Sources and hydrothermal alteration of organic matter in Quaternary sediments: A synthesis of studies from the Central Gulf of California

Deep sea drilling in the Central Gulf of California, a young and active spreading centre, shows that the high heat flow typical of these regions causes extensive alteration of sediment organic matter, especially near sills and above magma chambers where hydrothermal activity is concentrated. Even on the nearby passive margin, where there are no sills, heat flow is moderately high and hydrocarbon generation has begun in immature sequences. Migrating light hydrocarbons are detected especially where hydrothermal activity is concentrated. Thermogenic methane is more widespread, though not in the passive margin bordering the spreading centre. Despite the incidence of hydrocarbon generation and migration, the amounts of hydrocarbons involved are relatively small and apparently do not lead to commercially significant accumulations of petroleum.The organic matter in these sediments is mostly marine because the Gulf of California generally has low runoff from land and highly productive surface waters. Turbidites rich in terrigenous organic material are locally abundant in the mainly pelagic section in the Guaymas Basin. The highest concentrations of organic matter are found in laminated diatomites deposited on the Guaymas passive margin within the oxygen minimum zone.     

琼东南盆地乐东凹陷烃源岩热史及成熟史模拟

琼东南盆地乐东凹陷烃源岩热史及成熟史研究对乐东凹陷油气资源评价及勘探选区具有指导意义.在盆地地史和热史恢复的基础上,利用EASY％Ro模型模拟重建了琼东南盆地乐东凹陷崖城组烃源岩的成熟史.研究结果表明,琼东南盆地具有较高的古、今地温梯度,且古地温梯度多高于现今地温梯度,有利于烃源岩的成熟与天然气的生成.始新世—早渐新世...     

Relationship between statistical thermal alteration index and vitrinite reflectance for sedimentary rocks in northern Japan with reference to effects for unconformity,faulting, and contact metamorphism

Two organic maturity indices, the statistical thermal alteration index (stTAI) and vitrinite reflectance (R_O), are used to gain insight into the geological histories of sequences of Tertiary and Upper Cretaceous sediments in northern Japan that contain an unconformity and which are affected by faulting and contact metamorphism. The stTAI is based on the brightness, or gray level, of fossil pollen of Pinus, Podocarpus, Abies, and Picea species. Pollen brightness is measured using a transmitted‐light microscope equipped with a computer‐driven digital image processor. The stTAI represents the mean value of the modes for the complete array of indigenous pollen in rock samples. The stTAI indicates the level of organic maturation for Neogene sedimentary rocks of Japan, from incipient diagenesis to early catagenesis (R_O ≤1.0%). With the progressive diagenesis and catagenesis of sedimentary rocks, stTAI values generally show a progressive decrease, whereas R_O values increase. The effects of an unconformity and faulting are more clearly recorded in stTAI trends than in R_O trends. During early stages of organic maturation (R_O ≤0.7%), stTAI shows a rapid decrease, while R_O shows a rapid increase during the mature and post‐mature stages (R_O ≥0.8%). The occurrence of a range in R_O values for a given level of organic maturity makes it difficult to determine the influence of the unconformity on the increasing R_O trend. R_O values show a progressive increase toward an igneous dyke, but this trend is not apparent in stTAI values. Measurements of Tertiary and Upper Cretaceous rocks in Japan reveal that stTAI is more sensitive to heating duration than R_O, although R_O is more sensitive to heating temperature.     

锯缘青蟹大颚器的显微和超微结构

采用组织切片和电镜技术,对锯缘青蟹大颚器的形态结构进行了观察.大颚器位于大颚肌几丁质腱的后侧基部.腺细胞胞质丰盈,质膜大量内陷和作不同程度的卷绕,内含大量具管状脊的线粒体和滑面内质网.卵巢成熟时大颚器腺细胞内线粒体增多,少数线粒体形态异样,高尔基体易见,散布的小管状滑面内质网(tSER)少,高度汇合的tSER增多,大小形状不同的空泡数量明显增加,且近质膜分布,质膜内陷和卷绕程度加剧.表明此时锯缘青蟹大颚器的合成与分泌能力增强,其生理功能与卵巢发育密切相关.     

