Natural Gas Seepage On The South Texas Shelf

Abstract

ABSTRACT An area of extensive gas seepage has been found on the South Texas shelf. The seep area extends for a distance of approximately 25 miles on a transect out from the coast at latitude 26°10?N. Water depths range between 65 and 130 meters. During nine samplings over a three year period, elevated near-bottom concentrations of methane (up to 500 nl/L), ethane (0.6 nl/L), and propane (0.5 nl/L) compared to nearby surface and other near-bottom waters were observed. Anomalous gaseous hydrocarbon levels were also observed in the interstitial waters at three stations sampled in the region. Ethane and propane concentrations were typically one order of magnitude higher in these sediments compared to typical South Texas shelf sediments. The C1/(C2+C3) ratios in the sediments were as low as 14 indicating that the seepage may be of a petrogenic origin and not from shallow biogenic (microbial) sources. Significantly higher percentages of hydrocarbonoclastic bacteria were observed in the seep area. These may be a direct result of hydrocarbon seepage. No anomalous interstitial gas concentrations were observed in the 'Serendipity Gas Seep Area' of South Texas. INTRODUCTION As part of a Bureau of Land Management baseline survey on the South Texas shelf, water column low-molecular-weight hydrocarbons (C1-C4) were measured nine times over a three year period along four transects on this shelf. Interstitial sediment C1-C4 hydrocarbon concentrations were determined at 53 stations in this region. Fig. 1 shows transect positions and locations of all cores taken in this study. An extensive seep area on the South Texas shelf was identified in this program whose purpose was to establish ambient concentrations of these hydrocarbons prior to the development of offshore petroleum production operations. Dissolved hydrocarbon gases found in the marine environment are for the most part indicators of various biological processes or anthropogenic additions, but may in some instances be indicators of seepage from subbottom hydrocarbon reservoirs. Gas seepage appears to be a relatively common occurrence in the northwestern Gulf of Mexico. Early work indicated that bubbles rising from gas seeps can be readily detected at sea by standard sonar equipment. Over the past few years several thousand small bubbling seeps have been located along the continental shelf of the northern Gulf of Mexico by this method. Watkins and Worzel reported that over 19,000 seeps probably exist in a small area of about 6,000 sq km on the South Texas shelf (designated Serendipity Gas Seep Area). Cline and Holmes have identified what appears to be petrogenic seepage in Norton Sound, Alaska by anomalous concentrations of C2-C4 alkanes in the water column. If a large proportion of naturally occurring seepage were indicative of underlying petroleum and natural gas reservoirs, acoustical gas seep detection would provide an almost ideal method for offshore hydrocarbon exploration. However, Brooks et al. and Bernard et al. have reported that of 21 seep gases collected in the Gulf of Mexico only a relatively small fraction have a significant thermogenic component.