000 m. We found no obvious structural adaptations to life at extreme depths in the studied serpulid tubes and how serpulids are able to biomineralise and maintain their tubes below the CCD remains to be explained.     

Characterization of tar-like material and producible oil in Misoa C2 and C3 reservoir sands of the Area 2 South (Ceuta Field,Venezuela)

Here we characterized asphaltic oil occurrences in the Misoa C2 and C3 reservoirs within Area 2 South of the Ceuta Field (Lake Maracaibo Basin, NW Venezuela). To perform the geochemical and petrological characterization of the C2 and C3 sub-units, which cover an interval in which tar-like material was detected, we analyzed core (18) and cutting (123) samples using the Rock-Eval 6 technique in reservoir mode. Samples were studied under fluorescence light and scanning electron microscopy (SEM), showing the presence of two charges of oils and the relationships within silica cements and clay minerals. To evaluate vertical inter-reservoir communication and lateral intra-reservoir continuity, we also studied 12 oil samples from wells in Area 2 South. Biomarkers and isotopic signatures were examined using gas chromatography-mass spectrometry (GC-MS) and isotope ratio mass spectrometry (IRMS). The results suggest that oil and core extracts belongs to a single family derived from two pulses of hydrocarbon generation from the La Luna source rock, which was deposited in a marine reducing environment. The oil samples are a mixture of a paleobiodegraded oil charge and a later fresh, more mature oil recharge. The geochemical analyses also evidence that the formation of heavy oil tar zones can be related to biodegradation and in-reservoir oil mixing, though other formation mechanisms for the tar are also feasible. Finally, the GC fingerprints of the oils are indicative of lateral reservoir continuities across the F-1 and F-2 faults and vertical communication between the Misoa C2 and C3 sub-units in Area 2 South, suggesting that tar occurrences do not act as permeability barriers within the two reservoirs studied.     

Morphological indicators of structural control,relative sea-level fluctuations and platform drowning on present-day and Miocene carbonate platforms

The morphology of Carbonate platforms may be influenced by tectonic activity and eustatic variations. 3D seismic data and satellite imagery are used in order to investigate the morphological similarities between present-day carbonates platforms, East of Borneo Island and Miocene carbonate platforms of the South China Sea. The morphological similarities exhibit platform fragmentation, that could be caused by subtle faulting, sufficient to drown reef rims; platform contraction, which is a result of back-stepping of the reef margin during a relative sea level rise and polygonal patterns in internal lagoons, described as mesh reefs in modern platforms and possibly interpreted as karst in Miocene platforms.Vertical movements may trigger the formation of new geomorphological conditions that modify the distribution of coral growth with respect to the new hydrodynamic conditions in space and time. These movements (uplift and tilting) reduce and localize the space necessary for the coral ecosystem, explaining the contraction leading to drowning of parts of and, ultimately, the whole platform.     

黔中降起的油气勘探壁垒——兼论海相碳酸盐岩烃源岩

黔中隆起在地质构造上属于扬子地块的滇黔隆褶带．南华纪晚期初现雏形，震旦纪—奥陶纪为水下隆起，燕山运动使其整体抬高，喜马拉雅运动时急剧隆升，构造地貌长期相对隆起。地表油苗油样分析结果显示，在红外吸收谱图中缩合芳烃结构及含氧基团吸收明显，有机质热成熟度高，且遭受了氧化蚀变：震旦系以上地层大量暴露，地表水溶蚀下渗深度在黔西南地区可达3000-4000m；作为长期存在的古隆起，经历了多次抬升，直至挽近仍处于严重剥蚀阶段。地腹可能存在酸性岩浆热源使有机质过度热演化、保存条件不佳以及缺少勘探目的层，是黔中隆起油气勘探的三大壁垒。兼谈了碳酸盐岩烃源岩问题．认为形成大油气田的烃源岩主要是泥质岩系，“碳酸盐岩烃源岩”只是从属性的．分布在泥质烃源岩系的边缘或局部范围。     

孔隙衬里绿泥石的成因及对储层性能的影响

孔隙衬里绿泥石发育的砂岩大多属于优质储层,前人将其归功于绿泥石对孔隙的保护作用。按照孔隙衬里绿泥石形成的铁质来源,可划分为同沉积黏土膜转化型、富铁镁物质溶蚀再结晶型和转化与直接结晶混合型3种类型。通过国内外富孔隙衬里绿泥石砂岩实例的对比研究表明,孔隙衬里绿泥石可以通过抑制自生石英生长、提高抗压实能力、指示早期碱性溶蚀发育程度和堵塞孔喉等4种方式影响储层性能,且不同成因的孔隙衬里绿泥石对储层性能影响方式不同,同一成因绿泥石在不同成岩阶段的影响方式也不同。中成岩A期以前,转化型孔隙衬里绿泥石通过占据孔喉和提高抗压能力影响储层,而溶蚀结晶型孔隙衬里绿泥石主要指示火山物质发生碱性溶蚀的程度;只有砂岩成岩阶段达到中成岩B期以后,孔隙衬里绿泥石对石英生长的抑制作用才得到充分体现。