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Benefits of a Chronostratigraphic Framework, an example from the Papuan Fold Belt (new title 29Jan)


Lithostratigraphic correlations are unreliable at the regional or basin scale. It is preferable to employ a chronostratigraphic correlation that allows packages of rock to be correlated inside time lines defined by biostratigraphic (time) intervals. Early drilling of the Toro reservoirs in the Papuan Fold Belt from the mid 1980’s relied on lithostratigraphic characteristics to drive correlation. The current palynological zonation for Papua New Guinea presented by Horizon / MGP is the product of iterative fine tuning of the schemes of Davey (1987, 1999). This has been achieved by analysing significant events and trends exhibited by palynomorphs at both a field and basin wide scale. The Foldbelt wells have provided varying vintages of palynological data from a relatively low number of company/consultant palynologists. The early data is variously ascribed to the zonations of Helby, Morgan and Partridge (1987), Welsh (1990) as well as the Robertson Research scheme. The general high quality and consistency of the range chart distribution data enables reinterpretation with respect to the latest datum’s for these early wells. In this paper the authors present a chronostratigraphic chart relating biostratigraphic zones to a chronostratigraphic scheme based on the definition of significant surfaces such as sequence boundaries and maximum flooding surfaces. This chart, combined with regional well correlations and paleogeographic mapping, has revealed lateral facies changes of key reservoir intervals across the Papuan highlands. Before this study, lithostratigraphic correlations of the Toro Formation were based on the tripartite subdivision into Toro A, B and C informal units. The geological concept consisted of laterally continuous shoreface sands with little variation in thickness or character over tens of kilometres. Under the new chronostratigraphic scheme, sands in fields to the Northwest of the basin are time equivalent to shales in in fields to the Southwest of the basin. Top Toro A in some fields is actually a younger Alene sand (Ieru Formation) sitting unconformably on the Toro A. Employing this chronostratigraphic scheme, a greater understanding of the lateral variation of sand development across the Papuan Basin is gained. This insight is valuable for exploration purposes as well as enhancing our understanding of the reservoir systems across the developed fields.