Static and dynamic connectivity in bed-scale models of faulted and unfaulted turbidites

T. Manzocchi, J. J. Walsh, M. Tomasso, J. Strand, C. Childs, P. D. W. Haughton
2007 Geological Society Special Publication  
A range of unfaulted and faulted bed-scale models with sheet-like or lobate bed geometries and faults of comparable sizes to beds have been built and analysed in terms of bed connectivity and fractional permeability assuming permeable sands and impermeable shales and shale smears. A new method has been devised allowing amalgamation ratio to be included explicitly as model input and this property, rather than net:gross ratio, is found to be the dominant control on the connectivity of unfaulted
more » ... quences. At the geometrically representative scales considered (horizontal distances of > 1km for beds up to ca. 1m thick and faults up to ca. 5m throw), faulted sequences rarely have lower connectivities than their unfaulted sedimentological equivalents irrespective of whether fault rock properties are included. Models containing stochastically placed shale smears associated with each faulted shale horizon are generally better connected than if deterministic Shale Gouge Ratio cutoffs are applied. Despite the complex interactions between geological input and connectivity of the faulted sequences, the flow properties at representative scales are controlled by three geometrical variables describing connectivity, anisotropy and resolution. If two different faulted or unfaulted systems have identical values of these three variables they will have the same equivalent flow properties. End of Abstract of faulted turbidite reservoirs. Many turbidite reservoirs contain sandstones interbedded with low permeability shales, and in such cases the flow properties of the rock volume depend to a large extent on the connectivity of the sandstone beds. It has been suggested that for situations in which the shales can be assumed impermeable, many flow characteristics can be estimated very rapidly using semi-analytical scaling laws derived from percolation theory (e.g. King 1990; King et al. 2002; see also Stauffer and Aharony 1994; Sahimi 1995 for background discussion on percolation theory and its applications to flow). The important system parameters for this approach are a connectivity measure, which establishes how close the network of sandstone beds is to the percolation threshold; one or more anisotropy terms, which establish flow path tortuosity in different directions; and one or more resolution terms, which establish how closely the scale of interest (e.g. the inter-well spacing in a particular field development plan) approximates to the infinite systems for which results from percolation theory apply strictly. The central tenet to this approach is that the geological details of the system are not relevant per se, but only in as much as they contribute to the three parameters mentioned. In this study we examine faulted and unfaulted bed-scale models of idealised sheet-like turbidite geometries, and discuss geological controls on connectivity as well as whether and how these three more fundamental geometrical terms can be defined in anything other than the most simplistic idealisations of the reservoir geology. A glossary of the terminology used in this paper is given in the appendix. The first part of the paper concerns connectivity in unfaulted thin-bedded sheet-like or lobate bed geometries (e.g. Fig 1) , a correct representation of which is recognised as a significant challenge in turbidite modelling (e.g. Weimer et al. 2000; Browne & Slatt 2002). Despite strong vertical heterogeneity, these systems often appear laterally homogeneous at an outcrop scale owing to the high horizontal to vertical bed anisotropy. A compilation of width to thickness measurements from sheet-like systems (Fig 2a) indicates that turbidite deposits at all hierarchical scales from individual beds to complete systems are typically about 200 times longer than they are thick (+/-a factor of 10). For systems of beds of ca. 1m thickness or less, therefore, bed connectivity is a more significant control on inter-well flow (i.e. flow at length-scales of hundreds of meters to a few kilometres) than is the internal permeability distribution of the beds, provided the latter is small compared to the 2 Manzocchi et al: Connectivity in Faulted Turbidites permeability contrast between beds and shales. Bed connectivity can be recognised at outcrop as amalgamation surfaces (e.g . Fig 1b) , and in this study we make extensive use of the amalgamation ratio ( ; Chapin et al. 1994) as a connectivity measure. Departing slightly from the ambiguous definition of Chapin et al., we define amalgamation ratio as the fraction of sandstone bed bases that are amalgamated with the underlying sandstone bed when measured on a line sample. As noted by Stephen et al. (2001), this is equivalent to the total length (or area) of amalgamation normalised by the total length (or area) of sandstone bed bases in 2D (or 3D), and therefore is analogous to the "connectedness ratio" measured in the process-based fluvial models of Mackey & Bridge (1995) and Karseenberg et al. (2001). AR In the second part of the paper we examine the influence of faults on bed connectivity within geometrically representative systems. Previous studies (e.g. Bailey et al. 2002; James et al. 2004) have shown that the influences of fault system characteristics on connectivity are intimately tied to sedimentological characteristics, a recurring issue in our analyses. The focus of the analyses are on faults of comparable sizes to the principal sedimentological length-scales. Fault lengths and maximum fault throws range from a few times smaller to a few times larger than the length and thickness of the beds respectively. As we are principally considering beds thinner than ca. 1m, the faults are sub-seismic with maximum fault throws up to ca. 5m. The effects of fault rock properties are included using Shale Gouge Ratio cut-offs and by explicit stochastic shale smear modelling. In common with the assumption of a binary permeable / impermeable sedimentological system, we assume across-fault sand-on-sand juxtapositions are either permeable in the absence of a shale smear, or impermeable where one or more is present. We therefore do not address permeability decreases caused by cataclastic fault rock.
doi:10.1144/sp292.18 fatcat:627jsrawvrexfe2uhxuvln4x4y