Noble Gases in the Lunar Regolith

The most fundamental character of lunar soil is its high concentrations of solar-wind-implanted elements, and the concentrations and behavior of the noble gases He, Ne, Ar, and Xe, which provide unique and extensive information about a broad range of fundamental problems. In this paper, the authors studied the forming mechanism of lunar regolith, and proposed that most of the noble gases in lunar regolith come from the solar wind. Meteoroid bombardment controls the maturity of lunar soil, with the degree of maturation decreasing with grain size; the concentrations of the noble gases would be of slight variation with the depth of lunar soil but tend to decrease with grain size. In addition, the concentrations of noble gases in lunar soil also show a close relationship with its mineral and chemical compositions. The utilization prospects of the noble gas ^3He in lunar regolith will be further discussed.     

岩石圈深部温压条件下萜烷演化的实验研究

在密闭体系中700℃、压力高达3 GPa条件下进行褐煤加水的模拟实验,分析实验产物中的萜烷的变化规律,进而对高压高温下有机质演化进行研究.实验结果表明,相同压力条件下,温度升高有利于有机质的成熟演化;压力增加会抑制或延迟油气的生成和有机质成熟;五环三萜烷ββ生物构型转化为αβ地质构型所需能量比烯烃双键加氢饱和更高.高温超高压条件下,研究样品中C24四环萜烷存在4种同分异构体,含量由高到低依次包括10β(H)-降A-羽扇烷、10p(H)-降A-奥利烷、C24-17,21-断藿烷和10β(H)-降A-乌散烷.高碳数烷烃在地幔高压力条件下仍可以存在,这为认识超压盆地的油气资源与深层油气成藏及保存提供了新的思路.     

Contact-metamorphic effect on parameters for kerogen maturation

This study examines the effect of contact metamorphism by an igneous dyke on parameters for kerogen maturation, such as elemental composition (H/C, O/C, N/C), ¹H-NMR T₁, vitrinite reflectance and infrared spectra. Although elemental composition and ¹H-NMR T₁ of the kerogen changed only within 1.5 and 5.0 m from the dyke respectively, vitrinite reflectance of the kerogen was thermally affected beyond 10.0 m from the intrusion. It is concluded that vitrinite reflectance is the most sensitive of the parameters evaluated to assess thermal stress of kerogens.     

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province,southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant ¹⁴³Nd/¹⁴⁴Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO₂ at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.     

腐殖煤干酪根裂解气主生气成熟度上限探讨

烃源岩主生气成熟度上限的确定对含气盆地天然气资源量评价和主生气阶段的确定具有重要的意义。对不同成熟度的15个腐殖煤样开展了生气热解实验和氢、碳、氧元素含量测定,生气热解实验是在800℃高温下进行,结果表明低成熟度煤总产气率最高,随着成熟度的增加,产气率逐渐降低,在Ro为0.6%~2.3%范围内的煤产气率降低幅度很大,从61m3/吨.TOC降到11m3/吨.TOC,在Ro达到2.3%之后煤产气率很低,随着成熟度的增加,产气率缓慢降低。不同成熟度煤的H/C值也呈现相似的变化规律,在Ro<2.3%的煤中,H/C值随成熟度的增加快速降低,在此之后,H/C值缓慢降低,因此,认为Ro=2.3%为腐殖煤的干酪根裂解气主生气成熟度上限。     

Importance of the oxidation/maturation pair in the evolution of humic coals

The natural thermal evolution of type III coals (Humic origin) is expressed during diagenesis by a loss of oxygen as CO₂ and H₂O. Other phenomena such as oxidation can cause extensive geochemical modifications and may complicate the effects of simple maturation.Humic coals from the Jurassic in southeastern Utah were studied by elemental analysis, Rock-Eval pyrolysis and infrared spectroscopy. In a van Krevelen diagram (atomic H/C vs atomic O/C), the samples fall within the envelope defined by 860 reference humic coals covering the entire range of diagenesis. Nevertheless, various criteria (geochemical, petrographic, geological and microscopic) cast doubt upon the interpretation that such a distribution of coal composition results from thermal maturation.The same criteria indicate the intervention of redox phenomena. Comparison of our results with those from artificial and natural oxidation shows that these coals were subjected to an oxidation process different from ordinary late alteration. This process was probably due to circulation of highly oxidizing saline water causing oxygen fixation and the transformation of carboxyls into carboxylate anions. The cations that were fixed are oxygenated and certainly contain calcium, but also uranium and perhaps several other cations (V, Mo, Fe...). Emphasis is placed on possible mechanisms that cause such phenomena.