Geothermal
Convection: A Viable Mechanism for Early Burial Dolomitisation of Platform
Carbonates
Whitaker, Fiona F.1, Yitian
Xiao2, Corinne Haynes1 (1) University of Bristol,
Bristol, England (2) ExxonMobil Upstream Research Company, Houston, TX
Reaction transport models (RTMs) provide
a new means to quantitatively investigate carbonate diagenesis and its effects
on reservoir quality. RTM studies have demonstrated that geothermal (‘Kohout')
convection could drive replacement dolomitisation and also anhydritisation
during shallow burial, controlled by the balance between fluid flux and
temperature. However, extrapolation from earlier short-term (100ky) simulations
suggests complete dolomitisation requires at least 30-60 My.
Our more extended RTM simulations, using the same hydrogeological and
geochemical parameterization, indicate that
dolomitisation is likely considerably more rapid.
There are strong non-linearities in
dolomitisation rate over time as a function of the reactive surface area.
Although initial rates are indeed slow (~1 %/My), once
more than 5-10% of the rock is dolomitised, the process proceeds much more
rapidly. Beyond an undolomitised zone within 4 km of the margin where
temperatures are <18 oC, dolomite abundance increases both with
depth and towards the platform interior, forming with a broad-based,
flat-topped mushroom-shaped plume. In the interior complete dolomitisation can
occur within as little as 10-20 My. Across the
platform top a thin (<10 m) cap of up to 1~2% dolomite forms within 1 My, with magnesium sourced from overlying seawater. Assuming
a subsidence rate of 1 m/My, this process could
generate a “seed” dolomite and effectively half the time to complete
dolomitisation by geothermal convection. Syn-sedimentary fractures developed at
the platform margin short-circuit geothermal convection. The lower total flux
into the interior enables higher temperatures to be maintained inboard of the
fractures and thus facilitates more rapid dolomitisation.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California