Micro rhombic calcite of a giant barremian (Thamama B) reservoir onshore Abu Dhabi - Clumped isotope analyses fix temperature, water composition and timing of burial diagenesis

Micro porosity associated with micro-rhombic calcite provides volumetrically the largest pore volume in Middle Eastern Cretaceous carbonate reservoirs. With progressing reservoir development and depletion of macro-pore systems the low permeability micro pores have ever increasing importance as host to remaining hydrocarbons and for reservoir performance. A new technique is presented - Clumped Isotope Thermometry - which yields significant new insights on burial depth, timing of diagenesis and the nature of the diagenetic fluids. This in turn helps to better understand the widespread distribution of micro porosity in Abu Dhabi reservoirs. The genesis of micro-rhombic calcite has previously been placed into the shallow burial realm under the influence of meteoric or marine pore waters (Budd, 1989; Moshier, 1989) using oxygen stable isotopes and petrographic observations. However, the interpretation of oxygen isotope data is limited by the fact that the isotopic composition of calcite minerals is determined by two independent factors: the isotopic composition and the temperature of the precipitating water. The new analytical technique of clumped isotopes has established a way to independently determine one of the variables (temperature) thus allowing the other (water composition) to be fixed. The new technique has been applied to 15 samples from three cores of a giant Abu Dhabi reservoir. Samples are from the Barremian Thamama B reservoir unit. Stable isotope data exhibit a normal range for Thamama B calcite (δ¹³C: 2.8%o to 3.9%o; δ¹⁸0: 4.6%o to 5.9%o). Clumped isotope analyses indicate a most likely precipitation temperature of 65°C to 72°C and an isotopic composition of precipitating waters of 3.3%o to 4.9%o δ¹⁸0. Using a well calibrated basin model the temperature range places the depth of diagenesis between 0.65 km and 1.1 km and the timing of diagenesis into the Late Cretaceous (Turonian to Campanian). This coincides with the subsidence of the eastern Arabian plate margin caused by the opduction of oceanic crust (Semail ophiolite and associated marine sediments). The heavy oxygen isotopic composition of the diagenetic water of 3.3%o to 4.9%o δ¹⁸ O.corroborates a burial setting for diagenesis. Results imply that diagenesis was driven by a regional east to west burial flow system driven by compaction and convection with likely significant implications for the regional distribution of porosity & permeability.