Jurassic Fault Research Articles

Inversion tectonics in the Lyme Bay-West Dorset area of the Wessex Basin, UK

Abstract Inversion in Lyme Bay and West Dorset was characterized by the reversal of motion on major Jurassic faults, and regional uplift which resulted in the widespread loss of the mid-Cretaceous to Tertiary post-rift cover throughout the area. North-south contraction during the Tertiary resulted in the development of a major anticline in the hanging wall of the Lyme-Portland faults, which was accompanied by widespread folding within the footwall of these faults. Northeast-southwest sinistral and northwest-southeast dextral strike-slip faults were also developed throughout the Lyme Bay-West Dorset area and accommodated much of the north-south contraction. These features are considered to be a consequence of the mechanical effects of an extensive salt interval within the Triassic Mercia Mudstone Group of this area. Major Early Jurassic faults appear to have been directly linked with basement through the salt interval, although smaller faults within the footwall apparently detached on this horizon. The major faults were inverted in a complex manner during the Tertiary, commencing with simple reversal of motion on these faults, followed by detachment of the post-salt carapace as the footwall and hanging wall salt layers were brought into close proximity, thereby resulting in the widespread deformation observed in the footwall of these faults.

Oblique rifting and sequential faulting in the Jurassic development of the northern North Sea

Jurassic structural development of the northern North Sea has characteristics of oblique rifting. This is interpreted to result from an obliquity between Jurassic NW-SE-to WNW-ESE-directed extension and a preexisting N-S-trending zone of deformation related to Permo-Triassic rifting and thinning of the crust. The zone of maximum Jurassic extension i.e. the Viking and Sogn grabens, defines a narrow (25–40 km) depression which trends diagonally (NNE-SSW) across a broader (130–150 km) Permo-Triassic basin. The Jurassic rift zone consists of a system of ene´chelon rift segments, with changing symmetry along strike, bounded by faults oblique to the overall graben envelope.Sequential Jurassic fault development as well as the interference between N-S- and NE-SW-striking faults are explained within the context of oblique rifting. The early (Late Bajocian-Oxfordian) rift stage was dominated by reactivation of N-S-striking major faults, whereas a number of coeval but smaller faults have a NE-SW orientation. NE-SW-striking faults show increasing importance during the Jurassic stretching phase, and major faults defining this orientation accommodated most of the Kimmeridgian-Volgian extension. Faults striking NESW ] are observed to cross-cut earlier N-S-trending faults; they interfere with N-S-trending faults to define faultblocks with rhombohedral shape; and they are associated with partial collapse of crestal areas of N-S-oriented fault-blocks to form structural terraces.

Geophysical Modelling of Structure and Tectonostratigraphic History of the Longford Basin, Northern Tasmania

A new gravity survey combined with ground-based magnetic surveys have enabled detailed modelling of basement structures in the Longford Basin of northern Tasmania. This basin began to form in the mid-Jurassic, but contains only Cainozoic sediments. The models suggest that the basin has been formed by a series of predominantly northwest-trending Jurassic normal faults which have produced a series of half-grabens in which blocks were downthrown to both east and west with dips of up to 10° W.The basin blocks incorporate Palaeozoic to Mesozoic Parmeener Supergroup sedimentary rocks intruded by pre-to syn-fault Jurassic dolerites. These units overlie an older, geophysically distinct, Devonian fold–thrust terrane. Much of the tectonic activity from the Palaeozoic to the Cainozoic appears to have been controlled by the Tiers Fault system, a major gravity and magnetic lineament within the Tasmanian crust.In contrast to many other Tasmanian Mesozoic–Cainozoic basins, the Longford Basin is exclusively continental, with up to 800 m of Paleocene to Quaternary fluvial sediments having been drilled in two wildcat oil wells.The basin also contains intercalated Late Eocene to Pliocene basaltic volcanic rocks. Magnetic data indicate three eruptive centres localised in reactivated Jurassic faults. Massive flows, now lateritised, have infilled an Early Eocene to Pliocene drainage system.

