M. Rosenquist

Detecting hydrocarbon reservoirs from CSEM data in complex settings : Application to deepwater Sabah, Malaysia

Controlled-source electromagnetic (CSEM) field surveys offer a geophysical method to discriminate between high and low hydrocarbon saturations in a potential reservoir. However, the same geological processes that create the possible hydrocarbon reservoir may also create topography and near-surface variations of resistivity (e.g., shallow gas or hydrates) that can complicate the interpretation of CSEM data. In this paper, we discuss the interpretation of such data over a thrust belt prospect in deepwater Sabah, Malaysia. We show that detailed modeling of the key scenarios can help us understand the contributions of topography, near-surface hydrates, and possible hydrocarbons at reservoir depth. Complexity at the surface and at depth requires a 3D electromagnetic modeling code that can handle realistic ten-million-cell models. This has been achieved by using an iterative solver based on a multigrid preconditioner, finite-difference approach with frequency-dependent grid adaptation.

DETECTING HYDROCARBON RESERVOIR WITH SEABED LOGGING TM IN DEEPWATER SABAH, MALAYSIA.

SeaBed Logging (SBL), a technique that utilises Controlled-Sourced Electromagnetic (CSEM) fields to probe subsurface resistivity, has been applied in Malaysia to decrease the critical risk of having blown traps in thrusted anticlines. Integration of this technology with pre-drill prospect evaluation techniques has successfully de-risked the recent Alpha* discovery. Besides helping to add material reserves, this technology continues to de-risk nearby prospects and improves Shell’s drilling successes in the basin.

Integration of electromagnetic and seismic data to assess residual gas risk in the toe-thrust belt of deepwater Niger Delta

During the last couple of years, drilling activities in the deepwater thrust belt play of Nigeria (Figure 1) have shown mixed results with some wells finding pay but others coming across low-saturation gas and/or brine sands. In this frontier exploration context, Shell Nigeria E&P Co. (SNEPCo) has been particularly successful in a number of toe-thrust prospects where the key risk is trap integrity, related to both earlier thrust faults and later extensional faults.

Increasing the sensitivity of controlled source electromagnetics by using synthetic aperture

Controlled-source electromagnetics (CSEM) has been used as a de-risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. In this paper, we mainly show one type of synthetic aperture antenna whose field can be steered into a designed angle. Consequently, the field concentrates on the target reservoir and the airwave is reduced. We show a synthetic example and a data example to illustrate the increased sensitivity obtained by applying synthetic aperture CSEM source. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Aside from the applications to marine CSEM, synthetic aperture can be widely applied to other electromagnetic methods such as on land electromagnetics and bore hole electromagnetics.

The Next Generation Electromagnetic Acquisition System

We have compared field test data from a next generation node based CSEM acquisition system with data from a reference conventional system in a shallow water environment. The next generation system with much higher transmitter dipole moment and more sensitive receivers provides a step change improvement in the data quality, with clean data for all source frequencies out to 20 km offset compared to around 10 km offset for the reference system. The high data quality also provides clear improvements in the inversion results. This was demonstrated by improved imaging of a hydrocarbon accumulation under challenging conditions. We expect a maximal imaging depth, relative to the seabed, of up to 4500 m in future surveys with a commercial version of the next generation acquisition system.