AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration

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Innovative Approaches of Carbonate Diagenesis Characterization to Predict Thermochemical Sulphate Reduction (TSR) Occurrence in Sedimentary Basins: Contribution from Clumped Isotopes (Δ47) Thermometry, Laser Ablation U-Pb Chronometry and Fluid Inclusion Studies

Abstract

The reconstruction of paleo-fluid circulations in sedimentary basins is often under-constrained. This results from both the analytical challenge of performing the required analyses on the diagenetic mineral phases available in small quantities and the lack of tracers for some of the key diagenetic parameters (temperature, timing, fluid composition). Modern thermal reconstructions rely on various thermochronology methods such as fission-track, (U-Th)/He or K-Ar systems on U- or K-rich minerals, generally limited to siliciclastic lithologies. Carbonate rocks do not contain such mineral phases, limiting the possibilities to evaluate their thermal history. Given the widespread occurrence of carbonate lithologies, and their ubiquity in a variety of crustal and sedimentary settings, the development of a carbonate thermo-chronometer would open a new realm of applications in basin analysis. By coupling carbonate clumped isotope Δ47 thermometry, laser ablation U-Pb dating and fluid inclusion studies, new perspectives are opened for determining the temperatures of carbonate diagenetic phases together with the origin and composition of their parent fluids [1, 2, 3]. To validate these approaches on a well constrained case history, analyses were performed on carbonate specimens from 2000 m deep cores in a Middle Jurassic reservoir formation of the Paris basin (France). Laser ablation U-Pb dating was achieved on low U-bearing carbonates with an absolute uncertainty between 2.2 Ma and 16 Ma across a time span from 154 to 37 Myrs. These ages revealed successive phases of carbonates precipitated from early to late diagenetic conditions.The integration of these U-Pb data with Δ47 paleo-temperatures allowed defining time-temperature couples for each carbonate phase investigated that directly reveal the thermal history of the reservoir unit. This time-temperature path well agrees with the thermal scenario modelled on underlying shale layers and calibrated against organic matter maturity. This emerging carbonate Δ47/(U-Pb) thermo-chronometer has thus the ability to accurately and self consistently reconstruct thermal and fluid-flow histories of carbonate-bearing rocks within the oil window maturity zone (0-120°C). Then, this methodology was applied to constrain the occurrence of the Thermochemical Sulphate Reduction (TSR) reaction during the burial history of carbonate reservoirs from the Western Canada Sedimentary Basin. The study focused on the Devonian reefal carbonate reservoirs of the Nisku and Leduc formations, where some hydrocarbon fields have experienced TSR and contain up to 30% of H2S. Seven cores were chosen from areas of the basin having experienced different thermal histories and characterized by contrasting H2S production. The thermal information obtained from calcite fluid inclusions was combined with the burial-thermal history modelled for Devonian rocks of each of the investigated cores. This allowed to infer possible timing and fluid geochemistry for the occurrence of TSR reaction at basin scale.