Leman Sandstone Research Articles

Permo-Carboniferous plays of the Silver Pit Basin

The Silver Pit Basin is situated in the northern part of the southern Gas Basin, and is separated from the Sole Pit Basin to the south by the Outer Silver Pit Fault. Over much of the basin up to 400 m of Silverpit Formation interbedded shales and evaporites are present. The absence of the Leman Sandstone reservoir over much of the basin has meant that gas discoveries predominate in the Carboniferous (e.g. Caister, Murdoch) rather than in the Permian, as in the Sole Pit Basin. In the Silver Pit Basin and its immediate margins, hydrocarbons have been discovered in Paleozoic reservoirs ranging from Dinantian sandstones to Zechstein carbonates. Within the Carboniferous the most significant reservoirs are fluvio-deltaic sandstones within the Westphalian and Upper Namurian. The best quality reservoirs are generally found in the coarsest sandstones, though locally the development of fracture porosity and secondary porosity due to leaching can be significant. The reservoir potential of the Permian is only significant around the margins of the Silver Pit Basin. On the southern margin, at the feather edge of the main Rotliegend Leman Sandstone fairway, significant gas accumulations are present in aeolian and mixed aeolian/sabkha facies. On the northern margins the presence of Zechstein intra-basinal highs resulted in the local deposition of carbonates which may also have reservoir potential. The main source rocks are coals and shales within the Carboniferous, predominantly in the Westphalian, but also to a lesser extent in the Namurian and Dinantian. The main kitchen area is in Quadrant 44, where significant gas generation has occurred since the Cretaceous. Migration and hydrocarbon charging is relatively straightforward for the Leman Sandstone and Carboniferous sandstone reservoirs. For Zechstein carbonate reservoirs migration via basin margin faults facilitates hydrocarbon charging. The most significant regional seal is provided by the Permian sequence of Silverpit shales and overlying Zechstein evaporites. Carboniferous shales probably also act as seals on a local or semi-regional scale, supplementing the regional seal and contributing to deep intra-Carboniferous traps. Most discoveries have been made in reservoirs immediately beneath the Base Permian Unconformity and generally the prospective areas are best identified by mapping at Base Zechstein/Base Permian horizons. The most prospective structures are associated with the major northwest-southeast-trending structural highs, often as ‘pop-up’ features. Most accumulations are predominantly structurally closed; however, for the ‘feather-edge’ Rotliegend play, closure to the northwest can have a stratigraphic component.

A new lithostratigraphy and depositional model for the Upper Rotliegend of the UK Sector of the Southern North Sea

Abstract The lack of a pragmatic basin-wide lithostratigraphic subdivision for the Upper Rotliegend of the Southern North Sea Basin has presented numerous problems when attempting to correlate wells or implement reservoir zonation schemes within this important gas province. This deficiency has also made it extremely difficult to predict facies distribution, and thus reservoir quality, when generating or assessing new prospects. With this in mind, a detailed study of the cores and logs from 126 released wells from the UK sector of the gas province was undertaken. The results of this study suggest that it is possible to subdivide the Upper Rotliegend into five lithostratigraphic units (Units 1–5 from oldest to youngest) based on the recognition of drying-upward cycles. These drying-upward cycles are developed throughout the basin in both the Leman Sandstone and Silverpit Claystone Formations. They are considered to reflect long term fluctuations in the Rotliegend climate and thus they have important chronostratigraphic implications. In any given well the numbers of drying-upward cycles (or alternatively lithostratigraphic units) actually developed, the nature of their bounding surfaces and their component lithofacies are controlled by the position of that well within the depositional basin. Utilizing the new subdivision as a stratigraphic framework it has been possible to construct detailed lithofacies maps for the five units. These maps have revealed the complex tectonic and sedimentary evolution of the Rotliegend basin and they have also suggested a new depositional model for the fluvial facies association. The tectonic evolution of the basin has been traced by analysing the spatial and temporal distribution of the various units and their component facies. In areas such as the Sole Pit, where the onset of sedimentation was contemporary with early basin rifting, all five units (Units 1–5) can be recognized. However, in those areas where sedimentation was delayed due to progressive tectonic evolution (e.g. back-stepping of the Dowsing Fault Zone), one or more of the earlier units may be absent. The later post-rift sequences (Units 4 & 5) display the greatest areal distribution within the basin.

