Journal of Petroleum
Geology, vol. 20(3), July 1997, pp. 261-286
©Copyright 1997. Scientific Press, U.K. All rights reserved.
SEALING
CAPACITY OF THE MERCIA MUDSTONE GROUP IN THE EAST IRISH SEA BASIN: IMPLICATIONS FOR
PETROLEUM EXPLORATION
J. K. Seedhouse* and A. Racey**
In the East
Irish Sea Basin of the UK Continental Shelf, hydrocarbon accumulations in the Triassic
Sherwood Sandstone Group (SSG) are sealed by the Triassic Mercia Mudstone Group (MMG). The
MMG comprises alternating mud- and halite-dominated intervals: Where halites overlie the
SSG (e.g. in the Morecambe and Millom gasfields), seal integrity is excellent; however,
where the SSG is overlain by mudstones of the MMG (e.g. in the Douglas and Lennox
oilfields), seal integrity is variable and is controlled by the capillary entry pressure
of hydrocarbons into the pore network of the mudstones. Mercury porosimetry was
used to characterise the pore network of the Mercia Mudstone samples from several
localities in the East Irish Sea Basin. This data was used to assess seal quality, and
thus to calculate theoretical hydrocarbon column heights supportable for a given seal. It
appears that the sealing quality of the MMG mudstones and siltstones varies principally
with the nature of the pore-throat network. Fracture filling and pore plugging by bitumen
representing an earlier oil charge may also have improved seal integrity by blocking pore
throats.
