New Insights Beneath the Salt

Author Steven G. Henry, Rift Institute for Teaching and Training, Las Cruces, NM, Naresh Kumar, Growth Oil and Gas, Richardson, TX, and Sujata Venkatraman, Project Manager, ION-GX Technology, Houston, TX

ION's BasinSPANTM regional 2D programs feature surveys designed for the purpose of imaging the full crustal architecture. Their BrasilSPAN program is showing never before imaged sub-salt horizons and new stratigraphic and structural details of this frontier petroleum system.

Seismic section (A)

Seismic section A is an enlarged image from one of the regional lines situated across the 2007 Tupi discovery well. The yellow line represents the base of salt which is located 5 km below sea level (nearly 3 km below the ocean bottom). The well was designed to test the pre-salt Sag basin microbial reservoirs. An intermediate reservoir, named Coquina interval is also present in this well. The Sag interval is marked below the base of the salt in the seismic section. The interval marked "Rift" is shown as dipping beds under the Sag. Note that the faults shown in the seismic section terminate at the base of the Sag. The Lagoa Feia Formation source rocks are present in the Sag interval as well. The thick salt cover has effectively sealed off the generated hydrocarbons from migrating into the overlying section in this area forming large accumulations under the salt. In other parts of the basin, hydrocarbons can migrate across the salt layer, in which case a dry well, might result in the pre-salt. At the same time, these hydrocarbons can be trapped above the salt layer, and have formed large accumulations in other parts of Santos and Campos Basins.

Seismic Section (A)

Seismic section (B)

Seismic section B is an extended view from the same seismic line as section (A). This section is approximately 75 km across and shows the sub-surface sequences from a depth of 2.5 km to almost 10.0 km. It not only shows the location of the Tupi discovery, but also another discovery to the east, Jupiter. The highlighted yellow line represents the base of the salt with the top of the salt marked in green and appears as a series of diapirs across the section. Thickness of the layer ranges from 2.5 to 3.0 km. The Tupi well drilled through almost 2 km of salt before entering the target reservoir intervals. The Jupiter well, almost 30 km to the east, also drilled through approximately the same thickness of salt. The Jupiter location appears to be on a relative high and some of the faults disrupt the Sag layer, whereas, at the Tupi location, faults terminate at the base of Sag. A different tectonic history of each discovery can be implied, but the basic setting of hydrocarbon migration from the Lago Feia source rock, and accumulation in reservoirs below salt, remain the same. Well and test information, unavailable at this time, is needed to reconstruct the detailed geologic history of each discovery.

Seismic Section (B)

Seismic section (C)

Seismic section C is the regional view of the same east-west line in the central Santos Basin and is more than 600 km in length. Only the major features can be seen at the scale of this display, however the resolution of the Pre-Stacked Depth Migrated (PSDM) data, even at the regional scale, is very high. Although the line has been processed to 25 km in depth, only the uppermost 15 km are displayed. As shown on this line, thick Sag and Rift sections appear at the Tupi location and to the west. The base of the salt is relatively smooth in this segment of the line. Jupiter is on a structural high and the pre-salt intervals appear to be relatively thin. Just to the east of the Jupiter location, the pre-salt intervals appear to thicken again, and a layer of volcanics (shown in purple) appears to extend from the oceanic side at least 150 km from the eastern edge of the rift. We have interpreted the geometries on this line to imply that an early rift event (136 my) with overlying Sag deposits created the Tupi play. An eastward shift of the rift (125 my) creates the structure at Jupiter and farther east. At the eastern end of the line "normal" oceanic character with flat-lying sediments and uneven oceanic crust is seen. The eastward termination of the salt layer is clearly displayed.

Seismic Section (C) 

Section (D)

is an expanded image at "play" scale of the eastern end of the line where the salt layer terminates in a series of thrusts and seems to over-ride the flat lying sediments. This type of pre-salt setting has not been explored in Brazil but might have unrealized potential.   Modern seismic data such as ION's SPAN programs has allowed an unprecedented look in the subsurface that was not possible until recently. However, for successful exploration, seismic mapping has to be integrated with well information and regional tectonic history. Dry holes result when any piece of the puzzle is incomplete or incorrect.

Section (D)