Blina Oilfield, Canning Basin, case history

Abstract

The Blina Oil Field reservoirs consist of dolomitised limestones forming part of a buried Late Devonian reefal bank. The discovery well, Blina 1, flowed 905 bopd from the Fammenian Nullara Limestone. The field is estimated to contain over 5.7 million barrels of oil-in-place. The discovery of oil at Blina occurred after a quarter century search for productive buried reefs. Prior to 1957, drilling locations in the Canning Basin were determined by surface structure only. Early work on the Lennard Shelf consisted mainly of government-subsidised gravity, aeromagnetic and seismic surveys by West Australian Petroleum Pty Ltd (WAPET). Reefal features were associated initially with buried basement highs and hence gravity positives. The introduction of gravity and seismic surveys in the late 1950s resulted in a drilling location at Meda 1, on postulated buried reef, with associated oil and gas shows. The advent of Common Depth-Point Stack seismic acquisition techniques in the mid-1960s together with revised geological concepts of the reef-building process, resulted in more reliable maps of the buried reef and inter-reef structural complex. Conception of the Blina Field started with WAPET's acquisition of two lines as part of a loose grid of seismic control, and ended with Home Oil's (later Home Energy Company Ltd) interpretation of a new seismic programme, and the drilling of the discovery well. Velocity analyses and modelling of selected subsurface seismic features invoked the presence of carbonate platforms passing laterally into age- equivalent clastics. Two major reef-building phases were recognised ? the Pillara (Givetian-Frasnian) and Nullara (Fammenian) Cycles ? and integrated into the interpretation of the new seismic programme. Blina 1 penetrated a Nullara Limestone build-up over thick Frasnian clastics in an area of deep basement, in contrast to Meda 1 which had encountered apparent Pillara platform above shallow basement. Development mapping of Blina proceeded throughout the 1980s. Changes in perceived structure occurred as a result of improvements in seismic data quality and new well data. A depth map of the Top Nullara was constructed by overlaying a two-way-time map on its related map of average velocity tied to the wells. Depth maps of deeper reservoirs not associated with a reflection horizon were constructed by adding relevant thickness interval maps (as determined by well data alone) to the Top Nullara depth map. The reliable detection of lateral porosity variations within the limestone reservoir, prior to step-out drilling, remains unresolved.