UK Southern North Sea Research Articles

A buried granite batholith and the origin of the Sole Pit Basin, UK Southern North Sea

Modelled gravity profiles, based upon the post-Carboniferous geological structure revealed by reflection seismic data, have been used to predict the gravity variation across part of the Southern North Sea Basin. When compared to the observed gravity field, these calculated values reveal a significant residual gravity low located to the east of the Sole Pit axis. This low may indicate the presence of a large buried granite body; a sister to the granite previously proposed beneath the East Midland Shelf to the SW of the Sole Pit axis. During periods of extensional tectonics, major subsidence has occurred within the Sole Pit Basin between these two granite-cored stable areas.

Structural and sedimentological controls on diagenesis in the Ravenspurn North gas reservoir, UK Southern North Sea

The Rotliegend Lower Leman Sandstone Formation has been studied near the northern limit of its development in Ravenspurn North to determine the influence of structure, sedimentology and diagenesis on reservoir quality. The Lower Leman Sandstone in this area is about 300 ft thick and comprises six depositional cycles which reflect alternating periods of relatively dry and wet climate. Aeolian sands form the best reservoir and formed as small westward-migrating transverse dune fields and draa, at most 60 ft thick. These dunes were frequently reworked by southerly derived, strong fluvial sheetfloods which drained into the Silverpit playa lake. The Ravenspurn area is divided into main northwest-southeast-trending fault blocks which are markedly different in terms of their diagenetic evolution and reservoir performance. The northeasterly B structure contained gas earlier and was unaffected by mid- to late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization. A hierarchy of factors influenced the diagenesis. These were the primary lithological variation, burial history and the structural development of the Ravenspurn North area. Depositional processes also influenced diagenesis; the deposition of allogenic clay and the formation of early quartz, non-ferroan dolomite and anhydrite reduced the reservoir quality of fluvial sheetfloods. Burial diagenesis resulted initially in ferroan dolomite, kaolinite and later quartz precipitation in available primary and secondary porosity. Stable isotope and fluid inclusion studies indicate that ferroan dolomite and later quartz precipitated at about 100°C in Triassic-early Jurassic times from reduced fluids derived partly from the Carboniferous basement. Mixing of these fluids with more alkaline connate water was the main triggering mechanism for authigenic mineral formation. Gas accumulation took place first in the northeasterly B structure which had early closure; the last authigenic phase to precipitate here was poikilotopic siderite at temperatures of about 120°C. Elsewhere diagenetic fluids evolved to a more alkaline state and widespread illitization took place which particularly affected more permeable aeolian facies. The illitization reduced the reservoir quality of the Lower Leman Sandstone and contributed to diagenetic sealing (to the northwest) of the field. K–Ar dating indicates that peak illitization took place between 150–170 Ma (mid–late Jurassic). Subsequent periods of uplift in the late Cimmerian and particularly during the early Tertiary–Miocene produced the final structure of Ravenspurn North and the spillage of gas into this structure. The combination of structural and diagenetic events explains the differences in reservoir quality and well performance of the two structural blocks in the field.

Mesozoic structural evolution of the UK Southern North Sea: insights from analysis of fault systems

An understanding of the Mesosoic fault systems in the UK Southern North Sea is important in modelling the burial history of source and reservoir rocks, in determining the time of formation of structural traps in the Permian and older section and in mapping seismic velocity fields in Mesozoic rocks for accurate time to depth conversion. The evolution of Mesozoic fault systems in the Southern North Sea can be understood by examination of several faults with different amounts of movement across them, and by analysis of fault patterns. Structural analysis has been undertaken in selected areas where complications due to salt movement are minor. Local fault analysis is illustrated by balanced cross-sections and maps from Blocks 49/28,48/23 and 48/11. The faults studied are shown to have been active extensional faults during the Jurassic and early Cretaceous, but have in general undergone only limited movement during later Cretaceous and Tertiary phases of subsidence and uplift. The Mesozoic faults are detached from basal Permian and Carboniferous rocks by decollement in the Zechstein evaporites and are linked by transfer zones to form a basin-wide system of faults. Mesozoic fault systems overlie zones of weakness in pre-Zechstein rocks, but are not directly connected to them, so their orientation is controlled by Mesozoic forces rather than pre-existing structural grain.

