In offshore areas such as the deepwater Gulf of Mexico, it’s not uncommon for oil emanating from source rocks to bypass natural trapping mechanisms and leak upward to the seafloor.
Given that natural seals sometimes are flawed, entrapped oil also may rise to the surface via such conduits as fractures and faults. Identifying these natural seeps can prove invaluable to explorers looking for indications of a hydrocarbon source in their areas of interest.
It comes as no surprise that oil seeps have become a business in their own right.
An example of this was presented in at least one paper at the recent AAPG Annual Convention in Long Beach, Calif., when Marcio Mello , founder of High Resolution Technology & Petroleum (HRT) in Rio de Janeiro, Brazil, gave a talk on the effectiveness of high resolution geochemistry technology, satellite oil slick detection and direct geochemistry methods in assessing exploration risk in the deep and ultra-deepwater Gulf of Mexico.
“Natural oil seeps and high-resolution geochemical methods have historically provided invaluable information to oil explorers in frontier areas for assessment of lease and exploration basins,” Mello said.
“Foremost, they indicate the presence of active generative hydrocarbon source rocks,” he added, “without which there can be no hydrocarbon accumulations.”
More evidence of oil seeps’ growing profile comes from U.K.- based Infoterra, which recently completed mapping the offshore oil slicks in the Arctic area to add to its global offshore oil seeps database.
The database encompasses over 60 million square kilometersof offshore basins.
This database has been acquired via interpreting radar satellite data and screening offshore basins worldwide to a water depth of approximately 3,000 meters, according to Andy Wells, director of sales at Infoterra.
The database comprises more than 12,300 ERS (European Remote Sensing) satellite equivalent scenes.
“Oil seep identification is a complex process,” Wells noted, “due to the depth of water, variability in leaking petroleum systems and man-made pollution within the offshore frontier basins.”
The Infoterra team characterizes and ranks the slicks as probable natural seepage or man-made pollution and also maps rigs, platforms and ship traffic for a more complete picture of the controls on oil slick distribution.
Even though current oil prices north of $70 help to keep the drill bits turning, exploration costs have escalated along with commodity prices, making prospectors eager to embrace anything that reduces project risks – and oil seeps fit neatly into that category.
A ‘Feel-Good’ Factor
“Oil seeps give companies the feel-good factor when exploiting a frontier area,” said Martin Insley, an AAPG member and senior geologist at Infoterra. “If you see an oil seep, the idea is you have at least a petroleum play fairway.
“But that’s only the tip of the iceberg,” Insley said. “The real interest comes if you put that into a geological context with some seismic or other data – the first thing it tells you is there’s oil being generated in that region.
“If you have seepage, it’s usually from a reservoir rather than a source,” Insley added, “and some will say, ‘well, my reservoir’s breached and that’s not good news.’
“But you need a lot of oil, a substantial reservoir to create a long-term seep,” he noted, “so it depends if you’re upbeat about it or see it as a negative.”
Conversely, when you take a global perspective, there are areas with little or no seepage – which can present a quandary for the prospectors.
“The idea is you’ll either have no petroleum system in place in those areas,” Insley said, “or you’ll have a very efficient seal to the point where there’s no chance of oil escaping to the surface.
“One of the questions we face is whether it’s bad news if a company evaluates an area and finds no oil seeps,” Insley added. “But if you understand the geology of an area, then quite often you say there’s a very good seal in this region, and that’s why we don’t see any seepage.”
Risk in the GOM
Oil seep evaluation in the pricey, high-risk deepwater environment can provide an array of crucial information to explorationists.
“The high cost of offshore deep and ultra-deepwater exploration has made the identification of oil slicks and seeps a well-accepted risk assessment methodology in offshore basins all around the world,” said HRT’s Mello, who chaired AAPG’s highly successful 1998 international conference in Rio de Janeiro.
“In fact, the combination of high resolution geochemical methods and oil seeps can provide clues about hydrocarbon origin, type and age, and thermal evolution of the source rocks,” Mello added.
He noted that a recent project involving high resolution geochemical technology (HRGT) and oil slicks in the Gulf of Mexico allowed identification and characterization of oil mixing, extension of oil cracking to gas and quantification of accumulations in the subsurface.
The objective of the HRGT program, which was spearheaded by HRT, was to assess exploration risk in deep and ultra-deep waters of the northern Gulf. The project was based on RADARSAT-1 satellite data; it kicked off in January 2006 and concluded June 2007.
More than 1,000 seepage slicks were identified, with 727 of these thought to be representative of oil clusters. An oil cluster was interpreted as a group of RADARSAT-1 seepage slick polygons that share the same source point in a specific geographic space, according to Mello.
“These results indicate a 100 percent probability that an interpreted seepage cluster slick occurs as a result of the convergence of the optimum tectonic, temporal and environmental scenarios,” he said.
“And they confirm the presence of a prolific and active petroleum system in the subsurface of most of the investigated area.”
The slicks detected during the GOM project are geographically and geologically
associated with the regional salt complex trend in the area. They were
subdivided into four main exploration areas:
- Alaminos Canyon.
- Keathley Canyon.
- Garden Banks.
- Green Canyon.
Using technology from HRT associate Biomarker Technology, the project participants were able to analyze some oils using piston cores associated with the oil slicks identified in the sea surface.
The proprietary technology used included diamondoids (geochemical method to characterize percentage cracking from oil to gas and also to quantify amounts of gas in subsurface traps) and compound specific carbon-isotopic analysis of diamondoids, according to Mello.
The technology can identify oil type and link it to the original source rock.
When queried about the future of natural oil seeps and HRGT to evaluate hydrocarbons, Mello replied, “This is the most promising technology to help reduce exploration risk in deep and ultra-deep frontier areas all around the world.”
He noted HRT is currently applying this technique in a number of deep and ultra-deep areas besides the GOM, including locales in Brazil, Colombia, Peru and South Africa.
“It is very important to emphasize that the identification of the natural oil slicks in deep and ultra-deep water in any sedimentary basin is proof of the presence of an active source rock system,” Mello said.
“Without such a system, no oil accumulation can occur.”