Diagenesis and Basin Development (original) (raw)
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Chemical Geology, 2002
Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present.
Journal of Petroleum Geology, 1998
We report here on fracture and pore-filling cements in the Rotliegend Sandstones of the Sole Pit area, UK Southern North Sea. Standard thin-section petrography and scanning electron microscopy have been complemented b y fluid inclusion microthermometry. The main objectives of this paper are to establish the stratigraphy of the sandstones' cements and to describe the cements' characteristics. In addition, the combined use ofpetrography andfluid inclusion microthermometry is used to define the timing of cementation and gas migration in the Southern North Sea.
Marine and Petroleum Geology, 2010
Rock physical properties, like velocity and bulk density, change as a response to compaction processes in sedimentary basins. In this study it is shown that the velocity and density in a well defined lithology, the shallow marine Etive Formation from the northern North Sea increase with depth as a function of mechanical compaction and quartz cementation. Physical properties from well logs combined with experimental compaction and petrographic analysis of core samples shows that mechanical compaction is the dominant process at shallow depth while quartz cementation dominates as temperatures are increased during burial. At shallow depths (<2000-2500 m, 70-80 C) the log derived velocities and densities show good agreement with results from experimental compaction of loose Etive sand indicating that effective stress control compaction at these depths/temperatures. This indicates that results from experimental compaction can be used to predict reservoir properties at burial depths corresponding to mechanical compaction. A break in the velocity/depth gradient from about 2000 m correlates with the onset of incipient quartz cementation observed from petrographic data. The gradient change is caused by a rapid grain framework stiffening due to only small amounts of quartz cement at grain contacts. At temperatures higher than 70-80 C (2000-2500 m) the velocities show a strong correlation with quartz cement amounts. Porosity reduction continues after the onset of quartz cementation showing that sandstone diagenesis is insensitive to effective stress at temperatures higher than 70-80 C. The quartz cement is mainly sourced from dissolution at stylolites reflected by the fact that no general decrease in intergranular volume (IGV) is observed with increasing burial depth. The IGV at the end of mechanical compaction will be important for the subsequent diagenetic development. This study demonstrates that mechanical compaction and quartz cementation is fundamentally different and this needs to be taken into consideration when analyzing a potential reservoir sandstone such as the Etive Formation.
Diagenesis of a Deeply Buried Sandstone Reservoir: Hild Field, Northern North Sea
Clay Minerals, 1986
Calcite cementation and extensive dissolution of feldspar with formation of authigenic kaolinite, quartz cement and secondary porosity are the main diagenetic processes in the deeply buried Hild Field. Mineralogical and isotopic analyses, reservoir pressure and depositional environment suggest that these diagenetic processes occurred prior to burial at a depth of 1·5–2 km. The timing of the diagenetic sequence suggests that feldspar dissolution is related to meteoric water flow. Calcite occurs as an early diagenetic iron-poor cement, and as two types of later diagenetic (<120°C) ferroan calcite cements. The ferroan calcites are mainly an in situ dissolution-reprecipitation product of the early diagenetic phase. Extensive local dissolution of calcite was important for forming secondary porosity which is closely associated with a prominent ‘gas chimney’ in the area studied. A high CO2 content in the natural gases of the reservoir suggests that the solvent was carbonic acid formed f...
Clay Minerals, 1989
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 postdate kaolinite cementation, with illite K-Ar 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. 9 production well tOE Z~ o ~___~o Miles o tO 20Km~ FIG. l. Location map of Heather Field showing well locations and structural relationship to other Brent fields in the northern North Sea. platform 9 ~" fault ~A(~ O~" Top Brent structure contour
DIAGENETIC HISTORY AND RESERVOIR QUALITY OF A BRENT SAND SEQUENCE
The Etive Formation of the Middle Jurassic Brent Group in part of the Northern North Sea comprises dominantly clean, fine-to medium-grained sands, deposited as part of a barrier-bar complex. The overlying Ness Formation was deposited on supra-or intertidal fiats, and comprises silty channel sands with silts, muds and thin coals. The sands of both Formations are mainly quartz-rich, with up to 12% by volume of feldspar, and variable proportions of clayey matrix. Early carbonate cementation preceded a phase of quartz overgrowth, which continued during burial. Later dissolution of unstable grains, dominantly feldspars, was followed by precipitation of pore-filling kaolinite and minor late-stage mineral phases. Better permeability of the Ness sands (up to 500 mD) relative to the Etive (mostly < 10 mD) is mainly due to the effects of diagenesis on different lithofacies. Silty sands escaped intense quartz cementation and were thus more affected by acid groundwaters which improved permeability.
Depositional processes control initial porosity and permeability relationships in basinfilling sedimentary successions. These initial conditions then determine how diagenesis modifies the hydraulic properties of sediments as they undergo burial and lithification. Therefore, combining sedimentology, stratigraphy and diagenesis to understand the paleohydrologic evolution of sedimentary rocks throughout the evolution of basins is a necessary part of "basin analysis". Understanding the timing of fluid flow through sedimentary rocks is pre-requisite for the development of hydrologic models. Delineating temporal relationships requires integration of sedimentology and diagenesis through paragenetic relationships to constrain fluid flow associated with specific events, including those related to petroleum migration and formation of mineral deposits.
Applied Geochemistry, 2000
Calculation of the quantity and distribution of quartz cement as a function of time and temperature/depth in quartzose sandstones is performed using a coupled dissolution/diusional±transport/precipitation model. This model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and/or clay grains at stylolite interfaces within the sandstones, and the quantity of silica transport into and out of the sandstone by advecting¯uids is negligible. Integration of the coupled mass transfer/transport equations over geologically relevant time frames is performed using the quasi-stationary state approximation. Results of calculations performed using quartz dissolution rate constants and aqueous diusion coecients generated from laboratory data, are in close agreement with both the overall porosity and the distribution of quartz cement in the Middle Jurassic Garn Formation only after optimizing the product of the eective surface area and quartz precipitation rate constants with the ®eld data. When quartz precipitation rate constants are ®xed to equal corresponding dissolution rate constants, the eective surface area required to match ®eld data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0.008 cm À1 to 0.34 cm À1. In either case, these reactive surface areas are H2 to 4 orders of magnitude lower than that computed using geometric models.