What's new in exploration

	Vol. 227 No. 5
PERRY A. FISCHER, EDITOR

Tiny teeth take a bite out of Drake’s Passage. The great debate about the present and future of today’s global warming deals with only the past 2,000 to 400,000 years, and projects only into the next few centuries. The popular press usually forgets to mention the scientific fact that the Earth has been much warmer and colder than it is today, measured in geologic timeframes. One of the reasons for these climate swings are the Milankovitch Cycles, which are based on variations in Earth’s orbit. These include the planet’s tilt, orbital eccentricity and precession. These result in 100,000-year ice age cycles. Well, sort of.

Milankovitch Cycles don’t explain all of the ice age/ warming cycles; it’s more complex than that, with many cycles not matching, especially as you go back in time. Carbon dioxide and plate tectonics also play a role. There remains some debate on which is the largest contributing factor at a given time, as well as which is cause and which is effect.

A big effect of plate tectonics is how it can change global ocean currents. The Isthmus of Panama, the positions of Greenland and Australia are a few obvious examples. So is the continent of Antarctica, which, when it was connected to South America, had wet, boreal forests 35 to 40 million years ago. Cooling began during a very warm era, when levels of carbon dioxide were three to four times greater than today’s levels. Did ocean currents change and trigger the cooling? Tectonic regressions indicate that the tip of South America split off from Antarctica, i.e., Drake’s Passage, 15 to 49 million years ago. That date needed to be narrowed to settle a longstanding debate over when the Passage opened, because Antarctica is known to have been covered with ice by about 34 million years ago.

As good luck would have it, the Ocean Drilling Program had taken core samples from 1,000 ft below the seafloor at the Passage, some 20 years ago. Drs. Ellen Martin and Howie Scher at the University of Florida, knew that fish teeth are composed of the mineral apatite, which accumulates the rare-earth element neodymium. Its isotopic signature is different in Pacific fish teeth than in Atlantic fish teeth, so it can be used to determine when the land barriers between oceans broke down.

This characteristic led the researchers to look for tiny, half-millimeter fish teeth in the ODP core samples. When these were tested using a technique called thermal ionization mass spectrometry, their isotopic signature put the date that the passage opened at about 41 million years ago – more than enough time for a new ocean current to change the Antarctic climate form forest to ice. So, tectonic forces could indeed change ocean currents and be a climate force.

300 micron, 41-million year old fish tooth. Photo courtesy of Howie. D. Scher.

Slip sliding away. At the recent AAPG Annual Meeting held in Houston, Roy Dokka of Louisiana State University gave a presentation on a timely subject: The subsidence of the coastal US Gulf of Mexico. South Louisiana, Mississippi, Texas and Alabama are slowly being inundated. The prevailing model considers this subsidence to result from compaction and/or consolidation of young sediments, as well as human activities.

To see if this was true, Dokka used traditional geodetic measurements, as well as state of the art space technologies such as GPS, Lidar and radar methods to examine historic motions of benchmarks in the Michoud Fault area of Orleans Parish, Louisiana. This methodology allowed measurement of vertical change at different levels, over time, relative to a precise vertical datum. The result was that the new data does not support the prevailing model. That is, the dominant causes of the subsidence isn’t compaction, groundwater removal or oil and gas production, nor is it prevention of new sediments, rather, it is tectonics that dominate this subsidence.

During 1969 to 1971, tectonism was responsible for – 16.9 mm/yr loss, or some 73% of all subsidence; intermediate depth subsidence due to compaction of Pleistocene to middle Miocene strata was a constant – 4.6 mm/yr. During 1971 to 1977, at a – 7.1 mm/yr rate, 50% of the sinking was caused by tectonics, while the change in deep subsidence was caused by renewed motion along the large, normal Michoud fault.

Similarly, subsidence due to shallow processes, i.e., sediment compaction and groundwater off-take, was – 1.5 mm/yr and – 2.5 mm/yr, in the respective early and late periods. Since there was no oil and gas production near the fault, subsidence associated with that activity was not a factor.

Furthermore, fluvial and deltaic geologists accurately predicted the accelerated demise of the modern Mississippi River delta due to the construction of river levees. Geologists with backgrounds in biological processes have demonstrated that if natural processes were restored to the wetland areas, sediment productivity could very well match modern rates of subsidence and eustatic sea level rise. Unfortunately, until now, a similar level of understanding of the processes causing subsidence had not been reached due in large part to the lack of accurate and precise measurements of modern vertical change.

Best discovery of the month. Petrobras’ exploratory Well 4-ESS-164A hit pay in the offshore Espirito Santo basin. The new field of light oil is located 12 km (8 mi) northeast of the Golfinho field. The estimated volume of the new discovery is about 280 million barrels of oil equivalent. Preliminary data suggests that the oil is of excellent quality. The well was drilled by Diamond Offshore’s semisub, Ocean Winner.

Recently, Well 4-ESS-160, also in the Espirito Santo basin, resulted in the discovery of another field, adjacent to the Golfinho field, with estimated volumes of between 60 million and 80 million boe. This new find ramps up potential reserves in the greater field area to between 310 and 330 million boe. With these two discoveries, potential light oil reserves in the Espirito Santo basin are now up to about 600 million boe.