Brent Sandstone Research Articles

Conditions and timing of quartz cementation in Brent reservoirs, Hild Field, North Sea: constraints from fluid inclusions and SIMS oxygen isotope microanalysis

The conditions and timing of quartz cementation in the Brent sandstones in the Hild Field, Norwegian North Sea, were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence (CL) microscopy studies. Samples of the Tarbert and the Ness Formations were investigated in three different wells, presently lying at 3.8–4.3 km subsea and at 140–155°C. Quartz cement is present in all wells. Abundance varies from 3% to 26%, and tends to increase with depth of burial. Two distinct, separate episodes of quartz cementation, referred to as Q1 and Q2, are readily distinguished by CL microscopy. The two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses. The data indicate that Q1 quartz formed in the Early Tertiary (65–35 Ma) at 95–110°C and 2.3–3.0 km burial depth, and that Q2 mainly precipitated in the Oligocene–Miocene (40–10 Ma) at 125–135°C and 3.1–3.7 km. Q1 quartz precipitation follows the onset of overpressuring in the reservoir, while precipitation of Q2 seems to slightly precede and accompany the main phase of oil filling (35–10 Ma). Secondary ion mass spectrometry (SIMS) oxygen isotope microanalyses yield δ 18O values ranging from 17‰ to 25‰, averaging 23±2‰ for Q1 and 19.5±1.5‰ for Q2. Calculated δ 18O values of quartz-forming waters are similar for Q1 and Q2, ranging from 1‰ to 3‰, and are similar to the δ 18O value (≈2‰) of present-day formation water in the reservoir. Quartz cementation in the Brent reservoir of the Hild Field occurred during deep burial, at high temperature, from evolved 18O-rich basinal water most likely derived from the Viking Graben. Two episodes of enhanced quartz precipitation are recorded around 100°C and 130°C. No evidence was found for any significant contribution of 18O-depleted Jurassic meteoric water and/or low temperature precipitation of quartz cement.

Geochemical modelling of late diagenetic processes in the Brent Sandstone, Alwyn South area (East Shetland Basin, North Sea): 1. Estimation of the circulated fluids composition

This study simulates the third diagenetic stage of the Brent Sandstone in the Alwyn South area through modelling of water-rock interactions at temperatures of 120° and 140°C, as determined by the fluid inclusion method. Simulation results are compared to those from petrographic analyses. The major diagenetic events are recognized petrographically, but data on the chemical speciations of the invading fluid are not available. Thus, the geochemical simulations test both seawater and dilute water. A solid solution model is used in order to test the precipitated complex clay phase and its influence on the chemical composition of the reacting fluid. The best agreement with petrographic analyses is from rock interaction with seawater that is depleted in magnesium.

The brent sandstone reservoir (Alwyn-South area, North Sea): Apatite fission track, K-Ar and fluid inclusion analyses

The brent sandstone reservoir (Alwyn-South area, North Sea): Apatite fission track, K-Ar and fluid inclusion analyses

Polymer retention and inaccessible pore volume in North Sea reservoir material

Among the factors that govern the propagation of a polymer slug through porous media are retention and inaccessible pore volume (IPV). These quantities are sensitive to several parameters which often vary in coreflood experiments. Examples of these sensitivities include the core preparation procedure such as core cleaning and saturation restoration. From corefloods using Brent sandstone reservoir material and a Xanthan biopolymer, the retention of polymer is found to be critically dependent on the cleaning procedure adopted. Solvent cleaning with toluene and methanol gives much higher retention than a depolarized kerosene-brine cleaning routine. Solvent cleaned cores became water wet and the retention at residual oil saturation is 30 μg/g. The kerosene-brine washed core material became oil/mixed wet and the retention is very low, 5μg/g. The retention is not sensitive to the high clay content of 7–16% or variations in the amount of clay. The IPV is found to be insensitive to cleaning procedure, oil saturation, temperature and clay content and is determined to 17%.

