View source for Geological factors influencing gas production in the Tyne field (Block 44/18a), southern North Sea, and their impact on future infill well planning

== 1. Stratigraphy and correlation ==

The stratigraphy of the Tyne field is illustrated by well 44/18-2z (Tyne North), which contains one of the most complete sections through the sequence ([[:File:YGS_CHR_12_GEOL_FIG_02.jpg|Figure 2]]). Three units are recognized – Lower Ketch, Transition Sequence and Upper Ketch – with ages based on palynological criteria. The main producing reservoir is the Lower Ketch (Westphalian C). This reservoir is a high netto-gross fluvial sequence and has been informally subdivided into six reservoir zones. Chemostratigraphy has played a key role in reservoir subdivision and correlation, although its resolution is insufficiently detailed to identify individual reservoir zones.

The stratigraphy of the Late Carboniferous presented here differs slightly from that in O’Mara et al. (1999: fig. 3). No “Lower Schooner” is recognized, and biostratigraphical data (D. McLean, personal communication) show that the Lower Ketch, which has been dated palynologically as mid-Westphalian C, sits unconformably on a Coal Measures sequence of mid-Westphalian B age, termed the Westoe Coal Formation ([[:File:YGS_CHR_12_GEOL_FIG_02.jpg|Figure 2]]). The unconformity is regional (Besly 2002) and, in the Tyne field, the basal Lower Ketch sandstone (Zone 9) commonly contains reworked Namurian palynomorphs (D. McLean, personal communication), suggesting a deeper level of erosion elsewhere in the basin. Compared to the succession seen in the southern part of Quadrant 44 (Schooner Ketch area) and northern part of Quad 49, several hundred feet of late Westphalian B and early Westphalian C Coal Measures, including the Aegiranum Marine Band (equivalent to most of the Cleaver Formation of Besly 2002), have been eroded in the Tyne field area. This interpretation is supported by regional well-log correlations and seismic data, which locally show faulted Westphalian Coal Measures overlain by unfaulted Lower Ketch.

The top of the Lower Ketch (T-Horizon) is very sharp and in well 44/18a-5, where it was cored ([[:File:YGS_CHR_12_GEOL_FIG_03.jpg|Figure 3]]), it shows evidence of reworking, with small sandstone pebbles derived from the Lower Ketch scattered within the overlying mudstones at the base of the Transition Sequence. This Transition Sequence of Westphalian D age is also fluvial in origin, but in all wells the net-to-gross is considerably lower ({{anchor|DdeLink3393959258479}} [[:File:YGS_CHR_12_GEOL_FIG_02.jpg|Figure 2]], [[:File:YGS_CHR_12_GEOL_FIG_03.jpg|Figure 3]]). The channel-fill sequences are of similar appearance to those of the Lower Ketch and, like the latter, are believed to be of northerly derivation (see detailed discussion of depositional model below). However, the finer-grained non-channel sediments are geochemically distinctive and show the first evidence of the reworked low-grade metasediments that characterize the southern sediment source that predominated in later Westphalian times (Besly et al. 1993, Morton et al. 2001, Besly 2002, Pearce et al. 2002). It is because of these mixed geochemical signatures that the interval was termed the transition sequence (Chemostrat Consultants 1995), as it was unclear whether it correlated with the Lower or Upper Ketch of the UKOOA scheme (Cameron 1993). The Westphalian D age, together with the evidence for some southerly derived sediment, suggests a correlation with the newly defined Boulton Formation (Besly 2002: table 2).

The junction with the Lower Ketch (T-Horizon) is a distinctive chemostratigraphical marker in the Tyne field area. The time gap is small and the palynological data show no obvious missing section. However, well-log correlations ([[:File:YGS_CHR_12_GEOL_FIG_03.jpg|Figure 3]]) suggest that Zone 4 is probably missing in several of the wells in Tyne West, South and East. This fact, together with the reworking and sharp change in net-to-gross, suggests that there may be a small unconformity or non-sequence at this level.

The channel sandstones in the Transition Sequence are also gas-bearing. They were tested in well 44/18-2z ([[:File:YGS_CHR_12_GEOL_FIG_02.jpg|Figure 2]]) and there has been limited production from other Transition Sequence sandstones in the Tyne West field ([[:File:YGS_CHR_12_GEOL_FIG_01.jpg|Figure 1]]).

The Upper Ketch proper (as defined by chemostratigraphy in the Boulton field reference well 44/21-3), and now termed the Boulton Formation by Besly (2002), has been recognized at the top of the Carboniferous sequence in wells 44/18-2z ([[:File:YGS_CHR_12_GEOL_FIG_02.jpg|Figure 2]]) and 44/18a-5 ([[:File:YGS_CHR_12_GEOL_FIG_03.jpg|Figure 3]]). However, over much of the Tyne field, the general level of erosion at the base Permian level is such that this interval is very poorly preserved in all of the wells drilled. However, thick undrilled sequences are seen on seismic lines shot across downfaulted Carboniferous sections between the producing fields ([[:File:YGS_CHR_12_GEOL_FIG_04.jpg|Figure 4]]). In the limited thickness of succession drilled in wells, the sequence predominantly comprises claystone and siltstone, with thin gas-bearing sandstones. These sandstones contain low-grade metasedimentary rock fragments (Chemostrat Consultants 1995). These are of probable southerly derivation and compare closely with those in the 44/21-3 reference well (Pearce et al. 2005). Similar southerly derived sediments have been described by Morton et al. (2001).

The stratigraphical intervals recognized above can be correlated through all of the Tyne field wells. In some wells the deeper section (Zone 9) of the Lower Ketch is missing ([[:File:YGS_CHR_12_GEOL_FIG_02.jpg|Figure 2]], [[:File:YGS_CHR_12_GEOL_FIG_03.jpg|Figure 3]]). This is probably a result of the irregularity of the basal unconformity. Erosion of the older Coal Measures sequence created a slightly irregular topography, which was then filled in by the alluvial Lower Ketch sequence, with the oldest section (Zone 9) preserved in depositional lows. This process, together with the possible local unconformity at the T-Horizon, plus several variations in thickness within the other reservoir zones, leads to considerable thickness variations in the Lower Ketch, even in the small area of the Tyne field.