AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
A Hybrid Depositional-Diagenetic Approach to Building the Reservoir Model for the Unit 1 Platform of Tengiz Field (Republic of Kazakhstan)
(1) Chevron Energy Technology Company, San Ramon, CA.
(2) TengizChevroil, Atyrau, Kazakhstan.
(3) Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom.
Traditional rock typing combined with an inferred relationship to depositional environments forms the basis for a standard-approach, variogram-based reservoir model in carbonates. Using the Late Visean to Bashkirian portion of the Tengiz platform (Unit 1) as an example, we present a hybrid depositional-diagenetic approach to building the reservoir model that addresses the disconnect between depositional facies and reservoir quality due to diagenetic overprint.
In the Tengiz Unit 1 platform, reservoir properties of carbonates as defined by Petrophysical Rock Types (PRTs) are interpreted as the product of primary depositional facies and diagenetic modification that have separate and cross cutting spatial patterns, trends and interactions. Depositional cycles in the Unit 1 platform consist of a succession of generally shoaling lithofacies overlying a sharp base with evidence for subaerial exposure and/or flooding. Systematic study of the diagenetic products of several sequences across the platform using petrography, stable isotopes and CL reveal that the significant diagenetic modification includes early meteoric dissolution and subsequent cementation, late burial dissolution and late burial bitumen cementation.
PRTs are defined from a suite of well logs and calibrated by detailed rock studies using a variety of methods. The PRTs are designed to include spatial attributes of the combined stratigraphic, facies and diagenetic framework and form the basis for a Multiple Point Statistics and Facies Distribution Modeling (MPS/FDM) reservoir model of the Unit 1 platform. Of the six PRTs, one is linked to volcanic ash (PRT 1), one associated with bitumen (PRT 2) and four with increasing porosity (PRTs 3-6), wherein PRT 3 is tight and PRTs 4-6 represent increasing reservoir quality with PRT 6 the highest quality. PRT maps and vertical proportion curves were used to generate a Facies Probability Cube and convolved with Training Images, specifying the spatial interrelationship, to generate PRT simulations. The resulting model made critical improvements over previous “variogram” models as demonstrated by initial dynamic simulation and addressed the need for a refined understanding of the platform in preparation miscible gas injection. Future steps may include the separation of currently combined rock facies to improve heterogeneity prediction.