ALBERT

Description: 〈p〉Calcite cement is a major diagenetic phase in the marine sections of the Brent Group of the UKCS Heather oil field but is of negligible importance in the non-marine parts of the Brent succession. Calcite cement occurs as massive, stratabound horizons in excess of 3 m thick. Seismic attribute and wireline log analyses show that crestal areas of the reservoir seem to be largely free of carbonate cement, whereas downdip areas are locally heavily cemented. Early marine carbonate cement and bioclasts were probably the primary sources of the calcium in the calcite although dissolution, re-distribution via diffusion or flow and re-precipitation had major impacts on cement distribution. Poikilotopic calcite cement locally occludes all porosity. However, calcite cement locally surrounds late diagenetic quartz cement, and the presence of primary oil inclusions and high temperature primary aqueous fluid inclusions prove that some calcite grew late during burial. The occurrence of negative carbon stable isotope data reveal that calcite grew in the presence of CO〈sub〉2〈/sub〉 derived from biogenic sources. Input of CO〈sub〉2〈/sub〉 into the Brent reservoir facilitated dissolution and re-precipitation of carbonate minerals and permitted redistribution from dispersed early cement and bioclasts in the primary marine Brent sandstones into cemented horizons and nodules.〈/p〉

Description: The porosity and permeability of sandstone and carbonate reservoirs (known as reservoir quality) are essential inputs for successful oil and gas resource exploration and exploitation. This chapter introduces basic concepts, analytical and modelling techniques and some of the key controversies to be discussed in 20 research papers that were initially presented at a Geological Society conference in 2014 titled ‘Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction’. Reservoir quality in both sandstones and carbonates is studied using a wide range of techniques: log analysis and petrophysical core analysis, core description, routine petrographic tools and, ideally, less routine techniques such as stable isotope analysis, fluid inclusion analysis and other geochemical approaches. Sandstone and carbonate reservoirs both benefit from the study of modern analogues to constrain the primary character of sediment before they become a hydrocarbon reservoir. Prediction of sandstone and carbonate reservoir properties also benefits from running constrained experiments to simulate diagenetic processes during burial, compaction and heating. There are many common controls on sandstone and carbonate reservoir quality, including environment of deposition, rate of deposition and rate and magnitude of sea-level change, and many eogenetic processes. Compactional and mesogenetic processes tend to affect sandstone and carbonate somewhat differently but are both influenced by rate of burial, and the thermal and pressure history of a basin. Key differences in sandstone and carbonate reservoir quality include the specific influence of stratigraphic age on seawater composition (calcite v. aragonite oceans), the greater role of compaction in sandstones and the greater reactivity and geochemical openness of carbonate systems. Some of the key controversies in sandstone and carbonate reservoir quality focus on the role of petroleum emplacement on diagenesis and porosity loss, the role of effective stress in chemical compaction (pressure solution) and the degree of geochemical openness of reservoirs during diagenesis and cementation. This collection of papers contains case study-based examples of sandstone and carbonate reservoir quality prediction as well as modern analogue, outcrop analogue, modelling and advanced analytical approaches.

Description: :— Formation of microcrystalline quartz formation has proven to be effective at preserving porosity in deeply buried sandstone petroleum reservoirs, typically cemented by syntaxial quartz cement. There remains much uncertainty about what controls the growth of microcrystalline quartz and how it prevents syntaxial quartz overgrowths. Here, the Cretaceous Heidelberg Formation, Germany, provides a natural laboratory to study silica polymorphs and develop an understanding of their crystallography, paragenetic relationships, and growth mechanisms, leading to a new understanding of the growth mechanisms of porosity-preserving microcrystalline quartz. Data from scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) data illustrate that porosity-preserving microcrystalline quartz cement is misoriented with respect to the host grain upon which it grows. In contrast, ordinary quartz cement grows in the same orientation (epitaxially) as the host quartz sand grain, and typically fills pore spaces. EBSD and TEM observations reveal nanofilms of amorphous silica (~ 50–100 nm in thickness) between the microcrystalline quartz and the host grain. The microcrystalline quartz is interpreted to be misoriented relative to the host grain, because the amorphous silica nanofilm prevents growth of epitaxial quartz cement. Instead, the microcrystalline quartz is similar to chalcedony with [11–20] perpendicular to the growth surface and c axes parallel with, but randomly distributed (rotated) on, the host quartz grain surface. Development of pore-filling quartz growing into the pore (in the fast-growing c- axis direction) is thus inhibited due to the amorphous silica nanofilm initially and, subsequently, the misoriented microcrystalline quartz that grew on the amorphous silica.

