Depositional and lithological control on fractures in a steep, reefal carbonate margin: Lennard Shelf outcrops of the Canning Basin, Western Australia

Abstract

A system of exceptionally well-exposed, early-burial fractures in outcrops of Devonian carbonates on the Lennard Shelf, Canning Basin, Western Australia, serves as an analogue for fractures in deep reefal carbonate petroleum reservoirs in the Precaspian Basin of Kazakhstan. Data from early fractures were collected at Windjana Gorge, which crosscuts such a reefal depositional system. Less extensive evaluations were also made at three auxiliary sites along the Lennard Shelf.These early-formed, near-vertical fractures strike parallel or normal to the strike of the Devonian depositional margin. Fracture density correlates with the textural character of the host rock, with the highest values in boundstone-rich rock, lowest values in grain-rich rocks, and intermediate values in rocks of mixed assemblage, including breccia. Fracture density is also controlled in part by the mechanical character of bedding at the time the fracture system developed, syndepositionally or soon after. The environment of deposition (EoD) shows a weaker correlation with fracture density. At Windjana Gorge, fracture height correlates with the EoD, with the largest values in the reef core, progressing serially to shorter average height in the upper slope, middle slope, reef flat, and platform top/reef flat (a transitional EoD).To better understand the fracture size distribution within a reservoir that can influence fluid drainage, we introduce a new parameter, the intersected fracture height density (IFHD). This describes the aggregate fracture height connected directly to a traverse line or borehole. Because IFHD requires the measurement of fracture heights, it is measurable only at outcrop analogue exposures.