--> --> Reconstructing Deepwater Channel-lobe Depositional Environment in Highly Complex Multi-gravity Flow to Traction Flow Deposits using Borehole Image, Core and NMR Log, Yinggehai Basin, South China

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Reconstructing Deepwater Channel-lobe Depositional Environment in Highly Complex Multi-gravity Flow to Traction Flow Deposits using Borehole Image, Core and NMR Log, Yinggehai Basin, South China

Abstract

In recent years, operators have drilled more than 20 appraisal wells to evaluate and characterize a gas reservoir in the Upper Miocene formation in southern China’s Yinggehai basin. However, there is still no consistent understanding of the reservoir’s depositional environment. This area is thought to have been developed by a vertically and laterally superposing multi-gravity flow deposit; thus, previous deposits exposed frequent denudation, forming a highly complex and heterogeneous reservoir. Both gravity flow and traction flow sedimentary features were found, and inadequate interpretation of the depositional environment failed to unravel the petrophysical property differences and reservoir thickness variation. Therefore, reconstructing the depositional environment is imperative. We integrated borehole image, core, and nuclear magnetic resonance (NMR) log data to fully reconstruct the depositional environment in an adaptive approach. To efficiently achieve this, core and borehole images were calibrated to complement each other in analyzing lithology and sedimentary structure, then NMR and core data were calibrated to give an accurate petrophysical parameter. Results showed that the channel-lobe depositional environments were involved in multi-gravity flow (debris flow and turbidity flow) and traction flow (internal wave and tide). Based on the newly reconstructed depositional environment, channel lobes were reclassified into three subcategories: internal wave and tide, gravity flow, and reworked internal wave and tide. In addition, sedimentary sequences were clearly defined from the bottom up: internal wave and tide (reworked) deposit (S1), debris flow deposit (S2), internal wave and tide (reworked) deposit (S3), and pelagic shale deposit (S4). The reconstructed channel-lobe depositional environment is a new breakthrough; it is essential for the generation and significant improvement of a new geological model. Future field development will be greatly improved because horizontal well trajectories can be placed in more favorable sedimentary sequences.