Page 74 - An Atlas of Carboniferous Basin Evolution in Northern England

Play
fairway analysis
encountered in BP Minerals boreholes in this facies on the northern margin 53
of
the Bowland Basin (Fraser & Gawthorpe 1990).
Secondary carbonate plays

In addition to the main carbonate margin play, there are a number of other
plays within Dinantian carbonate depositional systems, the main ones being: (i)
carbonate buildups, (ii) Chadian grainstone shoals and (iii) shelf carbonates.

Carbonate buildups. A wide range of Dinantian carbonate buildups occur in
northern England, including Waulsortian buildups that range from isolated
mounds a few tens of metres high, to complexes hundreds of metres high that
cover several square kilometres. In general, the buildups are not reefal
frameworks and the core facies of all the buildups are largely micritic with
vuggy porosity and low permeability. However, the marginal facies are often
coarse-grained bioclastic grainstones that may act as potential reservoirs. By
analogy with well-exposed Waulsortian buildups of the Sacramento Mountains
in New Mexico (e.g. Kirkby & Hunt 1996), complex internal facies
architecture, controlled by a combination of sea-level change, bottom-water
anoxia, bottom currents and sea-floor bathymetry, is likely to result in marked
reservoir heterogeneity.
Waulsortian buildups of Chadian age (EC2) form the main play, with
basinal mudstones of Arundian age providing the top seal and the trap being
mainly stratigraphic. However, source rocks are a major problem given the
lack of potential source rock development in the overlying Arundian strata
(Fraser et al. 1990). Carbonate buildups are also developed in the Brigantian
(EC6), for example the Coalhills complex exposed along the southern margin
of the Derbyshire carbonate platform. Top seal to these Brigantian buildups is
provided by overlying and onlapping late Brigantian to early Namurian
Fig. 45. Composite 2D seismic line and interpreted geological cross section across the Widmerpool Gulf illustrating preservation of early syn-rift elastics in the footwall of the Hoton organic-rich mudstones which are also the source. Because of the major source
Fault (after Fraser et al 1990). These syn-rift elastics were deposited in the immediate hanging-wall of the Thringstone-Sileby Fault during earliest syn-rift times, but were subsequently problem with Chadian Waulsortian buildups, the Brigantian buildups are
preserved in a relatively shallow position, due to migration of fault activity onto the Hoton Fault. thought to be the more prospective.
The carbonate buildup play is thought to be prospective in off-shelf
locations in the Widmerpool Gulf, Hathern Shelf, Edale Gulf, Gainsborough
hummocky clinoforms into a series of parallel reflections. By analogy with The most prospective areas for this play occur where late Dinantian-early Trough and Humber Basin, and mounded seismic facies originally drilled as
exposures in the Bowland Basin (Gawthorpe 1986), the hummocky and Namurian source rocks, matured during Mesozoic burial, directly onlap reef prospects are now considered to be Dinantian carbonate buildups (e.g.
parallel reflections are interpreted to represent distal calciturbidite facies and Dinantian platform margins (Fig. 46). The four main areas where the Grove-1, Fig. 12). The majority of these areas have had access to oil generation
interbedded basinal mudstones and distal turbidites. The spatial distribution of carbonate margin play is developed are as follows (Fig. 48). during both Carboniferous and Mesozoic burial. Parts of the western Bowland
the carbonate platform margin play is limited to a 0.5-2 km wide fairway Basin and Goyt Trough may still retain some exploration potential, but
which rims the margins of the half graben (Figs 26-30 and 46). The narrowness Widmerpool Gulf (northern margin). Seismic and outcrop data have high- these areas rely solely on Carboniferous generation and underwent large
of the play fairway highlights the need to identify the margins on seismic, since lighted the presence of Asbian/Brigantian hanging-wall margins along the amounts of uplift in both the late Carboniferous-early Permian and during the
present well control is of insufficient density to map the location and extent of northern rim of the Widmerpool Gulf (Figs 12 and 47). Tertiary.
the platform margins in the subsurface.
Schofield & Adams (1985) and Gawthorpe (1987b) suggest that basinal Gainsborough Trough (north and south margins). Seismic and well data have Chadian grainstone shoals. Chadian grainstone shoals developed in proximal,
mudstones acted as sources of fluids for burial dolomitisation and secondary confirmed the presence of platform margins of Asbian and Brigantian age in above wavebase, portions of carbonate ramps. These facies can be observed at
porosity generation. Burial dolomitisation and leaching played a crucial role in both footwall and hanging-wall settings (Fig. 12). outcrop in the Bowland Basin south of the Askrigg Block and were proven to
controlling reservoir quality. Field and core petrographic studies indicate that, be oil bearing in Strelley-1 in the Widmerpool Gulf.
although primary porosity is occluded by early calcite cement, subsequent Commonly the grainstones are dolomitized and have good reservoir quality
dolomitisation and leaching created secondary porosity prior to hydrocarbon Edale Gulf. Field and seismic data suggest the presence of well developed with moderate to high visible porosity. Onlapping Arundian basinal mudstones
generation (e.g. Walkden 1987; Gawthorpe 19876). Onlapping late Brigantian Asbian/Brigantian margins surrounding the Edale Gulf (Fig. 17). Maturation and lateral facies changes into low permeability peritidal carbonates should
to early Namurian pro-delta mudstones not only provide a source of fluids for and migration of hydrocarbons into the margin are indicated by bitumen provide both top and lateral seal. Sourcing this intra-Dinantian play fairway is
dolomitisation and leaching, but are also the top seal and hydrocarbon source around Castleton (southern footwall margin). a problem; however, fault juxtaposition with the carbonate platform margin
rock for the play. The success of this play also requires lateral seal. This may be play fairway in Strelley-1 suggests a possible migration path from the basal
provided by the facies change from the high energy platform margin reservoirs Manx-Furness Basin (including Formby-Fylde). The presence of platform Namurian pro-delta source rock (Fig. 12).
into low porosity and low permeability shelf wackestones and peritidal margins to the north and south of the western Bowland Basin (i.e. Manx- A major problem with the Chadian grainstone play is in predicting its
mudstones, or by faulting against basal Namurian or Holkerian mudstone Furness) is inferred mainly from outcrop in the south of the Askrigg Block and distribution in the subsurface. Outcrop and well data suggest a very restricted
facies. boreholes such as Croxteth and Formby (Fig. 2). Residual hydrocarbons were fairway in terms of both its shoreline location and its apparent reliance on the