Triassic Sherwood Sandstone Group Research Articles

Contact Diagenesis: The Effect of an Intrusion on Reservoir Quality in the Triassic Sherwood Sandstone Group, Northern Ireland

Contact Diagenesis: The Effect of an Intrusion on Reservoir Quality in the Triassic Sherwood Sandstone Group, Northern Ireland

Fault distribution and timing in the Central Irish Sea Basin

Abstract Well data analysis and the interpretation of 2D and 3D seismic reflection data provide valuable insights into the distribution and timing of fault activity within the Central Irish Sea Basin (CISB). Structural and stratigraphic relationships have been used to constrain the timing of fault movements and to interpret the mapped fault patterns in terms of the tectonic evolution of the area. Four main fault trends are identified at the Top Lower Triassic Sherwood Sandstone Group level: I, a NE-SW fault trend that parallels the basin-bounding faults and is believed to be of Mesozoic age; II, a pervasive system of northsouth-trending faults that cross-cut the earlier NE-SW-trending faults, which manifests evidence of later Mesozoic extension followed by post-Oligocene transpressional fault reactivation; III, a NNE-SSW-trending, steeply dipping, fault set; IV, a WNW-ESE-trending conjugate extensional set that formed perpendicular to the NNE-SSW-trending transpressional faults during Late Tertiary dextral shearing. Early Tertiary axial centred basin inversion and regional exhumation have resulted in the elevation of the Sherwood Sandstone reservoir to shallow structural levels within the basin. Continued fault reactivation into Late Tertiary time has resulted in the compartmentalization of mapped structural closures and suggests that trap integrity is a major exploration risk factor in the CISB.

Evaluation of small-loop transient electromagnetic soundings to locate the Sherwood Sandstone aquifer and confining formations at well sites in the Vale of York, England

Shallow-depth transient electromagnetic (TEM) soundings have been performed at six borehole locations in an intensively farmed area in northern England to evaluate their usefulness in mapping geological formations under a thick cover of glacial drift deposits. The regionally important Triassic Sherwood Sandstone (SS) Group aquifer is directly overlain by Triassic Mercia Mudstone in the eastern two-thirds of the study area and by drift deposits in the west. Owing to the difficulty of deploying large loops and the overriding need to minimize lateral effects on the depth probes, square transmitter loops of 20, 40 and 50-m side-lengths were deployed in the central-loop configuration with the Geonics EM47 and PROTEM47/57 field equipment. Using a two-stage data interpretation technique, it is found that the effective depth of mapping ranged from about 8 to 150 m at most sounding locations. Comparison of inversion models with borehole data shows that the SS and some overlying sedimentary rocks may be discerned from the TEM soundings; there is a consistent pattern of resistivity distribution within each geological formation at all the borehole sites enabling a realistic identification of the key stratal units. However, a 7–11-m-thick upper layer is found in all the constructed models, which does not correlate with any known formation boundaries, but appears to be justified by comparison with sample dc resistivity soundings at two locations; it would also appear that the earliest time windows (<0.016 ms) are somewhat distorted by the band-limitation operation of the TEM instrumentation. This pilot study demonstrates that the TEM method is a potent tool for stratigraphic mapping in the region, but the upper 5–8 m remains largely inaccessible to the method using state-of-the-art equipment and conventional data processing techniques. It may therefore be necessary to combine TEM and short spread-length ( AB/2≤25 m) dc resistivity depth soundings to accurately map the near-surface in this glacial-covered terrain.

