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Doran, Helen1, R.S Haszeldine1, C Taberner2, A.E Fallick3
(1) The University of Edinburgh, Edinburgh, United Kingdom
(2) Institute of Earth Sciences, CSIC, Barcelona, Spain
(3) SUERC, Glasgow, United Kingdom

ABSTRACT: Fluid Migration around the Machar Salt Diapir UK Central North Sea: Impact on Porosity and Permeability of a Fractured Carbonate Reservoir

The Machar chalk oilfield, Central North Sea, forms a regional leak-off point from deeper geopressured sandstones. A concentric pattern of open permeable, and filled impermeable, fractures formed during salt diapir growth. The matrix chalk has 20% modal porosity with less than 150mD permeability
Cap rocks to the diapir are enriched in celestite (SrSO4), derived from outwith the salt. Mass-balance of pore-fluid required for Sr import into the celestite shows that several oilfield volumes have flowed through the present day reservoir.
Many individual fractures host multiple generations of carbonate, barite and fluorite growth. Fluid-inclusions in fracture-fills show maximum salinities (22 wt% NaCl eq), suggesting that fluids dissolved salt from the diapir. Maximum fluid-inclusion temperatures (150oC ) from wells spanning the crest and flank of the structure, are much hotter than expected for present depths (1900 to 2800m). Carbon isotope of fracture-filling calcites (+2 to +7  13C PDB ‰) exceed those of tightly constrained chalk matrix 0 to +3  13C PDB (‰). Thus at least some of the carbon was derived externally to the chalk. Oxygen isotopes from fracture filling calcites (16 to 25  18Osmow ‰) are depleted relative to chalk matrix values (24 to 29 18Osmow ‰).
The combined data sets indicate that fracture-filling material could not have formed from local chalk matrix pore-fluids. One possible explanation is pulsed expulsion of hot saline, and hydrocarbon, fluids from geopressured sandstones in the adjacent deep basin. Deep-derived fluids here cemented fractures at shallow depths.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.