Petroleum Geoscience - Volume 16, Issue 2, 2010

During the last decade several 3D digital reflection seismic datasets have been acquired in the Faroese sector of the Faroe–Shetland Basin which allow detailed seismic interpretation and mapping of parts of the area. This study presents mapping and seismic sequence stratigraphic interpretation of parts of a c. 450 m thick, mid-Eocene delta extending over some 3500 km2 in the Faroe–Shetland Basin. The delta built out into the basin from the Munkagrunnur Ridge and has been divided into nine seismic units. Isochore maps of these seismic units reveal a NW–SE depositional maximum trend, parallel to the slope front of the delta in most units. The dip of the slope front of these units is about 2.3–3.8°. The lowest unit has the outer form of a thin sheet, whereas the upper unit, a basinward divergent wedge, is mostly constrained to the shelf area.

Using the outer form of the seismic units and reflector terminations against the bounding horizons, the surfaces separating the nine mapped units are interpreted as either maximum regression surfaces (MRS) or surfaces representing the start of base level fall (BSFR) or a combination of these two types of surface. Surfaces of regression and erosion are observed within several units and, with additional information regarding the seismic character and nature of internal unconformities and systems tracts, the nine seismic units are divided into systems tracts deposited during eight cycles of relative sea level change.

Within the study area falling stage and lowstand systems tracts (FSST and LSST) are generally better preserved than transgressive and highstand systems tracts (TSST and HSST). Maximum flooding surfaces separating a TSST from the overlying HSST are not recognized, and in several sequences the TSST and HSST are either absent or so thin that they cannot be resolved in the seismic data.

The slope front of the delta was first developed in the lower, aggradational part of the delta and retained during the upper, progradational part, where the delta evolved as a small shelf margin delta.

Adding realistic thermal subsidence to the global sea level curve indicates that the Faroe–Shetland Basin was characterized by almost continuous relative sea level rise in the mid Eocene. Multiple stages of uplift of the Munkagrunnur Ridge during the mid-Eocene are considered to be the most likely explanation for the multiple cycles of sea level change inferred in the delta.

3D seismic and well data are integrated to determine the tectono-stratigraphic evolution of the SW margin of the Utsira High, northern North Sea rift system. During the Triassic, a series of minibasins formed due to passive diapirism of the evaporite-bearing, Upper Permian, Zechstein Supergroup. Subsequently, during the Jurassic, a series of secondary minibasins developed as the underlying salt walls collapsed. These minibasins were a few hundred metres deep, bound by sub-circular to elongate salt-cored structural highs and caused the development of complex subaerial topography and submarine bathymetry on the SW margin of the Utsira High. Salt withdrawal may have been related to: (i) partial dissolution of salt; (ii) differential erosion of the salt walls and adjacent Triassic-filled minibasins; or (iii) salt remigration caused by sub- or supra-salt extension or sediment loading. This study provides insights into the tectono-stratigraphic evolution of the SW margin of the Utsira High and has implications for (i) facies distribution of the Zechstein Supergroup within the northern North Sea rift system; and (ii) depositional system development, and thus reservoir distribution, within the Jurassic sedimentary succession.

The South Alamein study area is located in the Western desert of Egypt between the prolific petroleum trends of Razzak-Alamein and Baharriya. The petroleum system of the area has been evaluated within an exploration framework. Potential source rocks in the Cretaceous Alam El Bueib and Abu Roash Formations and the Cenozoic Apollonia Formation display good petroleum potential; however, they are thermally immature over a large part of the studied area. Compilation of existing data and new measurements have been carried out to determine the initial source potential of the Jurassic Khatatba Formation source rock as well as its kinetic parameters for hydrocarbon generation. Results show that maturation of the Jurassic source rock does not occur below 140 °C or a vitrinite reflectance of around 0.9%. This combination of temperature and vitrinite reflectance is not unusual for a mixed type II-type III source rock, but has crucial implications for exploration in the area. The main hydrocarbon kitchens are located northward of the study area and migration to the south began around 40 Ma ago. The hydrocarbons are primarily trapped in the crest of the Mesozoic tilted blocks. It is believed that both migration and trapping have been influenced by fault hydraulic behaviour. Since this behaviour is complex, a sensitivity analysis was carried out on the migration pathway compared with the fault sealing factor to understand the filling history of the structures.

Outcrop analogue studies provide key information for reservoir modelling which is difficult to obtain from traditional subsurface datasets (i.e. seismic data, well data). Terrestrial laser scanners or LiDAR (light detection and ranging), combined with digital photography, provide a new technique to create high resolution 3D digital outcrop models (DOMs). These DOMs generate exhaustive information which is used to build more realistic three-dimensional facies-based geocellular models and populate the different model zones.

This paper documents the use of an extensive dataset, which combines high resolution traditional field data and DOMs. We provide an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation. Geocellular facies association and connectivity models are well constrained by outcrop observations and a number of different techniques are used in the quality control of the final model. The study also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them. The workflow and results shown in this paper can be applied in similar analogue systems in order to help improve model building for subsurface reservoirs.

The east–west-trending Alaşehir graben is one of the prominent structures in western Turkey proved to have hydrocarbon potential. Previously published seismic profiles have been re-interpreted and they show that Miocene graben fill accumulated in the hanging wall of the east–west-trending first fault system, which has a well-developed wedge geometry. It is also clearly seen in the seismic profiles that the relatively younger, second and third fault systems are nicely merged with the first fault system, indicating that a rolling hinge mechanism was working in the Alaşehir graben.

Geological models of fluvial reservoirs are often constructed with object-based modellling techniques. This approach requires a specification of the distribution of channel sizes determined traditionally from core and well log data. The problem with these data is that they only show the apparent thicknesses of the channel objects: sometimes the thicknesses are too small because the well intersects the channel at the edge and sometimes the thicknesses are too large because channels are stacked. This paper proposes a new approach to determine reasonable fluvial channel sizes with indicator variograms. The relationship between the channel geometry and the variogram is established. The relationship is verified with synthetic examples. The calculated indicator variogram range is shown to be insensitive to channel width and width/thickness ratio. Two real reservoir examples show how the approach works in practice.

A series of tests were carried out employing a modified triaxial cell able to measure both elastic and electric properties simultaneously at reservoir conditions. Measured quantities such as porosity, permeability, resistivity and P- and S-wave velocities were cross plotted, and possible correlations were investigated. Identified experimental trends were then tested against a variety of rock physics models. A good correlation was obtained in most cases. Such results together with petrophysical logging data can be used to constrain possible joint inversion of controlled source electromagnetic data and seismic data. The simultaneous use of resistivity and acoustic measurements to monitor a water-substituting-oil process at reservoir conditions was also investigated. This study indicated that the electrical properties are much more sensitive to changes in saturation in the case of water flooding for enhanced oil recovery than the elastic properties.