VERSECKES, MICHAEL S., Halliburton Geophysical Services, Inc., Dallas, TX, and GEORGE WATTERS, Marshall & Winston, Midland, TX

ABSTRACT: Near-Surface Low-Molecular-Weight Hydrocarbon Gas Exploration: Strengths and Limitations

Near-surface low-molecular-weight hydrocarbon gas (LMWHG) data, when properly applied, can contribute to the effectiveness of many exploration and development programs. Like any exploration tool, LMWHG data is subject to limits and variables which can affect its efficiency. In order to maximize its benefits, it is important to understand LMWHG exploration and the factors which can affect its results.

The chief strength of LMWHG exploration lies in its ability to directly detect hydrocarbon seepage. LMWHG information cannot reveal the depth of an accumulation nor the accumulation's commercial viability. Because of these limitations, LMWHG data is most useful when integrated with other complementary techniques such as seismic or well control information. LMWHG's ability to indicate the presence of petroleum can help pinpoint the most promising direction or areas for field expansion or focus wildcat efforts to those targets with the greatest potential for hydrocarbon accumulation.

Other factors affecting LMWHG usefulness relate to the physical characteristics of the collection and analysis methods employed. Depth of sample, type of collection tool, and type of chromatographic detector used for analysis can have a direct impact on the sensitivity and noise level of the data. Sample program design can also impact the LMWHG results, especially when sample spacing is not sufficiently dense enough to account for the size and shapes of the exploration targets. Several case histories with LMWHG data collected along the Eastern shelf in west Texas demonstrate LMWHG success as well as some pitfalls encountered when LMWHG limitations were not taken into account.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.