Characterizing Complex Fluvial System Using Integrated Geological and Geophysical Analyses in the Jurassic Sediments in the Southern Part of the Northern Ustyurt Basin
Authors: Bashimova T. , Yessalina S. , Durmagambetov B. , Kuvanyshev A. , Kovyazin D. 
Published: November 2024

Abstract

Objectives/Scope
This work was initiated to discern the reservoir architecture of Jurassic payzones at one of the oil fields of the southwestern part of the North Ustyurt Basin for planning of an effective field development strategy. The structural and stratigraphic complexity of the field due to intense faulting, angular unconformities and fluvial-deltaic nature of the Jurassic sediments raised many questions related the reservoir extent, connectivity, and overall potential to optimize further drilling and development operations.

Methods, Procedures, Process
Well logs, seismic, core and cuttings data with regional literature have been analyzed to interpret geologic setting and depositional facies. Sedimentological analyses provided key control for interpretation of environment of deposition and vertical and lateral facies variations. Structural and stratigraphic framework have been built by detailed well correlation, followed by interpretation of seismic key horizons and fault system. Analysis of seismic attributes, served as the main background for mapping of geomorphological features away from wells. Among analyzed amplitude and frequency related seismic attributes, most important contribution was from the use of Exchroma, an amplitude expression enhancing attribute co-blended by RGB.

Results, Observations, Conclusions
Multiple stacked channel belts have been detected within 500-m-thick Middle Jurassic succession, extensive all across the study area. The geobodies have been at first traced based on well data. Wells with bell-shaped signature clustered into isolated geometries, akin to channelized systems. The sand bed thickness in some wells is less than 25 m. Wells apart of the channel belts showed abrupt sand beds disappearance. Scanning of the horizon slices attained by 3D seismic cube flattening to corresponding horizons confirmed presence of channel systems, remarkably correlating with well logs. The most prominent images have been attained by amplitude-related attributes, such as Exchroma, followed by Envelope, RMS and Sweetness. The highlighted areas are prominent as discontinuous strong amplitude seismic reflectors. The shape of the channel belts changes upward from straight, branched to meandering, indicating gradual decrease of gradient. The width of the channel belts ranges from 200 to 1500 m, with the greatest size observable in the lower part of Middle Jurassic. Mapping of channel belts away from the wells greatly facilitated well planning and had crucial impact on elevating rate of success in drilling. The further wells, targeted to mapped channel belts, showed 100% conformance with forecasted sweet spots occurrence.

Novel/Additive Information
This study represents a success story demonstrating application of integrated seismic attribute analysis for mapping complex fluvial system, that has important implications for reducing risks associated with drilling in highly heterogenous reservoir. Acquired output played a crucial impact on volumetric analyses and optimization of field development.