A NEW PERSPECTIVE ON THE ZONATION AND CORRELATION OF BARREN STRATA: AN INTEGRATED HEAVY MINERAL AND PALAEOMAGNETIC STUDY OF THE SHERWOOD SANDSTONE GROUP, EAST IRISH SEA BASIN AND SURROUNDING AREAS

The integration of data from high‐resolution heavy mineral and palaeomagnetic analyses of samples from nine East Irish Sea wells, and from onshore sections at St Bees Head and the North Cheshire Basin, has generated a temporally‐constrained heavy mineral stratigraphy and regional correlation within the Triassic Sherwood Sandstone Group. These results have shown that the combined application of these two techniques offers an effective tool for extending the zonation and correlation of barren strata into uncored wells. The lateral persistence of the heavy mineral zones, and the significant differences in the distributions of the heavy mineral varieties between individual zones, may reflect tectonic events and, to a lesser extent, climatic variations.Palaeomagnetic results indicate that the original magnetisation of the analysed samples was acquired as a detrital remanent magnetisation and that the characteristic remanent magnetisation in the red beds was acquired during and shortly after deposition. Magnetopolarities have been interpolated using the available data from which a composite reference section has been constructed.All well sequences exhibit a regionally persistent three‐fold zonation (Zones A, B and C, locally with subzones), defined by systematic changes in characteristic heavy mineral suites. Using a common sample set, the independently erected heavy mineral stratigraphy was tied to specific magnetostratigraphic Chrons. Heavy mineral Zone A coincides with a period of dominantly normal polarity, fixed by a 244 Ma Chron of Middle Scythian age. A change in polarity from normal to reversed at &gt; 243 Ma coincides with the boundary of Zones A/B. Zone B is Middle to Upper Scythian and shows a dominantly reversed polarity. A well‐defined Chron of 242.7 Ma has been detected close to the boundary of Zones B/C. Zone C is Anisian in age and is characterised by a dominantly normal polarity.

Fluid flow in faults: a study of fault hydrogeology in Triassic sandstone and Ordovician volcaniclastic rocks at Sellafield, north-west England

SUMMARY The structure and hydrogeological properties of subsurface faults in the Triassic Sherwood Sandstone Group and Ordovician Borrowdale Volcanic Group at Sellafield in west Cumbria were investigated in order to develop conceptual and numerical models for input to groundwater flow modelling at the possible site of an underground radioactive waste repository. Eighteen borehole intersections of faults were studied using core, borehole imagery and hydraulic test data (single and multi-well testing). The results were integrated with data and interpretations from seismic surveys and field studies in order to prepare conceptualizations of fault architecture and hydrogeology, suitable for taking forward into numerical modelling. Hydraulic test data indicate that fault zones in the Sherwood Sandstone Group are slightly more permeable than the host rock, and geological observations suggest that flow may be focused in the fault damage zones rather than in the faultrocks, which tend to act as flow baffles. Where present in sufficient numbers, granulation seams appear to be flow inhibitors. In the volcanic rock, hydraulic test data indicate that fault zones have little permeability contrast with the host rock. However, geological observations suggest that minor reactivation may have locally disrupted the baffling effect of faultrocks, thereby creating ‘leak points’. Overall, the study suggests that ground-water flow in faults at Sellafield is highly heterogeneous at the borehole scale, but at larger scales the hydraulic behaviour of faults is more homogeneous.

SEALING CAPACITY OF THE MERCIA MUDSTONE GROUP IN THE EAST IRISH SEA BASIN: IMPLICATIONS FOR PETROLEUM EXPLORATION

In the East Irish Sea Basin of the UK Continental Shelf, hydrocarbon accumulations in the Triassic Sherwood Sandstone Group (SSG) are sealed by the Triassic Mercia Mudstone Group (MMG). The MMG comprises alternating mud‐ and halite‐dominated intervals. Where halites overlie the SSG (e.g. in the Morecambe and Millom gasfields), seal integrity is excellent; however, where the SSG is overlain by mudstones of the MMG (e.g. in the Douglas and Lennox oilfields), seal integrity is variable and is controlled by the capillary entry pressure of hydrocarbons into the pore network of the mudstones.Mercury porosimetry was used to characterise the pore network of the Mercia Mudstone samples from several localities in the East Irish Sea Basin. This data was used to assess seal quality, and thus to calculate theoretical hydrocarbon column heights supportable for a given seal. It appears that the sealing quality of the MMG mudstones and siltstones varies principally with the nature of the pore‐throat network. Fracture filling and pore‐plugging by bitumen representing an earlier oil charge may also have improved seal integrity by blocking pore throats.