Burial diagenesis of Brent sandstones: a study of Statfjord, Hutton and Lyell fields

Abstract Brent sandstone compositions vary systematically with increasing depth and temperature in three North Sea fields. At Statfjord field ( c. 2500 m), sandstones are arkoses, lithic arkoses and subarkoses. At Hutton field ( c . 3050 m), sandstones are subarkoses and quartzarenites, whereas at Lyell field ( c . 3500 m), sandstones are mostly quartzarenites. Compositions are transformed by pervasive dissolution of plagioclase at relatively shallow depth, dissolution of K-feldspar at greater depth, and alteration of detrital mica to kaolinite or illite. Quartz cement is a minor constituent at Statfjord, mostly forming early during meteoric water flushing. Quartz cement abundance increases significantly at Hutton and particularly at Lyell, where quartz cement comprises up to 130f rock volume; the increase in quartz cement coincides with dissolution of detrital quartz along stylolites and microstylolites at temperatures greater than 90–100°C as well as the dissolution of feldspar. The dominant clay at Statfjord is kaolinite; at Hutton, illite forms as a pore-lining precipitate and, at Lyell, also by alteration of previously formed kaolinite. The transition from a kaolinite-dominated to an illite-dominated sandstone is probably controlled by several factors, among these the dissolution of K-feldspar, which may be due either to an influx of organic acids from the overlying Heather and Kimmeridge Clay Formations or to thermodynamic instability of the assemblage kaolinite-illite-K-feldspar at temperatures greater than 100°C. Elevated silica activity due to quartz dissolution on styloites and microstylolites may also induce illite formation. Chemical analysis of sandstone samples shows the Brent undergoes systematic loss of sodium and, to a lesser extent, potassium during burial diagenesis. The sodium loss is consistent with observed plagioclase dissolution. The potassium loss indicates that the potassium in illite is derived from the Brent itself, probably from dissolved K-feldspar, and not from an external source such as a basinal brine.

The petrophysical characteristics of the Brent sandstones

Abstract The commonly occurring ranges and the principal controlling factors of the porosity, permeability, capillary characteristics and fluid saturations of the Brent sandstones are presented. Whilst there is insufficient space to discuss these parameters in specific detail, their variability and the degree to which they are influenced by the geological characteristics of the Brent sandstones is reviewed. The paper shows, through the use of examples, that a thorough understanding of the petrophysical characteristics of a reservoir rock requires the integration of several types of data into a single model. The wireline log and laboratory core analyses provide a quantitative input, whilst geological factors such as mineralogy, diagenetic history, and sedimentary fabric provide a most necessary framework within which the pore geometry variability, detected by log and core data, can be examined. An appendix summarizes the principal quantitative techniques involved.

Geologic Aspects of Reservoir Souring--Brent Group Sandstones, North Sea: The Use of Conventional and Laser Extraction Techniques in Sulfur Isotope Studies of Authigenic Pyrite

The Middle Jurassic Brent Group sandstones are prolific oil reservoirs in the North Sea. Recently, a number of Brent wells have experienced increased levels of hydrogen sulfide contamination during production. A study involving the characterization of all iron-bearing minerals to understand the controls of mineralogy on the nature and timing of souring has been undertaken. The potential of iron minerals to scavenge hydrogen sulfide and hence delay/inhibit souring by precipitation of pyrite has been investigated by determining its distribution, quantity, and origin. Authigenic pyrite occurs as disseminated (framboidal, cubic, and octahedral) crystals and as a pore-filling cement. The pyrite has formed throughout the diagenetic history of the sandstones. however, most of the pyrite is considered to be a late cement formed during burial diagenesis. Conventional separation and sulfur isotope analysis of the authigenic pyrite was conducted on samples from oil- and water-bearing sequences to give a bulk signature for all pyrite present. Subsequently, laser sulfur isotope analysis was used to characterize the signature and origin of the different pyrite morphologies present. The enhanced level of sulfur isotope signature characterization can be used to improve the knowledge of the origin and timing of the different pyrite morphologies. This allows closer reconciliationmore » of the isotopic data to the diagenetic history of the Brent sandstones. Calculations have been made using the known quantities of iron in these minerals and the hydrogen sulfide concentrations present. This indicates the effect of sulfide precipitation on the hydrogen sulfide levels remaining in the reservoir.« less

