Imaging complex geology through challenging surface terrain – a case study from West China

Huiwen Xie, Chao Wu, Caiming Luo, WenSheng Duan,Yingjie Zhao, Xin Ji, Sherman Yang, Xiaolong Yao, Dingxue Wang, Zhengguo Jia and Qinglin Liu describe a reprocessing project combining four 3D datasets that improved the imaging of a deep subsalt thrustfaulted target in an area of variable topography and complex near-surface conditions. Oil and gas exploration is moving into areas of increasingly complex geology, where depth imaging – along with the associated need to build accurate velocity models – is becoming an ever more important tool in efficiently finding and developing oilfields. This article presents a case study of structural imaging from an area where both the subsurface geology and surface conditions are extremely complex. The project combined several vintages of land 3D seismic data from the Dabei block, located in the Kuche foreland basin area of the northern Tarim Basin, West China. Several previous attempts had been made to reprocess these datasets, but the results were all considered inadequate to meet the imaging quality and accuracy required for reliable well placement. The goal of the new reprocessing project was to deliver accurate imaging of hydrocarbon-bearing strata beneath a large anticline in subsalt thrust-fault blocks at depths of around 7000 m. Tomographic static correction using carefully picked first breaks outperformed previous field static solutions based on upholes. A multi-domain noise attenuation sequence included a newly developed method that uses true shot and receiver coordinates, thereby handling irregularities in acquisition geometry. A tilted transverse isotropic (TTI) velocity model was built using a combination of diving wave tomography (DWT) in the shallow part; geologically constrained reflection tomography in the remainder of the overburden; and multi-azimuthal tomography in the offset vector tile (OVT) domain in the salt and target zone. In areas of thick high-velocity conglomerate deposits, a seismic joint inversion (SJI) model building technique was applied to include information from non-seismic data to better define strong lateral velocity changes.