Hydrothermal dolomitization, mixing corrosion and deep burial porosity formation: numerical results from 1‐D reactive transport models

Abstract

Major corrosion has been found at depth in carbonate hydrocarbon reservoirs from different geologic provinces. Fluid inclusion microthermometry and stable isotopic compositions of carbonate cements, predating major corrosion, constrain the interpretation of the evolution of parental fluids during progressive burial and prior to the major corrosion event. Post-major corrosion mineral paragenesis includes pyrite (-marcasite), anhydrite, kaolinite (dickite) and fluorite. Although the post-corrosion mineral paragenesis represents minor volumes of rock, it may provide valuable insights into the post-corrosion brine chemistry.Using reactive transport numerical models, the roles of cooling and/or mixing of brines on corrosion have been evaluated as controls for dolomitization, deep burial corrosion and precipitation of the post-corrosion mineral paragenesis. Modelling results show that cooling of deep-seated fluids moving upward along a fracture may cause minor calcite dissolution and porosity generation. Significant dolomitization along a fracture zone and nearby host-rock only occurs when deep-seated fluids have high salinities (4 mol Cl kg )1 of solution) and Darcian flow rates are relatively high (1 m 3 m )2 year )1 ). Only minor volumes of quartz and fluorite precipitate in the newly formed porosity. Moreover, modelling results cannot reproduce the authigenic precipitation of kaolinite (dickite at high temperatures) by cooling.As an alternative to cooling as a cause of corrosion, mixing between two brines of different compositions and salinities is represented by two main cases. One case consists of the flow up along a fracture of deep-seated fluids with higher salinities than the fluid in the wall rock. Dolomite does not precipitate at a fracture zone. Nevertheless, minor volumes of dolomite are formed away from the fracture. The post-corrosion mineral paragenesis can be partly reproduced, and the results are comparable to those obtained from cooling calculations. Minor volumes of quartz and fluorite are formed, and kaolinite-dickite does not precipitate. The major outputs of this scenario are calcite dissolution and slight net increase in porosity. A second case corresponds to the mixing of low salinity deep-seated fluids, flowing up along fractures, with high salinity brines within the wall rock. Calculations predict major dissolution of calcite and precipitation of dolomite. The post-corrosion mineral paragenesis can be reproduced. High volumes of quartz, fluorite and kaolinite-dickite precipitate and may even completely occlude newly formed porosity.

Structurally controlled hydrothermal dolomites in Albian carbonates of the Asón valley, Basque Cantabrian Basin, Northern Spain

“…Petrography suggests that calcite precipitated after dolomite, although alternating precipitation was also observed. This could suggest late-stage calcite saturation in the fluid as a result of Mg exhaustion and/or excessive Ca liberation during the dolomitization process (Salas et al, 2007). Alternatively, at constant Ca/Mg ratio in the fluid, a drop in temperature can move a fluid from dolomite to calcite supersaturation (Gasparrini et al, 2006).…”

“…The difference in salinity between the pulses may be explained by: (a) variable water-rock ratios during salt-diapir leaching, and or (b) mixing between two fluids of different salinity such as a basinal fluid moving along a fault and interstitial water in the host, which might be modified seawater (Salas et al, 2007;Smith and Nyahay, 2008). The latter can provide additional Mg for the dolomitization process.…”

Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well, Lower Cretaceous, Valencia Trough, Spain)

“…The term exotic (Neilson & Oxtoby 2008) has been applied to this mineral paragenesis, which is commonly interpreted as being precipitated prior, or during, arrival of petroleum (Rossi et al. 2001; Esteban & Taberner 2003; Salas et al. 2007).…”

Advanced Workflows For Joint Modelling Of Sedimentary Facies And Diagenetic Overprint, Impact On Reservoir Quality

“…They are used to predict or discuss the impact of the factors controlling a diagenetic process, as host mineralogy or fluids geochemistry, and are still ongoing research topics (Caspard et al 2004;Whitaker et al 2004;Xiao and Jones 2006;Salas et al 2007). They also allow simulating sedimentary facies, geometries, and early diagenesis of isolated carbonate platforms in a sequence stratigraphic context (Patterson et al 2006).…”