Jon Gluyas

Duration of quartz cementation in sandstones, North Sea and Haltenbanken Basins

Abstract Evidence from fluid inclusions, petrography and burial history modelling is used to estimate the duration of quartz cementation in sandstones. The homogenization temperatures of primary fluid inclusions in quartz cements from samples from the Brent Group (North Sea) and Haltenbanken (offshore Norway) show variations (4σ) of 17.4 and 20.2°C. These are interpreted to reflect durations of less than approximately 9 and 17 Ma. Low variances are general features of such data sets and suggest that quartz cementation usually takes place as events with durations of the order of 10 Ma or possibly less. Confining the age of quartz cement growth using a combination of geological, petrographical and radiometric dating methods that are independent of fluid inclusions also suggests that quartz cements grow over periods substantially shorter than the age of the sandstone. It is not easy to reconcile this deduction with the suggestion that burial itself causes cementation.

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.

Model calculations of loss of porosity in sandstones as a result of compaction and quartz cementation

Abstract A simple mathematical model has been developed for the precipitation of quartz cements along a dipping sandstone bed as a result of the cooling of a quartz-saturated formation water. The model assumes that the sand suffers a substantial loss of porosity through compaction before quartz cementation occurs, and that the cementation takes the form of an event of 10 Ma duration. The results suggest that flow-rates of metres to tens of metres per year need to be sustained over the 10 Ma period for a significant amount of quartz cement to precipitate. Such flow-rates are generally thought to be characteristic only of artesian flow.

The filling and emptying of the Ula Oilfield: fluid inclusion constraints

Abstract The Norwegian sector Ula Field contains 430 million barrels of oil reserves trapped in a dip-closed Upper Jurassic sandstone reservoir and has multiple oil-water contacts (OWCs). Historically, the depth to the contacts has been the subject of some debate and development drilling has now proven the existence of at least six. The precise nature of some of these contacts remains uncertain because all were discovered after production start-up and determination of their origin is critical to future field development. The relationship of oil inclusion abundances to OWCs and rock properties (porosity, permeability) has been evaluated (a) to gain an insight into the filling history and mechanisms, and (b) to attempt to define pre-production OWCs. Oil inclusion abundances decrease with depth and towards present OWCs in Ula, but oil inclusions also occur below them. They indicate that Ula filled from the crest down, and that the filling was synchronous with cementation to at least 3500 metres subsea. Below this depth, cementation appears to have reduced permeability to such an extent prior to charge that the resulting filling geometry was irregular. Local abundance peaks within the oil column correlate with the tops of coarsening-upward cycles. Rare oil inclusions below the oil legs may represent either migration pathways updip from the source or invasion downdip from the main oil leg along higher permeability zones. Oil inclusions persist to greater depths in the east and south-east of the field which indicates that oil charged the field from these directions. A number of models explaining the variability of the OWCs have been ruled out by this work, thereby reducing the uncertainty in field development planning.

Element mobility during diagenesis: sulphate cementation of Rotliegend sandstones, Southern North Sea

Abstract Several wells in the Amethyst Gas field of the North Seas Southern Basin are poor producers and have been since they were drilled. The lack of gas flow from these wells is due to pervasive cementation of the Rotliegend sandstone reservoir by either anhydrite and/or barite. Both minerals precipitated late in the diagenetic history of the sandstones. Such cements from up to 20% of the total rock. Isotopic and geochemical evidence indicate that the source of the elements for these sulphate cements was outside the Rotliegend sandstone. The sulphur and oxygen isotope data for the anhydrite and barite are unlike those which could have precipitated in Lower Permian times from an evaporating marine basin. Nor are they like any sulphur and oxygen isotope ratios that might be derived from a provenance area for the Rotliegend Sandstone. However the sulphur and oxygen data are identical to anhydrite in the overlying Upper Permian, Zechstein carbonate and evaporate sequences. The Rotliegend sandstones did not contain much barium at deposition. However, unlike the sulphate, the Zechstein sequences are a highly unlikely source for barium, given the highly insoluble nature of barite. A more probable source for barium is the underlying Carboniferous Coal Measures. Precipitation of barite and anhydrite occurred during basin inversion. Specifically the cements are most abundant in gas wells that lie close to inverted normal faults that cut both the Permian and Carboniferous intervals. Both the timing and distribution of these cements is taken to indicate that faulting allowed, or indeed promoted, mixing of sulphate-rich and barium-rich formation waters derived from the Zechstein and Carboniferous, respectively.

Feldspar dissolution, authigenic clays, and quartz cements in open and closed sandstone geochemical systems during diagenesis: Typical examples from two sags in Bohai Bay Basin, East China

