Fluid-Flow Analogies in Submarine Fan Diagenesis at a Graben Margin, North Sea

Orla M. McLaughlin, R. Stuart Haszeldine

Reservoirs of the South Brae and Central Brae Oil fields are Upper Jurassic submarine fan sandstones and conglomerates deposited at the western margin of the Viking Graben. They have similar paragenetic sequences with one notable exception. Authigenic clays are more abundant in Central Brae, particularly kaolinite which is absent in South Brae.

Early calcite cementation is most abundant close to the west bounding fault and occurs in the same facies in both reservoirs. Because the ^dgr¹³C and ^dgr¹⁸O ranges are almost identical, it suggests they have precipitated in a comparable manner, possibly due to meteoric water mixing.

We have also noted that the late secondary porosity has formed in a similar spatial position in both fields and therefore may have been caused by the same mechanism. In the top reservoir layers commonly all the feldspar has been totally dissolved leaving large equigranular secondary pores. A consequent enhancement of the porosity by up to 8% is due to the expulsion of basinal fluids through this reservoir layer. South and Central Brae show a rapid loss of porosity and permeability with depth (up to 14% km^-1) but their crestal porosities are akin to those which would be predicted by sandstone compaction curves. Facies changes can partially explain this sudden loss in porosity, but the extent of diagenesis in these reservoirs is the process responsible for decreasing and incr asing reservoir quality. These processes have been controlled by the extent and focusing of diagenetic fluids into and out from the basin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995