Knut Bjørlykke

Hydrocarbon composition of authigenic inclusions: Application to elucidation of petroleum reservoir filling history

Abstract Geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the Ula Formation in the North Sea Ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir. Using microthermometry and the burial history as a dating tools, it is concluded that the petroleum in the K-feldspar inclusions was present in the more porous and permeable parts of the Ula Formation as early as 45–75 My Bp when the field was at a depth of about 1.0–1.5 km, as compared with the current depth of 3.4 km. This early petroleum, which was trapped as inclusions in authigenic K-feldspar, shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the Ula Formation, which was derived from the Mandal Formation source rock in late Neogene time. Molecular parameters show that the oil in the K-feldspar inclusions is significantly less mature than the crude oil in the present reservoir. The approximate 90°C temperature increase occurring after entrapment of the early petroleum in Kfeldspar (the field is currently at 143°C) appears not to have reset the low maturity signature of the oil in the K-feldspar inclusions. This could suggest that the temperature in the inclusions is too low for isomerization/selective thermal degradation to occur (lack of catalysts?), or that there are other controls on the ratio of some of these parameters. Still, parameters like the ratio of C21 to C28 triaromatic steroids, and those based on dimethyl- and trimethyl-naphthalenes, are comparatively similar in both the inclusions and in the reservoir crude. The oil inclusions in authigenic quartz and albite, formed from about 10 My BP (burial depth ≈ 2.5 km) until the present (burial depth = 3.4 km), are interpreted as representing a palaeo-petroleum charge having a composition intermediate between the oil found in K-feldspar inclusions and the oil charge in the present reservoir. Our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil, now found only in K-feldspar inclusions, with oil from the Mandal Formation. Analysis of solution gas in these inclusions suggests that the first petroleum-associated gas to arrive at the trap from the progressively maturing Mandal Formation had a much wetter composition than the current reservoir solution gas. To explain the present comparatively drier and isotopically lighter reservoir solution gas composition, the inference is made that there has been a recent ( During formation of K-feldspar inclusions (45–75 My BP), maturation of the Upper Jurassic Mandal Formation, the main source rock in this region, was limited possibly to the Graben axis. This suggests that the petroleum found in the K-feldspar inclusions was most likely derived from a deeper post Caledonian Palaeozoic formation.

Effects of burial diagenesis on stresses, compaction and fluid flow in sedimentary basins

Abstract Geomechanical properties of sedimentary rocks, compaction behaviour and strength characteristics, in-situ stresses and fracturing mechanisms are important aspects when evaluating strategies for well drilling and reservoir production, stimulation and monitoring. The main objective of this paper is to point out that the effects of chemical compaction, diagenetic processes and cementation must be considered in addition to the mechanical processes governed by effective stresses. Shales and sandstones may become brittle at shallow depth when carbonate cemented and may remain only lightly quartz cemented to a burial depth of 2–3 km with a low shear strength. Fractures are then only open if the fluid over-pressure exceeds the horizontal stress, and the effective stresses driving compaction must then be small. During deeper burial, tectonic fractures will gradually be sealed with cement. Below 2–3 km depth, compaction is mainly chemical involving dissolution and precipitation of minerals. Rapid expulsion of pore water from mudstones is not possible due to the low permeability and slow mechanical and chemical compaction rates. The high confining stress during progressive burial makes open fractures unlikely at normal pressures, often resulting in over-pressures and hydro-fracturing. Open extensional fractures typically develop during uplifts and basin inversion when there is no fluid expulsion from compaction or petroleum generation. Tectonically induced lateral stress may also cause mechanical and chemical compaction thus reducing plate tectonic stress in sedimentary basins. Local gravitational stress and differential compaction may account for some of the stress anomalies observed in basins like the North Sea.

Modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions

Abstract Mathematical calculations of thermal convection have been carried out using a porous three-player model to simulate pore-water flow in a sedimentary basin with layers of different permeabilities. The calculated flow lines demonstrate that even very thin layers ( eg , around salt domes and igneous or hydrothermal intrusions. Analyses of formation waters from sedimentary basins like the North Sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition. This is evidence suggesting that large scale convection, or other types of mixing, does not take place and positive salinity depth gradient may help to physically stabilize the formation water. The fact that many of the shallow reservoir rocks from the North Sea have formation water with low salinity and negative δ 18 O values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water. In the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross-sections. Diagenetic reactions will, therefore, normally be relatively isochemical during deeper burial.