Hyde: a proposed field development in the Southern North Sea using horizontal wells

The Hyde Field was discovered in 1982. It is a small gas field with a poor quality Lower Leman Sandstone reservoir, close to existing facilities. It may be typical of many future developments in the Southern North Sea. A development plan prepared for the field in 1989 was withdrawn because of the unacceptable economic risk. Since then, a revised plan has been prepared. It is proposed to produce the gas through three near-horizontal wells drilled close to the crest of the structure. These wells will target reservoir zones of moderate permeability ( c. 5 mD) formed from the deposits of large aeolian bedforms. The viability of the project was sensitive to the economics of the downside case. The risk was investigated by a statistical analysis of field performance. This involved assigning a range to each of the parameters controlling field GIIP and deliverability. These were then combined statistically to produce a probability distribution of economic performance which in turn was used in the project evaluation. Following this analysis, funds were obtained for the drilling of an appraisal well to test the major remaining uncertainty: the deliverability of a horizontal well. This well was completed on 20 February 1992. The planning, drilling, results and consequences are described below.

Structural and Sedimentological Controls on Diagenesis in the Ravenspurn North Gas Reservoir United Kingdom Southern North Sea

The Ravenspurn area is divided into two 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 Middle to Late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization. The deposition of allogenic clay and the formation of early quartz, nonferroan dolomite, and anhydrite reduced the reservoir quality of fluvial sheetflood. 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[degrees]C in the Triassic-Early Jurassic from reduced fluids derived partly from the Carboniferous basement. Gas accumulation took place first in the northeasterly B structure, which had early closure. Elsewhere diagenetic fluids evolved to a more alkaline state, and widespread illitization took place which particularly affected more permeable eolian 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 and 170 Ma (Middle-Late Jurassic). Subsequentmore » 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.« less

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.

Rotliegende Group Event Stratigraphy, Quadrants 42 and 47, Southern North Sea

The Easington Catchment Area is located in United Kingdom continental shelf Quadrants 42 and 47, 40 km offshore north Humberside, in the Southern North Sea basin. The area is located on the northern margin of the Rotliegende play fairway, where reservoir development, facies and quality is highly variable. Conventional lithostratigraphic nomenclature for the Lower Permian of the United Kingdom continental shelf Southern North Sea basin proposes a twofold subdivision of the Rotliegende Group with the boundary between the Leman Sandstone Formation (LSF) and the Silverpit Claystone Formation (SCF), often portrayed on wireline logs as a sharp contact. Analysis of core and log data from wells in the area demonstrates that the boundary is diachronous. Given the complexity of the depositional environment, the adoption of a sharp boundary is potentially misleading when correlating wells and predicting reservoir trends. This paper describes the development of a tectonic and climatically controlled event stratigraphy for the Rotliegende Group. In the absence of biostratigraphic data, sabkha and lacustrine intercalations have been interpreted as isochronous markers which equate to maximum flooding surfaces in marine sequences. Where sabkha intercalations are absent, laterally equivalent sand sheet packages or fluvial events have been used for correlation. A series ofmore » paleogeographic models and well correlations have been produced which are highly predictive in terms of facies distribution. The results of the study also highlight the diachronous nature of the LSF and SCF and the control of Carboniferous paleo-relief on facies and thickness variations in the Rotliegende Group. The results of the study have been used to predict Rotliegende Group facies and reservoir quality on a regional basis, a critical play element for successful exploration in this mature area.« less

The Age of Illite Cement Growth, Village Fields Area, Southern North Sea: Evidence from K-Ar Ages and 18O/16O Ratios

In this paper we describe K-Ar radiometric ages and oxygen isotope ratio measurements of illite cements from eolian sandstone samples of the Permian lower Leman Sandstone Formation (Rotliegendes Group). The samples come from a large (about 100 × 100 km) part of the Southern North Sea basin (Village Fields area) and from a range of burial depths (2.8-3.4 km subsea bed). Mean illite ages in the gas fields of Ravenspurn North (164 Ma ± 20.1 [2^sgr], n = 11), Hyde (161 Ma ± 12.3, n = 7), Cleeton (155Ma ± 20.2, n = 3), and Hoton (152 Ma ± 14.0, n = 14) fields, and in two wells located in Block 47/5A (162 Ma ± 16.1, n = 4) are all in the Middle to Late Jurassic, apart from two Early Cretaceous ages considered to represent unreliable analyses. The m an for all ages is 158 Ma ± 18.6 [2^sgr], n = 39. We largely avoided contamination with (old) detrital K-bearing minerals by careful sample selection and monitoring. To a first approximation, the assumptions of K-Ar dating appear to be fulfilled, so the measured ages are likely to date illite cement precipitation. Oxygen isotope ratios measured on the dated illite samples show small but significant variations among fields and a weak correlation with depth during the Middle Jurassic when illite grew. Both K-Ar dating and oxygen isotope ratios are compatible with a regional Middle to Late Jurassic phase of illite growth during which the mineral precipitated over a large area and over a range of temperatures governed by the depth of the formation at that time. The age of illite cementa ion coincides with rifting, which ultimately may have caused illite growth by driving pore-water flow.

