Evolution of Tengiz Rim/Flank Reservoir Quality: New Insights from Systematic, Integrated Core Fracture and Diagenesis Investigations

Daniel G. Carpenter¹, Sean A. Guidry¹, James D. Degraff¹, and Joel Collins^2
1 ExxonMobil Upstream Research Lab, Houston, TX
² ExxonMobil Development Company, Houston, TX

The Tengiz field is a Late Paleozoic isolated carbonate platform that developed on Early Paleozoic rift blocks. The distribution of facies, fractures, compaction, and subsequent porosity paragenesis control reservoir performance and pore types across three domains: platform, rim, and flank.

Early syn-depositional fractures controlled fluid flow, until an event that resulted in significant bitumen cementation (165 to 125 Ma) occluded porosity. Subsequent tectonic fractures, control present day fluid flow. Fractures exhibit variations in bitumen and calcite cement fill versus open pore volume. Core fracture measurements (spacing, aperture, continuity, cement, facies) highlight fracture density controls. Fracture networks are "diagenetic facilitators" for non matrix porosity in the Tengiz rim as evidenced dissolution along fractures, and lost circulation zones/bit drops which often correlate to PLT flow. Fracture properties vary according to stratigraphy: Unit 1 (Late Visean-Bashkirian) densities are 7.6 to 52.6 fractures/m, porosities (0.01-0.37%), apertures (0.05-0.55 mm) and open volume (1-60%); Unit 2 (Tournaisian-early Late Visean) densities are 2.91 to 9.04/m, porosities (0.01-0.06%); and volume (0-100%). Unit 3 (Famennian) densities are 0.08 to 4.14/m; porosities (0.03%) and volume (39-46%). Fracture density also varies by facies. Boundstones are heavily fractured, followed by grainstones, and mudstones. Central platform wells exhibit limited fracture networks, rare lost circulation, and matrix properties determine flow.

Diverese cements, pores, mineral assemblages, and diagenetic styles demonstrate several diagenetic processes operated in the Tengiz rim. Microbial boundstones were extensively cemented by marine cements during marine diagenesis. Subsequent vug formation in these boundstones occurred during multiple dissolution phases as indicated by their occlusion by a variety of early marine and burial cements. Fracture cements formed later in the paragenetic sequence. Late diagenetic burial alteration (e.g., dissolution) positively impacts reservoir quality and likely involves the interplay of fractures and chemically complex fluids, possibly produced off-structure during thermochemical sulfate reduction.