mid-Norwegian Continental Shelf Research Articles

Overpressure development in sedimentary basins induced by chemo-mechanical compaction of sandstones

Abstract Mechanisms of overpressure build-up resulting from chemically-induced compaction are investigated by considering that intergranular pressure-solution, controlled by temperature and effective stresses, is the dominant process of chemical deformation in sandstones. The numerical simulations are performed using a thermo-poro-mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling. At the material level, purely mechanical and chemo-mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components. Porosity data of the Middle Jurassic Garn Formation from the Haltenbanken area of the Mid-Norwegian Continental Shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions. Two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model. The results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure-solution compaction, whereas higher effective stresses associated with permeable depositional environments may lead to important chemo-mechanical deformation, resulting in low porosity sandstones and significant overpressure generation at later stages of basin history. An additional case is finally analyzed to investigate the consequences of pressure-solution inhibition due to diagenetic grain coating in a sandstone reservoir. The simulation resulted in porosity preservation and lower overpressure values, showing that a sedimentary basin submitted to an important level of chemo-mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place. The paper highlights the importance of integrating stresses, fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins.

Cold-water coral reefs and glacial landforms from Sula Reef, mid-Norwegian shelf

Cold-water coral reefs are commonly found associated with positive-relief glacial and other geological landforms such as shelf breaks, bedrock ridges and submarine slide terrains. Such reefs are generally restricted to ocean water temperatures of 4–12°C and water depths of 50–1000 m. Cold-water corals include stony corals (Scleractinia, including Lophelia pertusa ), soft corals (Octocorallia, including ‘precious’ corals, gorgonian sea fans and bamboo corals), black corals (Antipatharia) and hydrocorals (Stylasteridae) (Roberts et al. 2006). The coral Lophelia pertusa , which constitutes an important part of the cold-water coral reefs found in Norwegian waters, requires a hard bottom substrate for settlement, and growth is favoured in areas of relatively strong currents (Dons 1944; Mortensen et al. 2001). Glacial landforms such as iceberg ploughmarks, moraine ridges and streamlined glacial landforms (drumlins and mega-scale glacial lineations or MSGLs) can provide an elevated position relative to the surrounding seafloor, giving exposure to enhanced currents, and sometimes a relatively hard bottom substrate through consolidation and the winnowing of fines. The Sula Reef is located on the ENE–WSW-trending Sula Ridge on the mid-Norwegian continental shelf (Fig. 1a, e). The ridge is about 20 km long and up to 50 m high (Fig. 1a); ridge height decreases towards the NE. The Sula Ridge is made up of layered, resistant sedimentary rocks, with Quaternary glacial to Holocene hemipelagic acoustically laminated …

Skjoldryggen terminal moraine on the mid-Norwegian shelf

Terminal moraines are relatively large ridges of diamictic glacial debris produced at the outermost margins of past glaciers and ice sheets. Their identification on land is important in mapping the maximum extent of Quaternary ice sheets (e.g. Svendsen et al. 2004; Benn & Evans 2010). In marine environments where fast-flowing ice streams reach the shelf edge during full-glacial intervals, moraine ridges are not usually present and the seafloor is characterized by parallel-to-flow streamlined sediments (Ottesen & Dowdeswell 2009). Submarine terminal moraine ridges, by contrast, appear more typical of slower-flowing marine ice margins (Dahlgren et al. 2002; Dowdeswell & Elverhoi 2002; Dowdeswell et al. 2016). A very large ridge, about 150 km long and up to approximately 200 m high relative to the adjacent shelf, is located on the outermost part of the mid-Norwegian continental shelf edge between about 65° and 66° N (Fig. 1a). It is known as Skjoldryggen (‘skjold’ means ‘shield’ and ‘ryggen’ means ‘the ridge’ in Norwegian). Along most of its length, the ridge has a steeper offshore slope relative to …

Seismic geomorphological analysis and hydrocarbon potential of the Lower Cretaceous Cromer Knoll Group, Heidrun field, Norway

