Marine controlled-source electromagnetic of the Scarborough gas field — Part 3: Multicomponent 2D magnetotelluric/controlled-source electromagnetic inversions

Abstract

We carried out a multicomponent electromagnetic (EM) survey of the Scarborough gas field in 950 m water on the northwest Australian shelf. Magnetotelluric (MT) data, along with transmitter inline horizontal electric (Ey), vertical electric (Ez), and horizontal magnetic (Bx) field controlled-source electromagnetic (CSEM) data were collected. The Scarborough reservoir is a challenging EM target because it lies between a resistive overlying siltstone and a resistive basement. We carried out 2D inversions of various data combinations to determine how well they recover the expected geology. In particular, we examined the value of the vertical electric CSEM fields. Individual inversions of the Ey and Bx components generate almost identical models, suggesting that these two data sets do not carry independent data, although model studies suggest that this may not be the case in shallower water. Both models smear the siltstone, reservoir, and basement resistors together. The Ez-only inversion includes a resistor with a clear lateral extent at reservoir depths that is separated from basement, but when combined with other CSEM components, Ez provides only marginal improvements in resolution. Not surprisingly, an MT-only inversion is blind to the thin reservoir resistor but combined with CSEM data produces a clear separation of the reservoir from the basement. The combination of Ey, MT, and Ez also separates the siltstone horizon from the reservoir. The sensitivity of MT to horizontal conductivity makes it a powerful complement to the standard Ey CSEM data. The Ez CSEM component adds some value, but perhaps not commensurate with the logistical costs of data collection. The horizontal magnetic CSEM field appears to add little value at these water depths, but if simultaneous MT data are being collected, this component will be available at little cost.