Diagenesis of organic matter in a 400 m organic rich sediment core from offshore Namibia using solid state 13C NMR and FTIR

Although rates and mechanisms of early diagenesis have been well studied, the effects of microbial metabolism on the molecular composition of the sedimentary organic matter (SOM) over long periods of time need more investigation. In this study, we characterize the early diagenesis of marine SOM from organic rich sediments of the Ocean Drilling Program site 1082 located off Namibia, in the vicinity of the Benguela coastal upwelling system. We used both Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (¹³C NMR) to assess the quantitative partitioning of the organic carbon into major compound classes (aliphatic, aromatic, ester, carboxylic, amide and carbons from carbohydrates). Then, we calculate the SOM composition in the main biomolecules (proteins, carbohydrates, lipids and lignin) on the basis of previous ¹³C NMR based estimates of the molecular composition of the organic mixtures. Results show that the SOM is still labile at 7 m below the seafloor (mbsf) and composed of about 25% proteins and 15% carbohydrates. With increasing depth, the protein content exponentially decreases to 13% at 367 mbsf, whereas the carbohydrate content decreases linearly to 11%. The lignin and lipid content consistently represent around 10% and 40% of the SOM, respectively, and show an increase with depth, due mostly to selective enrichment as the more labile components are lost by degradation. Thus, these components of the SOM are considered refractory at the depth scale considered. The calculated remineralization rates are extremely slow ranging from 5.6 mol C m⁻³ ky⁻¹ at the top of the core to 0.2 mol C m⁻³ ky⁻¹ according to the organic carbon flux to the seafloor. Knowing the labile carbon losses, we propose a method to calculate the initial TOC before the diagenesis took place.     

火山岩储层质量预测方法探讨——以松辽盆地徐家围子断陷为例

松辽盆地徐家围子断陷深层火山岩储层具有埋藏深、 温度高、 压力大、 物性差、 非均质性强的特征。为了预测火山岩储层类型和质量在平面上的变化规律,本文讨论了影响火山岩储层物性的主要地质因素。结果表明,火山岩的物性主要受火山岩相（组）和所处对应碎屑岩成岩阶段的影响。距火山口越远的火山岩相组的火山岩储层,其孔隙度和渗透率越低。火山岩工业气层主要分布在晚成岩阶段A期以前的火山岩储层中。在晚成岩阶段B期,只有少量低产气层。本文充分考虑徐家围子断陷火山岩相和成岩作用对火山岩储层物性的影响与控制,应用成岩模拟软件,预测了成岩阶段的横向展布,通过叠合营三段火山岩相图和成岩阶段预测图,预测了火山岩储层的类型和质量,目前已发现工业气流的井主要分布在断陷中部的Ⅱ、 Ⅲ类火山岩储层中。     

储层成岩产物对于油气运移的指示意义——以塔里木盆地群苦恰克构造带东河塘组为例

泥盆系东河塘组下段砂岩储层是塔里木盆地群苦恰克构造带内重要的勘探层系,有关储层内油气充注方向与富集规律的研究目前仍未有统一认识。研究层段内碳酸盐胶结物(主要为白云石和铁白云石胶结物)分布较为广泛,其中作为早成岩阶段产物的白云石胶结物的含量在平面上呈现出自东向西依次增加的趋势; 而铁白云石胶结物则以交代先期形成白云石胶结物的形式产出于中成岩阶段,略滞后于储层内的油气充注,其胶结程度在平面上呈现出自东向西依次减弱的分布规律。铁白云石胶结物作为反映储层内较强还原环境的指示性矿物,其在储层内的分布规律也承担着判明油气运移方向或富集规律的"责任",而其上述分布规律为研究区储层内油气自东向西充注提供了全新的判定依据。     

鄂尔多斯盆地富县地区延长组成岩相特征及控制因素

通过野外剖面、岩心观察、薄片鉴定,结合扫描电镜与能谱分析等测试手段,在充分认识富县地区长6、长8油层组储层特征、成岩作用基础上,研究长6、长8油层组成岩相类型.主要成岩相包括有泥岩强烈压实相、砂岩压实(压溶)相、砂岩剩余原生粒间孔隙-次生溶蚀孔隙相、砂岩次生溶蚀孔隙相和微裂缝相等5种类型.结合沉积环境及沉积相平面展布特征,阐述了成岩相平面分布,结合储层发育特征主控因素深入研究成岩相特征主控因素,表明沉积环境为成岩相特征提供了物质基础,沉积物在后期成岩演化过程中受成岩作用控制,即成岩相为沉积物在成岩演化后的具体体现.     

Diagenesis of Clastic Reservoirs: Advances and Prospects

Along with the widely used experimental testing technology within 40 years,a number of significant knowledge and results have been achieved in diagenesis of clastic reservoirs an important branch of sedimentology and basin research,which has promoted the exploration and development of hydrocarbon bearing basins. This article encompasses the knowledge system to start an overview of the concept and research history of diagenesis, and to collate some representative classification viewpoints of clastic diagenetic stage. Several advances in recent theoretical and applied aspects of diagenesis, which have enriched the systems and technology series, have been introduced and summarized, including: ① improvement of alkaline diagenesis theory; ② increasingly widespread application in numerical simulation of clastic diagenesis; ③ rapid development of diagenesis within the framework of sequence stratigraphy or depositional facies; ④ preliminary investigation of structural diagenesis; ⑤ in-depth quantitative evaluation of diagenetic facies. Additionally, the research trends of the discipline system of diagenesis should be closely combined and discussed with diagenesis of tight sandstone reservoirs, diagenesis and its temporal properties of hierarchical systems, quantitative research of diagenesis, and integration of diagenesis and other fields. Frontier research and development directions have been prospected in order to bring forward further development of diagenesis, which is benefit for the forecasting and evaluation of the spatial and temporal distribution of hydrocarbon bearing clastic reservoirs.     

Influence of the volume loss of framework grains on the quantitative analysis of diagenetic modification of the original intergranular porosity

The evaluation of the original intergranular porosity loss due to compaction (COPL) and cementation (CEPL) is an important tool for understanding the contribution of diagenetic processes to porosity reduction and permeability retention of sandstones and carbonate grainstones. The previous approach for computing CEPL and COPL assumed that the volume of framework grains remained constant during diagenesis. However, such assumption will introduce error in COPL and CEPL if diagenetic processes reduced the volume of framework grains. New equations are presented that account for such situations. Comparably, the previous approach underestimates COPL and the compaction index, but over-estimates CEPL in the case of volume loss of framework grains by dissolution or intergranular pressure solution (DIPL). The deviation of CEPL and COPL caused by DIPL rises with the increase in the intergranular volume and DIPL. Analysis of three published case studies suggests that DIPL values of just a few percentages will significantly impact COPL and CEPL values. The effects of DIPL can be greater than the effects of uncertainty in original porosity estimates and uncertainty in point count data.     

福州盆地全新世沉积物的磁学性质及其对环境变化的响应

福州盆地位于海陆过渡地带,在海陆变迁过程中,沉积物记录了高分辨率的环境信息,是揭示沉积特征对环境变化响应过程及模式的理想区域.本文选择位于福州盆地的FZ5钻孔进行岩石磁学、环境磁学和古地磁学方面的研究,以期阐明该区域沉积物磁学性质对陆源碎屑输入、海平面变化和成岩作用的响应.岩石磁学结果表明钻孔沉积物以低矫顽力的亚铁磁性矿物为主体,但是在不同的环境变化阶段,磁性矿物的类型有较大变化.在9~3 cal. ka BP的海侵过程中,沉积物中以磁铁矿为主体,存在菱铁矿和铁硫化物等还原性矿物.硫化作用使细粒磁铁矿溶解形成胶黄铁矿和黄铁矿,其峰面随碎屑磁性矿物的浓度变化而迁移.但硫化作用没有完全消除磁铁矿携带的特征剩磁和陆源碎屑输入量以及海平面升降对该阶段沉积物磁性的控制.在~3 cal. ka BP以来随着海平面下降、沉积环境向陆相氧化环境转化,虽然早期还原作用仍然存在,但后期氧化作用使磁性矿物向高矫顽力的赤铁矿等矿物转变,氧化作用基本扰乱了磁铁矿携带的剩磁.沉积及其后期成岩作用过程中,发生在约~8.2、~7.7、~7.5、~2.7、~1.5、~0.5 cal. ka BP六次强烈的古氧化界面反映了福州盆地当时异常干旱或湿热的气候事件.     