The Heidrun field, located on the Halten Terrace of the mid-Norwegian continental shelf, was one of the first giant oil fields found in the Norwegian Sea. Traditional reservoir intervals in the Heidrun field lie within the Jurassic synrift sequence. Most Norwegian continental shelf fields have been producing from these Jurassic reservoirs for the past 30 yr. Production has since declined in these mature fields, but recently, exploration for new reservoirs has resurged in this region. The Jurassic rifted fault blocks form a narrow continental shelf in Norway, thereby greatly reducing the areal extent for exploration and development within existing fields. As the rift axis is approached farther offshore, these Jurassic reservoirs become very deep, too risky to drill, and uneconomical. This risk has prompted exploration in more recent years of the shallower Cretaceous, postrift stratigraphic succession. Cretaceous turbidites have been found in the Norwegian and North Seas, and the discovery of the Agat field in the Norwegian North Sea confirms the existence of a working petroleum system capable of charging Cretaceous reservoirs. These Cretaceous reservoirs were deposited as slope- and basin-floor fans within a series of underfilled rifted deeps along the Norwegian continental shelf and are thought to be sourced from the localized erosion of Jurassic rifted highs. We use three-dimensional seismic and well data to document the geomorphology of a deep-water, Lower Cretaceous wedge (Cromer Knoll Group) within the hanging wall of a rift-related half graben formed on the Halten Terrace offshore mid-Norway. Seismic attribute extractions taken within this Lower Cretaceous wedge reveal the presence of several lobate to elongated bodies that seem to cascade over fault-bounded terraces associated with rifted structures. These high-amplitude, elongated bodies are interpreted as deep-water sedimentary conduits that are time equivalent to the Cretaceous basin-floor fans in more distal parts of the basin to the west. These half-graben fills have the potential to contain high-quality Cretaceous sandstones that might represent a potential new reservoir interval within the Heidrun field.

New aeromagnetic and gravity compilations from Norway and adjacent areas: methods and applications

AbstractThe Geological Survey of Norway (NGU) has produced new aeromagnetic and gravity maps from Norway and adjacent areas, compiled from ground, airborne and satellite data. Petrophysical measurements on core samples, hand specimens and onin situbedrock exposures are essential for the interpretation of these maps. Onshore, the most prominent gravity and magnetic anomalies are attributed to lower crustal rocks that have been brought closer to the surface. The asymmetry of the gravity anomalies along the Lapland Granulite Belt and Kongsberg–Bamble Complex, combined with the steep gradient, points to the overthrusted high-density granulites as being the main source of the observed anomalies. The Kongsberg–Bamble anomaly can be traced southwards through the Kattegat to southern Sweden. This concept of gravity field modelling can also be applied to the Mid-Norwegian continental shelf and could partially explain the observed high-density rocks occurring below the Møre and Vøring basins and in the Lofoten area. Extrapolations of Late-Caledonian detachment structures occurring on the mainland can be traced on aeromagnetic and gravimetric images towards the NW across the continental margin. Subcropping Late Palaeozoic to Cenozoic sedimentary units along the mid-Norwegian coast produce a conspicuous magnetic anomaly pattern. The asymmetry of the low-amplitude anomalies, with a steep gradient and a negative anomaly to the east and a gentler gradient to the west, relates the anomalies to gently westward dipping strata. Recent aeromagnetic surveys in the Barents Sea have revealed negative magnetic anomalies associated with shallow salt diapirs. Buried Quaternary channels partly filled with gravel and boulders of crystalline rocks generate magnetic anomalies in the North Sea. The new maps also show that the opening of the Norwegian–Greenland Sea occurred along stable continental margins without offsets across minor fracture zones, or involving jumps in the spreading axis. A triple junction formed at 48 Ma between the Lofoten and Norway Basins.

Provenance of Late Quaternary ice-proximal sediments in the North Atlantic: Nd, Sr and Pb isotopic evidence

