EAGE Workshop on Non-Seismic Methods

Gravity meters are commonly used to map spatial changes in the earth's gravity. The application of gravity surveys to monitor change and recent improvements in gravity instrumentation will be summarized. The use of gravity to track the progress of a waterflood in Prudhoe Bay, Alaska, provides a 4D Gravity case history. Monitoring injected products using surface gravity measurements provides an effective "early warning system" for injection and sequestration projects. A borehole gravity survey in Hanford, Washington, demonstrates the ability to measure bulk densities using a borehole gravimeter. The applications include geotechnical studies, locating and monitoring thief zones in reservoirs, mineral exploration and grade control in mines. Long period movements of the earth can be recorded by gravity meters. The ground shaking in Luxembourg before and after an earthquake in Japan was recorded on a seismometer, a superconducting gravity meter and a gPhone gravity meter. The portable gPhone gravity meter provides the means to record earth movements along active faults using small earthquakes for signal sources. Airborne gravity systems have improved with better platform control, timing and GPS. Airborne gravity data acquired in the transition zones provide information for geoid mapping.

Magnetic and MT soundings are important non-seismic methods, used to image the subsurface basement structures in the northern part of the Gulf of Suez. The Pre-rift Nubia Sandstone reservoir is considered as a main target for hydrocarbon exploration in the offshore Gulf of Suez. It is unconformably overlies the Precambrian basement rocks. The seismic data in this region is very difficult to interpret below the salt - anhydritic formations due to the poor reflections received from the deeper sediments, resulting in a very obscure lithological succession. However, the magnetic data and the MT soundings synthetic models are integrated with the available geological information to delineate the basement structural framework of the study area. The geophysical data include a total field intensity aeromagnetic map, three MT soundings synthetic models, in addition to borehole data from available twenty five drilled wells in the area. Qualitative and Quantitative interpretation of magnetic data are carried out, including the RTP, Residual-Regional separation using lease square polynomial, and Second Vertical Derivative (SVD) techniques. Trend analysis of the magnetic maps reveals four major dominant fault trends N45o W (Suez or Clysmic), N5o - 20o W (East African), W 10o - 30oE (Aqaba) and N 80o - 90o E (Mediterranean) trends. Depth to the Precambrian basement surface is estimated on selected profiles. The Results indicated that the thickness of the sedimentary cover ranges between 2.3 and 3.8 Km. Magnetic modeling for three selected profiles confirms the basement structures in the area. Fifteen structural cross sections are constructed. MT synthetic models are produced and correlated with the available seismic sections and the geological column of the area. The results from the three MT models show perfect correlation with the geologic column. As well as, some of thin events (less than 20 meters), that are not clear or do not appear on the seismic sections have also been indicated with the MT models. Finally, after this integration the basement structures are outlined and the Nubia Sandstone structure contour map is constructed. The study revealed that the area lies above a faulted basement block tilted to the northeast and bounded to the west by zigzag fault pattern. Three door trap structures are found at the edges of the block (denoted 1 to 3) due to the intersection between clysmic and oblique faults. Another block bounded by west dipping fault is believed to exist in the area based on the interpreted magnetic data. Detailed investigation will help to identify the hydrocarbon potentiality of this block. Further, exploration is advisable for Nubia Sandstone to the west in the up dip direction, where western sealed fault traps are expected.

Improved reservoir reservoir management and production optimisation demands require accurate characterisation of reservoir properties and their changes through time. Advances in geophysical data acquisition and interpretation have led to significant improvements in the remote imaging of earth structure and properties. However, when only a single data type is considered, ambiguities in the interpretation can remain. Here we will concentrate on three contrasting methods: surface seismic, marine controlled source electromagnetic (CSEM) and well-log data. It is clear that a careful combination of all three data types can supply information that is not available, or is unreliable from any one data type alone. By integrating complementary sources of information and exploiting the strengths of each, estimates of rock and fluid properties such as gas saturation and porosity can be obtained with greater confidence than from any one data type alone. We illustrate this using an example based on the Luva discovery in the Norwegian sea.

This paper demonstrates an integrated geophysical approach to imaging Red Sea type salt structures. Density and susceptibly information from Gravity Gradient Imaging (GGI) and Magnetic Gradiometry acquired over a salt structure are integrated within a Seismic Velocity Modelling workflow. Combining a joint inversion modelling approach in this manner allows us to resolve the top and more importantly the base salt surfaces to a high degree of confidence.

The need to identify new hydrocarbon reserves and produce at optimal rates is steadily increasing in an era of greater worldwide demand. The need for additional focus on both exploration and production optimization is obvious. In recent years gravity and gradiometry have received increased scrutiny and consideration as a viable tool for E&P. Various sensor designs as well as deployment scenarios are receiving renewed attention from industry. Has gravity reached its full potential? Has the technology achieved required performance and acceptance? Are there operational or cost constraints that prevent full technology adoption? This paper will attempt to address some of these questions at a high level.

Geophysical monitoring tools are based on measurements of physical properties and their contrasts over the producing life of a reservoir. Each tool responds to contrasts in a physical property like electrical resistivity, magnetic susceptibility, density and elastic modulii. For reservoir monitoring a variety of seismic and non-seismic instruments are permanently installed along with other instruments in the total infrastructure. These measurements will image reservoir fluid distribution during the life of a field. Some tools are proven while many others are emerging. Electromagnetic or EM methods, on the other hand, rely on changes in electrical properties due to reservoir saturation changes as hydrocarbons are produced and water injected. Surface deformation caused due to pore pressure changes in reservoirs with production and injection activities can be continuously monitored using satellite radar interferometry (InSAR), ground-based GPS, and surface tilt meters.

In the fractured basement play of Block 18 a granitic basement is expected to have the best reservoir potential and the basic igneous rocks (diorite/gabbro) the least reservoir potential. The basement lithologies of Block 18 may be divided into three reservoir facies, i.e. granite, metamorphic and basic igneous which are likely to be lateral extensions of basement outcrops. Block 18 basement lineaments are expected to form complex networks and have developed over a number of phases. Local basement variations are influenced by a number of factors including basement lithology, the present day stress field and tectonic factors associated with rifting in the basin. 2.5-D integrated gravity and magnetic modeling and enhancement techniques resulted in an improved definition of the basement. We identified basement blocks and fault zones and proved the heterogeneity of the basement in Block 18. We identified igneous bodies within the sedimentary section and highly magnetic structures within the basement. Based on the modelled densities and susceptibilities, we are able to classify the basement types as generally granitic in the south, as likely metamorphic in the central north and as partly basic igneous in the northwest.

Geothermal energy is playing a larger role as an alternative energy source for both electricity generation and space heating. Our recent magnetotelluric and audio frequency magnetotelluric (MT and AMT) and gravity surveys in Iceland and Hungary have both characterized known geothermal reservoirs and identified new drilling opportunities. The success of these surveys has resulted in additional 2-D and 3-D MT/AMT and gravity data acquisition and the onset of a drilling program to evaluate the identified geothermal potential.

We investigate the feasibility of onshore hydrocarbon mapping using conventional resistivity methods. Three-dimensional modelling studies show, that standard methods such as DC-IP, CSAMT or LOTEM can sense a two by two kilometre reservoir at depths up to 1500 meter. If field noise levels are favourable, then well established EM methods (in acquisition and interpretation) could provide necessary information and consequently respected contractors may be used to de-risk terrestrial hydrocarbon licenses.

Land gravity data are still of significant value with respect to their high resolution compared to marine or airborne data. As well, only land data of microGal level accuracy can be considered for reservoir monitoring, where gravity signals amplitudes range between 10 - 100 microGal.