Abstract

Petroleum geologists working in carbonate plays are facing two common and inter-connected challenges linked to optimizing production. First, constraining the geometry, spatial distribution and inter-connectivity of reservoir geobodies is crucial as these properties can control the permeability anisotropy of reservoirs zones. This is difficult to do at the inter-well scale due to the limited resolution of seismic methods (20 meters or higher) compared to the size of typical reservoir geobodies (tens of centimers to meters and higher) and the very heterogeneous nature of carbonate reservoirs. Furthermore, diagenetic transformations are very important in carbonate reservoirs. Being able to fingerprint the process and timing of diagenetic transformation is crucial to a correct assessement of the distribution of cemented zones in the subsurface. The issue of diagenesis is also important for organic matter maturation and the timing of oil migration, and therefore the second challenge faced by reservoir geologists in carbonate plays is one of constraining as well as possible the thermal history of the targeted basin. This paper reports on the results of a major long-term research effort that addresses some aspects of this double challenge in the Middle East, and that focused on novel isotopic methods to constrain the thermal history of carbonate phases in the context of the geometry of geobodies measured at the outcrop. Geological work under the Qatar Carbonates and Carbon Storage Centre (QCCSRC), funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park, has as its long-term research goals to improve characterization of subsurface anisotropies in carbonate reservoirs, notably for CCS operations.

The overall approach of the QCCSRC team has been to do extensive fieldwork in the Middle East in order to constrain some aspects of the dimension of inter-well carbonate geobodies, but also to focus on fundamental research on applying the novel ‘clumped isotope’ method to subsurface relevant problems. The field of clumped isotopes paleothermometry is concerned with the state of isotopic ordering of carbonate minerals. More precisely, the clumped isotope paleothermometer relies on measuring the abundance of 18O-13C bonds within the lattice of a carbonate mineral and determining the offset between this abundance and a stochastic (random) distribution of isotopologues at high-temperature (nominally 1000°C, Wang et al., 2004). Both 18O and 13C are heavy, rare isotopes, and the ‘clumping’ of the two heavy isotopes into the lattice of a carbonate mineral is extremely rare (the natural abundance of the resulting CO2 of mass 47 is around 44 ppm, Eiler, 2007). But more importantly isotopic ‘clumping’ is governed by thermodynamic principles: at low temperature, ‘clumping’ of heavy isotopes is favored because the vibrational energy in a heavy-heavy bond is more than twice lower than the corresponding light-light bond, and the molecule consequently is more stable. At high temperatures, the effects of entropy mask the effects of clumping, giving rise to a stochastic distribution of isotopologues. The ‘clumping’ parameter is denoted D47 and is defined as:

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