Fluid inclusion and fission track thermochronology from the Brent sandstone reservoir (Alwyn area, North Sea)

Fluid inclusion and fission track thermochronology from the Brent sandstone reservoir (Alwyn area, North Sea)

Diagenesis, Porosity, and Fluid Evolution in the Brent Sandstone: United Kingdom North Sea: ABSTRACT

Diagenesis, Porosity, and Fluid Evolution in the Brent Sandstone: United Kingdom North Sea: ABSTRACT

Comment and Reply on "Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe"

Comment and Reply on "Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe"

Comment and Reply on "Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe"

Comment and Reply on "Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe"

Secondary Migration Routes in the Brent Sandstones of the Viking Graben and East Shetland Basin: Evidence from Oil Residues and Subsurface Pressure Data (1)

The Viking (Graben of the North Sea contains a major deltaic reservoir-the Brent Group. Within the Brent Group, the Etive Formation, a coastal barrier sand, is both areally continuous and has excellent porosity and permeability. It is sandwiched between the fine-grained micaceous sandstones of the Rannoch Formation below and the impermeable mudstones of the Ness Formation above. Consequently, the Etive Formation has acted as the most important regional conduit for secondary migration of Upper Jurassic sourced oils. Oil migration through time has left a heavy residue in the uppermost part of the formation. These residues are aromatic-asphaltic, but otherwise resemble locally reservoired oils. Migration-sensitive biological marker ratios obtained from the residues change wi h distance from source. Secondary migration route mapping, based on the movement of oil by buoyancy in well-defined, isolated pressure compartments, integrated with timing of oil generation, indicates that the Ninian field could be sourced from two areas-Late Cretaceous migration from the southeast in the Viking Graben and Tertiary migration from the west and southwest-explaining some of the contrasting reservoir and oil characteristics of the Ninian and Lyell fields.

Shale Diagenesis in the Bergen High Area, North Sea

AbstractIllite diagenesis in Tertiary and Mesozoic shales in the Bergen High area, northern North Sea, was studied using mineralogic, isotopic, and computerized thermal modeling techniques. The Tertiary shales are dominated by smectite, with lesser amounts of illite, kaolinite, and chlorite. At present burial temperatures of >70°C smectite is absent, and the shales contain abundant lath-shaped illite which yields a mixed-layer illite/smectite (I/S) X-ray powder diffraction (XRD) pattern. Transmission electron microscopy (TEM) indicates that the illite laths increase in abundance and thickness with increasing depth; XRD patterns indicate a progressive increase in the illite component of the I/S. The deepest samples were found to contain long-range ordered (R=3) I/S, which showed platy particle morphology with the TEM. K-Ar ages of most of the <0.1-μm-size illite separates imply that illitization was a relatively brief event affecting a thick sequence of sediments during late Cretaceous to early Paleocene time (65–87 Ma); however, measured ages were affected by trace levels of detrital Ar contamination and do not represent the true age of diagenesis.Several methods of quantifying Ar contamination were used to correct measured ages to obtain a reasonable estimate of the true age of diagenesis. The corrected ages are imprecise due to uncertainties in quantifying the levels of sample contamination, but generally suggest a Paleogene (38–66 Ma) period of illitization. In contrast, simple kinetic models of smectite-illitization suggest much younger ages of diagenesis (0–40 Ma at the Veslefrikk field; 0–60 Ma at the Huldra field). The timing of the diagenesis and the morphologic aspects of the authigenic illite suggest that illite precipitated before late Tertiary compaction and resulted in a decrease in fluid permeability. Low trapping efficiency of early Tertiary sediments, vertical escape of warm fluid from the Brent sandstone, and high heat flow may have promoted illite diagenesis in the shales prior to deep late-Tertiary burial.

