What's new in exploration

	Vol. 225 No. 8
PERRY A. FISCHER, EDITOR

A wealth of data. The MMS, part of the US Department of the Interior, released its Final Notice for Western Gulf Lease Sale 192, which will be held on Aug. 18, 2004, in New Orleans. The sale encompasses 3,907 unleased blocks comprising 21.2 million acres. Lease terms are 5, 8 and 10 years depending on water depths, which are <400 m, 400 – 800 m and >800 m, respectively. That part is routine. What is not routine is the amount of online information available to the public.

Interpretation (top) and seismic section (bottom) from Fairfield's Deep Shelf Reprocessed Post Stack Time Migration 3D Data Set.

MMS has placed a wealth of information that includes 18 of the largest prospects. Data on these include seismic time structure; seismic cross section and 3D volumes; play type; play description; play characteristics; depositional style; exploration history and discoveries; production history from similar plays; petrophysical data (porosity, water saturation, etc.); and much more.

There are even maps of fields with gas production from below 15,000 ft, for those operators considering taking advantage of the new tax relief for production from those depths. Additional data on several important fields, many of which are still producing, include production histories, sands completed and completion intervals in TVD, and reams of other data.

There's a lot for researchers and property bidders alike. The site is at: www.gomr.mms.gov/homepg/offshore/gulfocs/dgi_192/Sale_192_Intro.html.

The core of the matter. There are many issues in geology and associated earth sciences that remain uncertain, in contention, or just plain mysterious. Mantle and core processes are chief among them. Mantle plume origins and behaviors have been extremely difficult to explain in any framework that holds true across the spectrum of plate tectonics, seamounts, island chains and deep-earth seismic discontinuities. But a new mineral has just been discovered in the lowermost region of the earth's mantle that will likely change our view of these processes.

The so-called D-double-prime (D") layer has been very problematic for geophysicists for a long time. At 20 million psi and 2,100°F, if there is a hell on earth, D" is where you'll find it. It is an irregularly bounded layer, averaging about 90 miles thick, which exhibits very different physical properties to those lying above. This discovery may explain why the Earth's mantle/ core interface has such anomalous properties. Artem Oganov from the Laboratory for Cristallography at ETH Zurich, and Shigeaki Ono from the Japan Agency for Marine-Earth Science and Technology, have discovered a new form of perovskite.

Perovskite is a mineral comprised of calcium titanium oxide (CaTiO₃). Discovered by geologist Gustav Rose (1839) and named after Russian mineralogist Count Lev Aleksevich von Perovski, it typically exhibits a cubic (actually, slightly orthorhombic) crystal lattice.

Experimentation with minerals and pressure in the 1970s resulted in the discovery that many substances transform into a dense, perovskite structure when exposed to very high pressures. For example, Lin-gun Liu of the Australian National University demonstrated that the common mineral enstatite (MgSiO₃) can be used to synthesize a silicate perovskite. This same ability to transform is likely true of a many minerals believed to comprise the Earth's interior.

The obvious inference, as Liu reasoned, is that perovskites are likely to be abundant within the Lower Mantle and, more specifically, that silicate perovskite is the most abundant mineral on Earth, comprising about half of the planet's bulk. Due to such a possibility, perovskites have been of continuing interest to geologists for the clues they may hold regarding the planet's history. So, it makes sense to look to perovskite to explain the different seismic and other properties observed within these mantle layers.

Quantum-mechanical calculations and high-pressure experiments allowed Organov and Ono to show that perovskite transforms to a new, unnamed mineral, a post-perovskite phase under extreme pressure that is also strongly dependent on temperature. The new mineral has the same chemical composition as perovskite, but has an unusual layered crystal structure. This new crystal structure fits well with observed seismic and other anisotropic differences between the D" layer and the Lower Mantle lying above, which is composed of mostly iso-tropic perovskite.

Earth's interior showing D" layer.

Discovery of the new mineral should allow novel interpretations of earth's mantle processes. It is likely to have a significant influence on geophysical research. It is thought that the D" layer, for example, plays a key role in the creation of hot mantle plumes. There's much mystery left to be unraveled, but it looks like we've just discovered one of the keys.