In order to assess the isotopic characteristics of siliciclastic sediments delivered by the Laurentide, Greenland, Iceland and Fennoscandian ice sheets to the North Atlantic, the Nd, Sr, and Pb isotopic compositions were determined for twenty-six samples of Late Quaternary, fine-grained (<63 μm), ice-proximal glacimarine sediments throughout the northern North Atlantic. Along the eastern margin of the Laurentide Ice Sheet, ϵ Nd values of the glacimarine sediments vary from −23 to −29 in Baffin Bay and the West Basin of Hudson Strait to a high of −12.9 to −14.7 further south in the Gulf of St. Lawrence. The Sr and Pb isotopic compositions of these sediments are also variable ( 87Sr/ 86Sr=0.716–0.736; 206Pb/ 204Pb=16.81–18.75), with samples from Baffin Bay, Hudson Strait, and Cartwright Saddle having less radiogenic Pb than those from the Gulf of St. Lawrence. On the opposite side of the Atlantic fine-grained sediments from the Bear Island trough mouth fan and from the mid-Norwegian continental shelf have Nd, Sr and Pb isotopic compositions that are indistinguishable from sediments deposited in the Gulf of St. Lawrence. Glacimarine sediments deposited adjacent to northern and western Iceland have high ϵ Nd (5.7–6.2) and low 87Sr/ 86Sr (0.7042–0.7050), consistent with their derivation from Icelandic volcanic rocks, while sediments deposited nearby on the East Greenland shelf and slope trend towards lower ϵ Nd (−6.5–4.2) and higher 87Sr/ 86Sr (0.704–0.713). Overall, the isotopic variations observed for the ice-proximal glacimarine sediments mirror the variations that occur in immediately adjacent continental crust and hence represent the products of glacial erosion of the continental margins. A comparison of the isotopic compositions of the ice-proximal glacimarine and ice-rafted detritus (IRD) in the North Atlantic confirm that Hudson Strait is the probable source of low ϵ Nd (<−15) detritus in Heinrich events H1, H2, H4, and H5. Higher ϵ Nd (>−15) IRD comprising H3 and H6, and that was deposited before and after lower ϵ Nd IRD in other Heinrich events, could have been delivered to the North Atlantic from the Fennoscandian Ice Sheet, as previous workers concluded, but our data reveal that southeastern margin of the Laurentide Ice Sheet is a viable alternative source for the higher ϵ Nd IRD.

The role of grounding-line sediment supply in ice-sheet advances and growth on continental shelves: an example from the mid-Norwegian sector of the Fennoscandian ice sheet during the Saalian and Weichselian

We discuss the consequence of sediment accumulation at the grounding line of glaciers advancing across continental shelves. Sediment transfer through a deformable till bed and deposition at the grounding line act to decrease the water depth and adds support to the termini. Both are mechanisms that act to lower the calving rate and hence reduce mass losses during an advance of a grounded glacier. We use the geological record of the Saalian and Weichselian advances of the Fennoscandian ice sheet across the mid-Norwegian continental shelf to elucidate the consequence of this process. Based on seismic investigations and published maps, we infer that the development of a morainal bank fed and sustained through the advance(s) by means of a deformable bed was indeed important. This allowed the ice sheet to advance across a wide seaward dipping shelf, depositing thick diamicton (in places of more than 200 m thick) and a terminal morainal bank complex. We relate the reasons for this depositional mode, i.e. shelf and shelf edge accumulation rather than deposition of glacial debris flows on the continental slope, to slow and fast glacier flow regimes.

Upper Permian as a new play model on the mid-Norwegian continental shelf: Investigated by shallow stratigraphic drilling

A 750 m-thick, fully marine succession of sandstones, coarse-grained turbidites, shales, and reworked sabkha sediments has been cored on the eastern margin of the mid-Norwegian shelf. The succession has been dated as Upper Permian-Lower Triassic and is comparable to rocks of the same age exposed onshore East Greenland. These data demonstrate that the marine depositional basin between Greenland and Norway extended much farther east than previously thought. Reddish, shallow-marine sandstones in the lower 170 m of the cored succession probably represent reworking of older sedimentary rocks present to the east of the drill sites. This suggests that Upper Devonian-Lower Permian sediments were deposited on Caledonian basement east of the present-day limits of the basin. The cored succession also contains source rocks that can be stratigraphically correlated with the oil-prone source rocks of the Upper Permian Ravnefjeld Formation onshore East Greenland. Some of the cored sandstone intervals were stained with light, nonbiodegraded oil that most likely was sourced from Upper Permian or, alternatively, Lower Triassic mudstones. Reworked fragments of reef-building organisms in Upper Permian turbidites and Upper Permian carbonates encountered in an exploration well on the Nordland Ridge indicate that carbonate deposition and reef building occurred on structural highs on the Trondelag Platform. The observations from the cored successions are key elements in a new Paleozoic play model on the mid-Norwegian continental shelf and include the first evidence for the existence of an Upper Permian source rock in the area, with Upper Permian carbonates or sandstones or Triassic-Jurassic sandstones as reservoir. (Begin page 108)