Interaction of Permo-Triassic and Jurassic extensional fault-blocks during the development of the northern North Sea

The main structural elements resulting from Late Permian-Early Triassic extension have been resolved across the total width of the northern North Sea. The northern segment of the basin (north of 61°N), bounded to the west by the Hutton Fault and to the east by the Norwegian mainland, was occupied by four westerly tilted half grabens, 25–35 km in width, prior to Jurassic extension. The central segment of the basin (60°20′–61°N) exhibits a full graben beneath the Jurassic Horda Platform, flanked by half grabens tilted towards the basin margins. The change in Permo-Triassic fault-block configuration, defining structural domains, is largely constrained by a basement discontinuity which runs diagonally (NE-SW) across the basin. Maximum Jurassic extension is represented by several centres of subsidence, aligned NNE-SSW across a N-S-oriented and broader Permo-Triassic basin. Jurassic extension was concentrated within the Viking Graben proper south of 60°N, which in this area is represented by single westerly-tilted, en échelon half graben segments 20–25 km wide. The area affected by Jurassic extension becomes progressively wider to the north, again governed by the basement discontinuity, eventually encompassing the total width of the basin ( c. 200 km) north of 61°N. The central segment of the northern North Sea displays the greatest contrast in Permo-Triassic and Jurassic structural styles and basin geometry. Jurassic stretching and faulting was negligible to the east (Horda Platform), precisely the area which had been the site of maximum Permo-Triassic stretching and fault activity. It has generally been assumed that Jurassic fault configuration along the eastern margin of the central segment of the Viking Graben, to a large extent reflects the reactivation of major Permo-Triassic faults. This study demonstrates the interference of Permo-Triassic and Jurassic basin geometries in this area, due to major faults of the two generations dipping in opposite directions. As a consequence, the E-tilted Oseberg Fault Block of Jurassic origin and c. 20km in width, is underlain by a wider ( c. 35 km) and W-tilted Permo-Triassic fault-block.

Hydrocarbon-Related Diagenesis: The Key to Understanding Trap Integrity and Hydrocarbon Charge History in Australia's Timor Sea?: ABSTRACT

In the Late Miocene, the effects of continental collision partially to completely breached many charged, Mesozoic traps in Australia`s Timor Sea. An integrated geophysical and geochemical study has shown that the biodegradation of hydrocarbons leaking from these Mesozoic traps produced hydrocarbon-related diagenetic zones or HRDZs within overlying, Eocene aquifer sands. These HRDZs, which are principally defined by zones of intense carbonate cementation, are seismically-resolvable, with both their size and velocity being directly related to the amount of hydrocarbons that have passed through the aquifer sands. Consequently, the integration of observations on the seismic, velocity and isotopic characteristics of the HRDZs, the nature of present-day hydrocarbon seepage (as defined by sniffer and ALF data), and geometric relationships between the reactivation (Miocene) and trap (Jurassic) fault geometries, can provide a powerful predictive tool for evaluating and ranking undrilled structures and prospects within the Timor Sea.

Structure and tectonics of the northern North Sea: new insights from deep penetration regional seismic data

Abstract Recently acquired, deep penetration regional seismic data provide important new insights into the structure and tectonic evolution of the northern North Sea. Improved data quality permits better understanding of fault geometries and salt tectonics in the graben areas and reveals the pre-Tertiary geology of the surrounding platforms for the first time. This paper presents seismic data from five structurally distinct areas. (1) South Viking Graben . This relatively narrow graben formed the principal sedimentary basin during the Mesozoic period and contains a thick Triassic-Cretaceous succession. Low-angle, eastward-dipping faults developed at the western basin margin control a relatively simple half-graben geometry. The Beryl Embayment is notable for the local rotation of Triassic-Jurassic strata independent of the underlying basal Permian reflectors, indicating local Mesozoic salt movement in this area. Faulted and anticlinal highs east of the graben axis may also record limited salt tectonics. (2) Horda Platform . This area comprises a series of eastward-dipping rotated Mesozoic fault blocks. Mesozoic extensional faults developed as splays in the hanging wall of a major westward-dipping deep detachment surface. (3) East Shetland Platform . This area formed a structural high throughout the Mesozoic era. Regional seismic data provide a clear image of the geometry of a Palaeozoic basin, thought to represent the northward continuation of the Orcadian Basin recognized onshore in mainland Scotland, Orkney and Shetland. The sedimentary sequence is probably of Devonian (-Carboniferous?) age and contains major unconformities, which may permit subdivision of this succession into tectono-stratigraphic units. The contact with the underlying basement is characterized by a distinctive change in seismic character, traceable over the entire area. The deep structure of the East Shetland Platform is dominated by westward- or northwestward-dipping low-angle basement faults, interpreted as thrust surfaces. This structural grain is associated with the local development of eastward-vergent folds interpreted as hanging wall anticlines developed in the Upper Palaeozoic cover sequence. These structures provide evidence of a Late Palaeozoic deformation phase. (4) Unst Basin . Lying to the west of the East Shetland Basin and bounded by a faulted horst on its eastern margin, the Unst Basin is underlain by a spectacular low angle, westward-dipping detachment surface. Roll-over into this fault is interpreted as the result of extensional roll-back of an older (?Caledonian) thrust plane. The fault geometry may indicate structural analogies with the West of Shetlands Province. (5) East Shetland Basin . The structure of this area is characterized by the development of extensive, westward-dipping fault blocks underlain by low-angle, eastward-dipping extensional faults traceable to great depth. The seismic data indicate thickening of the sedimentary succession in the footwalls of these faults, consistent with Jurassic faulting of a pre-existing Triassic basin.

