Gravity gradiometry gains prominence

The last few years have seen full-tensor gravity gradiometry (FTG) become an important means of economically imaging structures quickly, resolving geological uncertainties, and – alongside 3-D and 2-D seismic data – developing a more complete picture of the subsurface.

Today the technique, which maps small density variations in underlying rocks by measuring the gradient of the earth’s gravity field, is being used onshore and offshore, in frontier exploration, for prospect generation, and in a wide variety of geologically challenging areas. One such area where FTG has considerable applicability is in rift systems – in this case, the East African Rift System.

The East African Rift System
The East African Rift System is the largest continental rift system on Earth. In simple terms, the rift can be thought of as a fracture in the earth's surface that widens over time.

A seismic reflection profiles Nkhata Bay to Likoma Islands on Lake Malawi from the original 1983 paper is shown.

The rift system extends from Lebanon in the north along the Red Sea and through Sudan, Uganda, Ethiopia, Kenya, Tanzania, and Mozambique. The rift is composed of two north-south rift trends, a western branch, and an eastern branch.

In terms of oil and gas exploration, according to energy analysts IHS Inc., “The only proven reservoirs are Paleocene, Oligocene, and Miocene clastics, but potential exists at lower levels in the Karoo, Jurassic, and Cretaceous.”

Today the rift is a target for exploration efforts, particularly for operators such as Tullow Oil plc in Uganda. The East African Rift System is underexplored, and to date, fewer than 200 wells have been drilled in an area of 888,000 sq miles (2.3 million sq km). In comparison, more than 4,600 wells have been drilled in the North Sea, in a 81,081-sq-mile (210,000-sq-km) area.

A seismic-stratigraph interprets the reflection profiles seen in the previous figure.

FTG already is being used to improve exploration success rate in the rift system – an indication of its success worldwide. For example, in 2009, Tullow Oil drilled nine wells in Uganda with eight successes and has an 87% global exploration success rate using FTG data alongside traditional sources such as 2-D and 3-D seismic data.

During 2009, for example, the company surveyed 1,042 sq miles (2,700 sq km) with FTG followed by more than 656 miles (1,700 km) of 2-D prestack time migration (PSTM) data and a little more than 5,390 sq ft (500 sq m) of 3-D PSTM. Today, the company has a highly active exploration program around Lake Albert, which includes the Kingfisher reservoirs and the Victoria Nile Delta play. Drilling in the Butiaba region – also part of the rift – began in April 2010.

Applying FTG data
Against this backdrop, ARKeX has developed an FTG model from a previously published paper, with the model highlighting the enhancements of the realism of the geology from the previously sourced seismic data.

The paper in question, “New Seismic Data and Implications for Future Research,” was published in 1983 by Bruce R. Rosendahl, then associate professor of geophysics at Duke University, and Daniel A. Livingstone, then professor of zoology at Duke. The paper examined the current seismic studies of the East African Rift and what conclusions could be drawn on rift basin development.

A 2-D-model illustrates the modeled gravity and gravity gradiometry profiles.

Taking the previous seismic data with the rift’s high-density contrasts (0.4 to 0.6 g/cu. cm) and shallow depths, FTG can provide geological data and a picture of the rift’s geology. FTG can precisely determine how many fault blocks there are, for example.

Together with a sparsely based 2-D seismic grid, FTG can provide a large amount of structural data based on which exploration wells can be drilled.

So, why does FTG generate such positive and clear results from the rift?

Primarily, it is due to the contrast between the basement and sediment cover. From this, FTG can pick up the architecture of the rift in considerable detail.

Gravity gradiometry is all about picking up on shapes associated with density contrasts; therefore, the bigger the contrast, the better the definition. The basement remains so rigid as it is rifting that it is defined by the uplifted fault blocks on the edge.

The combination of structural and stratigraphic mechanisms within the rift and structural traps that often develop when soft sediment drapes over the ridges and gets compacted lends itself well to FTG.

A 3-D model shows where horizontal offsets are introduced to create strike-slip faults.

Also, there are environmental benefits of using a non-invasive technology such as FTG. Exploration in the East African Rift System is challenging due to the vast areas, its environmental fragility, and the fact that much of the rift is covered by lakes. FTG, whether it is marine or airborne, allows surveys of vast and environmentally fragile regions to take place quickly, accurately, and efficiently.

FTG in Uganda
There is more to come from the application of FTG in the East African Rift System.

In July 2010, ARKeX announced it would carry out a gravity gradiometry survey for Tower Resources in Block EA5 in the Albertine Graben, which forms the northernmost part of the western arm of the East African Rift System.

The basin is relatively unexplored, but has good hydrocarbon potential. The survey will be used to map structures in the deeper part of the basin and assist in locating future seismic surveys. The airborne nature of the survey also will negate the logistical and environmental challenges of covering such a large area, which includes the Nile River and thousands of square miles of swamps.

A generated surface from the 3-D model includes modeled noise.

There are, of course, a number of caveats to FTG applicability in rifts. There will be areas, for example, where the density contrast will be less pronounced. There also is the possibility that certain areas of the rift do not have the necessary components for a working petroleum system.

It can be concluded, however, that rift systems such as in Africa are areas where FTG should be a component of data acquisition. Where there is a hydrocarbon system driven by basement topography, and where the overlying sediment has a large density contrast with that basement, FTG should be considered as an important screening tool in exploration.