The deep-water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts

The paper presents documentation on the distribution of, and damages to, deep-water reefs of the coral Lophelia pertusa in Norwegian waters. The reef areas have traditionally been rich fishing grounds for long-line and gillnet fisheries, and the coral habitat is known to support a high diversity of benthic species. Anecdotal reports claim that trawlers often use the gear, wires, chains and trawl doors to crush the corals and clear the area before fishing starts. To get an overview of the situation, information about the distribution and damage were collected from the literature, fishermen, and our own investigations. The results show that the corals are abundant particularly on the mid Norwegian continental shelf between 200 and 400 m depth. In general it seems that the largest densities are distributed along the continental break and at ridges of morainic origin. The reports from fishermen suggested severe damage to the corals and in situ observations using ROV confirmed the presence of mechanically damaged corals located on trawling grounds. A first estimate of the fishery impact indicates that between 30 and 50% of the reef areas are damaged or impacted. Fishermen claim that catches are significantly lowered in areas where the reefs are damaged. Potential ecological consequences of the destruction are discussed.

Compaction model for quartzose sandstones application to the Garn Formation, Haltenbanken, Mid-Norwegian Continental Shelf

The compaction of quartzose sandstones is described by a macroscopical visco–elasto–plastic model, derived from microscopical considerations. This model considers that the reduction of total porosity (the relative pore volume) is related to the changes in the relative cemented volume and the relative intergrain volume. Furthermore, we consider that the variations of these two petrophysical parameters result from two physical mechanisms, mechanical compaction and closed system chemical compaction.The model has been calibrated for the Middle Jurassic Garn Formation from the Haltenbanken area of the Mid-Norwegian Continental Shelf. Around 300 data points from 17 wells have been used in this study. The viscous parameters have been optimised against the present day data. With total porosity data the best fit is obtained for an activation energy between 14 and 16kJ/mole and a macroscopical viscosity at 15°C between 39 and 46GpaMa (1.21024–1.41024Pa.s). If quartz cement data are used, the best fit is obtained for an activation energy between 16 and 18kJ/mole and a macroscopical viscosity at 15°C between 47 and 56GpaMa (1.51024–17.1024Pa.s).The visco–elasto–plastic compaction model has been tested in the Haltenbanken area of Mid Norway, with the result that observed porosity trends and pore pressures can be modelled. Using multiple 2-D basin modelling simulations the results of this model and its implications for fluid flow and petroleum systems analysis can be seen. The compaction method has significant impact on the simulated fluid flow and petroleum migration pattern.

Radiocarbon and seismic evidence of ice-sheet extent and the last deglaciation on the mid-Norwegian continental shelf

Reconstruction of the ice extent and glacier chronology on the continental shelf off mid-Norway has been severely hampered by the lack of dates from the glacial deposits. Seismic interpretation and new accelerator mass spectrometer radiocarbon dates show that the ice sheet extended to the edge of the continental shelf at the last glacial maximum. The two youngest till units near the shelf edge were deposited about 15,000 and 13,500 BP. The results indicate that the ice sheet partly reached the shelf break as late as 13,000 BP, followed by a deglaciation of most of the continental shelf in less than 1000 years.

Late Cretaceous–Cenozoic intraplate contractional deformation at the Norwegian continental shelf: timing, magnitude and regional implications

Contractional structures (large anticlines and synclines, reverse faults and inverted centres of deposition) of assumed Late Cretaceous–Cenozoic age are common in Cretaceous–Tertiary basins of the northwestern European margin. The similarities in style, orientation and timing of these structures are striking. The present detailed analysis of one anticline (the Ormen Lange Dome) of the mid-Norwegian continental shelf indicates that the total contraction is moderate (less than 2–3%), and that the analysed anticline has been growing almost continuously since its initiation in Eocene till Present. Inversion in the Barents Sea started already in the Late Cretaceous. This episode is suggested to be related to far-field effects of active plate-margin processes, and transfer of stresses across the plate as a consequence of the sub Hercynian and Paleocene `Laramide' event of the Alpine Orogeny. The development of co-axial structures was facilitated by stress focusing along pre-existing, high-relief N–S- and NE–SW-trending fault complexes. Far-field plate tectonic stresses originating mainly from the Alpine Orogeny seem to have been the most important cause of contractional deformation on the NW European shelf. In addition, ridge push from the North Atlantic spreading may have contributed significantly, particularly during the Neogene.

