Mass transport deposits and processes in the north slope of the Xisha Trough,northern South China Sea

Triple mass-transport deposits （MTDs） with areas of 625, 494 and 902 km^2, respectively, have been identified on the north slope of the Xisha Trough, northern South China Sea margin. Based on high-resolution seismic reflection data and multi-beam bathymetric data, the Quaternary MTDs are characterized by typical geometric shapes and internal structures. Results of slope analysis showed that they are developed in a steep slope ranging from 5° to 35°. The head wall scarps of the MTDs arrived to 50 km in length （from headwall to termination）. Their inner structures include well developed basal shear surface, growth faults, stepping lateral scarps, erosion grooves, and frontal thrust deformation. From seismic images, the central deepwater channel system of the Xisha Trough has been filled by interbedded channel-levee deposits and thick MTDs. Therefore, we inferred that the MTDs in the deepwater channel system could be dominated by far-travelled slope failure deposits even though there are local collapses of the trough walls. And then, we drew the two-dimensional process model and three- dimensional structure model diagram af the MTDs. Combined with the regional geological setting and previous studies, we discussed the trigger mechanisms of the triple MTDs.

Predict sandstone distribution by integrated study of deformed shale using borehole image and seismic data,a case study from northern Gulf of Mexico

Many Upper Tertiary reservoirs from the Gulf of Mexico(GoM)are sandstones deposited either in channel-levee systems or lobe systems in the intra-slope deepwater environment.One of the major uncertainties about those reservoirs is their distribution,which is likely controlled by salt tectonics.The current salt structure,however,does not represent the salt structure when the sands were deposited.It is difficult,if not impossible,to restore the salt history based on current salt structures.Salt movement resulted in a great amount of deformed shale in GoM.Borehole images on the other hand can be used to characterize the internal structure or texture of deformed shale;and the dips of deformed shale from borehole images may be used to define the paleo slope direction,which controls the movement of deformed shale.The internal structure or texture of deformed shale,therefore,may provide some information about the history of salt movement,which may also control the sand distribution.In this integrated study,all the available data,including borehole images,seismic volumes,and other petrophysical logs,were used to characterize the reservoir sands and shales.The reservoir sands are mainly composed of amalgamated sand,layered sand,and laminated sandstone.Based on dips from borehole images,paleo flow directions of reservoir sands were defined.The shales are categorized as hemipelagic shale and deformed shale.The hemipelagic shale has relatively lower and consistent dips;whereas deformed shale has relatively higher variable in both dip magnitude and dip azimuth.The integrated study suggests the main reservoir of the field is submarine lobe sands deposited above an allochthonous salt in the basin.The evacuation of the salt body below the basin created small geographic lower area for sand lobes to accumulate.The dips from the deformed shale provided information about the center of the small(or mini)basin,thus established a relationship between the dip pattern and sandstone distribution.If this relationship is valid for the other upper Tertiary deformed shale in the GoM area,a new method can be developed,which may enable us to predict the sandstone distribution using borehole images and provide guidance for petroleum evaluation and field development in the future.