Structural development and hydrocarbon occurrence of the Carboniferous in the UK Southern North Sea Basin

Recent Carboniferous gas discoveries in the UK Southern North Sea display a pronounced structural alignment. Previous efforts to decipher Carboniferous structural development have been hampered by limited well control and poor quality seismic data. The recent increase in exploration activity in the southern area of Quadrant 44 has facilitated a detailed analysis of the Carboniferous. Interpretation of spectral gamma ray logs and biostratigraphic data from more than 40 deep wells have permitted re-correlation of the Carboniferous section. These new well correlations and VSP data from key wells make it possible to relate seismic reflection character to stratigraphy for several Carboniferous units. More than 2000 km of seismic data were interpreted throughout the southern part of Quadrant 44 to map the distribution of these Carboniferous seismic packages below the angular Base Permian Unconformity. This interpretation project has resulted in a new understanding of Carboniferous structural development and its relationship to hydrocarbon occurrence. East–west-trending hangingwall roll-overs, formed during Carboniferous crustal extension, were subsequently transected by northwest-trending regional anticlines, generated during the Variscan orogeny. Hydrocarbons have been discovered in both the regional anticlines and localized roll-over structures.

Thermal history reconstruction using apatite fission track analysis and vitrinite reflectance: a case study from the UK East Midlands and Southern North Sea

Abstract Apatite fission track analysis (AFTA) is a method of obtaining thermal history information in sediments. In sections that have been hotter in the past, AFTA provides estimates of maximum palaeotemperatures, and the time at which a sedimentary section began cooling from maximum palaeotemperatures. It is useful for studying the thermal history of sequences containing source rocks, and can provide information critical to the understanding of the timing of oil generation. Integration of AFTA and vitrinite reflectance (VR) data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining AFTA and VR data. This approach allows definition of the major facets of thermal history based on directly measurable parameters, rather than on assumed values of palaeo-heatflow, and provides rigorous estimates of the amount of uplift and erosion. The case study demonstrates the application of this approach to the East Midlands Shelf region. Here AFTA data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 Ma. Combined AFTA and VR data show that this cooling was due to kilometre-scale early Tertiary uplift and erosion under near normal geothermal gradients. Our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion, but was regional in extent. This has implications for the evolution of source rock maturity in this region and in other basins in around the UK where AFTA has indicated early Tertiary heating and where the extent and amount of Tertiary uplift and erosion may have also been underestimated by previous workers.

Ryazanian to Barremian mineral stratigraphy of the Speeton Clay in the UK southern North Sea Basin

SUMMARY Analysis of a late Ryazanian to early Barremian Speeton Clay section in a cored North Sea borehole has revealed a pattern of changing mineral composition that is believed to represent changes in sea-level, climate and palaeogeographical configuration. The lithological sequence can be broadly matched with that of the onshore Speeton Clay section, with precise onshore-offshore correlations being provided by sporadic bentonite layers. Because the factors controlling changes in the mineral assemblages are likely to have been of regional effect, the mineral stratigraphy established for the borehole sequence may prove to be of use not only in correlating within the North Sea marine realm, but also in relating the marine sequences to the paralic to non-marine Wealden sequences of southern England.

The Esmond, Forbes and Gordon Fields, Blocks 43/8a, 43/13a, 43/15a, 43/20a, UK North Sea

Abstract The three separate fields, Esmond, Forbes and Gordon form the Esmond Gas Complex in Quadrant 43 in the UK Southern North Sea. The Bunter Sandstone Formation is the reservoir in each of the separate, seismically defined, simple, anticlinal structures. The Bunter Sandstone correlates well across the three structures and is composed of a 400-500 ft thick interval containing individual channel sandstones deposited in an arid environment on an alluvial fan. Porosity and permeability are controlled by original texture and by subsequent diagenesis. Reservoir communication within the Bunter reservoir is good though locally tortuous. First gas production started in July 1985. Four offshore installations have allowed gas to be produced at full contract (plateau) rates of 200 MMSCFD without major interruption. Ultimate sales gas reserves are approximately 530 BCF.