Cenozoic sequence stratigraphy of the central and northern North Sea Basin: tectonic development, sediment distribution and provenance areas

The Cenozoic succession in the central and northern North Sea has been investigated to establish a regional sequence stratigraphic framework. Changes in sediment distribution indicate a complex pattern of regional vertical movements along older Palaeozoic and Mesozoic structures in Cenozoic times. These vertical movements, mainly related to tectonic processes along the continental margin to the north and north-west, were responsible for the generation and removal of provenance areas for sediments delivered to the North Sea basin through Cenozoic time. During the early through middle Palaeogene, sediments were mainly sourced from areas in the west and from the Atlantic margin in the north. Uplift in the northern North Sea in the late Eocene was, in the earliest, Oligocene followed by marked basin subsidence and tectonic uplift of southern Norway. A similar pattern of tectonic movements resulted in subaerial exposure of the northern North Sea in the early Miocene, followed by significant tectonic subsidence in the basin and along the Atlantic margin in late Miocene-Pliocene times. At the same time, southern Norway was uplifted and became a major sediment source. After the Miocene-Pliocene subsidence-uplift events, glacial processes in southern Norway and fluvial processes in a large drainage area east and south-east of the North Sea were responsible for the main sediment influx to the North Sea. The Cenozoic depositional sequences in the North Sea developed in close interaction with regional tectonic movements and changes in provenance areas. Tectonic movements in north-west Europe overprinted global sea-level changes, so that the generation of depositional sequences and sequence boundaries apparently occurred independently of the rate of eustatic sea-level change.

Fluid flow in sedimentary basins

Abstract The pore waters in sedimentary basins are ultimately derived from sea water, meteoric water, mineral bound water, or from the underlying basement. Geochemically these waters initially have different signatures; however, the pore water compositions are easily altered by reactions with minerals or by mixing with other fluids. Meteoric water is driven downwards into sedimentary basins by the gravitational potential, which approximately corresponds to the elevation of the ground water table above the sea or lake level. It is difficult to model such flow, however, because the permeability on a large scale is almost impossible to represent realistically. The continuity of confined sandstone and limestone aquifers plays an important role in determining the flux of meteoric water received by rocks in different parts of the basin, but this is again difficult to predict. Pore water flow driven by compaction typically has velocities several orders of magnitude lower than what is commonly found in meteoric water flow regimes. The average rate of upwards flow is lower than the rate of subsidence and the difference is the rate of incorporation of sea water in the topmost layer at the sea floor. The salinity of formation water provides important constraints on fluid flow in sedimentary basins. In the North Sea Basin and the Gulf Coast Basin, these types of data indicate that vertical mixing by compaction-driven flow and convection is limited. Compaction-driven flow obeys Darcys Law but there are complex interdependencies between pressure, permeability and compaction. Given the low compressibility of water, the flow which can result from pressure release alone is very limited, except on a local scale along permeable faults and fractures. The main flow of pore water during burial is driven by compaction, which is a slow process, even when overpressure is released abruptly with a resulting increase in the net effective stress. In the case of high permeability faults, the flow into the fault from low permeability sedimentary rocks at depth may be rate-limiting. The displacement of pore water in adjacent sediments near the top of the faults may also slow down fluid flow on fractures that do not reach the sea floor. Modelling of fluid flow in sedimentary basins should be based on permeability distributions which are mostly determined by primary facies, diagenesis and the tectonic history of the basin. Extensional tectonics produce a predominantly vertical fracture network which may serve as conduits for pore water flowing deep into the crust. Upward flow of hot fluids in basement fractures must have a high velocity in order to produce large thermal anomalies. Concentrations of dissolved components precipitate at the surface and form deposits of quartz and ore minerals. If basement fractures are overlain by a thick sequence of soft sediments, the rate of flow on the fractures will be reduced because of the low permeability in the sedimentary cover, and conductive heat transport will dominate. Flow of water at high enough velocities to bring hot water to the surface and produce hot springs is most likely to occur in fractured basements rocks or well-cemented sedimentary rocks. Concentrated precipitation of dissolved components will occur near the surface, where the rate of cooling is highest. Also, sediment-hosted ores precipitated from flow of hot water on fractures are therefore likely to form before accumulation of thick overlying sedimentary sequences.

