Discussion and Reply 'A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs:Jebel Ousselat,northern Tunisia' by E.Vennin et al.

摘要

Simon J.Beavington-Penney and Andrew J.Burnett write:Vennin et al.(2003) have provided an interesting study of the Ypresian (Eocene) El Garia Formation of Tunisia.This unit has been the subject of numerous studies,largely because it reservoirs hydrocarbons within the Gulf of Gabes (e.g.Loucks et al.1998;Racey et al.2001;Jorry et al.2003).Because of the economic significance of this unit,its correct interpretation,especially in terms of the continuity and correlation of depositional sequences (and,therefore,reservoir layering) is crucial.Using the well-studied outcrops to the west of Kairouan in north-central Tunisia,Vennin et al.(2003) present a sequence stratigraphic model of the El Garia Fm.Their high-resolution model recognizes three orders of depositional sequences (third to fifth),with their 'small-scale' cycles occurring as individual decimetre- to metre-scale beds (which,they suggest,fall within the range of the duration of Milankovitch cycles) and groups of these beds which form metre- to decametre-scale,fifth-order packages.Vennin et al.(2003) claim that this hierarchy of cycles was produced by a nested hierarchy of eustatic sea-level change.We would like to suggest that these cycles (and the invoked causal sea-level change) are debatable,not least because they contain no unambiguous evidence of relative sea-level change (i.e.subtidal deposits capped directly by subaerial exposure surfaces).The El Garia Fm.was deposited over a period of approximately 2.8 Ma (Racey et al.2001),during which time several third-order sea-level changes might be expected to have occurred.However,earlier studies from both outcrop and subsurface (offshore Tunisia borehole core) have not noted widespread surfaces that can be correlated within the El Garia Fm.(such as the Fe-rich hardgrounds which Vennin et al.(2003) record as bounding their fifth-order cycles) and have not identified any prominent hierarchical cyclicity (e.g.Moody et al.2001;Racey et al.2001;Beavington-Penney 2002;Jorry et al.2003).Whilst one study (Loucks et al.1998) did identify several,poorly defined cycles in some boreholes,these could not be correlated between wells.