Paul M. (Mitch) Harris
Carbonate Reservoir Consultant
ChevronTexaco Energy Technology Company
Special Earth Science Seminar, "Developing the Geologic and Reservoir Model for Tengiz Field – A Steep-Sided, Carboniferous Isolated Platform, Precaspian Basin"
Wednesday, February 6, 2013
to 6:00 PM
100 Keith-Wiess Geological Laboratories
6100 Main St
Houston, Texas, USA
Tengiz field is an isolated carbonate buildup in the southeastern Precaspian Basin, containing a Late Famennian to Early Bashkirian platform succession, and is one of the world’s largest hydrocarbon reservoirs. This presentation will review the step-by-step "workflow" that was used to pull together seismic, well, and analog data and provide a robust geologic model for this important carbonate reservoir.
Building the Geologic Model
Platform backstepping resulted in approximately 800 m of relief above the Famennian platform, followed by up to 2 km of Serpukhovian progradation. The upper Visean, Serpukhovian, and Bashkirian form the main hydrocarbon-bearing interval at Tengiz. Visean and Serpukhovian platform cycles, several to 10’s of meters thick, are laterally continuous and have predictable facies. In contrast, icehouse-driven, m-scale Bashkirian platform cycles show significant lateral facies heterogeneity. The distribution of reservoir rock types in platform facies is determined by burial diagenetic modification of an earlier reservoir system that included meteoric alteration and porosity enhancement below major sequence boundaries, and reduced dissolution along higher-order sequence boundaries associated with the presence of volcanic ash. The burial diagenetic overprint included corrosion and cementation phases followed by bitumen emplacement and associated corrosion.
The Serpukhovian progradational margin (slope) consists of in-situ upper slope microbial boundstone, and middle and lower slope breccias containing microbial boundstone clasts. Periodic margin failure during both Serpukhovian and Bashkirian time resulted in a high degree of lateral facies discontinuity. Solution-enlarged fractures, large vugs, and lost circulation zones enhanced mainly during late diagenesis form a high-permeability, well-connected reservoir in the margin and slope. This diagenetic overprint is associated with the presence of bitumen, and extends upward into overlying Serpukhovian and Bashkirian platform facies and inward into adjacent late Visean platforms, where it has substantially altered reservoir properties that remained after early diagenesis related to cyclic depositional processes.
Building the Reservoir Model
Reservoir properties of carbonates as defined by Petrophysical Rock Types (PRTs) are the product of primary depositional facies and diagenetic modification that have separate spatial trends and interactions. Depositional cycles in the Tengiz platform (Late Visean to Bashkirian) are made up of a succession of generally shoaling lithofacies overlying a sharp base with evidence for subaerial exposure and/or flooding. Key diagenetic overprint that is linked to the stratigraphic and depositional patterns includes: 1) the combined effect of late burial calcite cementation and (later) dissolution and, 2) late burial bitumen cementation.
PRTs are designed to include spatial attributes of the combined stratigraphic, facies and diagenetic framework form the basis for the Multiple Point Statistics and Facies Distribution Modeling (MPS/FDM) simulation of the platform reservoir model. Of the six PRTs, one is linked to volcanic ash (PRT1), one associated with bitumen (PRT2) and four with increasing porosity (PRT3-6) where PRT3 poorest reservoir and PRT6 represents high quality. PRT maps and a facies probability curve were used to generate the probability cube and convolved with training images, specifying the spatial interrelationship, to generate a PRT realization. Fractures are a key aspect of reservoir in the slope, so special steps are required to identify open fractures and distribute them within the model.
The revised sequence stratigraphic framework and integration of novel concepts in modeling the diagenetic overprint addressed the need for a refined understanding of the platform and slope. In addition, the extensive use of MPS/FDM modeling approaches has resulted in a more realistic integration of both depositional and diagenetic trends.
Biography of Paul M. (Mitch) Harris:
Paul M. (Mitch) Harris, is a Carbonate Reservoir Consultant with ChevronTexaco Energy
Technology Company in San Ramon, California, performs carbonate research, supports
technical projects, and provides consulting and training for the various operating units of
ChevronTexaco. His work during the last 25 years has centered on facies-related,
stratigraphic, and diagenetic problems that pertain to carbonate reservoirs and exploration
plays in most carbonate basins worldwide. Mitch received his B. S. and M. S. degrees
from West Virginia University and Ph.D. from the University of Miami, Florida. He has
published numerous papers, edited several books, and is active in AAPG and SEPM. He
has been a Distinguished Lecturer and International Distinguished Lecturer for AAPG,
and was awarded Honorary Membership from SEPM. Mitch is also adjunct faculty at
Rice University, the University of Miami, and the University of Southern California.