Indications about Subvolcanic Rocks from a VSP Survey and Hydrocarbon Geochemistry of the Lopra-1 Well, Faeroe Islands, North Atlantic

Liselotte Kiorboe and Troels Laier

The Faeroe Islands consist of tholeiitic basalts extruded during the break- up of the Northeast Atlantic in the early Tertiary. In 1981, the stratigraphic Lopra-1 well (2.2 km of basalt) was drilled through an antiform, but did not penetrate the base of the basalt. A downhole extension of the Lopra-1 well is planned for early 1996.

The VSP-technique, was used to explore the Lopra-1 area using a zero- offset VSP, a walk-away VSP and a refraction seismic survey from 1988-89. Combining the separate profiles, it was possible to overcome the problems of strong S-waves. A strong negative impedance coefficient was found on the zero-offset VSP, a few hundred metres below TD in accordance with a low-velocity zone found in the refraction seismic profile. Reflections below TD are correlated between the different seismic profiles. A velocity model and estimates of attenuation are presented. Velocities vary with basalt-bed thickness and velocity anisotropy is caused by the vertical fracturing of the basalts.

Water and gas (1:16 vol/vol) containing traces of oil flowed out of the original well. The gas consisted of methane (72%), nitrogen (27%) and higher hydrocarbon gases. Isotopic analyses indicate a thermogenic origin of the gas, ^dgr¹³C of the individual gases being -41.5^pmil (C₁); -32.4^pmil (C₂) and -26.5^pmil (C₃). An oil dominated by n-alkanes C₁₃-C₆₀ was isolated from organic solvent extracts of the water. GC and GC-MS analyses indicate that this oil was most likely derived from a thermally mature type II kerogen. An oil with identical composition was recovered, during a new VSP survey in 1994.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California