In this issue…

WE LEAD THIS ISSUE with an interesting assessment (by J. K. Seedhouse and A. Racey of British Gas) of the sealing quality of the Triassic Mercia Mudstone Group in the East Irish Sea Basin. Hydrocarbons here are reservoired in the fluvial and aeolian sandstones of the Lower Triassic Sherwood Sandstone Group, a syn‐rift succession up to about one km thick. Both natural gas and crude oil are sourced from underlying Namurian shales, and are produced commercially at a number of relatively small fields located offshore NW England. A regional top‐seal is provided by the post‐rift Mercia Mudstones; however, the quality of this seal is very variable across the basin. In terms of general stratigraphy, the Mercia Mudstone Group is composed of alternating halite‐and mudstone‐rich units, which together reach a maximum thickness of over 3,000 m. Where a halite‐dominated unit directly overlies the reservoir sandstones, a very efficient seal is formed. However, halite‐rich units are limited in their areal distribution; where they are absent, mudstone‐dominated intervals may overlie the reservoir section, providing a seal of very variable quality. This paper reports on the factors which control the variation in seal integrity of the mudstone‐dominated units.The authors make use of downhole pressure data from a number of wells to give a first indication of Mercia Mudstone Group seal integrity. Pore pressures at the top of the oil column are seen to increase markedly, as would be expected, if the overlying sealing unit is halite‐dominated. The pressure build‐up is more variable where mudstones overlie the reservoir interval. In a second stage of the investigation, cores of the sealing interval from two wells and two onshore boreholes were logged in detail. Mercury‐injection porosimetry was used to assess the pore framework of selected core‐plug samples. The results indicate that variations in capillary entry pressures are the most important control on seal integrity in the mudstone‐dominated units. The authors also briefly assess other controls, including the presence of fractures in the seal unit, variable hydrocarbon saturation in the mudstones' pores, and variations in seal thickness across the basin.

The Lennox oil and gas Field

Abstract The Lennox oil and gas Field is located in Block 110/15 of the East Irish Sea, approximately 10 km west of Southport, Lancashire. The field was discovered in 1992 with well 110/15-6 drilled by the P791 BHP group. The Lennox structure is a rollover anticline in Permo-Triassic sediments formed in the hanging wall of the Formby Point Fault. The reservoir comprises the Triassic Sherwood Sandstone Group, and is characterized by excellent permeability aeolian and fluvial sandstones. The structure is sealed by the overlying shales and evaporites of the Mercia Mudstone Group. To date four wells have been drilled on the Lennox Field and have delineated a free gas cap in communication with a constant thickness oil rim. The hydrocarbons have been sourced from Namurian Holywell shales and reflect complex migration into the Lennox structure. Reservoir studies indicate likely hydrocarbons-in-place of 463 BCF of gas and 218 MMBO of oil. Development approval for the Lennox Field was granted in 1993 with production due to commence in late 1995 from a series of horizontal oil producers. Gas produced during the early years will be recycled and re-injected into Lennox via a crestal gas well to help maximize the oil production. During the later years, production will switch to sales gas. Lennox will be developed as an integral part of BHP’s Liverpool Bay Development.

