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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,

Petroleum spills often create persistent “source zones” of hydrocarbon‐impacted soils that are either in direct contact with ground water or in indirect contact through soil moisture infiltration and dissolution. Understanding the natural attenuation of source zones is critical to anticipating their longevity and the future chemical composition of their associated dissolved ground water and vapor plumes. A companion paper to this work presents an approach for site‐specific assessment of source zone natural attenuation (SZNA), based on three levels of data collection and analysis oriented toward answering common questions of interest. Use of this SZNA assessment approach is illustrated here through application to the 3000‐acre former Guadalupe Oil Field in California, where numerous petroleum source zones are present within a dune sand aquifer system. SZNA processes and mass loss rates are assessed through use of data from geochemical profiles of continuous cores, nested ground water wells, and soil‐gas probes. Mass loss rates are estimated by quantifying fluxes of dissolved electron acceptors and degradation products in ground water and in soil gas. Under current conditions, gas transport processes coupled with aerobic and anaerobic biodegradation acting on exposed portions (above the water table) of source zones are the most significant loss mechanisms. Depending on the depth of contamination and other factors, the mass loss rate per unit surface area for those processes varies from approximately 0.1 to 1.0 kg total petroleum hydrocarbons per m2/year. In comparison, mass losses from the submerged part of the source zone and involving ground water transport processes (i.e., dissolution and biodegradation) were estimated to be about approximately 2 orders of magnitude lower. Once aerobically degradable hydrocarbons above the water table are depleted, the slower mass loss processes operating on the submerged parts of the source will control source zone mass loss.

This work focuses on the site‐specific assessment of source zone natural attenuation (SZNA) at petroleum spill sites, including the confirmation that SZNA is occurring, estimation of current SZNA rates, and anticipation of SZNA impact on future ground water quality. The approach anticipates that decision makers will be interested in answers to the following questions: (1) Is SZNA occurring and what processes are contributing to SZNA? (2) What are the current rates of mass removal associated with SZNA? (3) What are the longer‐term implications of SZNA for ground water impacts? and (4) Are the SZNA processes and rates sustainable? This approach is a data‐driven, macroscopic, multiple‐lines‐of‐evidence approach and is therefore consistent with the 2000 National Research Council’s recommendations and complementary to existing dissolved plume natural attenuation protocols and recent modeling work published by others. While this work is easily generalized, the discussion emphasizes SZNA assessment at petroleum hydrocarbon spill sites. The approach includes three basic levels of data collection and data reduction (Group I, Group II, and Group III). Group I measurements provide evidence that SZNA is occurring. Group II measurements include additional information necessary to estimate current SZNA rates, and group III measurements are focused on evaluating the long‐term implications of SZNA for source zone characteristics and ground water quality. This paper presents the generalized site‐specific SZNA assessment approach and then focuses on the interpretation of Group II data. Companion papers illustrate its application to source zones at a former oil field in California.

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