Diagenesis in Okavango fan and adjacent dune deposits with implications for the record of palaeo-environmental change in Makgadikgadi–Okavango–Zambezi basin, northern Botswana

This work considers the spatial distribution and ages of western MOZ basin siliclastic sediments prior to providing insights into the diagenesis of degraded dune and alluvial fan sands. Previously published and new TL/OSL ages imply that extensive over-washing of dune sands took place at least 100 ka ago while ages on Okavango floodplains imply that the fan was formed ca. 40 ka and has since undergone periods of higher and lower flood regimes. Sediment analyses indicate that both dune and fan sands contain a diagenetic matrix of clay-enhanced amorphous silica (CEAS) which bonds weakly formed aggregates. The time of formation of diagenetic matrix products is inconclusive but may have been accelerated during or shortly after events dated using OSL/TL techniques. Hence earlier dune over-washing may have led to greater porewater of an acidic to near neutral nature which in turn promoted smectite formation and silicic acid precipitation > 100 000 years ago. The relatively abundant CEAS matrix in floodplain sands implies more recent semi-continuous flood events again of an acidic-near neutral nature leading to the formation of smectite. In this case the floodplain sediments are dated as having been deposited around 40 and 11 ka, when porewater content may have accelerated clay formation and silica dissolution. The dual nature of the CEAS in the islands reflects a changing environment from smectite-dominated flooding events to sepiolite-dominated desiccation events. Flooding may also correspond to TL/OSL ages over the past 40 000 years which contributed to accelerated CEAS formation. The sepiolite is associated with a Ca-rich matrix implying desiccation which may relate to drying events over the 40 000 year period or to riparian tree root pumping and selective salt accumulation. This work shows that sedimentation in incipient rifts is complex and rarely explained totally in terms of primary depositional events. The implications of different stages of sand diagenesis may be significant in enhancing palaeo-environmental interpretations in semi-arid fluvial environments.     

Impact of sedimentology and diagenesis on the petrophysical properties of a tight oolitic carbonate reservoir. The case of the Oolithe Blanche Formation (Bathonian,Paris Basin,France)

The Oolithe Blanche Formation was studied in three quarries, located at the south-eastern edge of the Paris Basin (France). Heterogeneities in reservoir properties were assessed through a sedimentological, diagenetic and petrophysical study. The relationships between depositional settings, diagenesis and petrophysical properties were analysed using detailed petrographic studies, image analysis, Nano CT-scans and petrophysical measurements.The carbonate reservoir pore network is mainly controlled by intraparticle microporosity which ensures the connectivity with interparticle meso- and macroporosity.Early cementation vs. early compaction processes (mainly grain interpenetration) may have considerable influence on fluid-flow properties and parameters such as permeability, acoustic velocities and tortuosity. Better reservoir properties are found when compaction processes begin before cementation.From statistical analyses, e.g. Principal Component Analysis and Linear Discriminant Analysis, a sedimentological/diagenetic and petrophysical model is proposed that is in a good agreement with the geological model developed from field work.     

Diagenesis and bioavailability of mercury in the contaminated sediments of Ulhas Estuary, India

Sequential extraction of Hg was performed in a core collected from intertidal area in Ulhas Estuary in order to characterize the downward distribution and diagenetic behavior of Hg in a polluted estuary. Concentration of total Hg ranged between 0.46 and 6.40 μg g⁻¹ with significant decrease in surficial sediment as a result of closing of two Hg-cell based chlor-alkali plants. Results of sequential extraction showed that >65% Hg was strongly bound to organo-sulphur and inorganic sulphide species that are not bioavailable. Flux of only 18.8% of Hg was found compared to its value deposited on sediment–water interface. Hence, it is concluded that there is no significant diagenetic remobilization of Hg in Ulhas Estuary. This core also has been analyzed for ²¹⁰Pb geochronology. The estimated sedimentation rate is 0.31 cm y⁻¹ in the present sampling region.