The geology and hydrocarbon prospectivity of the North Channel Basin

Abstract The geology and hydrocarbon prospectivity of the offshore North Channel has been evaluated, integrating data from a number of onshore wells with seismic, gravity, magnetic and onshore outcrop studies. Interpretation of these data suggest that the North Channel can be divided into two sub-basins, separated by the offshore extension of the Southern Uplands Fault Zone (SUFZ). The narrow, southern, Portpatrick sub-basin has a NNW-SSE trend and consists of a series of simple tilted fault-blocks which dip east, towards the basin-bounding Portpatrick Fault. The northern sub-basin widens considerably into the Larne-Firth of Clyde basin complex, which has a predominantly Caledonide (NE-SW) orientation and is an extension of the Scottish Midland Valley. This sub-basin is characterised by westerly dipping fault blocks, the reversal in basin polarity apparently accommodated along the SUFZ. The primary reservoir interval is expected to occur in the Lower Triassic Sherwood Sandstone Group, although secondary targets may exist in the Permian and Carboniferous. Shallow coring and seismic facies analysis suggests that the Mercia Mudstone Group subcrops Quaternary-Recent deposits at sea bed and overlies the Sherwood Sandstone. Halites and shales within the Mercia Mudstone Group should provide an effective regional top-seal, except in areas where the Sherwood Sandstone is close to the surface. Large tilted-fault block and rollover anticline traps have been identified on seismic data, although the Carboniferous source rock potential and burial history are poorly understood and exploration risk is high. Regional outcrop patterns and the compaction state of the Mesozoic strata, together with vitrinite measurements suggest the basin was uplifted in the early Tertiary. In common with the East Irish Sea, maturity was probably reached prior to the end of the Cretaceous. This implies that source rocks within the basin will not currently be generating hydrocarbons, having effectively been ‘switched’ off by Tertiary uplift. Charge mechanics are uncertain but must involve early pre-uplift migration or remigration.

The development of the North Morecambe gas field, East Irish Sea Basin, UK

The North Morecambe Gas Field in the East Irish Sea Basin was discovered by well 110/2-3 in 1976 and contains reserves of over 1TCF. The structure is fault-closed on three sides, and dip-closed to the north. Development is by 10 conventionally deviated high-angle wells, from a not-normally-manned platform, through a dedicated pipeline to a new terminal at Barrow. The Triassic Sherwood Sandstone Group reservoir is composed of sandstones deposited in a semi-arid,fluvial and aeolian setting. Thin, aeolian sandstones dominate flow into the well-bore. Platy illite reduces the permeability by 2-3 orders of magnitude in the lower illite-affected zone of the reservoir. RFT measurements from the first development well proved that the free water level was 25 ft higher than expected, giving a maximum gas column of 975 ft. Re-mapping after drilling has shown that 56% of the GIIP is contained in the high permeability illite-free zone.

The chemistry and origin of groundwater in Triassic sandstone and Quaternary deposits, northwest England and some UK comparisons

The Triassic Sherwood Sandstone Group is a major UK aquifer. In this study the groundwater geochemistry of a 60 km 2 area has been investigated to improve the understanding of the hydrogeology of the area and to determine the origin of groundwater. Archived analyses of groundwater samples were interpreted to determine the dominant geochemical processes and subsequently samples were taken from selected sites for stable and radio-isotope measurements. Near areas of direct recharge of the Sherwood Sandstone, groundwaters are unsaturated with respect to carbonate minerals. Sulphate and chloride are contributed to the groundwater in the Sherwood Sandstone from the overlying Quaternary glacial drift deposits and Mercia Mudstone. The groundwater chemistry and isotopes of sulphate indicate that the source of sulphate is gypsum in the Triassic sequence and the Triassic-derived glacial deposits. A significant proportion of the calcium in groundwater is shown to be exchanged for sodium in the aquifer matrix. However, in the more mineralised waters, carbonate minerals are precipitated because of contributions to the groundwater of calcium from dissolution of gypsum. Up piezometric gradient, groundwaters are modem and have relatively high 14C and tritium contents. The oldest water which was sampled for 14C is estimated to be about 4000 years older than the modern waters which were sampled from near recharge zones. The groundwater regime in the study area may be compared and contrasted with Triassic sandstone aquifers elsewhere in the UK. Incongruent carbonate dissolution occurs, as described in the Triassic of the East Midlands, the Lower Mersey Basin and southwest England. Exchange reactions are important in determining the groundwater chemistry as found elsewhere in the Triassic of northwest England, and in contrast to the East Midlands and southwest England.

Hydrocarbon prospectivity of the Central Irish Sea Basin with reference to Block 42/12, offshore Ireland

Abstract The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration &amp; Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized. Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion. Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data. The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.

Diagenesis of Triassic Sherwood Sandstone Group rocks, Preston, Lancashire, UK: a possible evaporitic cement precursor to secondary porosity?

