Quantification and classification of a giant fluvial-distributive system - the Triassic Mungaroo Formation, NWS, Australia

Abstract

When producing reservoirs in fluvial deposits, it is notoriously difficult to define pore volumes connected to completed well intervals from sparse subsurface data. It is equally difficult to forecast production behaviours like initial delivery rates and long-term drawdown curves. These prediction challenges reflect the wide variety of potential fluvial reservoir geometries, from isolated narrow shoestring sands to broad labyrinth sheets, and complex patterns of intra-reservoir heterogeneities. Improved subsurface predictions require detailed characterization of the fluvial system to define the scale and magnitude of grain size variations across a hierarchy of depositional scales. A fluvial reservoir architecture classification is used to improve subsurface characterization and is applied to the Triassic Mungaroo Formation, Northwest Shelf, Australia. Integration of a substantial database of 3D seismic and records from hundreds of well penetrations provides an exceptional opportunity to characterise a large fluvial system across the full hierarchy of scales from source to sink.The fluvial hierarchy and channel-belt classification, developed specifically to aid in subsurface modelling of fluvial systems, was partially created and tested using observations from the Mungaroo Formation. The fluvial hierarchy is defined as having progressively larger-scale depositional units termed: 1) Bed, 2) Bed set & Storey, 3) Element, 4) Complex, and 5) System. A channel-belt element is the typical subsurface modelling object composed of an amalgamation of channel bar and abandonment fill deposits (storeys) formed between episodes of river avulsion. Channel-belt elements are classified into four different channel-belt types with decreasing stream power: A to D. A-type channel-belts are high powered and dominated by downstream accretion deposits. B-type belts are moderately powered and feature a mix of down and lateral accretion deposits. C-type belts are moderately-low powered and dominated by lateral accretion deposits. D-type belts are low- to very-low-powered and feature oblique to downstream accretion deposition.High-quality 3D seismic data covering large parts of the basin allowed the mapping and interpretation of individual channel-belts over hundreds of kilometres in length to document downstream changes. Integration of seismic, core and well logs allow measurements of channel-belt width, thickness, and channel-belt edge rugosity geometry to estimate river bend wavelength. Mungaroo Formation C-type and D-type channel-belts average 1100 m wide (ranging from 150 to 3000 m) and 22 m thick (ranging from 5 to 68 m), which indicates these deposits formed in very large rivers. Along individual stratigraphic intervals, thicker, wider channel-belts with longer edge rugosity wavelength in the proximal part of the system pass down depositional dip into narrower, thinner, and lower wavelength rugosity belts. These spatial changes in channel-belt geometry are used to define changes in the fluvial system across a broad fluvial distributary system spanning hundreds of kilometres, including the branching of the trunk river into smaller distributaries that are typical of a distributive fluvial system.Channel-belt scaling relationships and detrital zircon provenance data indicate a continental-scale fluvial catchment comparable in size to large-discharge modern coastal systems like the Mississippi, Orinoco, and Parana rivers. The interpretation of the Mungaroo Formation as a large, low-gradient distributive river system, has significant reservoir modelling and hydrocarbon production implications.