SUBSURFACE STUDIES

The Injected Sand Group, over the course of the two-years project, has been studying several case-studies proposed by the supporting companies. The case-studies mainly come from the Paleogene deep-water reservoirs of the North Sea, but the project had the chance to embrace also examples from the Angolan offshore Mid-Cenozoic deep-water channels.

Some of the fields or discoveries, which have been studied, are displayed in the above picture. They are all characterised by extensive remobilisation features:

1. Grane; 2.Gryphon, Leadon; 3. Jotun; 4. Alba
 
 
 

(core pictures of injected sands - courtesy of Kerr-McGee)

The most obvious characteristic of injected sand units in cores is the cross-cutting relations with the background fine-grained sequence. Other common features which may help in the recognition are the sharp, fracture-defined boundaries, the deformation of surrounding shales and the occurence of heterometric, angular, often platy shale-fragments. This example comes from the Gryphon Field (K. Purvis, in publication), one of the first fields in the North Sea where the relevant effect of sand remobilisation was recognised. The appearence of injected sands can be very different: brecciated units or tiny sndstones with ptygmatic folds are as common as this example and indicate different conditions at the moment of injection.

Reservoirs characterised by large-scale fluidisation and injection are often difficult to image, even on 3D seismic data. However, recent  acquisition of multi-component 3D seismic data using ocean bottom cable (OBC) technology have vastly improved the subsurface image of the Alba Field, leading to revised reservoir models and development plans (see fig. on the left; from MacLeod et al. 1999). Hence, the application of OBC technology may provide a key to properly imaging injected reservoir sands elsewhere. In any case, it would seem that wide-angle stacks should be interpreted alongside the conventional 3D cube for improved reservoir imaging. Interpretation of converted-wave seismic data reveal substantial modifications by remobilisation and injection of the original depositional geometry of the reservoir, some of which may also be visible on conventional 3D seismic or wide-angle stacks. These features include mounds, lateral wings, ridges and partly detached sand bodies, which cross-cut original stratigraphic relationships.

(courtesy of Chevron U.K.)

(courtesy of PGS)

The increasing amount of 3D seismic and well data from the NW European Atlantic margin has led to an increased recognition of injection features, especially in the Faeroe-Shetland Basin. Although less well studied,  this example from the Atlantic margin shows that large-scale remobilisation and injection are common phenomena, also outside the North Sea. The injected features interpreted on seismic data are often high amplitude or "bright". This could be related to higher impedance of the disturbed sands, relative to in situ sands and shales, confirming the well logs and core observations, but other factors such as pore-fluid or tuning effects could also be in effect.

The lower Paleogene turbidite reservoirs of the North Sea frequently display a typical stratigraphic organization which is represented in the conceptual diagram on the right. Gamma-ray log and lithological section are completed with representative core photographs of remobilization and injection features. The thin ("ratty") sands above the main reservoir are mainly thought to be of injected (dykes and sills) rather than depositional origin. Core picture of dykes and sills is from Purvis et al. (2002). Injection breccias are usually found close to the top of an underlying massive sand, while homogenized sands, giant pillars and over-steepened laminae are found in the top of the massive sand. Each core section is 3 feet (~0.9 m long)

Models of sand distribution and geometries vary depending on data quality and interpretation mind set: Upper model shows a massive sand confined to an erosional scour with "ratty" sands on gamma-ray logs interpreted as thin-bedded turbidites in the overburden shales. The lower model shows a massive sand with no apparent confinement and mounding due to differential compaction. The "ratty" sands are interpreted as sand injectites in the overburden shales, whilst wing-like reflections at the edges of the sand are interpreted as low-angle sand dykes and sills/extrusions. The lower model is fully applicable to the mound shown in the previous figure. Over the past decade, increased quality of core and 3D seismic data has changed the interpretation of many fields of the North Sea Paleogene from the upper to the lower model.