Abstract Boreholes at Kirkham [SD 4324 3247] and Weeton Camp [SD 3888 3603] in Lancashire penetrate Triassic sandstones of the Sherwood Sandstone Group that occur near the eastern margins of the Manx-Furness (East Irish Sea) Basin. Petrological analysis of sandstone samples from cores was undertaken to determine diagenetic characteristics. Some very porous sandstones with porosities of the order of 25–30% have cement dissolution textures indicating that earlier extensive cements have largely been removed by aggressive pore fluids, probably during the Triassic. These early cements were calcite and non-ferroan dolomite, and possibly later evaporitic cements (anhydrite, gypsum, halite) introduced during the period of deposition of the Mercia Mudstones. Non-ferroan dolomite post-dates and locally replaces early calcite. There is minor ferroan dolomite, late calcite and pyrite. Extensive early quartz overgrowths and cements have developed in some quartz arenites. Porosity values are about 10 to 13%, but permeabilities are likely to be low. These quartzcemented quartz arenite units may correlate between the boreholes. The clay mineral assemblages of the sandstones of each borehole are significantly different. The Kirkham sequence is characterized by illite with Fe-rich chlorite and minor smectite, whereas the Weeton Camp sequence is dominated by kaolinite with minor illite and smectite. These differences are attributed to different pore fluid histories, possibly structurally controlled.

Identification and significance of aeolian deposits within the dominantly fluvial Sherwood Sandstone Group of the East Irish Sea Basin UK

Abstract The Triassic Sherwood Sandstone Group sediments of the East Irish Sea Basin are over 4000 ft thick and comprise medium- to coarse-grained sandstones and rare thin mudstones. Facies models developed during production drilling on the South Morecambe Field show that deposition occurred in a braided fluvial setting with minor aeolian episodes. The major facies associations present are: (A) major channel fill; (B) ephemeral channel fill; (C) non-channelized sheetflood deposits; (D) and (E) non-reservoir fines (abandonment and playa respectively); (F) aeolian dune and sand-sheet. Volumetrically, the major channel (A) and sheetflood (C) facies dominate, and the alternation of these facies associations is correlatable across the field. The aeolian sandstones form units varying in thickness from a few grain diameters up to 3 m thick. The sand-sheet and dune deposits can be correlated over considerable distances and show pressure communication on RFT logs. Although the aeolian sandstones comprise only 5–10% of the reservoir they have very high porosities and permeabilities (up to 30% and 10 darcies) and make disproportionate contributions to flow into the wellbore. The depositional criteria used to differentiate between aeolian and fluvial deposition in the South Morecambe Field are pinstripe lamination, good sorting and lack of rounded clay clasts, but these criteria are not definitive. As a result of the dissolution of an early diagenetic cement, aeolian sandstones have very high porosities compared to fluvial sandstones. This high porosity is reflected in high sonic transit times and allows aeolian sandstones to be identified tentatively in uncored wells by use of sonic logs.

Controls on reservoir quality in the Triassic Sherwood Sandstone of the Irish Sea

The Lower Triassic Sherwood Sandstone Group constitutes the principal reservoir target in the East Irish Sea Basin. Interpretation of sedimentary facies allows recognition of major low sinuosity braided fluvial channel, minor ephemeral fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and playa lake deposits that are discussed in terms of facies associations. Reservoir quality is highly variable, with porosity and permeability data revealing significant differences between fluvial, sheetflood and aeolian facies associations. The latter, in particular, preserve very high reservoir quality reflecting an open grain fabric and paucity of blocky cements. Variations in porosity between the facies associations relate primarily to the distribution of quartz cement which is significantly more abundant in fluvial channel sandstones than other facies. Authigenic illite, while severely limiting permeabilities in some reservoir zones, is not facies specific and does not contribute to the observed variations. It is suggested that the disparity in the abundance of quartz cement between facies associations relates to two distinct grain types that correlate broadly with fluvial and aeolian associations, respectively, and probably derive from different provenances. The evolution of the basin, in controlling the distribution of the major facies, influenced the distribution of the two grain populations and, thus, also the reservoir quality of the sandstones.

