Vertical effective stress as a control on quartz cementation in sandstones

Temperature-controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones. Here, we integrate quantitative petrographic data, high spatial resolution oxygen isotope analyses of quartz cement, basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress-sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones. We present data from highly overpressured sandstones in which, despite the current burial temperature of 190 °C, quartz cement occurs in low amounts (4.6 ± 1.2% of bulk volume). In situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 Ma and a temperature range of 80–150 °C, during which time high fluid overpressures resulted in consistently low vertical effective stress. We argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

Geological repositories: scientific priorities and potential high-technology transfer from the space and physics sectors

The use of underground geological repositories, such as in radioactive waste disposal (RWD) and in carbon capture (widely known as Carbon Capture and Storage; CCS), constitutes a key environmental priority for the 21st century. Based on the identification of key scientific questions relating to the geophysics, geochemistry and geobiology of geodisposal of wastes, this paper describes the possibility of technology transfer from high-technology areas of the space exploration sector, including astrobiology, planetary sciences, astronomy, and also particle and nuclear physics, into geodisposal. Synergies exist between high technology used in the space sector and in the characterization of underground environments such as repositories, because of common objectives with respect to instrument miniaturization, low power requirements, durability under extreme conditions (in temperature and mechanical loads) and operation in remote or otherwise difficult to access environments

Sandstone Diagenesis in Sediment–lava Sequences: Exceptional Examples of Volcanically Driven Diagenetic Compartmentalization in Dune Valley, Huab Outliers, Nw Namibia

At the base of many flood basalt sequences and along volcanic rifted margins, volcanism can compete with the existing sedimentary environments, resulting in interbedded sequences of volcanic rocks and sediments. Here we report on sediment interlayers that are found in the lowermost volcanic units of the Etendeka flood basalts in NW Namibia (Twyfelfontein and Awahab formations), part of the much larger Paraná–Etendeka Igneous Province. The sandstone bodies, predominantly eolian dunes, are isolated in a sequence of Lower Cretaceous (∼ 134 Ma) lava flows. The uppermost part or where sediment deposition and lava emplacement is observed to interact is characterized by barchanoid dunes, which were actively migrating during the emplacement of the lava flows. The fossil (isolated by lava) barchan dunes studied in Dune Valley show three characteristically different diagenetic styles. In Dune Valley, each dune body is completely encapsulated by lava, with additional igneous intrusions cutting through some bodies. We recognize three distinct styles of diagenesis: Type 1: fossilised dunes that are red in color and lack major authigenic mineralization, with grain compaction and subsequent porosity loss being the dominant diagenetic process. Type 2: dunes that have been bleached white, which have undergone a more complex diagenetic pathway. Type 2 dunes have abundant calcite, kaolinite, and böhmite as authigenic phases and lack hematite grain coatings. Detrital plagioclase is absent in white dunes (XRD analysis), with pseudomorphs of kaolinite common. This diagenetic assemblage results in the white dunes having lower porosity and permeability compared to the red dunes. The observations are probably due to a flux of carbon dioxide (CO2), hydrogen sulfide (H2S) and/or hydrogen (H2)–rich hydrothermal groundwater derived from igneous intrusions below. Type 3: “hot contact” effects at lava-flow contacts, where the unconsolidated dunes were rapidly indurated during lava emplacement (volcano-eogenesis). Type 3 diagenesis is restricted to << 1 m depth below lava contacts and common in dunes displaying both Type 1 and Type 2 diagenesis. The distribution of diagenetic Types 1 and 2 is dune specific, and throughout Dune Valley approximately half of the dunes have been bleached (e.g., Type 2 diagenesis), whereas diagenetic Type 3 is a hot contact phenomenon and is therefore found along all basal lava and dike contacts. This work has relevance to understanding the development of sediment–lava systems, to hydrocarbon exploration and development in preserved sediment–lava sequences, and the hydrothermal process described provides an example of natural CO2 sequestration

Impact of Grain-Coating Clays on Porosity Preservation in Paleocene Turbidite Channel Sandstones: Nelson Oil Field, UK Central North Sea