Diagenesis and Hydrocarbon Accumulation, Brent Sandstone (Jurassic), Bergen High Area, North Sea

Diagenesis of Brent Group sandstones in the Bergen High area was studied and related to the history of hydrocarbon maturation and pore-fluid environment. Calcite and kaolinite are low-temperature diagenetic precipitates that formed as meteoric pore water entered the Brent at the end of Brent deposition and later, during late Cimmerian erosion. A major period of feldspar dissolution associated with quartz and illite cementation occurred at Huldra field. Precipitation of diagenetic illite in Brent sandstones was closely linked with hydrocarbon maturation in the adjacent Viking graben and accumulation of generated fluids in traps at structural highs. At Huldra field, illite K-Ar ages suggest onset of illite diagenesis in the latest Cretaceous, initial reservoiring of hydroca bons during the latest Paleocene to early Eocene (58 Ma), and filling of the reservoir by early Oligocene (38 Ma). Pore-water character during illite diagenesis, interpreted from oxygen isotope and quartz overgrowth fluid inclusion data, was warmer than predicted by burial history analysis and one and a half to two times seawater salinity. The incursion of saline, isotopically evolved fluids at Huldra suggests replacement of paleometeoric water by basin compaction fluid. Illite K-Ar ages from Veslefrikk field indicate a minimum age of hydrocarbon accumulation at about 30 Ma. Modern pore-water chemistry in the Brent at Veslefrikk is unlike the suggested water chemistry at Huldra and indicates that migrating compaction water did not completely displace older fresh connate water. This sugge ts that hydrocarbons migrated into Veslefrikk as a discrete phase, without great volumes of associated compaction water.

Geochemical Evidence for the History of Diagenesis and Fluid Migration: Brent Sandstone, Heather Field, North Sea

The diagenesis of Brent Group sandstones at Heather Field was studied to reconstruct time-dependent variations in reservoir quality, hydrodynamic history, and oil emplacement. Depositional facies, isotopic and trace-element composition of authigenic minerals, and present-day formation-water chemistry indicate several major changes in porewater chemistry related to both gravitational and compactional flow systems that significantly impacted diagenesis. Early cementation by calcite was related to influx of meteoric water and completely occludes porosity in certain areas of the Field, especially in lower reservoir zones. Geochemical, petrographic, and structural evidence indicate that average calcite precipitated at low to moderate temperature from reducing isotopically-depleted water having high levels of radiogenic Sr (40°–50°C, δ 18O = −4 to −6‰, 87Sr/86Sr > 0·71). A major period of kaolinite precipitation and feldspar dissolution followed calcite cementation. The isotopic composition of pore-filling kaolinite shows Field-wide uniformity (δ 18O average 13·8‰, δD average −53·2‰), suggesting thorough flushing of the reservoir by meteoric water and precipitation at low to moderate temperature (45°–60°C). Tectonic, burial, and thermal histories suggest that meteoric flushing occurred during the late-Cimmerian sea-level low, possibly in response to gravitational flow of meteoric water from exposed parts of the adjacent East Shetlands Platform. Illite and quartz diagenesis post-date kaolinite cementation, with illite K-Ar ages indicating precipitation through much of the Paleogene (55–27 Ma), coincident with migration of hydrocarbons from neighbouring sub-basins of the East Shetlands Basin. Illite stable isotopic data indicate precipitation in a system resulting from partial mixing of trapped meteoric pore-fluids with saline compaction water. The intensity of sandstone diagenesis is influenced by differences in the fluid migration history, content of detrital K-feldspar, and the time of hydrocarbon emplacement and results in spatial and temporal variations in reservoir quality.

Constraining Diagenesis Using Fluid Inclusions in Cements from Petroleum Basins: Examples from North Sea and Middle East: ABSTRACT

