Note on the importance of hydrocarbon fill for reservoir quality prediction in sandstones

Oil emplacement retarded the rate of quartz cementation in the Brae Formation deep-water sandstone reservoirs of the Miller and Kingfisher fields (United Kingdom North Sea), thus preserving porosity despite the rocks' being buried to depths of 4 km and 120degreesC. Quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs. Important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time (greater than 15 m.y.). The earlier the hydrocarbon fill, the greater is the degree of porosity preservation. Failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg, potentially leading in turn to poor decisions about the need for and placement of downflank water injectors. During exploration, the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement

Sandstone cementation and fluids in hydrocarbon basins

Are porewater flow or stasis exclusive hypotheses? We think there is an intermediate view. Processes governing sandstone cementation in the deep sub-surface are elusive, case-specific and difficult to model in general terms. Combining techniques from petrography, isotopic and ion microprobe analyses with basin modelling one can narrow the possibilities towards unique hypotheses. Examples are given, predominantly from the North Sea basins, where palaeo-porewaters in different settings may evidence: (1) meteoric, compaction, or convection origins; (2) overpressured vertical leakoff; and (3) stasis and gt 100 m 'diffusion', helped by flow dispersion. Geochemical interaction transfers K and Al to muds, C to sands and forms secondary porosity by feldspar loss at depth, late carbonates, and hairy illite Chat can date oil charge