Pearl River Mouth Basin Baiyun Sag Zhuhai Turbidite Fan Research

Baiyun sag, where is deep water environment and only one exploration well was drilled presently, is the biggest hydrocarbon generation depression located in the continental slope of northern South China Sea. In recent years, the deep water fans in Zhujiang Formation of Miocene have been widely and deeply studied, but related researches on sedimentary facies of Zhuhai Formation of Oligocene is still quite few. Although deltas have been identified in this formation, nobody has ever mentioned the turbidite fan at present.This paper investigates the Zhuhai Formation, in Baiyun sag, Pearl River mouth basin, using theories such as seismic stratigraphy, petroleum geology, et al., based on seismic data, with the help of well drilling nearby, adopting the technique methods of macroexaminations combined with microexaminations. The turbidite now has being the main sandstone reservoir and one of the major exploration objectives during our current exploration activity. Here we show our recent finding of turbidite-fan complex in Zhuhai Formation, Southern sag of Baiyun depression, Pearl River mouth basin. The turbidites are detected for the first time from seismic profiles by applying Berg's geological model and seismic indicators about the prograded delta and turbidite depositional system through the analysis of seismic faces. Under the parallel reflection horizon representing the delta plain and delta front, prodelta seismic subfacies, presenting as tangential oblique and complex sigmoid-oblique seismic reflection pattern, are visible wildly. At the toe of (or under) the foreset beds, there are low-angle oblique reflections (comparing with overlying delta foreset beds), which prograde following the progradation of the delta. And there are the aggregates of some mound seismic reflections and brachy-diagonal seismic reflections, located in the center zone of south sub-sag. These reflections can be easily recognized as a typical turbidite-fan complex.Based on the study of distribution and evaluation for this turbidite-fan complex and the analysis of sand body distribution of this turbidite-fan complex, it is pointed out that the source was supplied by main stream channels developed within some deltas, and it is a huge one, rich in sandstone. Moreover the composite fans system may be composed of three kinds of turbidite fan (i. e., the turbidites associated with prograded delta, present as serial wedges; the mounded turbidite fan above the sigmoid slope associated with aggraded delta; the deposits may be derived from a channel,the basin turbidite, displayed as a mound, is the strike section of upper fan, the deposits may be derived main northern slope through a canyon) and developed by multicyclic deposition.The analysis of entrapment of hydrocarbon for this turbidite-fan complex shows that it appears to be one of the major exploration objectives of the future in this area because it is preferable in hydrocarbon preservation conditions.