Refilling Oil Fields

Surprisingly under water, and deeper underground, scientists have found hints that gas and oil deposits are refilling naturally.   Although it sounds too good to be true, increasing evidence from the Gulf of Mexico suggests that some old oil fields are being refilled by petroleum surging up from deep below, scientists report. That may mean that current estimates of oil and gas abundance are far too low.   Recent measurements in a major oil field show "that the fluids were changing over time; that very light oil and gas were being injected from below, even as the producing [oil pumping] was going on,"   The injection of new oil from deeper strata is of any economic significance, whether there will be enough to be exploitable. The discovery was unexpected, and it is still "somewhat controversial" within the oil industry. It is now clear that gas and oil are coming into the known reservoirs very rapidly in terms of geologic time. The inflow of new gas, and some oil, has been detectable in as little as three to 10 years. In the past, it was not suspected that oil fields can refill because it was assumed the oil formed in place, or nearby, rather than far below.   According to marine geologist Harry Roberts, at Louisiana State University, "petroleum geologists don't accept it as a general phenomenon because it doesn't happen in most reservoirs. But in this case, it does seem to be happening. You have a very leaky fault system that does allow it to migrate in. It's directly connected to an oil and gas generating system at great depth."   What the scientists suspect is that very old petroleum -- formed tens of millions of years ago -- has continued migrating up into reservoirs that oil companies have been exploiting for years. But no one had expected that depleted oil fields might refill themselves.   Now, if it is found that gas and oil are coming up in significant amounts, and if the same is occurring in oil fields around the globe, then a lot more fuel than anyone expected could become available eventually. It hints that the world may not, in fact, be running out of petroleum.     The first sketchy evidence of this emerged in 1984, when Kennicutt and colleagues from Texas A&M University were in the Gulf of Mexico trying to understand a phenomenon called "seeps," areas on the seafloor where sometimes large amounts of oil and gas escape through natural fissures.   "The first discovery was with trawls. It was an area of massive seepage, and was expected that the oil seeps would poison everything around" the site. But it was found just the opposite.   "On the first trawl, two tons of stuff was brought."They were long strawlike things that turned out to be tube worms.     The biologists know what to make of it. "The experts immediately recognized them as chemo-synthetic communities," creatures that get their energy from hydrocarbons -- oil and gas -- rather than from ordinary foods. So these animals are very much like, but still different from, recently discovered creatures living near very hot seafloor vent sites in the Pacific, Atlantic and other oceans.   The animals living around cold seeps live on methane and oil, while the creatures growing near hot water vents exploit sulphur compounds in the hot water.   The discovery of abundant life where scientists expected a deserted seafloor also suggested that the seeps are a long-duration phenomenon. Indeed, the clams are thought to be about 100 years old, and the tube worms may live as long as 600 years.   The surprises kept pouring in as the researchers explored further and in more detail using research submarines. In some areas, the methane-metabolizing organisms even build up structures that resemble coral reefs.   It has long been known by geologists and oil industry workers that seeps exist. In Southern California, for example, there are seeps near Santa Barbara, at a geologic feature called Coal Oil Point. And, Roberts said, it's clear that "the Gulf of Mexico leaks like a sieve.     Although the oil industry hasn't shown great enthusiasm for the idea -- arguing that the upward migration is too slow and too uncommon to do much good -- the search for new oil and gas supplies already has been affected. Now, companies scan the sea surface for signs of oil slicks that might point to new deposits.   "People are using airplane surveys for the slicks and are doing water column fluorescence measurements looking for the oil. "They're looking for the sources of the seeps and trying to hook that into the seismic evidence" normally used in searching for buried oil.   Similar research on known oil basins in the North Sea is also under way, and "that oil is very interesting. There are absolutely marvelous pictures of coral reefs which formed from seepage [of gas] from North Sea reservoirs,"   Analysis of the ancient oil that seems to be coming up from deep below in the Gulf of Mexico suggests that the flow of new oil "is coming from deeper, hotter formations" and is not simply a lateral inflow from the old deposits that surround existing oil fields. The chemical composition of the migrating oil also indicates it is being driven upward and is being altered by highly pressurized gases squeezing up from below.   This upwelling phenomenon, fits into a classic analysis of the world's oil and gas done years ago by geochemist-geologist John Hunt. He suggested that less than 1 percent of the oil that is generated at depth ever makes it into exploitable reservoirs. About 40 percent of the oil and gas remains hidden, spread out in the tiny pores and fissures of deep sedimentary rock formations.   And "the remaining 60 percent," Whelan said, "leaks upward and out of the sediment" via the numerous seeps that occur globally.   Also, the idea that dynamic migration of oil and gas is occurring implies that new supplies "are not only charging some reservoirs at the present time, but that a huge fraction of total oil and gas must be episodically or continuously bypassing reservoirs completely and seeping from surface sediments on a relatively large scale,"   So far, measurements involving biological and geological analysis, plus satellite images, "show widespread and pervasive leakage over the entire northern slope of the Gulf of Mexico,"       As a test, the researchers attempted to drill down into a known fault zone that was thought to be a natural conduit for new petroleum. The drilling was paid for by the U.S. Department of Energy.     In addition to the drilling effort and the inspection of seeps the three-dimensional seismic profiles of the underground reservoirs commonly show giant gas plumes coming from depth and disrupting sediments all the way to the surface.