--> Abstract: Prospect Failures in a World Class Hydrocarbon System: Definition, Distribution and Possible Causes of Low Gas Saturated Sands in the Deepwater Gulf of Mexico, by O. P. Gross and K. C. Hood; #90937 (1998)

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Abstract: Prospect Failures in a World Class Hydrocarbon System: Definition, Distribution and Possible Causes of Low Gas Saturated Sands in the Deepwater Gulf of Mexico

GROSS, OLIVER P., and KENNETH C. HOOD, Exxon Exploration Company

The greatest risk in deepwater Gulf of Mexico (GOM) amplitude prospecting is the occurrence of low gas saturated (LGS) sands. This study analyzed the results from all geologic prospects drilled in greater than 1350' of water in which logs were available. Of the 174 prospects, 103 are amplitude anomaly prospects and their results are listed below:

^bull 67% were successful (>15' net HC in a zone)

^bull 18% failed due to LGS sands

^bull 10% found shows of HC (<15' net HC in a zone)

^bull 3% were caused by unusually strong lithologic variation

^bull 2% contained HC in a non-reservoir zone

LGS zones are characterized by their lower sonic velocities (slower by 25-50 microseconds/ft) than other nearby non-amplitude producing water sands. Key examples are shown illustrating how LGS is seen and defined on sonic, density and resistivity logs as well as seismic. Maps document tim regional variation in LGS sands and the GOM hydrocarbon system.

On a predrill basis LGS sands cannot be distinguished from gas and oil pay zones. All three reservoir types have average impedances of 15 to 18% based on log calculations. Since how gas saturated sands are often drilled in a good trap position, these failures are due to the HC system in one or more of three ways:

a) inadequate source (Thermogenic or Biogenic) to fill the trap

b) inadequate migration to fill the trap

c) inadequate seal to effectively trap the HC's

Preliminary observations indicate biogenic methane may be the leading cause of LGS zones.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah