Burial Cementation and Dissolution in Carboniferous Platform-Top, Slope and Basinal Facies, Tengiz Field, Kazakhstan

Collins, Joel F.; Katz, David A.; Harris, Paul (Mitch); Narr, Wayne

Tengiz Field is an isolated carbonate buildup in Kazakhstan that produces hydrocarbon from Carboniferous platform-top grainstones, slope microbial boundstones, and basinal breccias occupying a progradational buildup margin 800 to 1000 m. thick within the oil column. Hydrocarbon production from this margin is controlled by non-matrix permeability (enlarged fractures and caverns), much of which was affected by burial dissolution. Corrosive fluids were distributed by early fractures associated with depositional instability, and compactional fractures formed during shallow burial.

The distribution of non-matrix porosity and dispersion of corrosion into adjacent matrix varies according to facies. Upper slope boundstones feature extensive destruction of original matrix porosity by early calcite cement, with burial dissolution initiated mainly along early fractures. Dissolution operated over sufficient time to form a network of small caverns along connected fractures. The overlying platform-top facies had early matrix porosity partly destroyed by burial calcite cement and bitumen and has fewer caverns because it contains fewer large fractures. The basinal breccias were too deep for gravitational fractures to be present, thus late dissolution occurred along stylolites and burial fractures. Reduced early cement volumes resulted in local preservation of matrix porosity that was enhanced by late burial dissolution. Cavernous porosity is predicted to be less important also in this facies, due to absence of the early fracture system.

Burial dissolution occurred in two stages. An early stage of in-situ corrosion followed a reservoir pressure decline and temporary evacuation of hydrocarbons that formed bitumen and burial cement. Cathodoluminescence, stable isotopes, and clumped isotope data show that burial cements include inorganic geothermal and hydrothermal calcite up to 225^oC. During re-pressurization, calcite corrosion occurred along fracture walls and adjacent matrix in areas where bitumen is present, forming low-permeability pathways through the reservoir. A later stage of dissolution likely involved large-scale circulation of a corrosive fluid. Migration of an external fluid and lateral confinement by fractures explains a general vertical pattern of local matrix dissolution in basinal facies, dominantly fracture/cavern permeability in the slope facies, and partial matrix porosity occlusion by late calcite cement and bitumen in the platform-top facies.

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