Lateral migration pathways of petroleum in the Zhu III subbasin, Pearl River Mouth basin, South China Sea

摘要

The discovery of the WC13-1 and WC-13- oil fields in the Zhu II subbasin of the Pearl River Mouth basin in the South China Sea has led to debate as to the oils on Horst Qionghai (HQH) are sourced from Half-graben Wenchange-A(HGWC-A) or Half-graben Wenchang-B (HGWC-B). Using the previous finding of sequence stratigraphers, this paper further analyzes the sedimentological characteristics of the Zhuhai and Zhujiang Formations and points out that the sandstones deposited on the unconformities (basement, T_6 and T_7) possess good porosity, permeability and lateral connectedness for hydrocarbon migration. The depositional systems and facies for the major reservoir rocks had gradually changed from fan-delta and semi-enclosed bay via barrier islands and washover to tidal flats, tidal channels and tide-influenced shoreface in response to the rise in sea level in the early Miocene. The sandy carrier beds of the transgressive systems tract in the Zhujiang Formation, especially in Zhujiang Formation II, act as the principal migration conduits for the WC13-1 and WC13-2 oil fields. Measurement of sandstones in drilling holes shows an increase in porosity in porosity in the Zhujiang Formation II and implies that the oil had ever passed through. Furthermore, the structural morphologies of T_5 and T_4 demonstrate that there are two salient structural noses connecting HQH with the HGWC-B and plunging into HGWC-B. T_5, a regional muddy seal, seals the porous standstones underneath and accounts for the key role played by the sandy carrier beds in transporting the oils generated in HGWC-B to HQH. Correlation of oils in HQH and HGWC-B supports the plausibility of this migration pathway.