Cements in petroleum reservoirs commonly contain inclusions filled with brine, hydrocarbon, or mixtures of both. The physicochemical conditions of cementation as well as the quality of the inclusion oil can be derived from microthermometric and fluorescence microspectrophotometric analysis of fluid inclusions. These data, when integrated with petrographic, geochemical, engineering, and burial history data, can constrain the timing and nature of both diagenetic events and oil migration. Brine and oil-bearing inclusions occur in quartz overgrowths from the Brent Sandstone, Northwest Hutton field, North Sea. Fluorescence analysis indicates that the inclusion oil is similar to the reservoired oil. Inclusion data define the reservoir conditions that existed during the initial stages of oil accumulation and concurrent quartz cementation. The abnormally high pressure of inclusion entrapment suggests that the reservoir was overpressured. Paleo-overpressuring probably helped preserve reservoir porosity by slowing compaction-related loss of porosity in coarse-grained reservoir sands. Brine, oil, and three-phase (brine, oil, and vapor) inclusions occur in burial-related calcite and dolomite cements from a Lower Cretaceous (Thamama) carbonate reservoir in the Middle East. Inclusion data indicate that these cements were precipitated from and simultaneously trapped an invading basinal brine that contained an immiscible component of hydrocarbon. The intersection of brine and oilmore » isochores in pore-throat space allows a unique determination of the time and depth of initial reservoir fill-up. The fill-up appears to coincide with the onset of peak oil generation from the source rocks believed to have supplied these reservoirs.« less

Temporal and Spatial Variation in Diagenesis and Reservoir Quality: Brent Sandstone, Heather Field, North Sea: ABSTRACT

Brent Group sandstones in the Heather field show extreme inter- and intrafacies heterogeneity in reservoir quality as a result of diagenetic variation. Diagenetic patterns varied spatially and temporally as a result of variations in paleofluid chemistry, the time of hydrocarbon accumulation, and detrital grain composition. Important diagenetic cements are poikilotopic calcite, kaolinite, quartz, and illite. Geochemical, petrographic, and structural evidence indicate that calcite precipitated in the Late Jurassic (approximately 150 Ma) at a low temperature (40/degrees/-50/degrees/C), from reducing water of partial meteoric derivation (/delta//sup 18/O water = /minus/4 to /minus/6 /per thousand/ SMOW) that contained highly radiogenic strontium (/sup 87/Sr//sup 86/Sr > 0.71). Calcite distribution was partially controlled by local erosion of the Brent immediately following its deposition. Subsequently, a major period of kaolinite precipitation and feldspar dissolution occurred. Isotopic and tectonic/thermal history data suggest that these events were caused by thorough meteoric flushing (/delta//sup 18/O water = /minus/6 to /minus 8/ /per thousand/ SMOW) during the mid-Cimmerian sea level low (ca. 140 Ma), but not via recharge at the mid-Cimmerian unconformity immediately above the structure. Quartz precipitated as a result of feldspar dissolution, pressure solution, and fluid movement up fault zones over a long period of geologic time. Inmore » the vicinity of major faults, quartz fluid inclusions indicate invasion of hot, saline brines.« less

Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe

Significant amounts of feldspar have been dissolved from Middle Jurassic sandstone oil reservoirs in the North Sea (northwest Europe) during burial diagenesis Sandstones of the Middle Jurassic Brent Group become increasingly quartzose with increasing burial depth. At Statfjord field (2500 m depth), sandstones are arkose to subarkose; at Hutton field (3050 m depth), they are subarkose to quartzarenite, and at Lyell field (3500 m depth), sandstones are typically quartzarenite. In spite of extensive feldspar dissolution, the abundance of secondary porosity due to feldspar dissolution is similar for all fields, averaging 2.9% of the total rock volume. Thus, far more feldspar has been dissolved than is recorded as secondary porosity. The limit on preservation of secondary porosity may be largely the effect of the mechanical strength of the rock. An excessive number of large secondary pores lowers the rock strength below the point at which the rock can withstand overburden stress, thus causing collapse of some of the secondary pores.