Description: Porous rocks in the subsurface are now used for carbon capture and storage (CCS), to help ameliorate the effects of greenhouse gas emissions. These porous reservoir rocks require a caprock to retain the CO 2 . It is important to characterize caprock quality and its stability in the presence of elevated partial pressures of CO 2 . Lower Triassic sandstones are common in the UK and NW Europe and are being considered for future CCS projects. The caprock to these sandstones is the Middle and Upper Triassic Mercia Mudstone Group. We have studied the Mercia Mudstone using mineralogy, petrology and mercury injection porosimetry to assess its caprock quality. Detrital minerals are dominated by quartz, K-feldspar, illite and chlorite; diagenetic pore-filling minerals are dominated by calcite, dolomite and gypsum. In samples with abundant clay minerals, there are only small quantities of pore-filling diagenetic cements. Porosity is broadly uniform for both clay-rich and clay-poor samples. The cleaner (clay-poor) samples had their initial pore spaces filled with early diagenetic pore-filling carbonates and gypsum. Despite the broadly uniform porosity, mean pore throat diameter displays a strong inverse correlation with clay content whereas threshold capillary entry pressure shows a strong positive correlation with clay content. The more clay-rich samples represent much better caprock than the coarser-grained samples that contain abundant pore-filling cement. The samples could support potential column heights of supercritical CO 2 between 70 and 540 m and have calculated permeabilities between 10 –20 and 10 –19 m 2 ; leakage would occur only on geological time scales, assuming that these samples are representative of the whole caprock. Because caprock quality correlates with illite content and illite will be relatively immune to elevated partial pressures of CO 2 , the Mercia Mudstone probably represents a durable caprock for future Lower Triassic sandstone CCS projects.

Description: A bstract :  Whole-rock, inorganic, geochemical data have previously been used to aid characterization and stratigraphic correlation of fossil-poor clastic sedimentary deposits, although this approach has not yet been calibrated by testing chemostratigraphic correlations against physical stratigraphic correlations from outcrop. Chemical data from a well-exposed siliciclastic succession in a Tortonian (late Miocene) submarine slope channel system in the Tabernas Basin of southern Spain were generated using X-ray fluorescence supplemented by X-ray diffraction, SEM analysis, and light microscopy. Amongst the 〉 60 elements measured, nearly a quarter of them have proved to be useful or reliable for geochemically based correlation at the tested scales (~ 10 m, ~ 50 m, ~ 100 m). These elements are SiO 2 , K 2 O, Al 2 O 3 , CaO, and TiO 2 , as well as Zr, Nb, Th, Rb, Cs, As, and the rare earth elements, La, Nd, and Ce. High-resolution, bed-to-bed geochemical correlation is feasible over short distances (~ 10 m). Geochemical data from sections in beds that are traceable suggest that a given stratigraphic level seems to be geochemically homogeneous with respect to some key elements. Geochemical data also provide a useful aid for high-resolution sedimentary correlation, at distances of several tens of meters up to at least 100 m. Since each individual bed seems to be broadly homogeneous in composition, at a larger scale it is possible to discern a provenance signal in geochemical data. Chemical stratigraphy thus seems to work even in stratigraphically complicated submarine channel complexes.