Late Cenozoic sedimentation and uplift history on the mid-Norwegian continental shelf

Four Mid(?)/Late Cenozoic sedimentary units are recognised on the mid-Norwegian continental shelf: (1) A thin Miocene unit, (2) a “deltaic complex” variously dated to be of Oligocene or Early Pliocene age; (3) A Late Pliocene-Early Pleistocene low angle prograding unit and (4) a Mid/Late Pleistocene blanketing diamicton. The deltaic complex which extends along the Norwegian coast from the Lofoten Islands in the north to at least 62°45′N, is inferred to be the result of regional uplift of Fennoscandia during Oligocene or Early Pliocene times. As a result of further uplift and regional glaciations during the Late Pliocene-Early Pleistocene, a system of low angle prograding sequences of mainly glacigenic sediments were deposited. Inferred glacial erosional forms suggests periods of grounded ice on the continental shelf. The sedimentary regime was dominated by subglacial sediment input to the shelf edge and later reworking by gravity flows down the continental slope. At first most of the sediments within the study area were derived from the Lofoten area in the northeast. Later the progradation had a more east-west direction. During the Late Pliocene/Early Pleistocene period the shelf edge prograded about 100 km, and an average sedimentation rate of 42 cm/10 3 years is found.

Soil investigations, offshore mid Norway: A case study of glacial influence on geotechnical properties

Fine-grained glaciomarine and glacial deposits on the outer Mid Norwegian continental shelf show complex variations in shear strength and degree of consolidation. At the Smørbukk Sør field (approx. 65°N, 7°E) about 80 m of variably overconsolidated clayey till is found on top of normally- or possibly underconsolidated glaciomarine and marine sediments. A high gas content is found below 60 m, and the porewater in the rather soft sediments in the lower part of the borehole may have been partly trapped by gas hydrates when the overlying hard till was deposited. The variations in geotechnical properties of the 80 m thick till section are suggested to result mainly from compaction below grounded ice during the last glaciation, and we interpret the strength variations in the till to reflect shifts between freeze of sediment porewater onto the base of the ice causing highly overconsolidated intervals, and thermal equilibrium or melting at the ice sole resulting in intervals of softer till. A large ridge northwest of Smørbukk (Skjoldryggen) is probably partly formed by ice push during ice flow oscillations at the same time as the described till at Smørbukk was deposited. This ridge retarded the ice flow and hence the frictional heating at the end of surges, creating pulses of net freezing base ice and consolidation of the substratum. The described process has possibly affected a wide area up-ice of Skjoldryggen, and may be common in areas with a complex glacial history and a fine-grained substratum.

High-Frequency Depositional Sequences and Stratal Stacking Patterns in Lower Pliocene Coastal Deltas, Mid-Norwegian Continental Shelf

Extensive deltaic and coastal progradation occurred along the mid-Norwegian continental shelf during the early Pliocene. Thirty-eight well-developed, high-frequency (fourth-order) sequences are identified within the deltaic complex on multifold seismic data. The fourth-order sequences are arranged in four oblique progradational and two sigmoid progradational sequence sets. Deposition of the high-frequency sequences and their stacking patterns probably were in response to high-frequency cycles of relative changes in sea level cycles produced by variable rates of subsidence and uplift, superimposed on high-frequency eustatic cycles within a lower frequency eustatic system. The mixed aggrading/prograding sequence sets are interpreted to represent increased space-added accommodation rates and deposition within third-order highstand systems tracts. Conversely, the progradational sequence sets are interpreted to represent decreasing space-added accommodation rates and deposition within the third-order lowstand systems tracts. The recognition of multiple sequence sets likely reflects the effect of long-term relative fall in sea level (tectonic uplift?) superim osed on high-frequency eustatic cycles. The terminal sequence boundaries of the sequence sets are formed by conjunction of third- and fourth-order falling inflection points on the relative sea level curve. These sequence boundaries represent the timing of maximum subaerial erosion and may represent times of greatest potential for the deposition of reservoir sands.