Porosity loss in sand by grain crushing: experimental evidence and relevance to reservoir quality

The compaction of loose sands has been tested up to high effective stresses (50 MPa). More than 50 test runs were made on five different types of sands, which were of well-sorted fine- to coarse-grained, mono-quartz and lithic compositions. The compaction curves are related directly to observations made from thin sections prepared at different stress levels. The image analyses of the thin sections show that the degree of grain fracturing increases continuously as a function of stress level. Fracturing is more intense in coarse- than in fine-grained sands and a lithic sand, fractures more readily than a mono-quartz sand in a given grain size category. Petrographic observations and grain size analyses show that, although the average grain size reduces with increasing stress level, grain fracturing is most effective in producing grains between 50 and 200 μm, which also reduces the sorting of the sands. Grain size reduction and porosity losses are higher in coarse-grained and lithic sands than in fine-grained and mono-crystalline quartz dominated sands. Fractures similar to those experimentally produced are also seen in the deeply buried Jurassic reservoir sandstones from Haltenbanken area. The experimental compaction data may provide a basis to predict reservoir quality prior to extensive quartz cementation.

Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea

Abstract The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. ≈ 2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis.

Open or closed geochemical systems during diagenesis in sedimentary basins: Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs

Descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events (epigenesis), involving mineral dissolution and precipitation, which have been interpreted to have occurred during the burial history. Published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial. Near-surface diagenetic reactions may be open, involving significant changes in the sediment composition and formation of secondary porosity caused by high pore-water flow rates of meteoric water or reactions with sea water near the sea floor. Calculations show that the bulk chemical composition of the sediments below the reach of high pore-water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore-water flow. Mass transport between shales and sandstones is also limited because the pore water is, in most cases, buffered by the same minerals so that the concentration gradients are low. Recent studies show that silica released from clay-mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones. If the geochemical constraints for mass transfer during burial diagenetic reactions are accepted, the chemical reactions involved in diagenesis can be written as balanced equations. This offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance. Large-scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested, but because such changes cannot be explained chemically and physically, no predictions can be made. Burial diagenetic processes are, in most cases, not episodic but occur as slow adjustments to increased stress and temperature, driving the sediments toward increased mechanical and thermodynamic stability. As a result, the porosity of a single lithology must decrease during progressive burial, but each lithology has a different porosity curve. This article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced. This provides a theoretical basis for the modeling and prediction of reservoir quality.

Enhanced pressure solution creep rates induced by clay particles: Experimental evidence in salt aggregates

Pressure solution is responsible for mechano-chemical compaction of sediments in the upper crust (2–10 km). This process also controls porosity variations in a fault gouge after an earthquake. We present experimental results from chemical compaction of aggregates of halite mixed with clays. It is shown that clay particles (1–5 microns) greatly enhance the deformation by pressure solution in salt aggregates (100–200 micron), the strain rates being 50% to 200% faster in samples containing 10% clays than for clay-free samples. Even the presence of 1% clay increases the strain rate significantly. We propose that clay particles enhance pressure solution creep because these microscopic minerals are trapped within the salt particle contacts where they allow faster diffusion of solutes from the particle contacts to the pore space and inhibit grain boundary formation.

Diagenetic Albitization of Detrital K-feldspar in Jurassic, Lower Cretaceous, and Tertiary Clastic Reservoir Rocks from Offshore Norway, I. Textures and Origin

ABSTRACT The origin of albitized grains in sandstones has been the topic of considerable discussion in recent years. Some workers believe that the albitization of feldspar can occur in situ during burial diagenesis, but others argue that albitization is a feature inherited from source clastic grains. Our petrological and chemical studies of subsurface samples of the Jurassic, Lower Cretaceous, and Tertiary reservoir sandstones from offshore Norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis. Criteria suggesting that the albitization has occurred during burial diagenesis include 1) euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces; 2) generally untwinned albitized grains, mostly riddled with minute br wnish inclusions; 3) lack of cathodoluminescence in albite; 4) homogeneous and pure albite composition (> 99% Ab); 5) absence of albitized grains in carbonate-cemented zones; and 6) increase in the percentage and degree (partial to complete) of albitized K-feldspar with depth. Under SEM, albitized grains display two types of fabrics. Type I albitized grains are composed of numerous tiny albite crystals (1-30 µm) growing along cleavage planes in parent grains, and they have abundant dissolution porosity, while Type II albitized grains show blocky albite crystals (5-80 @m) forming pseudomorphs of detrital K-feldspar, and they lack any dissolution porosity. Type I albitized grains are found commonly at shallower depths (between 2.2 and 3 km and temperatures varying from 65° to 90°C), whereas Type II albitized grains dominate at greater depths (> 3.5 km and at temperatures > 90°C). Albitization proceeds by a dissolution-precipitation mechanism. At shallower depth, dissolution of the parent K-feldspar is more rapid than the precipitation of albite, and rates of albite precipitation increase with depth, favoring pseudomorphic replacement of K-feldspars. Other factors that influence albitization processes are aNa+/aK+, permeability reduction, and surface-dissolution kinetics of the parent grain.