The diagenesis of Brent Group sandstones at Heather Field was studied to reconstruct time-dependent variations in reservoir quality, hydrodynamic history, and oil emplacement. Depositional facies, isotopic and trace-element composition of authigenic minerals, and present-day formation-water chemistry indicate several major changes in porewater chemistry related to both gravitational and compactional flow systems that significantly impacted diagenesis. Early cementation by calcite was related to influx of meteoric water and completely occludes porosity in certain areas of the Field, especially in lower reservoir zones. Geochemical, petrographic, and structural evidence indicate that average calcite precipitated at low to moderate temperature from reducing isotopically-depleted water having high levels of radiogenic Sr (40~50~ 61so ---4 to -6~, sTSr/S6Sr > 0.71). A major period of kaolinite precipitation and feldspar dissolution followed calcite cementation. The isotopic composition of pore-filling kaolinite shows Field-wide uniformity (61so average 13"8~00, 6D average -53.2~), suggesting thorough flushing of the reservoir by meteoric water and precipitation at low to moderate temperature (45*-60~ Tectonic, burial, and thermal histories suggest that meteoric flushing occurred during the late-Cimmerian sea-level low, possibly in response to gravitational flow of meteoric water from exposed parts of the adjacent East Shetlands Platform. Illite and quartz diagenesis post-date kaolinite cementation, with illite KAr ages indicating precipitation through much of the Paleogene (55-27 Ma), coincident with migration of hydrocarbons from neighbouring sub-basins of the East Shetlands Basin. Illite stable isotopic data indicate precipitation in a system resulting from partial mixing of trapped meteoric pore-fluids with saline compaction water. The intensity of sandstone diagenesis is influenced by differences in the fluid migration history, content of detrital K-feldspar, and the time of hydrocarbon emplacement and results in spatial and temporal variations in reservoir quality.The goals of this study were to evaluate temporal and spatial variations in the diagenesis of Brent Group sandstones at Heather Field and to relate them to the hydrochemical and hydrocarbon migration history of the area. The results are based on detailed petrographic and geochemical characterization of major diagenetic phases. Geochemical evidence of diagenetic environment was integrated with geohistory analyses and present-day porewater chemistry to help interpret pathways of fluid migration and controls of porosity and permeability. As will be shown, Brent sandstone diagenesis in Heather Field is broadly similar to other North Sea Fields but differs in the relative timing and intensity of events (Jourdan et al., 1987;Thomas, 1986;Bjorlykke et al., 1986). Isotopic data on calcite,

The north-central United States preserves one of the best continental records of late Pliocene and early-middle Pleistocene glaciations in the Northern Hemisphere. The glaciogenic sequences of this region consist of multiple tills interbedded with paleosols and volcanic ashes. Here we present results on paleomagnetic measurements and till compositional data from glacial sedimentary sequences in Iowa, Nebraska, Kansas, and Missouri. Periods of normal polarity (Brunhes Chron) and reverse polarity (Matuyama Chron) were identified in the sedimentary sequences investigated. This chronology is further constrained by the presence of three volcanic ashes derived from dated eruptions of the Yellowstone caldera, which indicate that the oldest till was deposited Ͼ2.0 Ma. Based on these results we identify three groups of tills representing at least seven pre-Illinoian glaciations: two older groups of reverse-polarity tills containing low and intermediate proportions of crystalline clasts, respectively, and one younger group of normal-polarity tills enriched in crystalline lithologies. The clay mineralogy of the reverse-polarity tills is enriched in kaolinite and depleted in expandable clays relative to the normal-polarity tills, which are also characterized by a minor amount of chlorite. The silt fraction of tills also shows min- † eralogical contrasts whereby the normalpolarity tills are characterized by increases in calcite, dolomite, and feldspar, whereas the older tills show depletion in these minerals and a relative enrichment in quartz. These petrographic and mineralogic changes are indicative of an increase with time in the areal distribution of unweathered igneous and metamorphic source bedrock.

