Revealing new hydrocarbon potential through Q-compensated prestack depth imaging at Wenchang Field, South China Sea

Abstract

Wenchang Field in the South China Sea contains a welldeveloped fault system, resulting in complex subsurface geology. Imaging the complex fault system plays an important role in hydrocarbon exploration in this area since the fault system forms a link between the source rocks and reservoirs. However, it is di\x8dcult to obtain a high-quality depth image of the fault system due to the e\x90ects of complex velocity and seismic absorption. Inaccurate depth velocities lead to fault shadows and structure distortions at the target zone. Absorption e\x90ects further deteriorate seismic imaging as they cause amplitude attenuation, phase distortion, and resolution reduction. We demonstrate how a combination of high-resolution depth velocity modeling and Q imaging work together to resolve these challenges. \xadis work\x80ow provides a step change in image quality of the complex fault system and targeted source rocks at Wenchang Field, signi\x82cantly enhancing structure interpretation and reservoir delineation. A couple of commercial discoveries have been made, and several other potential hydrocarbon reservoirs have been identi\x82ed based on the reprocessed data, which reveal new hydrocarbon potential in this region. Introduction Wenchang Field, situated in the Pearl River Mouth Basin in the northern part of the South China Sea, is an area of active hydrocarbon exploration and production (Figure 1a). \xade Pearl Lin Li1, Lie Li1, Tao Xu1, Min Ouyang1, Yonghao Gai1, Yitao Chen2, Xiaodong Wu2, Yongxia Liu3, and Jason Sun2 River Mouth Basin is a continent-marginal and extensional basin that developed Wenchang continental faulted sedimentation in the rifting stage of the Paleogene period. In Wenchang Field, the middle-deep lacustrine source rocks consist of taupe mud and shale, which contain abundant C30 4-methyl steranes (Chen and Cunmin, 1993). \xadese types of source rocks are of high quality, as they were deposited in a lacustrine-weak oxidation-weak reduction sedimentary environment (Jiang et al., 2015). \xadese source rocks bearing oil-generating potential are characterized by abnormally high amplitudes and dominant low frequency on the seismic image, and they have been validated by recent drilling in the Wenchang 19 Depression. \xaderefore, it has been hypothesized that high-quality middle-deep lacustrine source rocks are also present at the junction of Wenchang 14 and 19 depressions (Figure 1b), which, if validated, would signi\x82cantly increase the exploration potential of Wenchang Field. Above the deep source rocks lies a complex intraformational fault system, the Flower Fault System, which developed in the postrifting stage. \xadere are many favorable traps with high exploration potential in the area. However, complex velocities, strong multiples, and low signal-to-noise ratio (S/N) in the target zone have compromised imaging quality. Figure 2a shows a typical section from legacy prestack depth migration (PSDM) processing located at the junction of Wenchang 14 and 19 depressions. Two important questions have persisted for years without receiving 1CNOOC, Zhanjiang, Guangdong, China. E-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]. 2CGG, Singapore. E-mail: [email protected]; [email protected]; [email protected]. 3CGG, Beijing, China. E-mail: [email protected]. https://doi.org/10.1190/tle38080604.1. Figure 1. (a) Wenchang Field location. (b) Survey map of Wenchang 14 and 19 depressions and the Qiong Hai Tu Qi Survey in Wenchang Field.