The Camelot Fields, Blocks 53/1a, 53/2, UK North Sea

Abstract The Camelot Gas Fields (Camelot North, Northeast and Central-South) lie in Blocks 53/la and 53/2 in the Southern North Sea, some 30 miles (48 km) east of Great Yarmouth. Initial sub commercial discovery wells were drilled in 1967, 1969 and 1972. Further exploration and appraisal drilling was carried out in 1987 and 1988. This paper covers the Field history up to the 53/la-10 appraisal well in June 1988. The Lower Permian, Leman Sandstone Formation is the reservoir, with the gas accumulations trapped in tilted fault terraces. The Leman Sandstone Fm. in the Camelot area is 800 ft thick with a gas column up to 200 ft. Development of the fields will be in two phases. Phase I will consist of 5 wells deviated from the Camelot 'CA' platform to produce reserves from the Camelot North and Central-South Fields. Production commenced in October 1989. Phase II scheduled for 1991/92 will tie-in the Camelot Northeast Field. Gas is exported from the unmanned 'CA' gathering platform via pipeline to Amoco's Leman 'A' complex, from where the gas is transported to shore via the existing Amoco pipeline from Leman to Bacton. Total recoverable reserves for the Camelot Fields are estimated at 215 BCF.

The Clipper Field, Blocks 48/19a, 48/19c, UK North Sea

Abstract The Clipper Gas Field is a moderate-sized faulted anticlinal trap located in Blocks 48/19a and 48/19c within the Sole Pit area of the southern North Sea gas basin. The reservoir is formed by the Lower Permian Leman Sandstone Formation, lying between truncated Westphalian Coal Measures and the Upper Permian evaporitic Zechstein Group which form source and seal respectively. Reservoir permeability is very low, mainly as a result of compaction and diagenesis which accompanied deep burial of the Sole Pit Trough, a sub-basin within the main gas basin. The Leman Sandstone Fm. is on average about 715 ft thick, laterally heterogeneous and zoned vertically with the best reservoir properties about the middle of the formation. Porosity is fair with a field average of 11.1%. Matrix permeability, however, is less than 1 millidarcy on average and is so low that some intervals in the field will not flow gas unless stimulated. Steep dipping zones of natural fractures occur in certain areas of the field; these commonly allow high flow rates to be achieved from large blocks of low-permeability matrix. Expected recoverable reserves from the most favourable part of the field are 558 BCF and Clipper Field is now being developed in conjunction with part of the adjacent Barque Gas Field. Later development of the remainder of Clipper Field will depend upon reservoir performance in the initial development area.

The Barque Field, Blocks 48/13a, 48/14, UK North Sea

Abstract The Barque Field is associated with some of the earliest gas discoveries in the southern North Sea. In the Sole Pit area the reservoir, the Rotliegend Group Leman Sandstone Formation of Lower Permian age, occurs between Carboniferous Coal Measures, which source the gas, and Zechstein evaporites which form an excellent seal. Primarily aeolian, the sandstone has very low permeability resulting from deep burial of the Sole Pit Trough. The deepest burial and hence the maximum diagenetic damage to the reservoir was achieved in the early Late Cretaceous prior to the two-fold main phases of inversion in Late Cretaceous and Mid-Tertiary. Compaction and diagenesis reduced reservoir permeability to such an extent that parts of the field would be non-productive were it not for the presence of effective natural fracture zones and well stimulation by hydraulic fracturing techniques. Though appraisal and evaluation have been relatively extensive, remaining uncertainties dictate a conservative development in conjunction with the adjacent Clipper Field. A selected initial area was developed for first gas in October 1990. Good reservoir performance may lead to later development of the whole field.

Ravenspurn South Field, Blocks 42/29, 42/30, 43/26, UK North Sea

Abstract The Ravenspurn South Gas Field is located in the Sole Pit Basin of the Southern North Sea in UKCS Block 42/30, extending into Blocks 42/29 and 43/26. The gas is trapped in sandstones of the Permian Lower Leman Sandstone Formation, which was deposited by aeolian and fluvial processes in a desert environment. Reservoir quality is poor, and variations are mostly facies-controlled. The best reservoir quality occurs in aeolian sands wth porosities of up to 23% and permeabilities up to 90 md. The trap is a NW-SE-striking faulted anticline: top seal is provided by the Silverpit Shales directly overlying the reservoir, and by Zechstein halites. Field development began early in 1988 and first gas was delivered in October 1989. Production is in tandem with the Cleeton Field, about 5 miles southwest of Ravenspurn South, as the Villages project. Initial reserves are 700 BCF and field life is expected to be 20 years.