Description: The occurrence and distribution of minerals in modern sedimentary systems hold many clues to help unravel the origin and distribution of reservoir quality-controlling minerals in ancient and deeply buried sandstones, but few quantitative studies have been undertaken. Here we have used a range of techniques including X-ray diffraction, scanning electron microscopy and fully automated mineralogical QEMSCAN analysis to provide a comprehensive understanding of mineral composition and distribution within the post-glacial, clastic sediments of the Ravenglass Estuary, NW England. The Ravenglass Estuary is fed by two main rivers: one drains a granite-dominated hinterland, the other drains a hinterland that contains andesite and Triassic red bed sandstones. The granite-supplied arm has slightly more quartz-rich and Fe mineral-poor sediment than the andesite- and red bed-supplied sediment. The provenance signals are muted for feldspar and mica minerals heavy-mineral garnet populations seem to be sensitive to provenance. Detrital K-feldspar grains are preferentially associated with illite-dominated clay mineral coats, whereas all plagioclase mineral grains are preferentially associated with kaolinite-dominated clay mineral coats. This can be explained by rapid early diagenesis in the sediment with K-feldspar grain surfaces replaced by illite and plagioclase grain surfaces replaced by kaolinite. The andesite- and red bed-supplied sediment contains twice the amount of Fe minerals, which are dominated by chlorite, than the granite-supplied sediment. Chlorite rarely is associated with grain coatings on feldspar grains, possibly because it is predominantly a detrital mineral. Detrital Fe minerals seem to be locally replaced by pyrite due to bacterial sulphate reduction, suggesting that some early diagenetic processes may serve to lock away iron and prevent it from creating Fe-rich clay minerals.

Description: By controlled experiments that simulate marine depositional environments, it is shown that accelerated weathering and clay mineral authigenesis occur during the combined process of ingestion, digestion and excretion of fine-grained sediment by two species of annelid worms. Previously characterized synthetic mud was created using finely ground, low-grade metamorphic slate (temperature approximately 300°C) containing highly crystalline chlorite and muscovite. This was added to experiment and control tanks along with clean, wind-blown sand. Faecal casts were collected at regular intervals from the experimental tanks and, less frequently, from the control tanks. Over a period of many months the synthetic mud (slate) proved to be unchanged in the control tanks, but was significantly different in faecal casts from the experimental tanks that contained the worms Arenicola marina and Lumbricus terrestris. Chlorite was preferentially destroyed during digestion in the gut of A. marina. Both chlorite and muscovite underwent XRD peak broadening with a skew developing towards higher lattice spacing, characteristic of smectite formation. A neoformed Fe-Mg-rich clay mineral (possibly berthierine) and as-yet undefined clay minerals with very high d-spacing were detected in both A. marina and L. terrestris cast samples. We postulate that a combination of the low pH and bacteria-rich microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay mineral precipitation processes during digestion. These results show that macrobiotic activity significantly accelerates weathering and mineral degradation as well as mineral authigenesis. The combined processes of sediment ingestion and digestion thus lead to early diagenetic growth of clay minerals in clastic sediments.

Description: Phengite and chlorite have undergone decomposition during pyrometamorphism caused by the intrusion of a dolerite feeder pipe into Dalradian greenschists in Argyllshire, Scotland. All reaction products are extremely fine grained. Transmission electron microscopy has revealed that phengite pseudomorphs consist of biotite, spinel, mullite, sanidine and phengite, and that chlorite pseudomorphs consist of combinations of chlorite, spinel, orthopyroxene, magnetite, cordierite and biotite. Although the reactions were short-lived and did not go to completion, microprobe analysis and phase diagram analysis have revealed that there has been significant chemical interaction between the phyllosilicates and the surrounding rock. Numerous orientation relationships exist between the original minerals and their reaction products; the close-packed planes in the precursor phyllosilicates were inherited by their reaction products.