Facies and Structural Controls from Reservoir Quality, Latest Devonian-Early Carboniferous Fairfield Group, Lennard Shelf, Northern Canning Basin, Australia
The mixed carbonate-siliciclastic Fairfield Group (latest Famennian-Early Visean) is a major ramp complex that evolved following the demise of Late Devonian reef-rimmed platforms. Formations within the Fairfield Group are variously reservoirs, seals and source rocks in several producing and uneconomic oilfields. Surface exposure is very poor but the group is intersected in about 40 wells, some of which are cored, across the outer Lennard Shelf. This study focuses on re-evaluation of the Fairfield Group within a sequence-stratigraphic framework to establish a geological model in which to examine the role of depositional facies and diagenetic modification on reservoir quality. Carbonate-dominated and mixed carbonate-siliciclastic facies associations are interpreted to represent peritidal to intertidal inner ramp settings. Carbonate-dominated facies associations deposited in lower energy, mid to outer ramp settings are also recognized. Turbiditic outer ramp sandstones-siltstones potentially record relative sea-level lowstands. Facies distribution was controlled by relative sea level, underlying topography, siliciclastic input from the Precambrian landmass and proximity to major fault systems.
Accommodation zones related to the extensional history of the region created compartments along the Lennard Shelf and may have influenced the types of reservoirs generated in the Fairfield Group. Reservoir units within compartments show lateral continuity and their distribution can be traced using well logs. Subtidal to peritidal depositional settings were important in reservoir development. The most productive reservoirs in the Fairfield Group are dolostones with moldic and/or vuggy porosity developed on structurally controlled paleo-highs (e.g. Blina 1). Other uneconomic reservoirs are known from dolomitic/calcareous sandstones (e.g. Meda 1). The outer ramp turbiditic sandstones deposited on the hangingwall of major faults near the basinward-limit of the Lannard Shelf may represent potential reservoirs. These deeper water sandstones are potentially significant given that several small producing reservoirs on the Lennard Shelf are in sandstones of the overlying Anderson Formation and Grant Group.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009