The Rough Gas Storage Field, Blocks 47/3d, 47/8b, UK North Sea

Abstract The Rough Gas Field is operated by British Gas as the world's first offshore gas storage reservoir. This relatively small field (original reserves 366 BCF) is located on the western margin of the Southern North Sea Basin. The reservoir, the Rotliegendes Gp (Leman Sandstone Fm.) average thickness 95 ft, is developed in a basin-margin facies association in which aeolian and wadi-related fluvial processes have interacted. Small-scale dune and interdune sandstones, with generally excellent permeability, alternate with wadi facies sandstones with variable but mainly poorer permeability. The vertical distribution of these facies permits a three-fold zonation of the reservoir. Flow profiles obtained from Production Logging Tools show a very strong correspondence between sedimentary facies and productivity/injectivity. The Carboniferous subcrop includes several very low-permeability sandstone intervals which contact the Rotliegendes at the angular Base Rotliegendes unconformity. A small amount of gas from these sandstones seeps across the unconformity surface into the Rotliegendes. The Rough Field was discovered by Gulf in 1968, and was operated by Amoco on a conventional depletion basis with 6 development wells from 1975 until the early 1980s. In 1980 British Gas secured 100% ownership of the Field and its facilities, and converted it to storage mode. Excess summer supplies are injected into the reservoir, to be produced during the winter to meet the peak demand. Conversion to storage mode has necessitated the installation of substantial new facilities, and the drilling of 23 further wells. First injection of gas was achieved in 1985.

The West Sole Field, Block 48/6, UK North Sea

Abstract West Sole is located in the Sole Pit area of the Southern North Sea Permian Basin in UK Block 48/6. The field was discovered in 1965 and was the first commercial discovery in the UK Continental Shelf. Gas Production commenced in 1967. Initial reserves are 1.873 TCF of which 1.335 TCF had been produced by the end of 1989. Gas is trapped in aeolian sandstones of the Permian Lower Leman Sandstone Formation. Three depositional facies are recognized, comprising aeolian dune, fluvial and sabkha. The aeolian dune facies form the principal reservoir sandstones, in units up to 40 m (131 ft) thick. However, permeability is reduced due to pervasive illite cementation, such that it averages 3 md in the dune sandstones. Productivity is enhanced in the southern part of the field by 'open' gas-filled fractures, generated during the Alpine inversion. The trap was also amplified at this stage and comprises a faulted inversion anticline trending NW-SE. The source rock is the Westphalian Coal Measures, lying directly beneath the reservoir.

The Cleeton Field, Block 42/29, UK North Sea

Abstract The Cleeton Gas Field is located in the Sole Pit Basin in the Southern North Sea in UK Block 42/29. The gas is trapped in sandstones of the Lower Permian Lower Leman Sandstone Formation, which was deposited by wind and occasional fluvial action in a desert environment. In contrast to nearby Ravenspurn South, the sands have excellent reservoir properties, particularly in the aeolian sandstones, with porosities around 22% and permeabilities between 10 and 100 md. The trap is a NW-SE-striking faulted anticline, in which the top seal is provided by the early Permian Silverpit shales directly overlying the reservoir. The field has been producing since October 1988 and its use in the Villages Field Project is as a peak shaving producer to the Ravenspurn South Field main output. The initial reserves are 280 BCF and the field life is expected to be 9 years.

The Victor Field, Blocks 49/17, 49/22, UK North Sea

Abstract The Victor gas field lies in the Southern North Sea Gas Province on the eastern flank of the Sole Pit Basin. The field straddles Blocks 49/17 and 49/22, and is situated approximately 140 km off the Lincolnshire coast. Victor was discovered in April 1972 and is operated by Conoco (UK) Ltd on behalf of BP, Mobil and Statoil. The structure is an elongated tilted fault block, trending NW-SE. The reservoir sands are contained in the Leman Sandstone Formation (Rotliegendes Group) of Early Permian age, and consist mainly of stacked aeolian and fluvial sands with a gross thickness of 400-450 ft across the field. Porosities vary from 16-20%, with permeabilities ranging from 10 md to 1000 md in the producing zones. Initial gas in place is estimated at about 1.1 TCF with recoverable reserves of the order of 900 BCF. The field was brought on-stream in October 1984, and the five producing wells deliver, on average, 200 MMSCFD through the Viking Field 'B Complex' to the Conoco/BP terminal at Theddlethorpe in Lincolnshire

The Thames, Yare and Bure Fields, Block 49/28, UK North Sea

Abstract The Thames, Yare and Bure Fields, discovered in 1973, 1969 and 1983 respectively, are located in the UK sector of the Southern Gas Basin. The reservoir is made up predominantly of aeolian dune sandstones of the Rotliegendes Leman Sandstone Formation. The fields lie on the upthrown side of NW-SE-trending fault blocks in the central part of Block 49/28. The Thames complex came on production in October 1986 and is estimated to have a field life of 10 years. Current estimates put gas in place for the three fields at approximately 350 BCF, with recoverable reserves of 240 BCF.

The Ravenspurn North Field, Blocks 42/30, 43/26a, UK North Sea

Abstract The Ravenspurn North Field is a gas accumulation located in the Southern North Sea, Permian Gas Basin which was discovered in October 1984. It has undergone four years of appraisal well drilling culminating in the approval of the development plan in 1988. Development wells are currently being drilled and three offshore installations are planned; first gas production began in July 1990. The Ravenspurn North Field is a combined structural and stratigraphic trap. The reservoir is fault closed along a series of anastomosing oblique strike-slip and normal faults. Seals along the faults are provided by the Silverpit Formation mudstones and Zechstein Group evaporites. The reservoir deteriorates to the northwest because of thinning, facies change and increasing authigenic clay content. The Lower Leman Sandstone Formation of the Rotliegendes Group forms the reservoir. It consists of a sequence of aeolian dune, fluvial sheetflood, fluvial channels and lake margin sabkha deposits. Non-reservoir intervals are formed by playa lake mudstone sequences. Fluvial and sabkha facies dominate in the northwest while aeolian facies dominate in the southeast parts of the Field. Reservoir quality was initially controlled by lithofacies distribution. Subsequent diagenesis further modified the reservoir rock resulting in variations in the porosity and permeability. Deliverability is a function of variable permeability with two areas identified; the high deliverability area where gas wells have tested sufficient quantities for commercial production without artificial stimulation and a low deliverability area where gaswells require hydraulic fracture stimulation before significant commercial production rates are achieved.

The Amethyst Field, Blocks 47/8a, 47/9a, 47/13a, 47/14a, 47,15a, UK North Sea

Abstract The Amethyst gas field was discovered in 1970 by well 47/13-1. Subsequently it was appraised and delineated by 17 wells. It consists of at least five accumulations with modest vertical relief, the reservoir being thin aeolian and fluviatile sandstones of the Lower Leman Sandstone Formation. Reservoir quality varies from poor to good, high production rates being attained from the aeolian sandstones. Seismic interpretation has involved, in addition to conventional methods, the mapping of several seismic parameters, and a geological model for the velocity distribution in overlying strata. Gas in place is currently estimated at 1100 BCF, with recoverable reserves of 844 BCF. The phased development plan envisages 20 development wells drilled from four platforms, and first gas from the 'A' platforms was delivered in October 1990. A unitization agreement is in force between the nine partners, with a technical redetermination of equity scheduled to commence in 1991.

Geology and Petrophysics of the Victor Field

Summary The Victor Gas Field is located in the Southern North Sea Gas Province some 140 km east of the Lincolnshire coast. The Lower Permian Leman Sandstone Formation (Rotliegendes Group) forms the reservoir and consists entirely of sandstones with occasional silty shales in the basal section. It rests uncomformably on the Carboniferous, which was uplifted, gently folded and eroded during the Variscan Orogeny.

The Leman Sandstone Formation in U.K. Block 49/28 Sedimentation, Diagenesis and Burial History

Summary Block 49/28 lies on the northeastern side of the Sole Pit Inversion. The Leman Sandstone Formation increases in thickness from 280 ft to 702 ft southeast wards across the block. The section can be subdivided by facies and by the recognition of major bounding surfaces between aeolian sequences. Transverse dunes up to 100 ft in height have been identified. Early secondary porosity development accompanied by growth of mixed layer clays, feldspar, dolomite and silica cements with minor anhydrite was followed by burial related illitisation of clay minerals and late kaolinite authigenesis. The most important control on reservoir quality in the block is the development of illite, which reduces permeability. The extent of fibrous illite growth was controlled by pre-Cretaceous burial. Late kaolinite, related to the migration of pore waters accompanying gas, is restricted to gas bearing structures. The relative timing of authigenic mineral growth and the structural history of the area can be combined to pinpoint the timing of gas accumulation as Middle to Late Jurassic.