Rb-Sr and K-Ar Dating of Clay Diagenesis in Jurassic Sandstone Oil Reservoir, North Sea

Rb-Sr and K-Ar isotopic determinations were combined with scanning electron microscopy (SEM) observations and x-ray diffractometry (XRD) controls to date the diagenetic formation of clay minerals from oil-bearing sandstones of the Middle Jurassic Brent Sandstone in the North Sea. The separated clay fractions often contain detrital components, especially K-feldspars, which are reduced to sizes smaller than 0.4 µm either by intense in-situ alteration or by destructive sample preparation. Direct Rb-Sr and K-Ar isotopic dating of diagenetic illite therefore was not possible. Isotopic data obtained on mixtures of detrital and newly formed components showed, however, that illite formed about 40 to 45 Ma. This value also was obtained directly on clay fractions separated by a gentle method of disaggregation. Illitization is related to emplacement of hydrocarbons and associated waters into the formation. Aqueous and hydrocarbon fluids simultaneously trapped in quartz-overgrowth inclusions indicate late silicification contemporaneous with this fluid migration. Minimum trapping temperatures of at least 110°C for these fluid inclusions are too high for the present burial depth. Hot fluids are assumed to have originated in deep source rocks beneath the Brent reservoirs and migrated upward during the middle Eocene.

Thistle Field Development

Nadir, F.T.;* SPE, BNOC (Development) Ltd. Summary A mathematical reservoir model of the Thistle field was used to explain the rapid pressure decline in some early development wells. This paper describes (1) how evidence was gathered to confirm the hypothesis that the Thistle reservoir is divided into three fault blocks and (2) the changes made to the development plan to arrest and reverse the field's pressure and plan to arrest and reverse the field's pressure and productivity decline. productivity decline. Introduction The Thistle field is located about 125 miles (201 km) northeast of the Shetland Islands (Fig. 1) in the very prolific northern part of the Viking graben. The prolific northern part of the Viking graben. The field, discovered in July 1973 with the completion of Well 211/18-2 is mainly in U.K. Offshore Block 211/18 but extends into Block 211/19. The participants in Block 211/18 are the British Natl. Oil participants in Block 211/18 are the British Natl. Oil Corp. (BNOC), Deminex, Santa Fe, Tricentrol, Burmah Oil, Charterhouse, and Ultramar. Block 211/19 participants are Conoco, Gulf, and BNOC. Blocks 211/18 and 211/19 were awarded, respectively, in the third and fourth rounds of U.K. offshore licenses. BNOC (Development) is operator for the unitized field.Nadir and Hay presented a comprehensive history of Thistle field in which they discussed the initial development plan and its objectives and compared the reservoir thickness and quality found in the first seven development wells with what was predicted. This paper updates the earlier work by predicted. This paper updates the earlier work by emphasizing reservoir engineering studies conducted to pinpoint the reason for the rapid pressure decline and, hence, production decline. It describes modifications to the development plan that allowed water injection wells to be located correctly to repressurize the reservoir and reverse the trend of declining production. Geology The Thistle Field is an easterly dipping Middle Jurassic Brent sandstone reservoir (Fig. 2) with an average net pay thickness of 380 ft (115.8 m). The formation is subdivided into four reservoir sands: D, C, B, and A. Stratigraphy and the reservoir quality were described in detail by Nadir and Hay and updated by Hallett. There have been few changes to the original geological model; the main change is some evidence of erosion between the Sands D and C, less Sand D erosion, and the possibility of syndepositional fault movement. The quality of the sand is slightly better than originally expected. Reservoir Modeling Production from Thistle field started in Feb. 1978, Production from Thistle field started in Feb. 1978, and the eighth development well was completed in Aug. 1978 (Fig. 3). The wellhead pressures of the producing wells in the crestal area of the field were producing wells in the crestal area of the field were declining faster than anticipated, which could have been a result of localized pressure sinks and/or a small or nonexistent aquifer. The completion of Well 08A showed Sand-B pressure to have been depleted by 1,200 psig (8274 kPa), which dispelled any localized pressure sink concepts. The search for a solution started.A black-oil mathematical reservoir model was used to match field performance. The matching parameter was pressure. Wellhead flowing pressures, measured with dead-weight gauges, were converted to bottomhole flowing pressures by Orkiszewski's method and to equivalent grid-block pressures by Peaceman's method. Bottomhole pressure buildup Peaceman's method. Bottomhole pressure buildup surveys were used as spot checks to confirm the reliability of the resultant grid-block pressure values. Repeat formation tester (RFT) pressures identified the different pressure regimes in the various sands at a specific point in time at some distance from producing wells. producing wells. JPT P. 1828