Stratigraphy of the oil and gas reservoirs: UK Continental Shelf

The petroliferous sedimentary basins of the UK Continental Shelf are remarkable for the diversity of their reservoir strata. Reservoir rocks in fields currently in production range in age from Devonian to earliest Eocene, but significant hydrocarbon discoveries have also been made in rocks as as young as the mid-Eocene. The reservoirs are predominantly siliciclastic rocks, with facies ranging from continental fluvial and aeolian, to marine gravity flow deposits from sub-wave base environments. In this paper stratigraphic context of the producing horizons in the UK Continental Shelf (UKCS), principally the North Sea, is reviewed, and the sedimentation of the reservoir strata placed in an outline geological history. The main producing horizons are described in summary. Matters of stratigraphic terminology and correlation both between fields and between basins are discussed. A lithostratigraphy for the UK southern North Sea was established by Rhys (1974), and for the central and northern North Sea by Deegan & Scull (1977). Although these schemes have proved to be fairly robust, in the last 13 years the acquisition of new data plus a proliferation of new terms not fully documented in the public domain, argue strongly for a comprehensive revision and rationalization which is beyond the scope of this paper. Attempts in the public domain to standardize nomenclature across international boundaries in the North Sea, pursued by Deegan & Scull (1977) for the UK and Norwegian sectors, have lapsed for the most part in subsequent years. Economic basement in the UK North Sea can be regarded at present

A buried granite batholith beneath the East Midland Shelf of the Southern North Sea Basin

Published BGS gravity data of the UK Southern North Sea Basin show the presence of a gravity low trending to the ESE across the northern half of Quadrant 47. Two simplified geological cross-sections have been constructed across this feature with sedimentary structure based upon well information and reflection seismic data. Gravity modelling of these sections demonstrates that the gravity anomaly is unlikely to be produced by the presence of thick, low density sediments. Consequently, the presence of a large, buried granite batholith is postulated. The buoyant influence of such a granite may have exerted considerable tectonic control during the development of the Southern North Sea Basin. In particular, its presence may help to explain the development of the East Midland Shelf, the associated complex fault movements along the Flamborough Head and Dowsing Fault Zones, and the areal distribution and thickness of the Rotliegendes sand.

LOWER CRETACEOUS LITHOSTRATIGRAPHY NE OF THE SOLE PIT AREA IN THE UK SOUTHERN NORTH SEA

Seven boreholes from block 49, UK sector have been divided lithostratigraphically to illustrate the degree of comparison between the Dutch and British lithostratigraphical schemes for the Lower Cretaceous of the southern North Sea.The two schemes differ in scope and finesse, with the Dutch scheme offering more subdivisions. Both are applicable to boreholes in the British as well as the Dutch sectors of the southern North Sea. Borehole 49/25‐1 (Shell/Esso) is designated as the reference borehole in the British sector to illustrate the Dutch scheme.The Upper Holland Marl Member is considered to be equivalent to the Red Chalk formation while the Speeton Formation is divided into the equivalent of the Middle Holland Shale Member, Lower Holland Marl Member and the Vlieland Shale Member of the Dutch sector scheme.

LOWER CRETACEOUS LITHOSTRATIGRAPHY NE OF THE SOLE PIT AREA IN THE UK SOUTHERN NORTH SEA

LOWER CRETACEOUS LITHOSTRATIGRAPHY NE OF THE SOLE PIT AREA IN THE UK SOUTHERN NORTH SEA

Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit areas of the UK southern North Sea

Early diagenetic features of the Rotliegendes are related to deposition in a desert environment. Wadi sands suffered early calcite cementation as the sediment dried. Aeolian sands were cemented with halite and gypsum where the water table was close to the surface and all grains became coated with ferric oxide and clay after passing below the water table. Later diagenesis is recognised by the presence of pore-destroying cements (recrystallized clays, authigenic feldspar, quartz, dolomite and anhydrite). Their occurrence is related to deep burial followed by faulting and late Cretaceous uplift. In Leman Bank and Sole Pit, the top Rotliegendes reached depths of about 11000 and 14000 ft (3350 and 4250 m respectively) prior to maximum uplift of 6000 ft (1800 m).