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars. We examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality. One of the reservoirs is the Eocene fan delta sandstone buried 2.5–4.0 km (1.5–2.5 mi) below sea level (BSL) in the Gaoliu (GL) area of the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km (1–2.8 mi) BSL in the Shengtuo (ST) area of the Dongying sag. Both sandstones consist mainly of lithic arkoses and feldspathic litharenites, and have secondary porosity formed by dissolution of feldspars. In the GL sandstones, the absolute amounts of authigenic clays and quartz cements (generally 125°C [257°F]). The low abundance of authigenic clays and quartz cements, and low pore-water salinity indicate that much of the , , and released from leached K-feldspars were exported from the GL sandstone system. And the extensive feldspar dissolution enhanced much porosity and permeability. In contrast, the ST sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays (kaolinite and illite) and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (3.1 km [2 mi] BSL) where temperature exceeds 125°C (257°F). The presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the ST sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars. Secondary porosity in thin sections can be up to 3%, but little porosity ( The diagenetic difference between the GL and the ST sandstones can be interpreted by assessing pore-water evolution in these two areas. The current pore waters with low salinity and negative hydrogen isotopic compositions in the GL sandstone system indicate the significant impact of meteoric water, whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the ST sandstone system indicate little trace of meteoric water. Access of meteoric freshwater to the GL area probably occurred during the late Oligocene to Neogene through widely developed faults in the Paleogene and Neogene strata. The low-salinity water could have been responsible for flushing of solutes derived from feldspar dissolution. As such, diagenesis in the GL sandstones is considered to have occurred in an open geochemical system, whereas with limited faults and high water salinity, the ST sandstones acted as a closed geochemical system where precipitation of kaolinite, illite, and quartz cements occurred following dissolution of feldspars.

Diagenesis of the Rotliegend Sandstone: the answer ain't blowin' in the wind

Abstract Aeolian, Rotliegend sandstones from the Village Fields area of the southern North Sea were cemented by illite and kaolinite during the latter part of the Jurassic coincident with a phase of active rifting in the basin. The components for the formation of the clays could not have been supplied from the original detritus, which was rich in quartz and poor in feldspar and lithic fragments. Instead, large quantities of aluminium, potassium and silicon must have been supplied to the sandstone from elsewhere. The source for the potassium is likely to have been either the laterally equivalent Silverpit Claystone Formation or the overlying Zechstein evaporite sequences which were plumbed into the Rotliegend during the late Jurassic rifting. The source of silica and alumina has not been uniquely identified.

The late field life of the East Midlands Petroleum Province; a new geothermal prospect?

Modification of existing oilfield infrastructure could deliver a cost-effective way to extend the economic life of depleted onshore oilfields. Naturally warm connate and injection water contained within these fields could be initially co-produced with remaining oil reserves and used to deliver clean, cheap, non-intermittent heating. The East Midlands Petroleum Province contains over 30 fields with a production history spanning 95 years, and we have chosen to examine the Welton field in detail. Well data for the Welton field have been analysed to ascertain extractable heat within both oil- and non-oil-bearing (water-bearing) strata within the field. Production rates were calculated to be 728 m3 d−1 oil and 854 m3 d−1 water. These values also include productivity of intervening largely water-bearing intervals. Target formation temperature at 1500 m was determined to be 52.5°C, allowing an extractable heat energy calculation to be undertaken for a range of temperature differentials. For a 30°C depletion in temperature, 1.6 MWt extractable heat is available within the Welton field alone. This equates to 14040 MWh of heat energy available for consumption by the domestic market or within commercial greenhouses.

Simulation of muon radiography for monitoring CO2 stored in a geological reservoir

Current methods of monitoring subsurface CO2, such as repeat seismic surveys, are episodic and require highly skilled personnel to acquire the data. Simulations based on simplified models have previously shown that muon radiography could be automated to continuously monitor CO2 injection and migration, in addition to reducing the overall cost of monitoring. In this paper, we present a simulation of the monitoring of CO2 plume evolution in a geological reservoir using muon radiography. The stratigraphy in the vicinity of a nominal test facility is modelled using geological data, and a numerical fluid flow model is used to describe the time evolution of the CO2 plume. A planar detection region with a surface area of 1000 m2 is considered, at a vertical depth of 776 m below the seabed. We find that 1 year of constant CO2 injection leads to changes in the column density of ≲1%, and that the CO2 plume is already resolvable with an exposure time of less than 50 days.

An assessment of the footprint and carrying capacity of oil and gas well sites: The implications for limiting hydrocarbon reserves

We estimate the likely physical footprint of well pads if shale gas or oil developments were to go forward in Europe and used these estimates to understand their impact upon existing infrastructure (e.g. roads, buildings), the carrying capacity of the environment, and how the proportion of extractable resources maybe limited. Using visual imagery, we calculate the average conventional well site footprints to be 10,800m2 in the UK, 44,600m2 in The Netherlands and 3000m2 in Poland. The average area per well is 541m2/well in the UK, 6370m2/well in The Netherlands, and 2870m2/well in Poland. Average access road lengths are 230m in the UK, 310m in The Netherlands and 250m in Poland. To assess the carrying capacity of the land surface, well pads of the average footprint, with recommended setbacks, were placed randomly into the licensed blocks covering the Bowland Shale, UK. The extent to which they interacted or disrupted existing infrastructure was then assessed. For the UK, the direct footprint would have a 33% probability of interacting with immovable infrastructure, but this would rise to 73% if a 152m setback was used, and 91% for a 609m setback. The minimum setbacks from a currently producing well in the UK were calculated to be 21m and 46m from a non-residential and residential property respectively, with mean setbacks of 329m and 447m, respectively. When the surface and sub-surface footprints were considered, the carrying capacity within the licensed blocks was between 5 and 42%, with a mean of 26%. Using previously predicted technically recoverable reserves of 8.5×1011m3 for the Bowland Basin and a recovery factor of 26%, the likely maximum accessible gas reserves would be limited by the surface carrying capacity to 2.21×1011m3.