PETROLEUM GEOLOGY OF THE MERLINLEIGH SUB-BASIN, WESTERN AUSTRALIA

Abstract

Esso's recent drilling program in the Merlinleigh Sub-basin, onshore Carnarvon Basin, represents the culmination of the first phase of concerted exploration activity in the area since the WAPET era of the 1960s. The region is unusual among Australian petroleum provinces in having excellent exposures of reservoir, source and seal rocks of Palaeozoic age. While both Esso wells (Burna 1 and Gascoyne 1) failed to encounter hydrocarbons in the primary Wooramel Group play, encouraging potential still exists. The reservoir in the Wooramel Group play is the Early Permian Moogooloo Sandstone, a fluviodeltaic to nearshore sheet-sand facies with porosities to 23 per cent and permeabilities in excess of 100 millidarcys. Likely hydrocarbon sources are siltstones in the overlying Byro Group, with total organic carbon contents averaging 3 per cent, and calcilutites in the subjacent Callytharra Formation with similar organic content. Locally, the Jimba Jimba Calcarenite Member (Billidee Formation) and the Cordalia Sandstone also provide rich source units. The least certain aspects of the Early Permian play are fault and top seal, and reservoir quality at depth. Notwithstanding the relatively shallow depths to source strata in the area, vitrinite reflectance analyses from drill cores indicate that maturation is attained as shallow as 900 m on the folded and faulted western margin of the sub-basin, and at an approximate depth of 1200 m in the depocentre beneath the Kennedy Range. This can be related to high regional heat flow, and to erosion of some 1500-2000 m of sediments prior to the regional Early Cretaceous transgression.

Older plays which have been identified in the area remain to be adequately evaluated. Potential reservoir sands are present in the Silurian Tumblagooda Sandstone, the Middle and Late Devonian Nannyarra and Munabia Sandstones, and the Early Carboniferous Williambury Formation. Possible source rocks include carbonates of Middle Devonian and Early Carboniferous age. One of the objects of current research has been to locate areas where seal, provided by the glacigene Lyons Formation of Late Carboniferous-Early Permian age, is sufficiently thin to permit economic drilling.