The Forties Sandstone Member is an important deep-water reservoir in the Central North Sea. The role of depositional characteristics, grain-coating clays, and diagenesis in controlling the reservoir quality of the sandstones is poorly understood. The main aim of the study is to understand the role of depositional characteristics, grain-coating and pore-filling clays, and diagenesis in controlling the reservoir quality evolution of turbidite-channel sandstones. The study employed a multi-disciplinary technique involving thin section petrography and scanning electron microscopy (SEM) to investigate the impact of grain size, clay matrix content, mode of occurrence of grain-coating chlorite and illite, and their impact in arresting quartz cementation and overall reservoir quality in the sandstones. Results of our study reveal that porosity evolution in the sandstones has been influenced by both primary depositional characteristics and diagenesis. Sandstones with coarser grain size and lower pore-filling clay content have the best reservoir porosity (up to 28%) compared to those with finer grain size and higher pore-filling clay content. Quartz cement volume decreases with increasing clay-coating coverage. Clay coating coverage of >40% is effective in arresting quartz cementation. Total clay volume of as low as 10% could have a deleterious impact on reservoir quality. The Forties Sandstone Member could potentially be a suitable candidate for physical and mineralogical storage of CO2. However, because of its high proportion (>20%) of chemically unstable minerals (feldspar, carbonates, and clays), their dissolution due to CO2 injection and storage could potentially increase reservoir permeability by an order of magnitude, thereby affecting the geomechanical and tensile strength of the sandstones. Therefore, an experimental study investigating the amount of CO2 to be injected (and at what pressure) is required to maintain and preserve borehole integrity. The findings of our study can be applied in other reservoirs with similar depositional environments to improve their reservoir quality prediction

Role played by clay content in controlling reservoir quality of submarine fan system, Forties Sandstone Member, Central Graben, North Sea

Proximal to distal fan change in grain size, clay matrix content, and grain-coating clays have been identified as key contributing factors for eservoir quality evolution of submarine fan turbidite sandstones. This study evaluated the role played by grain-coating and pore-filling clays, depositional facies, and diagenesis in reservoir quality evolution of the Paleocene Forties submarine fan sandstones (Central North Sea) from proximal to distal fan settings. To help provide a comprehensive understanding of the role played by pore-filling and grain-coating clays in destroying and preserving reservoir quality, respectively, in turbidite sandstones, we have used a multi-disciplinary approach including petrography, burial history, scanning electron microscopy, and stable isotopes analysis. Results of the study showed that reservoir quality is influenced by both depositional facies and diagenesis. The proximal-fan, amalgamated sandstones facies have the best reservoir quality due to coarser grain size, lower pore-filling clays, and lower amount of ductile grains. In contrast, the distal-fan, mud-prone heterolithic facies have the poorest reservoir quality due to finer grain size, higher pore-filling clays, and higher amount of ductile grains. Pore-filling clays between 10 and 30% have a deleterious effect on reservoir quality, reducing porosities and permeabilities to generally <10% and <1 mD, respectively. Based on the relatively shallow, present-day burial depths of the studied Forties Sandstone Member (2200–3100 m TVDSS), the percentage of clay-coating coverage to significantly inhibit quartz cementation ranges from 40 to 50%. Detrital, grain-coating smectites, probably inherited from the shelf/continental environments and/or emplaced through sediment dewatering, have transformed into chlorite, illite, and illite-smectite. Calcite and siderite, where well-developed, have arrested mechanical compaction and also occluded porosity, thereby rapidly degrading reservoir quality in the sandstones; however, their dissolution by acidic pore fluids could potentially create secondary intergranular porosity, enhancing reservoir quality of the sandstones. Evidence presented demonstrates that, high quality reservoir sandstones that deviate from normal porosity-depth trends for submarine fans sandstones can be attributed to facies changes (composition and grain size) with a complex interplay of mechanical compaction, detrital clays and authigenic clay coatings inhibiting quartz cement precipitation

The importance of facies, grain size and clay content in controlling fluvial reservoir quality – an example from the Triassic Skagerrak Formation, Central North Sea, UK

Clay-coated grains play an important role in preserving reservoir quality in high-pressure, high-temperature (HPHT) sandstone reservoirs. Previous studies have shown that the completeness of coverage of clay coats effectively inhibits quartz cementation. However, the main factors controlling the extent of coverage remain controversial. This research sheds light on the influence of different depositional processes and hydrodynamics on clay-coat coverage and reservoir quality evolution. Detailed petrographic analysis of core samples from the Triassic fluvial Skagerrak Formation, Central North Sea, identified that channel facies offer the best reservoir quality; however, this varies as a function of depositional energy, grain size and clay content. Due to their coarser grain size and lower clay content, high-energy channel sandstones have higher permeabilities (100–1150 mD) than low-energy channel sandstones (<100 mD). Porosity is preserved due to grain-coating clays, with clay-coat coverage correlating with grain size, clay-coat volume and quartz cement. Higher coverage (70–98%) occurs in finer-grained, low-energy channel sandstones. In contrast, lower coverage (<50%) occurs in coarser-grained, high-energy channel sandstones. Quartz cement modelling showed a clear correlation between available quartz surface area and quartz cement volume. Although high-energy channel sandstones have better reservoir quality, they present moderate quartz overgrowths due to lesser coat coverage, and are thus prone to allowing further quartz cementation and porosity loss in ultra-deep HPHT settings. Conversely, low-energy channel sandstones containing moderate amounts of clay occurring as clay coats are more likely to preserve porosity in ultra-deep HPHT settings and form viable reservoirs for exploration