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Title: Hydrostratigraphic control on early dolomitisation : new insights from process-based modelling
Author: Al-Helal, Anwar Bader
ISNI:       0000 0004 2739 1996
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2012
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ABSTRACT This study used "TOUGHREACT" to simulate the interactions between subsurface multiphase fluid and heat flow and solute transport resulting from water-rock interaction in carbonate platforms. A high-resolution 2D RTM constructed to evaluate the sensitivity of geothermal convection and brine reflux to variations in extrinsic and intrinsic controls. The model was also modified to investigate the diagenetic potential of "latent reflux". Simulations suggested that refluxing brines rapidly restricts geothermal convection to the platform margin, with flow focussed in the more permeable shallow carbonates. Complete dolomitisation occurs beneath the brine pool then extends downward forming partial dolomitisation. This enhanced porosity at shallow depth, however, anhydrite precipitation occluded porosity within the partial dolomitisation area and limited the depth of reflux circulation. In contrast, geothermal:c~nvection forms a smaller partially dolomitized body over a longer time. Higher rates of fluid flow in circular platform increased the depth of alteration beneath the brine pool compared to linear baseline. Temperatures vanations affected dolomitisation rate to different extends. For accurate estimation of the diagenetic potential of different salinities brines, two versions of TOUGHREACT code were used with pre-estimating ionic strength of brines. The HKF model used for brines characterized by < 3 molal ionic strength, while for> 3 the HMW model was used. "latent reflux" caused only minor dolomitisation due to prior Mg2+ consumption at shallow depth. With geothermal circulation re-established, dolomitisation became focussed towards the platform margin. The sedimentary textures variation in strati graphic sequence, secular variation of seawater and precursor mineralogy were key factors affecting dolomitisation and were explored through a ID high resolution model that represent a complex strati graphic sequence of upper Jurassic Arab-D Formation. Observations indicate preferential alteration of the finer-grained beds within the stratigraphic sequence. Simulations indicate that this phenomenon representative of individual carbonate cycles formed in response to eustatic sea level rises.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available