TheChimneyCube® is a new exploration tool (Statoil/dGB patent pending) that reveals vertical hydrocarbon migration paths. Practically, chimney cubes are used as indicators of where hydrocarbons originated, how they migrated into a prospect, and how they spilled or leaked from this prospect and created shallow gas pockets, mud volcanoes or pockmarks at the sea floor. Note that TheChimneyCube® is one product of dGB's object detection, interpretation, and visualization software package, d-Tect. Other seismic features that can be detected include faults, 4D effects, fractures, and salt bodies.

TheChimneyCube® uses a 3D volume of stacked seismic data with other prior information such as the interpreter's insight, well information and other geologic data, to highlight vertical zones of chaotic seismic character, usually associated with gas chimneys (Heggland et al. 2000). Seismic data are input into a neural network and a chimney cube is generated as its output. The chimney data needs to be integrated with regional, well, and prospect data to obtain optimum results.

The predictions of hydrocarbon phase, hydrocarbon charge and seal effectiveness are critical risk factors in petroleum exploration. Historically, most resources are devoted to delineating the structural geometry and reservoir potential of a prospective lead. Yet, in rank exploration wells, typically over 50% of dry holes are due to either charge or seal failure. A chimney volume used in conjunction with other data, can address these key risk factors. Current applications of TheChimneyCube® include detecting shallow gas hazards, distinguishing between charged and non-charged prospects or fault segments, supporting or refining basin models, constraining seal risk, and predicting hydrocarbon phase. Some of these applications are highlighted in Aminzadeh et al. (2001).

Detect shallow gas hazards: TheChimneyCube® concept was originally developed for this application, following a blowout in the North Sea. Often it is difficult to distinguish lithologic from hydrocarbon related anomalies in the very shallow subsurface. Gas chimneys used in conjunction with shallow amplitude or AVO anomalies have been proved to be a useful tool in detecting shallow gas drilling hazards.

Distinguish between charged and non-charged prospects or fault segments: The hydrocarbon system is undercharged in many hydrocarbon producing basins, such as the Gulf of Mexico. Thus determining which fault segments and reservoir intervals are receiving preferential hydrocarbon charge is critical. Chimney cube data have been used in the GOM to high-grade prospective fault segments and reservoir intervals that are up-dip of the chimneys. Similarly the chimney cube can also distinguish between faults that are major hydrocarbon pathways and faults that have not received any hydrocarbon charge.

Support and refine basin models: Chimney cube data can be used to refine 2D basin models, by defining which faults may be more active, and suggesting areas that may be more prone to vertical migration of gas. The basin models can subsequently be modified and thus give more meaningful results. Often chimney data can also provide clues about charge efficiency.

Constrain risk on fault seal, top seal, and lateral seal: Chimney data can be a useful tool for distinguishing between leaking and nonleaking faults, and pinpointing concerns about either top-seal or lateral-seal risk.

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Prediction of hydrocarbon phase: is critical in areas such as deepwater Nigeria and the northwest shelf of Australia, where marketing of gas is much more costly. The traditional tools employed to predict hydrocarbon phase, including source rock facies variations and structural timing variations, and geophysical modeling are often inconclusive. Understanding trap integrity and its resultant effect on gas chimney character is a promising approach. Our model, based on Sales (1997) and O'Brien et al. (1998), divides traps into three main categories:

1. High Integrity Trap (gas prone) Seal capacity for gas and oil is greater than closure, thus traps spill oil and trap gas.
2. Moderate Integrity Trap (oil prone) Seal capacity is less than closure, thus trap leaks gas and minor oil.
3. Low Integrity Trap (dry hole) Seal capacity is insufficient for an economic accumulation.

The category in which a prospect falls is often difficult to predict predrill. The character of the gas chimneys is often a key clue to make this determination. Distinguishing between active and relict seeps can be significant. By combining chimney cube data with trap geometry, most likely hydrocarbon fill (from amplitude anomalies, pressure data, or regional data), and piston core or other surface geochemical data, semiquantitative predictions of the hydrocarbon phase can be made. A summary of the model is shown in the table below.

Conclusions: Chinmey cube data, when used with fault cube and other geologic data (structural model, fault seal analysis, basin models, pressure data, piston core and geochemical data) and knowledge of the area, has proven to be a useful tool in quantifying seal and charge uncertainty and detecting shallow gas hazards.

References

Aminzadeh, F., de Groot, P., Berge, T., and Valenti, G., Using Gas Chimneys as an Exploration Tool, World Oil, 2001.

Heggland, R., Meldahl, P., de Groot, P., and Aminzadeh, F., Seismic chimney interpretation examples from the North Sea and the Gulf of Mexico, American Oil and Gas Reporter, 2000.

O'Brien, et. al., 1998. Evaluating Trap Integrity in the Vulcan Subbasin, Timor Sea, Australia, using Integrated Remote-sensing Geochemical Technologies. In Purcell & Purcell (ed.) The Sedimentary Basins of Western Australia 2: Proceedings West Australia Basin Symposium Perth Western Australia, 1998, p. 237-254.

Sales, J. K., 1997, Seal Strength vs. Trap Closure - A Fundamental Control on the Distribution of Oil and Gas, In Surdam, ed., Seals, Traps and the Petroleum System, AAPG Memoir 67, p. 57-83.

Table 1. Summary of the model.

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