Kaolinized, potassium-leached zones at the contacts of the Garn Formation, Haltenbanken, mid-Norwegian continental shelf

The Middle Jurassic Garn Formation consists mainly of subarkosic arenite in which potassium feldspar is several times as abundant as plagioclase. In cores from eight wells, the top of the formation (upper 6–12 m) appears to have undergone relatively intense alteration of potassium feldspar to kaolinite, with the resulting loss of bulk postassium content. This could be explained as a result of subaerial weathering of the top of the sandstone body, except for the fact that in two of the wells the same pattern of alteration is observed at the base of the formation. An alternative explanation is that alteration resulted from the introduction of acid released by the maturation of organic matter in adjacent shales. In all instances the altered zones display a gradual increase in kaolinite content and a decrease in the ratio of bulk-rock potassium: aluminium over a distance of several metres approaching the formation boundary. This pattern indicates that the active species responsible for the alteration entered vertically from the formation contacts rather than being introduced by lateral flow along the sandstone bed. The cores studied are tentatively interpreted as recording different stages in the following depth-related sequence of diagenetic events: (1) early kaolinization throughout the formation by meteoric water flushing; (2) formation of additional kaolinite near the formation contacts by the influx of acid from surrounding shales at approximately 2.7 km depth below the sea floor; and (3) extensive alteration of kaolinite to illite at approximately 3.7 km depth. Illitization has been impeded in the leached zones near the formation contacts due to a local deficiency in reactive potassium; here kaolinite survives unaltered, whereas kaolinite throughout the rest of the formation has been extensively replaced by illite. This implies that the illitization of sandstone can occur by short-range diffusion in a chemically closed system, solely in response to increasing thermal exposure at greater depth.

Structure and seismic stratigraphy of the innermost mid-Norwegian continental shelf: an example from the Frohavet area

The Frohavet Basin, a newly discovered sedimentary basin on the innermost mid-Norwegian continental shelf, occurs in an area 60×20 km and contains a stratal sequence up to 1000 m thick. Dating by ice-transported erratic blocks suggests a Jurassic age. The sequence is preserved in half-grabens downthrown to the north-west, and is cut by numerous faults, some of which are interpreted as syndepositional. The sequence reflects an upward change from continental deposition, with the Frohavet area acting as a local depocentre, to more widespread continental and marine deposition. The predominant sediment transport direction was towards the north-west. The Frohavet sequence was deformed during a phase of extensional tectonism, probably in Late Jurassic—Early Cretaceous times.

Potassium-Leached Zones at the Contacts of the Garn Formation, Mid-Norwegian Continental Shelf: Evidence for the Role of Shale in Sandstone Diagenesis: ABSTRACT

Potassium-Leached Zones at the Contacts of the Garn Formation, Mid-Norwegian Continental Shelf: Evidence for the Role of Shale in Sandstone Diagenesis: ABSTRACT

Sandstone Permeability Viewed as a Function of Intergranular Macroporosity: Garn Formation, Haltenbanken, Mid-Norwegian Continental Shelf: ABSTRACT

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Relationship Between Diagenesis and Reservoir Quality in Sandstones of the Garn Formation, Haltenbanken, Mid-Norwegian Continental Shelf(1)

A quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the Middle Jurassic Garn Formation. Porosity variation is controlled by the combined effects of compaction and quartz cementation. In general, more porosity has been lost by compaction than by quartz cementation, but compactional porosity loss appears to have occurred relatively early in the burial history (before burial below 2 km below the sea floor). Thus, progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation. Permeability is a function of the abundance of intergranular macroporosity, which is interpreted to be a measure of porosity. At depths less than 3.5 km below the sea fl or, intergranular macroporosity generally comprises more than 50 percent of the total porosity, but at greater depth, as total porosity decreases below about 16 percent, the ratio of effective to ineffective porosity drops below one. This depth corresponds to the onset of extensive illitization and grain dissolution, which results in major reorganization of the pore system. Generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the Garn Formation, but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis.