Tectonic Controls on Tengiz Rim/Flank Reservoir Quality: New Insights from Integrated Log, Regional Stress, and Far Field Stress Modeling
Daniel G. Carpenter1, Michael C. Tsenn1, James D. DeGraff1, Joel Collins2,
and Sean A. Guidry1
1 ExxonMobil Upstream Research Lab, Houston, TX
2 ExxonMobil Development Company, Houston, TX
Regional stress orientations of the Miocene-Recent Alpine/Zagros orogeny are consistent with modern earthquake seismicity, borehole breakouts, and elongate cavern distribution in Cretaceous/Jurassic carbonates at Krubera caves (Caucasus Mountains). Recent fracture development at Tengiz field including NE-SW extensional and conjugate fracture orientations appear consistent with regional stress. Well pulse test anomalies and PLT flow/fracture orientations also support recent tectonic control.
Interior platform facies exhibit rare fracturing compared to prolific rim/margin fracture/dissolution networks. Various mechanisms have been proposed to explain fracturing at platform margins (gravitational collapse, differential compaction, etc.) however, this study shows the importance of mechanical stratigraphy in a far field stress regime on fracture distribution. Understanding margin fracture controls is important to prediction of fluid flow and diagenesis within fracture networks and is needed to model fracture distribution. Core and log fracture data demonstrate penetrative fracturing of the platform margin up to 2 km inboard of margin escarpments. New core lithology/fracture descriptions are used to develop a mechanical framework for Tengiz rim/flank and adjacent platform facies.
Stress modeling produces stress concentrations consistent with observed open fracture networks and PLT anomalies. The Tengiz rim/flank (brittle boundstone and cemented grainstone facies) effectively shields interior platform facies (poorly cemented grainstones) from stress. This relationship is observed in core analysis as well as in lost circulation/bit drop trends. Conventional theories loosely explain spatial fracture distribution as a result of syndepositional gravitational collapse and decreasing fracturing toward platform interiors. However, mechanical models go further by demonstrating how tectonic partitioning of stress fields occurs between brittle rim/flank and plastic platform interior facies. Models also help explain preferential NE-SW flow trends across rim parallel orientations, how recent fracture networks evolved, and how the networks post date cementation and are open to fluid flow and diagenetic modification.