Abstract:

Late Eocene to Early Oligocene tectonic-sedimentary evolution in the Wenchang B sag (Pearl River estuary basin) is highly complex. Multi-phase and multi-source fan delta deposits and braided delta were formed, which were continuously affected by the Zhu Ⅲ south fault activity. This led to the characteristics of frequent vertical overlays and strong lateral reservoir heterogeneity. By studying the tectonic-sedimentary evolution, we analyzed the effect of fault activity time and mechanism and differences in the sedimentary framework, provenance and reservoir distribution. Combining with the complexity of local sedimentation, we analyzed the vertical facial change and the planar distribution regularity of the reservoir. Based on the sedimentary sequence and facial control, we discussed the genesis of low permeability reservoir and evaluated mid-deep formations. The study shows that there are two provenance systems from the Wenchang B sag, the fan delta that presents mélange deposits of acid extrusive volcanic rock (parent rock) in steep-slope zone, and the braided delta that presents traction current deposits of granite (parent rock) in gentle-slope zone.Physical properties of the latter are clearly better because of the source changes, the traction current mechanism, and the weakly compressive tectonic stress.Besides,there are four phases of fan delta along the sag, whose (from bottom to top) provenance change progressed from east to west.We found that both the sedimentary hydrodynamic conditions and the differential activity of the Zhu Ⅲ south fault are important factors for the planar heterogeneity of the reservoir. In the southern zone (with weak fault activities), the fan delta deposits are featured by their wide sedimentary range, the full handling and elutriation, and the low common shale content, which altogether give better physical properties. Consequently, seismic facies differences and detailed contrast analysis of sedimentary facies and lithofacies can be conducted to study the local sedimentation and low-permeability formation mechanism. This allows better constraint of seismic data and the redefinition of deposition mechanism and phase difference, which can significantly influence reservoir properties. Based on this system, we build on previous knowledge to finely recognize and portrait the scope of distribution and evolution characteristics of the deltaic sedimentary system of each deposition stage, and predict the exploration direction of “sweet point” reservoirs.On the basis of the understanding of hydrocarbon accumulation conditions, we successfully guide the discovery of oil and gas exploration and new horizons in the western Pearl River estuary basin, and increase the confidence of continuously expanding the exploration to the northern zone in the mid-deep formation of the Wenchang B area.

Key words: Wenchang B sag, Late Eocene-Early Oligocene, fan delta, sedimentary insider, mechanism of low permeability, “sweet point” reservoir