Petroleum Geoscience - Volume 15, Issue 4, 2009

Interpretation of a large, well-calibrated 3D seismic data volume in the UK Southern North Sea (SNS) suggests that the occurrence of two little known and hitherto poorly documented carbon dioxide (CO2)-rich gas discoveries is strongly controlled by the style and timing of deformation and the presence of a regional Upper Permian (Zechstein Supergroup) evaporite super-seal. It can now be shown that the CO2-rich accumulations are limited to Rotliegend Group, Leman Sandstone Formation (LSF) reservoirs located on the western edge of a major, extensional fault block, the Fizzy Horst, that lies on the eastern flank of a through-going NNW-striking, partially-inverted depocentre, termed the Brown Graben. Significantly, unlike other structures, which experienced Cenozoic compressional reactivation, the traps containing the CO2 are located adjacent to deep-seated faults upon which contractional reactivation occurred only during the Late Cretaceous suggesting a spatial and temporal control on its occurrence. The structural results provide a robust, unifying and testable structural model through which to assess the inherent exploration risk of drilling unwanted, CO2-prone traps in this part of the prospective basin. Conversely, the fact that CO2 was evidently sealed over geological time-scales shows the significance and long-lived (c. 50 Ma) effectiveness of the Zechstein Supergroup evaporite canopy in retaining CO2, as well as larger and less mobile methane (CH4) molecules. The results thus highlight the potential that traps containing LSF reservoirs have as future sites for CO2 storage (carbon sequestration) in the SNS.

Deep-imaging reflection seismic profiles offshore the Gulf of Guinea, West Africa constrain the structure and composition of a major fracture zone accommodating obliquely divergent intra-continental rifting and eventual break-up along the Gulf of Guinea margin. Interpretation of the seismic data reveals a c. 70 km wide fracture zone comprising fault-bounded blocks of hybrid ‘proto-oceanic’ crust. Gravity modelling of the seismic profiles allows us to propose a testable interpretation in which the fracture zone is composed of a central block of oceanic crust between blocks of partly serpentinized mantle. A synoptic model for the middle Cretaceous break-up of this margin suggests it was accommodated by a left-lateral transtensile shear zone in which fault-bounded blocks underwent continuous counter-clockwise rotation. Serpentinization of the upper mantle was promoted by the embrittlement and fracturing that resulted from progressive stretching and thinning. Domino-style back-rotation of early-formed faults within the shear zone meant that they attained progressively gentler dips before eventually locking up. Consequently, overprinting of faulting led to later faults dissecting – and translating in their hanging walls – a mixed assemblage of stretched continental crustal material and partly serpentinized mantle peridotite. With progressive strain, oceanic crust was produced at releasing bends within the shear zone and a leaky transform evolved.

Flood basalt successions cover many potentially prospective sedimentary basins world-wide, and a few instances exist of intra-basalt petroleum discoveries. However, little is known about the architecture and rock properties of the lava flows, intrusions and other lithologies that make up these successions. We present a simple, effective method of obtaining information from borehole data on the different volcanic facies within a flood basalt succession. Our aims are: (1) to provide a means of determining proportions of different volcanic facies without detailed examination of borehole data or where borehole data are limited; (2) to explore the relationship between onshore and offshore observations. The facies classification scheme providing the framework for this research includes tabular-classic lava flows, compound-braided lava flows, hyaloclastites and intrusions. We show how this scheme can increase our knowledge of the offshore succession and can be useful in hydrocarbon exploration.

In the Faroe Islands, three different basalt formations display a range of facies onshore. Boreholes have been drilled through these, and several kilometres' depth of log data collected. The proximity of these boreholes to onshore observations allows the identification of different facies within the wireline log data. This work demonstrates that histograms of P-wave velocities provide an efficient method of identifying the different facies, and we also explore why these distributions are so different. When applied to borehole data from published ODP wells and one commercial well, it is possible to estimate proportions of the different volcanic facies using the velocity distributions alone.

The largest non-associated gas reserve of the world is hosted by the Upper Dalan–Kangan (Upper Khuff equivalent) Permo-Triassic carbonate–evaporite successions. Detailed characterization of these strata in the South Pars field has shown that the reservoir properties are a function of both sedimentary and diagenetic processes at the field scale. Facies analysis of the studied units indicates that the sediments were deposited in the inner regions of a homoclinal carbonate ramp and were subsequently subjected to shallow diagenesis and minor burial. The vertical distribution of the facies shows cyclic patterns that impact reservoir quality.

The rock type classes have been grouped on the basis of the dominant pore spaces, and have enabled distinct fields to be identified. This approach has demonstrated a relationship between poroperm values and rock type groups. Diagenetic overprinting has significantly affected the reservoir properties.

Although the original poroperm heterogeneities in the studied reservoir are inherited from the Upper Dalan–Kangan palaeoplatform, they have been modified strongly by diagenetic overprinting. Consequently, tentative correlation may be possible between facies types and reservoir properties based on diagenetic effects. Therefore, for precise characterization of the Upper Dalan–Kangan reservoir properties it has been necessary to integrate both the depositional history and diagenetic features.

We evaluated the relationship between geological parameters and the flow behaviour of channelized reservoirs with the aid of an experimental design approach. The geological parameters included geometrical properties, such as channel dimensions and sinuosity, petrophysical parameters, such as permeability and net-to-gross ratio, and a derived property: connectivity. The reservoir flow behaviour was characterized with various metrics based on simulated production data generated with the aid of reservoir flow simulations. In the first part of our study, we found a weak correlation between the geometrical parameters and the reservoir flow behaviour, whereas we found a strong correlation between connectivity and the flow behaviour. In the second part we demonstrate how to use the strong correlation between connectivity and flow behaviour to make a selection of models with similar production response from a large ensemble of reservoir model realizations.

Velocity behaviour in chalks is determined by a combination of their depositional conditions and subsequent diagenesis. Based on rock physics data from three drill sites on the Ontong Java Plateau, a strategy for velocity modelling is proposed where geological information is imposed via pore structure models.

The Reuss and time-average relations define upper and lower bounds of the velocity data and the self-consistent approximation was applied to find direct links between velocity and changes in microstructure. The latter approach provides an estimate of the dynamic changes of the pore structure as a result of diagenesis in chalks. Depositional pore-models of chalks were determined from environmental conditions and can be defined from textural information. Mechanical compaction and cementation decrease porosity as a function of depth, but may increase the velocity by different rates as the pore structure changes differently. Discrimination between sediment stiffness and pore structure stiffness enables us to justify low velocity for indurated sediments and high velocity for soft sediments. Our results indicate the relevance of the pore structure to velocity interpretation because some variations in velocity data may result from pore structure differences rather than changes in fluid or porosity.

Thermal history is an important control on generation of gas and oil in source rocks. Therefore, a realistic treatment of thermal conductivity in rocks is essential for hydrocarbon generation modelling in sedimentary basins. We present two different techniques for thermal conductivity estimation: (1) upscaling, based on arithmetic, geometrical and harmonic mean values of averaged conductivity from anticipated mineral composition; (2) infometric regression of thermal conductivity and anisotropy from measurements in rocks of known porosity and mineral composition. The accuracy of the estimate depends on the technique and we expect that the most precise way to predict thermal conductivity of sedimentary rocks leads to considerable improvement in the accuracy of basin modelling and hydrocarbon prospect identification.

Various thermal conductivity models based on porosity and lithology are used for modelling temperature history for sensitivity purposes. Models where matrix conductivity is determined from measured thermal conductivities based on the mineralogy give considerably lower strata temperatures than models where geometrically averaged matrix conductivity are determined from mineralogy. This is assumed to be caused by deficiency in the mixing laws not taking into account the texture effect of the sediments. The temperature histories estimated by the three mixing law models – arithmetic, geometrical and harmonic mean – are considerably different. We show that the mixing law models give a temperature history and predicted hydrocarbon maturation far off even if we have temperature measurement at the same location. For example the harmonic model predicts the onset of the maturation of the Jurassic formations 20 million years later than does the model based on measured conductivities.