AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
3D Seismic Interpretation of Permian-Carboniferous Carbonates in Offshore Northern Europe: Faulting and Leakage Analysis
(1) Exploration, REPSOL, Madrid, Spain.
In some areas of northern Europe, the Paleozoic contains potential carbonate reservoirs in the Carboniferous-Permian section. Seismic interpretation focused on both the evaluation of possible compartmentalization, and potential fault-leakage, as the only relevant well had only oil shows.
Mapping of key horizons was made on a 3D seismic survey. The main objective was the location and reservoir quality of organic buildups normally present at the top of the carbonate sequence. Our interpretation concludes that organic buildups or mounds are not isolated elements, but rather interconnected structures forming a polygonal network with internal depressions. In addition, based on their internal geometry these bodies record different stages of growth, which are not due to vertical stacking, but to shifts and local progradations. These issues complicate volumetric estimates because complete stratigraphic closure is required. Considering all these factors, it was imperative to identify any structural compartmentalization in order to define potential prospects.
In order to support our structural interpretation, several attributes of discontinuity were calculated, being the “structure cube attribute” the one that gave the best results. Unfortunately, horizon slices on top of the carbonate sequence do not yield the expected results, because the stratigraphic features completely mask the structure. However, the combination of several 3D rendering techniques, such as opacity, transparency, different types of display, etc. permitted the definition of structural compartments within the studied area.
A likely explanation to the dry hole is the existence of leakage due to faulting. In order to document this hypothesis, a series of detection techniques were used, which showed isolated shallow bodies of high amplitude that may represent migrated hydrocarbons. The volume of discontinuity attribute indicates a clear correlation of these anomalies with the direction of fault planes. Additionally, the seabed was examined for leaks associated with the aforementioned structures, but their impact is minimal. On the other hand, the location of "paleopockmarks" in the Upper Triassic section is very intriguing, and it may provide clues into basin modeling.
Seismic interpretation and attribute analysis permit to conclude that the polygonal network that comprises the organic buildups can be divided in multiple-sized compartments. This favors the existence of both stratigraphic and structural traps, which facilitates the calculation of volumetrics. The results confirm thus, the presence of leakage in this zone, which is critical to define areas of lower risk.