The sedimentology and reservoir characteristics of selected Upper Carboniferous sandbodies.
Outcrop data are increasingly used in reservoir simulation models for
extrapolating well-data into interwell volumes and predicting subsurface
porosity and permeability heterogeneity. This thesis has developed a
method of core and outcrop description that provides quantifiable
geological information for input into such models.
Four Upper Carboniferous fluvio-deltaic sandbodies (two at outcrop in
northern England and two in southern North Sea core) have been described
using eight type lithofacies, "L1 to L8": Intraclast conglomerate;
Pebbly sandstone; Large scale cross-bedded sandstone; Small scale crossbedded
sandstone; Ripple cross-laminated sandstone; Heterolithic; Sandy
mudstone). These have been mapped onto
photomontages of outcrops and used for core logging. Each sandbody has
been subdivided into lithofacies associations, characterisable by their
internal lithofacies geometries and vertical relationships.
Outcrop studies have shown that sheet-like lithofacies geometries
predominate in distributary channels. Previously described as a
distributary channel, the Table Rocks Sandstone contains lenticular
geometries and has been reinterpreted as a proximal distributary mouth
bar Sandbody. Wells 44/28-2 and X represent comparatively varied
alluvial/fluvial to fluvio-deltaic sequences, with sporadic primary reddening.
Lengths of lithofacies units generally vary with grain size and overall
depositional energy, Most mouth bar shales represent bedfarm drapes
("topset shales"), which are short (up to 4m) and relatively abundant.
Fewer, longer "toeset shales", formed by amalgamation of fines at
bedfarm bases, are typical in channel sandbodies.
Core-plug measurements and petrographic estimates of poroperm
characteristics of L1 to L8 have shown that grain size is a main control
of permeability, Gridding of lithofacies maps has enabled the modelling
of depositional permeability anisotropy, Each grid-square (lm by 0,5m)
has been assigned a permeability "indicator" (relative value), based on
its main lithofacies. L2 has been given the maximum permeability
indicator, followed by L3, L4, L1 and L5.
Semivariogram analysis of lateral (depositional) permeability anisotropy
of a distributary channel Sandbody has shown that extrapolation of core
or wireline log poroperm data would not be reliable for more than 25-30m
around the borehole,