Offshore Trinidad Survey Identifies Hydrocarbon Microseepage

Abstract

ABSTRACT A successful on shore microbial exploration technology which identifies hydrocarbon microseepage has been modified for the offshore marine environment. This new technology expands offshore exploration programs by delineating hydrocarbon microseepage from the seafloor muds. Specific microbial populations measured in marine sediments identify light hydrocarbon microseepage concealed by the complex geology of offshore Trinidad. Positive microseepage anomalies were located in significant alignments and are associated with known oil and gas condensate production in Amoco's eastern offshore Trinidad leaseholds. INTRODUCTION In 1989, Amoco Trinidad Oil Company conducted a sea floor coring operation off the east coast of the island of Trinidad to test for indications of oil and gas seeps. An evaluation was currently under way in preparation for the 1989 offshore lease sale using traditional geologic techniques.1 The coring operation was intended to high grade seismic leads on the open blocks.2 Active seafloor seeps had been identified over the known oil and gas fields using side-scan and sub-bottom profiler surveys during geologic hazard assessment. It was felt that sea floor cores would effectively and economically locate additional seeps as direct indicators of hydrocarbon reservoirs. Sixty-two cores were analyzed with a new microbial technique which measures specific microorganisms associated with light hydrocarbons. Large populations of these microorganisms would indicate anomalous hydrocarbon microseepage. Control samples were collected over known Amoco production, dryholes, and hydrocarbon macroseeps, but the majority of the samples were collected in extensive unexplored areas, covering approximately 10,000 sq. km. Correlation of microbial data with geologic and geophysical data would enhance marine exploration efforts and help distinguish between productive and non-productive leaseholds. BACKGROUND The island of Trinidad, located off the northeast coast of Venezuela, has a long and impressive history of oil production. Over 2 billion barrels of oil has been produced since commercial development began in 1902.3 Wells were located near active oil seeps onshore in the vicinity of Pitch Lake on the southwest peninsula of Trinidad as early as 1867. The tar from a large oil seep near Pitch Lake has been utilized for years. Sir Walter Raleigh used this tar to caulk his ships in 1595.4,5 The mud volcanos of Trinidad are a continuing point of geologic interest and are at least in part related to hydrocarbons. This was dramatically illustrated in 1911 when a mud island off the southern coast burst into flames 300 feet high.6 In addition, active oil seeps make an interesting feature in the front yards of many homes near the city of San Fernando. Offshore of Trinidad hydrocarbon macroseeps have been noted for many years and tar balls are often found on the beaches which are likely the result of natural oil seeps. Anecdotal accounts of active seeps are reported by members of the Amoco service vessels that routinely visit the offshore platforms. In the vicinity of the producing oil and gas fields, and drilled and un-drilled prospects, geologic hazard surveys have identified numerous features indicating active or historic hydrocarbon seeps.