Abstract--Illite diagenesis in Tertiary and Mesozoic shales in the Bergen High area, northern North Sea, was studied using mineralogic, isotopic, and computerized thermal modeling techniques. The Tertiary shales are dominated by smectite, with lesser amounts ofillite, kaolinite, and chlorite. At present burial temperatures of > 70~ smectite is absent, and the shales contain abundant lath-shaped illite which yields a mixed-layer illite/smectite (I/S) X-ray powder diffraction (XRD) pattern. Transmission electron microscopy (TEM) indicates that the illite laths increase in abundance and thickness with increasing depth; XRD patterns indicate a progressive increase in the illite component of the I/S. The deepest samples were found to contain long-range ordered (R=3) I/S, which showed platy particle morphology with the TEM. K-Ar ages of most of the < 0.1-#m-size illite separates imply that illitization was a relatively brief event affecting a thick sequence of sediments during late Cretaceous to early Paleocene time (65-87 Ma); however, measured ages were affected by trace levels of detrital Ar contamination and do not represent the true age of diagenesis.Several methods of quantifying Ar contamination were used to correct measured ages to obtain a reasonable estimate of the true age of diagenesis. The corrected ages are imprecise due to uncertainties in quantifying the levels of sample contamination, but generally suggest a Paleogene (38-66 Ma) period of illitization. In contrast, simple kinetic models of smectite-illitization suggest much younger ages of diagenesis (0-40 Ma at the Veslefrikk field; 0-60 Ma at the Huldra field). The timing of the diagenesis and the morphologic aspects of the authigenic illite suggest that illite precipitated before late Tertiary compaction and resulted in a decrease in fluid permeability. Low trapping efficiency of early Tertiary sediments, vertical escape of warm fluid from the Brent sandstone, and high heat flow may have promoted illite diagenesis in the shales prior to deep late-Tertiary burial.

Feldspar dissolution is an important process affecting the reservoir quality of Brent Group sandstones in the northern North Sea and shows a strong relationship to reservoir temperature and pore water chemistry. In low temperature, brackish, shallow to moderately buried reservoirs (2400–3700 m, reservoir temperature less than 100–120°C) feldspar is generally a major detrital component whose abundance is affected by local variations in facies, extent of early meteoric leaching, and kaolinitization. In deeply buried Brent reservoirs (3700–4700 m) where present burial temperatures exceed 130–140°C detrital K-feldspar is absent and sandstones are characterized by major illitization of early formed kaolinite, precipitation of fibrous illite, extensive quartz cementation, and variable albitization. K-feldspar dissolution apparently supplied elements necessary for illitization, which, according to K-Ar evidence, occurred during early Tertiary burial, associated with initial stages of hydrocarbon accumulation and brine migration. Illite K-Ar ages are unaffected by feldspar contamination in deep reservoirs and the narrow range of measured ages for widely ranging illite size-separates indicates that the major dissolution/illitization reaction was a geologically rapid process. In contrast, illite K-Ar ages from shallow and intermediate-depth reservoirs range from Palaeocene to Early Miocene and often have considerable size dependency due to small amounts of feldspar contamination in coarser fractions. Variable contamination and its effect on measured K-Ar ages often precludes establishing the history of illitization and a well-defined relationship to hydrocarbon accumulation in intermediate-depth reservoirs. Although some feldspar is dissolved during early burial history by low temperature circulation of meteoric water, only deeply buried Brent reservoirs are characterized by the complete diagenetic removal of feldspar; however, the secondary porosity generated by late dissolution is of little benefit to reservoir quality because accompanying cementation results in major loss of permeability.

Basalt alteration from different pedo‐microenvironments in western Oregon was studied using petrographic, electron microscope, X‐ray diffraction, and differential thermal techniques. Initial alteration was similar in each environment and consisted primarily of mineral dissolution associated with the formation of etch pits and hairline cracks and the development of isotropic domains within feldspar phenocrysts. The stage of alteration characterized by secondary mineral formation was distinctly microenvironment dependent. Smectite formed as the initial crystalline weathering product of basalts in the Oregon Coast Range, but was metastable with respect to hydrated halloysite along fracture zones in the weathering crust where more leaching micro‐environments occureed. Smectite altered to chloritic intergrade and halloysite in cyclic wet and dry pedoenvironments. The occurrence of gibbsite was related to the alteration of glassy basalt lithologies under acid leaching conditions. Goethite formed form the alteration of an initial non‐crystalline iron oxide phase.