Probing reservoir-scale migration, entrapment in the Gulf of Mexico

Pinpointing migration pathways, leaky compartments

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The Integrated Reservoir Investigations Group (IRIG) at Texas A&M University has focused on a complex, faulted Shell field, located in Ship Shoal Blocks 274 and 293 in the US Gulf of Mexico. Geophysical analyses include interpretation of regional structure from a three dimensional (3D) seismic dataset and seismic attribute mapping of three producing sands (D, F, and G), which yielded refined maps of fault-bounded compartments.

Analysis of oil and gas samples from across the field showed great diversity in hydrocarbon type - laterally and vertically. Geological research resulted in characterization of faults and shale seals. Examination of production data suggested that some faults seal, while others leak. However, it was by integrating these analyses that the most significant breakthroughs in predicting hydrocarbon migration and entrapment were made. Two examples are discussed below.

Migration pathways

Heterogeneity of hydrocarbon geochemistry in Ship Shoal 274/293 can be explained by a simple migration history. An early stage of oil and gas charge was followed by late stage gas migration from a different source. The late gas resulted in fractionation and field-scale remigration of oil.

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Map of hydrocarbon geochemistry projected onto schematic structure map of the G sand, Ship Shoal 274/293. Inset shows the general location of the field area in the Gulf of Mexico. Purple indicates parent oil and gas; medium green indicates fractionated oil, and light green indicates condensate. Latestage gas is shown in pink.
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The two pulses of hydrocarbon charge probably track the structural evolution of the field. Early oil and gas was introduced while fault A was active or earlier. Late-stage gas migrated up the more recently active fault B.

Geochemical diversity correlates well with the mapped structure. Parent oil and gas are widespread, whereas fractionated oil and condensate are focused along secondary faults parallel to fault B. Temperature and salinity data suggest hotter fluids recently traveled along the edge of salt and up fault B, then into the field along secondary faults.

By mapping geochemical data onto structure and tracking the migration history, we have been able to explain reservoir problems such as why gas is found structurally below oil in Ship Shoal 274/293. Also, we are able to pinpoint certain structures that are favored pathways for reservoir charge, such as antithetic faults to large-displacement faults.

Untested compartments

Structure interpreted from seismic attribute mapping of producing sands indicates Ship Shoal 274/293 is pervasively cut by small displacement faults (15-30 meters or less). Detailed structure maps raise two critical questions.

  • Is there a way of verifying maps of small-offset faults?
  • How well do small-offset faults compartmentalize producing sands?

Both these questions were answered by comparing fault-bound compartment size (estimated from structure maps) with drainage areas (calculated from production data). Compartment size compares favorably with drainage area for relatively thin sands (thickness less than local fault offset). For thicker sands drainage area is typically greater than predicted compartment size, suggesting fluid communication across compartment boundaries.

Type-curve analysis of production data independently confirms that excessively large drainage areas correlate to leaky fault boundaries. The most likely explanation is that small offset faults juxtapose sand on sand.

In summary, integration of geological, geophysical, and engineering datasets verifies detailed structure maps, also broadly predicts which sands are likely to have leaky compartments, and pinpoints untested compartments.

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Schematic structure map of the G sand showing the relationship between mapped compartment size and drainage area calculated from production data (shown as circles centered around analyzed well). "L" designates a compartment whose boundaries leak, "N" designates acompartment with non-leaking boundaries, and "M" indicates that the data are non-conclusive. Untested compartments are shaded.
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Ongoing research

Some key questions remained to be answered. First and foremost among these is: Are the integrative "tools" developed with data from Ship Shoal 274/293 applicable to other fields? We are in the process of acquiring new datasets to test these integrative analyses.

Other basic questions are: What time scales are appropriate for reservoir charge? In a system of stacked fault-bound reservoirs, which ones will be charged and which will be bypassed? IRIG geoscientists and engineers use computer modeling to answer these questions. Key results of our models include:

  • Reservoirs can fill quickly - in many cases, less that 20,000 years
  • Fault permeability probably determines migration pathways
  • Capillary pressures of faults and reservoirs determine reserves
  • GOR (gas-oil ratio), fault and reservoir properties, structure, and charge rate can all affect the final distribution of oil and gas, sometimes resulting in gas-charged upper reservoirs and oil-charged lower reservoirs.

Summary

By integrating geological, geophysical, geochemical, and engineering analyses, and field data with modeling results, the IRIG is finding answers to key problems of reservoir-scale hydrocarbon migration and entrapment. Students and staff are studying shale and fault-zone core from locations across the Gulf of Mexico to better understand variability in seal properties, and test for correlation with depth and proximity to salt.

Ongoing research is aimed at improving development and production efforts for industry sponsors, thereby reducing costs, and at educating students to work in a modern petroleum industry.

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