The geology of the North Morecambe Gas Field, East Irish Sea Basin

The North Morecambe Gas Field lies in the East Irish Sea Basin close to the producing South Morecambe Field, from which it is separated by a very deep WSW–ENE graben. It is a shallow structure, crest slightly above −3000 ft, with bounding faults to the south and east, but dip-closed to the north and west. The GWC is at −3950 ft. The Triassic Sherwood Sandstone Group reservoir was deposited under semi-arid continental conditions and comprises a complex interplay of channel-fill sandstones, non-channelized sheetflood sandstones and high-permeability aeolian and reworked aeolian sandstones. The vertical alternation of these facies forms the basis for differentiating nine facies-defined reservoir units. Palaeocurrent analysis indicates flow from NE to SW. A complex diagenetic history is recognized, with several phases of dolomite and quartz cementation. Differential compaction is also a significant control on the disposition of reservoir properties. The greatest control on permeability (but not porosity) is platy illite which precipitated beneath a palaeo-gas/water contact at an early stage in the growth of the structure. This gives rise to a diagenetic layering of the reservoir into a high-permeability illite-free layer and a deeper, low-permeability, illite-affected layer. The Mercia Mudstone Group which provides the seal, comprises four mudstone–halite cycles. Thickening of these cycles on the north flank of the structure indicates very early development of the northerly component of dip. The imposition of northerly dip entirely pre-dates the growth of platy illite, and this conclusion has a major impact on the manner in which the surface which separates the illite layers has been mapped on the north flank. The gas, sourced from Westphalian/Namurian sediments, is enriched in CO 2 compared with South Morecambe. This, together with contrasts in vertical cement distribution between the two fields, indicates that the breaching and loss of the first gas charge which is believed to have occurred on South Morecambe did not occur on North Morecambe. Furthermore, although separated from South Morecambe by only a narrow graben, the aquifer pressure is 140 psia less than that of South Morecambe, indicating that the major basin faults are full seals. Proved reserves are 1.08 TCF. The field is currently being developed for first gas in 1994.

An investigation of metal enrichment in Triassic Sandstones and porewaters below an effluent spreading site, West Midlands, England

Abstract Soil and sandstone samples were taken from below a sewage effluent spreading site at Whittington Hall, near Stourbridge. The sandstones belong to the Triassic Sherwood Sandstone Group, whole rock samples were analysed for major and trace elements, and porewaters (extracted by high speed centrifuge) for trace metals. The elements found to be enriched in the sediments, at shallow depths (generally not greater than 1 m) were, in order, Zn, Pb, Cu, Cr, Ni and Sr. Systematic decreases with depth were not recorded for the elements Mn, V, Rb, Y and Zr. Retention of the metals is believed to be a combination of filtration of particulate matter and reaction with the sediment, particularly the oxyhydroxide material and possibly smectites. The observed metal enrichment corresponds with the effluent composition and in the case of Zn there would appear to be an approximate balance between the total input into the system and the amount retained in near-surface sediment. The borehole samples were frozen on site in order to suppress microbial changes in the porewaters. Metal concentrations in the porewaters were not, in general, affected but significant differences were noted for Fe, Mn and Ca. At depth in the unsaturated and saturated zones porewater concentrations approach the ‘natural’ background concentration. These decreases in concentrations take place below the zone of sediment-porewater reaction based on whole-rock analyses and are therefore attributed to progressive dilution of the porewater and groundwater.

Thin section and S.E.M. textural criteria for the recognition of cement‐dissolution porosity in sandstones

ABSTRACTCement‐porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high free‐surface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small ‘v’‐shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore‐filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface.

