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Reservoir Implications of Clumped Isotope Investigation and Micro-CT Imaging of Early Dolomite


Dolomitization can improve or alter carbonate reservoir quality. Its understanding is therefore critical for oil and gas industry. Common methods used in tracing the origin of dolomite within reservoirs include fluid inclusions analyses and stable isotopes measurements. However, fluid inclusions in early dolomite crystals are usually too small to be measured. Isotopes measurement methods are plagued by the fact that the bulk isotopic composition of the dolomite depends on both the bulk isotopic composition of the parent fluid and the temperature of precipitation. It is therefore challenging to constrain numerical models with temperature fields or diagenetic fluid chemistry, which leads to major uncertainties in the prediction of dolomitization-related reservoirs heterogeneities. Here, we use the clumped isotopes thermometry (Ghosh et al., 2006; Eiler, 2007) to independently determine the temperature of dolomite precipitation or recrystallization. We apply this temperature measurement to dolostone samples of the two carbonate platforms of the Marion Plateau (NE Australia). It is an excellent analogue to dolomitized Miocene reservoirs producing hydrocarbons in South East Asian fields. It is also a natural laboratory to study the effect of platform geometry and carbonate facies on the dolomitization. The clumped isotope thermometry preliminary results show that the temperature of dolomite precipitation varies laterally and through depth. It illustrates the complex history of different dolomitization episodes in the two platforms, including recrystallization events. An intriguing result is that the temperatures measured with clumped isotopes are cooler than the 60-70-degree isotherm where most of the dolomitization occur in reactive transport models (Whitaker, 2004). In addition, we investigated a range of partially dolomitized samples at the pore scale using a 3D XCT scanner. Results show that image segmentation techniques allow a 3D qualitative and quantitative analysis of the pore network, showing the dolomite nucleation sites, quantifying the minerals surface areas and also revealing the connectivity pattern in 3D. These properties are key inputs in reactive transport models. It suggests that microporosity played a negligible role in controlling the flow of the dolomitization fluid in the Marion Plateau. The results of this study will help to better constrain dolomitization models and thus reduce uncertainties in the characterization of dolomitized reservoirs.