Challenges in Shallow Water CSEM Surveying: A Case History from Southeast Asia
Authors: Chandola Sandeep Kumar , Karim Rashidah Bt. A. , Mawarni Amy , Ismail Russikin , Shahud Noreehan , Rahman Ramlee , Bernabe Paul , Brauti Ketil 
Published: February 2016

Summary
CSEM survey in marginal water depths and complex geological setups poses several challenges due to the interference of airwave with electromagnetic field and the background resistivity variations. Recently, PETRONAS conducted a pilot marine CSEM survey in Southeast Asia in relatively shallow water depths. We present here the challenges encountered and the methodology adopted in analysis of CSEM data and the value addition achieved through the survey.

Introduction
Controlled Source Electro-Magnetic (CSEM) surveys have proved to be useful in de-risking the hydrocarbon prospects in the deep water environment, due to their capability to distinguish between the brine and hydrocarbon saturated reservoirs. However, the diffusion of EM waves through the sub-surface is a complex process and this complexity is compounded by the airwave effect and background resistivity variations. Under such conditions, simplistic interpretation schemes might lead to wrong estimation of the sub-surface resistivities.
In the year 2006, PETRONAS conducted a pilot CSEM survey in one of its offshore block in Southeast Asia with the following objectives:To understand key risks of two hydrocarbon prospects prior to drilling by integrating seismic and CSEM data.To evaluate the strengths and limitations of the CSEM technique for its future application in shallow water depths and complex geological setups.We adopted an objective driven workflow for modeling, acquisition, processing and interpretation of CSEM data to address various issues likely to affect the data.Geological Setup and Challenges
The two hydrocarbon prospects identified in the survey area allowed rigorous testing of known limitations and challenges in CSEM survey and the interpretation of data. One of the prospects (Prospect-A) is a faulted anticline structure formed in Late Pliocene with sandstone reservoirs as the primary and secondary targets. The challenges posed by the CSEM survey over this prospect included complex geology, proximity of primary target to a resistive basement, marginal water depths (200–500 m) and rugged sea-bed topography (Fig.1). The other prospect (Prospect-B) is a thrusted duplex structure, with a four-way dip closure generated in the Middle Miocene. The main reservoir objective is the Early Miocene platform carbonate. Water depths over this prospect range from 500–700 m which is well within the known limits of the CSEM technique. This prospect, however, has a conceptual geologic model built on seismic data with no immediate well control which required intensified workflows for modeling and interpretation. Also, two shallow bathymetric humps present to the east of the prospect could affect the EM response and therefore needed detailed analysis (Fig. 2).
Early CSEM surveys demonstrated that the method is effective in areas of relatively simple geological structures, including deepwater turbidites and channel systems. However these settings represent only a small proportion of potential exploration regimes. The survey area does not fall into the category of relatively simple geological regimes due to the factors mentioned above. In shallow water depths, 'airwave', i.e. signals that have interacted with the extremely resistive air can have a severe impact on the recorded signals and can dominate the CSEM response at source-receiver offsets which are sensitive to resistivity structure at the depths of reservoirs. In addition, the effects of rugged sea-bed and resistive basement mentioned above, were also expected to pose a challenge in interpreting the CSEM response and hence needed to be understood well (Fig. 3).