Size fractions (<4 and 0.4–1.0 μ) of Brent Group sandstones from the northern North Sea contain mostly illite-smectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the south-central North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking.
The K-Ar data of the fractions less than 0.4 μm of Brent Group samples plot on two arrays in a 40Ar/36Ar vs. 40K/36Ar diagram that have isochron characteristics with ages of 76.5 ± 4.2 and 40.0 ± 1.5 Ma, and initial 40Ar/36Ar ratios of 253 ± 16 and 301 ± 18, respectively. For the Fulmar Formation samples, the data points of the fractions less than 0.2 and less than 0.4 μ also fit two isochrons with ages of 76.6 ± 1.4 and 47.9 ± 0.5 Ma and initial 40Ar/36Ar ratios of 359 ± 52 and 304 ± 2, respectively. Some of the coarser 0.4–1.0-μ fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 μ. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure.
Illite is not as well crystallized as expected for potential crystallization temperatures above 160°C measured by fluid-inclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea. ±
A recent story about Shell’s withdrawal from a long-lived project on in situ production of shale oil from oil shale was an interesting example of over-interpretation of a small dataset.
Stealth apparently can come in all sizes so some countries are hesitant to accept the use of UAVs for research. Fortunately Norway is one of those places that have permitted geoscientists to drone on.
The Netherlands is renowned for its giant Groningen gas field, which contains about 100 TCF of gas and has a total life expectancy of about 100 years.
Sometimes the best seismic data acquisition system for a job is a combination of two.
Seismic acquisition crews in southwest France attempt to gather data without disturbing the locals.
The fifth annual Fabric of the Land art and science exhibit is proving valuable to geologists - and artists around the world.
AAPG members are being asked to help create an educational research facility to study a part of England's Jurassic Coast.
The fact that velocity models based on seismic reflection surveys commonly do not consider the near-surface geology necessitates filling the gap between the top of a velocity model and the surface of the Earth. In this study, we present a new workflow to build a shallow geologic model based exclusively on borehole data and corroborated by laboratory measurements. The study area is in Chemery (France), located at the southwestern border of the Paris Basin, where a large amount of borehole data is publicly available. The workflow starts with identifying lithologic interfaces in the boreholes and interpolating them between the boreholes. The three-dimensional (3-D) geometry of the lithologies then allows interpretation of the position, orientation, and offset of fault planes. Given the importance of the fault interpretation in the modeling process, a combination of different approaches is used to obtain the most reasonable structural framework. After creating a 3-D grid, the resulting 3-D structural model is populated with upscaled velocity logs from the boreholes, yielding the final near-surface P-wave velocity model. To better constrain the velocity model, we conducted laboratory measurements of P- and S-wave velocities in dry and water-saturated conditions on all lithologies in the model. The laboratory data were used to populate the 3-D near-surface model with VP/VS ratio values. The presented workflow accounts for one-dimensional borehole data and is much more iterative and time-consuming than workflows based on two-dimensional seismic sections. Nevertheless, the workflow results in a robust 3-D near-surface model allowing for structural interpretations and revealing the 3-D seismic velocity field.
Anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the Triassic Skagerrak Formation in the central North Sea at burial depths greater than 3200 m (10,499 ft), from which hydrocarbons are currently being produced. The aim of our study was to improve understanding of sandstone diagenesis in the Skagerrak Formation to help predict whether the facies with high porosity may be found at even greater depths. The Skagerrak sandstones comprise fine to medium-grained arkosic to lithic-arkosic arenites. We have used scanning electron microscopy, petrographic analysis, pressure history modeling, and core analysis to assess the timing of growth and origin of mineral cements, with generation, and the impact of high fluid pressure on reservoir quality. Our interpretation is that the anomalously high porosities in the Skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the Late Triassic onward, in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats. Increasing salinity of pore fluids during burial diagenesis led to pore-filling halite cements in sustained phreatic conditions. The halite pore-filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases. Fluid flow associated with the migration of hydrocarbons during the Neogene is inferred to have dissolved the halite locally. Dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35% at current production depths.
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