The Tectono-Stratigraphic Development and Hydrocarbon Habitat of the Carboniferous in Northern England

Over 70 years of exploration in the Carboniferous of northern England has resulted in the discovery of recoverable reserves totalling 75 million barrels of oil and 27 billion cubic feet of gas contained in 36 oil and gas fields. During this time the petroleum industry has amassed a substantial quantity of borehole and reflection seismic information. Integrating this essentially subsurface database with information derived from surface exposure has permitted a hitherto unachievable understanding of the Carboniferous in terms of its structural and stratigraphic development and hydrocarbon habitat. One of the key objectives of this study has been to attempt to answer the question: "Is the East Midlands unique as an oil province in northern England?" To achieve this, data from an extensive inventory of 'successful' intra-Carboniferous tests has been synthesized and used to identify and calibrate the key factors controlling the specific hydrocarbon environments. These factors have in turn been used to assess the remaining 10-15 sub-basins in northern England where Carboniferous sediments were deposited and where the play may be potentially developed. Regional seismic lines tied to borehole data and exposure studies from several northern England Carboniferous basins have been interpreted using a sequence stratigraphic approach. This has been used to develop a consistent tectono-stratigraphic framework in tying between different basins of the northern England Carboniferous. The strong NW- SE and NE-SW structural trends which control the basin geometry were inherited from the Late Palaeozoic, Caledonian orogeny. These fault trends were consistently reactivated throughout the Carboniferous in both an extensional and compressional sense. The main influence on Carboniferous basin evolution in northern England was the Variscan collision type orogeny. The Variscan plate cycle controlled the development of syn-rift (extension), post-rift (thermal subsidence), and inversion (compression) megasequences or cycles from Late Devonian to Early Permian times. Depositional sequences developed within these Carboniferous megasequences are primarily controlled by episodic rifting and periodic fault reactivation with eustatic sea level changes providing only minor control at the subsequence level. The main hydrocarbon source rocks (pro-delta shales) are confined to isolated, syn-rift depocentres. Syn-rift siliciclastic reservoirs (sandstones and conglomerates) are also restricted to the rifted half-graben. Carbonate grainstone reservoirs (limestones) rim the margins of the deeper half-graben where terrigenous input has been limited. Delta top channel and mouth bar reservoirs (sandstones) are best developed where they axially infill remnant syn-rift bathymetry. The late Carboniferous-early Permian culmination of the Variscan orogeny is seen to be the main trap forming event. All hydrocarbon discoveries to date display some element of Variscan inversion in their geometry. Variscan tectonics have also exerted a subtle but important control on play fairway evolution. Mesozoic burial ensured hydrocarbon generation which post-dated Variscan trap formation. Still later Tertiary thermal uplift inverted and modified many basins and traps. These now represent the main controls on the present day distribution of hydrocarbons in the Carboniferous of northern England. Several key areas where Carboniferous source rocks have generated significant hydrocarbons during the Mesozoic have been identified; the East Midlands being the most significant in terms of discovered hydrocarbons and perceived future potential