Reservoir Geology of the Morecambe Field

Summary The Morecambe Gas Field, located in the Irish Sea Basin, came on stream on 1 January 1985 and is to be used as a seasonal storage facility. Structurally the field is a northwest-southeast trending faulted anticline divided into two parts by a narrow southwest-northeast graben system. Gas is produced from the upper part of the Lower Triassic Sherwood Sandstone Group. The reservoir sandstones were deposited predominantly by the action of low sinuosity braided streams during periods of high stage flow. Facies associations recognised from core and wireline log data include: Channel Sandstones, Secondary and Ephemeral Channel Sandstones, Sheetflood Sandstones, Floodplain fine member and Channel abandonment deposits. Reservoir quality is dependent on grain size, shape, sorting, facies control and the distribution and abundance of authigenic cements. Quartz and dolomite cements are a major factor in causing porosity reduction, whilst authigenic illite controls effective permeability. Platy illite only occurs below an identifiable datum believed to represent a ‘paleo’ gas-water contact. Below the illite datum permeability is impaired by up to one hundred fold. A change in illite crystal morphology in the lower parts of the reservoir results in a basal zone which exhibits improved quality. The main producing horizons are stacked sandbodies within the Keuper Waterstones and Keuper Sandstone Frodsham Member. On the basis of facies modelling a three dimensional reservoir model has been developed for simulation purposes. A simplified three layer model which incorporates the influence of authigenic illite has also been erected and will control future field development. A phased development programme is planned for the field over the next ten years. The first stage is currently underway, utilising slant drilling technology to economically drain the reservoir. Maximum gas production from the field will be 1200 million cubic feet per day.

Patterns of diagenesis in the Sherwood Sandstone Group (Triassic), United Kingdom

AbstractThe Triassic Sherwood Sandstone Group comprises a complex of continental red beds deposited by a major fluvial system flowing dominantly down a northerly inclined palaeoslope. Sedimentation took place in several distinct, tectonically active basins with varying maximum burial depths, ranging from shallow (&lt;1 km) to deep (&gt;3 km). Despite proximal to distal variations in stream type, a distinct suite of early diagenetic events can be recognized throughout all the depositional basins, which is related to the depositional environment. These events are best preserved in those basins with shallow burial histories, and show many similarities to the processes recorded from modern red beds of the Sonoran Desert, Baja California, although a more advanced grade of diagenesis has been reached in the Sherwood Sandstone. On the margins of the Irish Sea Basin in Cumbria, where burial was shallow, these early diagenetic textures are well preserved. The detrital grains underwent changes aimed at reaching equilibrium with the near-surface chemical environment. Unstable silicates were dissolved or replaced and the released ions were capable of precipitating authigenic phases, typically mixed-layer illite-smectite, K-feldspar, non-ferroan carbonates and hematite. Lateral variations in the early diagnetic assemblages reflect chemical and spatiotemporal changes ofinterbasin depositional and diagenetic environments. Deeply buried equivalents in the Irish Sea reached a maximum burial depth in excess of 3 km towards the end of the Mesozoic. Superimposed on the early diagenetic fabric are a series of depth-related changes. In the absence of early cements, compaction reduced porosity to low levels. Mixed-layer illite-smectites were converted to highly crystalline illites with low Fe and Mg contents. Early framework-preserving non-ferroan carbonates were extensively dissolved, generating widespread secondary porosity. Late pore-filling carbonate cements comprise ferroan dolomites and ankerites with compositions up to Ca(Ca0·02Mg0·43Fe0·53Mn0·02). Following late Mesozoic burial, inversion of the Triassic basins resulted in the re-exposure of basin margin sequences and shallow burial basins. Modern groundwaters are typically very dilute, with essentially neutral pH, and are capable of dissolving carbonate, sulphate and halite cements. In the Wessex Basin, however, the Triassic sandstones of the South Devon aquifer have been extensively modified by the percolation of post-inversion acidic groundwaters. These low-pH waters have resulted in thein situbreakdown of feldspar to produce abundant authigenic kandite, widespread dissolution of carbonate cements and removal of early-formed iron oxide cements. In contrast, the concealed, deeply buried eastern margin of the Wessex Basin exhibits a diagenetic evolution resultant of deep burial and has no development of authigenic kandite.