Technology - Oil & Gas Exploration

Seismic imaging is a powerful tool that has helped increase success in petroleum exploration. However seismic does have some limitations including its inability to image thin or obscured deep reservoirs, difficulty seeing through volcanic layers or thick salt sequences, difficulty imaging steeply dipping strata, high cost per unit area, and general inability to determine reservoir charge.

The current worldwide restricted supply of acquisition capacity makes alternative non-seismic tools of greater interest and importance. Using amplified geochemical imaging where seismic is ineffective, or combining seismic with this technique can dramatically increase exploration productivity. Hydrocarbon charge in reservoirs as deep as 7500 m, and as shallow as 300 m, have been confirmed using amplified geochemical imaging even through thousands of feet of volcanic deposits or evaporites.

With petroleum exploration, it is critical to look for nanogram quantities (10-9 gram) of about 80 hydrocarbons and sulfur compounds ranging from ethane (C2), to Phytane, (C20). This range, broader than all other geochemical techniques, combined with its very high sensitivity allows chemical fingerprinting of near surface microseepage of petrochemicals ranging from dry gas to moderate/heavy oil (Viforeanu). Plotting of pristane/phytane ratios, C17/phytane ratios, and alkane/alkene ratios can also provide insight to the reservoir character and petroleum phase.

Amplified geochemical imaging has been used in over 130 basins in more than 50 countries in all terrains including desert, jungle, plains, tundra, and offshore. Applications include frontier exploration, prospect evaluation, in-field development, and looking for by-passed pay.

FRONTIER
The use of amplified geochemical imaging in frontier applications allows evaluation of very large blocks (8000-plus km2) in a relatively short time for costs of less than 10% of 3D seismic costs. In this application, it is used to validate the petroleum system, make decisions on areas of the block to keep or drop, evaluate leads, and to focus a seismic program to areas of identified charge.

PROSPECT EVALUATION
Amplified geochemical imaging can be used to validate and map charge, making it useful in setting drilling priorities. Where relatively fresh productive or dry wells are available in the basin, it is preferable to place 10-15 samplers around them to collect a chemical fingerprint to be used as models for the statistical analysis. This improves the accuracy of prediction especially in more complicated settings with multiple sources.

The phase type can be differentiated between dry gas, condensate and oil and, with adequate modeling, differentiate oils with substantially different composition or specific gravity.

With hundreds of wells drilled, amplified geochemical imaging is 95% accurate in predicting dry holes (Potter et al., 1996). When combined with other G&G, routine users claim it doubles their overall success rate.

This technique generally can’t determine the thickness of a reservoir or its permeability and hence can’t determine economic attractiveness of a prospect. But, it is about 90% accurate in defining some level of charge, which can substantially raise POS calculations and reduce risk.

When used extensively in a given basin with many wells, such as the Woodbine Trend of Polk County, Texas, the Anadarko basin in Oklahoma, USA, and the San Jorge basin in Argentina, it has been shown to correlate to initial production rates or the product of reservoir thickness and permeability, helping to determine prospect economics.

With proper sample spacing, amplified geochemical imaging can often locate offset faults or fracture swarms, so critical in unconventional gas plays or compartmentalized reservoirs.

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