March 3, 2000
Peter Flemings
442 Deike
Building
Department of
Geosciences
Penn State
University
University Park, PA.
16802
814-865-2309
Alan Huffman
Manager Seismic
Imaging Technology
Exploration Production
Technology
5 RWA -1000 South
Pine
Ponca City,
OK
580-767-5405
On March 3, Joint Oceanographic Institutions/U.S. Science Support Program (JOI/USSSP) and Conoco are
co-sponsoring a workshop in Houston entitled GeoFluids of Overpressured Strata in the Gulf of Mexico. The workshop will (1) define
the important technical problems in geopressure for basic and applied science, and (2) define how the D/V JOIDES Resolution
could be used to solve these problems. The problems defined can easily be carried to the next generation of Ocean Drilling Program
(ODP) research. A core purpose of this meeting is to unite the industry and academic community in a problem of importance to both
communities. The meeting will be held on March 3rd at the airport Marriott Hotel in Houston, TX. There will be approximately 40
participants.
Fundamental and applied questions remain unresolved
as to the state and evolution of pressure and stress in rapidly deposited sedimentary basins. The shallow subsurface (less than 1000
meters below the sea floor) in the deepwater Gulf of Mexico is an ideal location to study these processes and the D/V Joides
Resolution is a superb platform for conducting experiments to examine these processes. The geopressure problem is of
fundamental interest because overpressure in sedimentary sequences records fluid flow. In turn, understanding how fluids are
transported in sedimentary basins is an important component of understanding the global hydrologic cycle. Furthermore, overpressure
may play an important role in controlling slope stability, the location of sea floor seeps, and driving fluid flow along faults. The
problem is of applied interest because a better understanding of pressure and stress will assist a major problem in the oil industry,
which is how to drill stable wells in pressured strata. In addition, observations made in the shallow, low-effective stress, zones
have fundamental implications for how trapping and migration of fluids occurs in the deeper earth.
At the recent
ODP-Academic-Industry Partnership Workshop held in the fall of 1999 in Houston, Texas, the problem of fluids and faults was
recognized as a fundamental one for both academia and industry. This workshop proposal arises from that meeting.
It is largely accepted that rapid deposition of low permeability sediments results in the generation of overpressures (pressures greater than hydrostratic). Overpressures are observed around the world where there has been recent and rapid sedimentation of fine-grained strata. While there is broad agreement that overpressures exist and that rapid sedimentation is sometimes responsible, there is disagreement as to the precise details of how overpressure is generated and how fluids migrate in overpressured formations. For example, some authors propose that overpressure begins at the sea floor while others argue that this is not possible because sediments near the sea floor are so porous that the permeability is high enough to bleed off any pressures present. Some workers believe that porosity can be used as a proxy for pressure (because there is a relationship between porosity and effective stress) while others do not believe this is possible. Perhaps the greatest debate focuses on understanding the the actual permeability of these mud-rocks.
At a broader level, how are overpressures driving fluid flow? Flow is anisotropic and can be focussed along high permeability layers and along fault planes. An understanding of pressure and permeability distribution will provide insight into how fluids migrate in sedimentary basins and into how they are expelled at the sea floor. By understanding how and where geopressures form, we can begin to understand the hydrologic cycle. In turn this effects other earth behavior. For example, overpressures control the location of seeps, positions of slope failure, etc.
Understanding the origin, mechanisms, and processes
of overpressure is extremely important to the petroleum industry. Overpressures encountered at shallow depths have proven to be
extremely costly. Because the shallow zone is drilled without circulation, when geopressured fluids are encountered they are
extremely difficult to control. The phenomenon has been labeled 'shallow-water flow' and great effort has been invested in limiting
its occurrence. A second basic need is to predict pressure from other measurements. For example, a common technique is to use
porosity, or density measurements to predict fluid pressure. This approach has always been subject to ambiguity. A third basic need
is to understand the rheological properties of sands and muds in the first 1000 meters below the mudline. This zone is historically
under-sampled. Measurements are made in the shallower zone (0-100 meters) for the purposes of installing platforms. Measurements
are made in the deeper zone (thousands of meters) as exploration targets are approached. In this intermediate zone (0-1000 meters),
there is little understanding of basic rock properties, pressure, and stress state. Finally, this intermediate zone is of interest
because it is a low effective stress zone which is similar to many deeper overpressured zones. Understanding the permeability,
rheology, and stress state within this zone will assist the understanding of trap, seal, and migration in the deeper subsurface.
The D/V Joides Resolution is an ideal platform
that could be used to make significant inroads into these problems. A fantastic way to make advances on these problems would be to
acquire a series of hydrogeological and geotechnical measurements in the first few thousand feet (0-1000 meters) of the sediment
column in an area where overpressures exist. Possible experiments that could be performed include: 1) acquisition of whole core for
determination of rock rheology; 2) wireline log suite to establish ties to rock properties; 3) direct pressure measurements (rft and
piezoprobe); 4) direct stress measurements (mini-stress tests); 5) biostratigraphic analysis to determine age and accumulation rate
of sediments; Deformation experiments on whole core to determine rheological properties; pore fluid chemistry analyses.
The goal of the workshop is to bring together
representatives from the academic and industry community focus on two issues: 1) define the important technical problems in
geopressure for both the basic and applied science; 2) define how the D/V JOIDES Resolution could be used to solve these problems; 3) begin to
develop a specific drilling proposal to address these problems.
Things are moving very rapidly. The date for the meeting is March 3, 2000 at the airport Marriot in Houston, TX. There will be approximately 40 participants.