Characteristics of the turbidite fan in the WenchangFormation of the Enping Sag, Pearl River MouthBasin, China and its hydrocarbon significance

A turbidite fan in the Eocene upper Wenchang Formation in the Enping Sag, Pearl RiverMouth Basin (PRMB) has been studied using seismic, logging and borehole data. The fan is characterizedby parallel progradation on the dip seismic profile and is mound-shaped or lenticular-shaped on the strikeseismic profile. The study of the core and logging data from well EP17-3-1, which is located in the frontside of the turbidite fan, shows that this fan is a set of normal grading sand beds, interbedded withinthick dark grey mudstones of semi-deep to deep lake deposits in the Wenchang Formation. The fan isinterpreted as a sand/mud-rich turbidite fan that has an area of over 140 km2 and a maximum thicknessof over 340 m. Combined with a study of the regional geological background and previous provenanceanalysis of the Eocene Wenchang Formation, the main potential provenances for the turbidite fan areconsidered to be the Panyu low-uplift and northern fault terrace zone. The Enping Sag is considered tobe a half graben-like basin whose north side is faulted and whose south side is overlapped. Basementsubsidence in the Eocene was mainly controlled by boundary faults which dip relatively steeply on thenorth side, causing the subsidence center of the Enping Sag in this stage to be close to the north boundaryfaults. Sustained faults developed in the Enping Sag during the Eocene caused an increase of the relativeheight difference between the north and the south uplift zone in the Enping Sag. Affected by the secondepisode of the Zhuqiong movement (39-36 Ma) in late Eocene, sediments which had accumulated on thePanyu low-uplift zone were triggered and moved toward the subsidence center of the Enping Sag andformed the turbidite fan. The second episode of the Zhuqiong movement is the most important triggeringfactor for the formation of the turbidite fan in the Wenchang Formation. Seismic attribute characterizationshows that the low frequency energy is enhanced and high frequency energy is weakened when seismicwaves propagate through the oil-bearing zone in this fan. Amplitude versus offset (AVO) anomaliesare observed in the seismic data and abnormally high pressure is encountered. The turbidite fan in theWenchang Formation has provided important information for sedimentary evolution in deep layers of theEnping Sag and pointed to a new direction for the hydrocarbon exploration in the study area.