Abstract The Elswick Field is located within Exploration Licence EXL 269a (Cuadrilla Resources Ltd is the operator) on the Fylde peninsula, West Lancashire, UK. It is the first producing onshore gas field to be developed by hydraulic fracture stimulation in the region. Production from the single well field started in 1996 and has produced over 0.5 bcf for onsite electricity generation. Geologically, the field lies within a Tertiary domal structure within the Elswick Graben, Bowland Basin. The reservoir is the Permian Collyhurst Sandstone Formation: tight, low-porosity fluvial desert sandstones, alluvial fan conglomerates and argillaceous sandstones. The reservoir quality is primarily controlled by depositional processes further reduced by diagenesis. Depth to the reservoir is 3331 ft TVDSS with the gas–water contact at 3400 ft TVDSS and with a net pay thickness of 38 ft.

Abstract The dating of ore deposition in low temperature epigenetic sediment-hosted mineral deposits has long proven problematical. A potential means of dating such deposits is basin evolution modeling. The data required to undertake such a study on part of the Bowland Basin of NW England is presented and used to demonstrate the technique and its limitations. The Bowland Basin is a Carboniferous half graben. Initiated in the late Devonian, the basin actively extended during the Carboniferous until the late Westphalian, when it underwent inversion (uplift) until the early Permian. Renewed subsidence commenced in the Permian and continued until inversion in the Cenozoic. During its evolution the basin accumulated a sedimentary succession that is preserved as syn-rift elastics, Dinantian age limestones and mudstones, Namurian and Permo Triassic clastic sediments. The Cow Ark-Marl Hill Moor district lies on the flank of the present basin inversion axis. Here Fe-Pb-Zn-S mineralization occurs within a complex multi-generation vein sequence hosted by Dinantian limestones. This mineralization has been shown to postdate occlusion of primary and secondary porosity within the host-limestones and also tectonic stylolitization caused by end-Carboniferous basin inversion. The mineralization is present as two distinct episodes, iron-disulfide dominated Period 1 and galena dominated Period 2, paragenetically separated by multiple vein carbonates and hydrocarbon fluid inclusion generations. The mineralizing fluids were impure NaCl-MgCl 2 -CaCl 2 -H 2 O brines with salinities >15wt% NaCl equivalent and homogenization temperatures of 85°-115°C and 95°-130°C for Periods 1 and 2 respectively. The Bowland Basin's burial history was modeled for positions on and flanking the inversion axis, using surface temperatures of 5° to 20°C and a steady state heat flow of 30°C km -1 . Modeling results imply that the Bowland Basin was able to generate hydrocarbons from the Dinantian to present and that mineralization temperatures were attained in the early Carboniferous and have been maintained since. Direct and indirect methods for determining mineralization timing from the heat flow and subsidence model are summarised and applied to the Bowland Basin. The results imply Period 1 mineralization to be Triassic and Period 2 mineralization to be Jurassic or Cretaceous in age. These dates are consistent with the modeled timings of hydrocarbon generation within the Bowland Basin.

Abstract The Ribblesdale fold belt, representing the Variscan inversion of the Bowland Basin, is a well-known geological feature of northern England. It represents a crustal strain discontinuity between the granite-underpinned basement highs of the northern Pennines and Lake District in the north, and the Central Lancashire High/southern Pennines, in the south. Recent seismic interpretation and mapping have demonstrated that the Ribblesdale fold belt continues offshore towards Anglesey via the Deemster Platform, beneath the Permo-Triassic sedimentary cover of the southern part of the East Irish Sea Basin. The Môn–Deemster fold–thrust belt (FTB) affects strata of Mississippian to late Pennsylvanian age. Variscan thrusts extend down into the pre-Carboniferous basement but apparently terminate at a low-angle detachment deeper in the crust, here correlated with the strongly sheared Penmynydd Zone exposed in the adjacent onshore. Up to 15% shortening is observed on seismic sections across the FTB offshore, but is greater in the strongly inverted onshore segment. Pre-Carboniferous thrusting post-dates formation of the Penmynydd Zone, and is probably of Acadian age, when basement structures such as the southward-vergent Carmel Head Thrust formed. Extensional reactivation of the Acadian structures in early Mississippian time defined the northern edge of the offshore Bowland Basin. The relatively late brittle structures of the Menai Strait fault system locally exhume the Penmynydd Zone and define the southern edge of the basin. The longer seismic records from the offshore provide insights to the tectonic evolution of the more poorly imaged FTB onshore.

Abstract One aspect of ore deposit studies is their potential application in prospecting for new deposits. That application has been considered in this volume (e.g., Leventhal and Giordano, 2000; Wood, 2000) and elsewhere. Heroux et al. (1996), for example, integrated organic reflectance and clay mineralogy to delineate alteration zones associated with mineralization. Both ore and petroleum deposits are a result of fluid migration, and from this viewpoint it can be expected that organic and inorganic fluids may use similar lithological pathways (Giże and Barnes, 1994). This chapter will briefly introduce organic maturation modeling as an approach to predicting both the age and relative timing of ore and petroleum fluids. The thermal maturation of sedimentary organic matter is primarily a function of time and temperature. Simplistically, if two of the parameters (organic maturity, temperature, and time) are known, then the third can be derived. If organic maturity can be determined (using optical properties such as vitrinite or bitumen reflectance, or using geochemical parameters such as isomer ratios or elemental ratios), as well as temperature (fluid inclusions), then time (e.g., duration of heating event) can be estimated. A close association between organic matter and some ore deposits has been noted throughout this and other volumes. The association may reflect genetic links (e.g., reduction or complexing), or maysimply reflect genetically unrelated aqueous and hydrocarbon fluids using the same aquifer. Petroleum, or petroleum-derived bitumens, have been reported as inclusions in ore minerals from many ore deposits (Roedder, 1984). If the time-temperature dependence of organic matter can be used to estimate when the petroleum stage of organic maturation occurred, then a potential dating method for the age of the ore deposit is also established. The use of organic modeling of the petroleum stage of organic maturation is shown for the Carlin (Nevada) disseminated gold deposit and the Bowland basin, United Kingdom, an historical district of renewed interest following the discovery of the Irish base metal deposits. The Carlin deposit provides an example of the use of organic modeling as a means of ascertaining whether or not organic matter was mobile at the time of mineralization, thus providing evidence to support or refute specific genetic concepts. The Bowland basin example will show that the integration of modeling, fluid inclusion data, and field observations can provide constraints on the probable age of mineralization.