Evolution of Greenhouse-to-Icehouse Meteoric Diagenesis in an Isolated Carbonate Platform and its Effects on Porosity and Permeability Networks in Subsurface Reservoirs, Tengiz Reservoir, Kazakhstan

Katz, David A.; Hillbun, Kelly; Playton, Ted; Harris, Paul M.; Humphrey, John; Hsieh, Jean

Near surface diagenesis of isolated carbonate platform tops can significantly modify the primary porosity and permeability, and in turn impact the distribution of diagenetic products in the deep burial environment. We document a subsurface example from the Tengiz Field, a Devonian-Carboniferous isolated carbonate platform reservoir in Kazakhstan. By integrating carbon and strontium isotope chemostratigraphy, biostratigraphy, and sequence stratigraphy, we successfully unraveled linkages between reservoir quality distribution and distinct phases of diagenesis.

Departures of the C-isotope values in late Visean-early Serpukhovian platform-top sediments compared to the seawater secular C-isotope curve identified the occurrence of early meteoric cementation which is coincident with low porosity-permeability pathways in the reservoir. Conversely, upper Serpukhovian-Bashkirian stratigraphy isotopically records a more dominant marine signature and petrographic evidence shows abundant dissolution by freshwaters and cementation in the burial environment; together these characteristics are correlated with higher porosity-permeability trends in the reservoir. Sediments of the late Visean-to-early Serpukhovian transitional climate had more stable low-Mg calcite mineralogies, experienced less meteoric exposure events, and greater residence times of fresh water lenses. In contrast, sediments of the late Serpukhovian-Bashkirian Icehouse climate were characterized by more soluble aragonite mineralogies, higher frequency of meteoric exposure, and shorter fresh water lens residence times related to high frequency high amplitude sea level changes. Therefore abundant volumes of freshwater low-Mg calcite cements affected the late Visean-early Serpukhovian strata and created persistent low permeability baffles in the Modern reservoir, whereas late Serpukhovian-to-Bashkirian sediments developed a paragenetically early and high permeability pore network system from early dissolution which were available to a variety of cementing and corrosive burial fluids.

Porosity and permeability networks from shallow carbonate platform environments are established paragenetically early and strongly effect the distribution of reservoir quality in subsurface reservoirs. Separation of the seawater and diagenetic C-isotopic values and their relationship to the resultant pore network has implications for integrating predictive models of diagenesis into subsurface reservoirs.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013