ABSTRACT: Fault-controlled carbonate cementation in the Middle Jurassic Legendre Formation, Dampier and Beagle Sub-basins, Northern Carnarvon Basin, Western Australia

Smalley, P. Craig¹, Julian C. Baker², Neil B. Thompson³, and Travis Enman³
(1) BP Amoco, Middlesex, United Kingdom 
(2) Reservoir Solutions Pty Ltd, Brisbane, Qld, Australia 
(3) Woodside Energy Ltd, Perth, WA, Australia

Reservoir sandstones of the Legendre Formation are extensively cemented by dolomite in Lynx-1A and ankerite in Picard-1, two exploration wells at near-crestal locations. The cementation is seriously detrimental to reservoir quality; a key question is thus whether the cements are widespread throughout these prospects, or whether the phenomenon is limited only to structural crests. Dolomite and ankerite in these wells, and elsewhere in the Legendre Formation, are remarkably similar in terms of their habit, paragenetic timing and stable isotopic composition, indicating a common mode of formation. Dolomite and ankerite formed at 90º-100ºC (2200-2500m burial depth), incorporating cations most likely derived from dolomite and/or smectitic clays in underlying marine mudrocks of the Murat Siltstone and Athol Formation. Carbon was generated by thermal decarboxylation of organic matter. The absolute timing of dolomite/ankerite cementation varies regionally as a result of the Legendre Formation entering the temperature window for dolomite/ankerite precipitation at different times in different parts of the basin. It is proposed that the unusually high content of carbonate in Lynx-1A and Picard-1 is due to repeated fluctuations in pCO₂ caused by pressure venting along major fault systems close to these two wells. The model predicts extensive carbonate cementation in the vicinity of valving faults, but much lower background levels of carbonate cement elsewhere in the Legendre Formation.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia