Constraints on the Burial History of Tengiz Field From Clumped Isotope Paleothermometry

Abstract

Burial diagenesis is a primary control on reservoir quality in many carbonate reservoirs. To predict the distribution of reservoir quality, it is often necessary to constrain the timing and conditions of burial diagenetic events. Clumped isotope paleothermometry provides an avenue to assess the evolution of reservoir temperatures and the oxygen isotopic composition of pore fluids, sediments, and cements during diagenesis. In Tengiz Field, Republic of Kazakhstan, the Unit 1 margin and slope reservoir were modified by a protracted burial diagenesis. Flow is dominated by natural fractures and solution enlarged mega-porosity. Multiple fracturing, dissolution, and cementation events occurred, beginning during deposition and continuing through deep burial. Our study focuses on sediments and the cements that occlude fractures and vugs. Samples of host-rock indicate that sediments recrystallized during shallow burial. Burial cements (isopachous, acute-terminated calcite, followed by coarse, blocky spar) formed at 80-130°C, corresponding to burial depths of ∼2.5-4.5 km. These cements occlude much of the macroporosity, indicating that a large, open pore system existed long into the burial history. Locally, spar-filled fractures cross-cut one another, indicating that some fractures also formed during burial. The present burial depth and temperature of precipitation are positively correlated with paleo-fluid δ18O and negatively correlated with mineral δ18O. Temperatures and paleo-fluid δ18O are on-trend with modern formation waters. These data suggest fluids and rocks have interacted in a partially closed system, whereby minerals forming at ever-increasing temperatures locked up isotopically lighter δ18O, leaving pore fluids to evolve heavier δ18O. This scenario is consistent with fluid-rock interaction models, which reproduce the evolution of pore fluids under variably closed conditions. The results of this study demonstrate that the Tengiz slope evolved to a partially closed system. Assumptions of closed-system behavior have been used to argue for the ineffectiveness of burial dissolution in carbonate reservoirs. Yet burial dissolution is common in the Tengiz flank, where it is a significant control on reservoir quality. With multiple fracturing, dissolution, and cementation events, the Tengiz flank illustrates the potential for significant burial modification of reservoir quality, and clumped isotope paleothermometry proved effective at elucidating that history.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016