A Multistep Reactive Convection Model for the Prediction of the Dolomite Bodies Distribution in the Karachaganak Field - Kazakhstan

Abstract

In the Carboniferous reservoir of the Karachaganak field the occurrence of replacive dolomite improves the reservoir quality. Petrographic and geochemical data on core samples suggest a geothermal convection along the slope as the main dolomitizing mechanism. Despite the significant number of wells, inferring the 3D distribution of these dolomitized bodies from lithological logs analysis only is quite challenging. To improve the understanding on the dolomite spatial distribution, Reactive Transport Modelling (RTM) at field scale has been performed with an in house developed software (MuFloT). MuFloT is a graphical environment designed to perform non isothermal multiphase flow and reactive transport models through a customized version of the TOUGHREACT commercial code. Dolomitization by geothermal convection in the shallow burial domain was simulated on a 3D grid at the field scale in three discrete steps, following the stratigraphic architecture consisting of an aggrading phase (Layer 1), a prograding phase (Layer 2) and an upper prograding unit dominated by cyclic platform deposits (Layer 3). The geometry of the three layers was obtained by decompaction and tilting of the present-day horizons from the reservoir static model. The grid was populated with five rock domains characterized by different porosity and permeability, which were inferred by extrapolating present-day values to depositional conditions. The specified heat flux at the bottom boundary in each step generates a heterogeneous temperature distribution with higher temperatures up to 47 °C reached in the subsurface of the platform interior. The temperature difference between the warm platform interior and the colder flanks generates convective half cells causing a seawater influx from the flanks of the platform. This geothermal convection triggers the replacement of calcite by dolomite. Dolomite formed in each step acts as a seed for additional dolomite formation during the subsequent episodes of geothermal circulation. Simulation of all the three Layers (4 Myr in total) drives complete dolomitization on the slope areas in Layers 3 and 2, in agreement with observations from well logs and cores. The modelled spatial distribution of dolomite provides trends and rules for the distribution of lithology and reservoir properties in the reservoir model.