Inversion structures and hydrocarbon occurrence in Argentina

Abstract Positive inversion features are a common structural ingredient in broad regions of western and central Argentina. They show up in several geological provinces distributed from Salta at 24°S down to Tierra del Fuego below 54°S. Considerable variety in inversion style and intensity developed due to regional positioning within the Andean orogenic belt, near the mountain edge, or well within the South American foreland. Weakly inverted grabens, doubly plunging buckles and compressional anticlines may site as far as 600 km away from the present day oceanic trench and 300 km east of the Cordilleran front. Most of the studied examples are genetically linked to high-angle or listric normal fault families that developed across southern South America, induced by pervasive Triassic to Cretaceous extensional stress conditions. Old extensional structures involved in the inversion process include: segments of Triassic successor basins linked to extensional collapse of the Late Paleozoic orogenic tract, Jurassic faults belonging to a multicomponent rift system developed after incipient Basin-and-Range style deformation of Patagonia, segments of a network of linear Jurassic-Neocomian troughs and half-troughs related to pre-breakup fragmentation of the South American slab, and roughly linear Cretaceous-Palaeogene fault-bounded troughs of the arc-crest type induced after supracrustal extension driven by easterly subduction along the Pacific edge of the plate. As in other deformed areas around the world the inversion process did not advance as a wave from the orogenic front (‘bulldozer’ mode), but apparently occurred as a more or less synchronous reactivation of faults distributed across the inverted tract (‘accordion’ mode). In the Neuquén basin middle Jurassic to Cretaceous reversal from block subsidence to local uplift occurred along an intra-plate megashear aligned with the inboard projection of the South Atlantic oceanic transforms. Protracted strike-slip motion and related syndepositional block-inversion favoured pre-migration structuring and stratal anomalies prone to provide sedimentary and combination traps. Over most of the oil-producing Argentine basins inversion took place late, during the Cenozoic. Thus timing, with respect to the local maturation histories and onset of regional hydrocarbon charge, was a key control on field size and oil richness in each region. Late tectogenesis and late regional migration combined in the non-marine Cuyo basin to provide fields in the 250 MMBO range, out of inverted Triassic half-grabens. Less favourable convergence of inversion and migration is recorded in the western San Jorge basin, where regional contraction occurred 20–40 Ma after regional oil emplacement, and late inversion resulted in dispersive hydrocarbon reaccommodation. Finally, migration in the southern Magallanes basin happened at the time when burial beneath a thick foreland wedge had reduced reservoir properties and thermally degraded the source sections into the gas generation stage.

Discussion on palaeoecology and sedimentology across a Jurassic fault scarp, NE Scotland

J. R. Underhill writes: In their recent paper, Wignall & Pickering (1993) provide an excellent analysis of the sedimentological and palaeontological aspects of the Upper Jurassic exposures on the East Sutherland coast. Their work clears up many outstanding problems relating to the area and deserves wide recognition. However, there are a few minor points relating to the structural aspects of their study or to offshore correlations which may warrant qualification or further clarification. Controls on Allt na Cuile Sandstone sediment dispersal. Wignall & Pickering contend that supply of the Allt na Cuile Sandstone during the early Kimmeridgian was via a transfer fault zone between two separate segments of the Helmsdale fault. Although an offset in the trace of the Helmsdale fault is supported by onshore mapping, little evidence exists for a connecting fault between the two strands which would be classified as a transfer fault zone. Instead, field relationships could be taken to suggest that the faults overstep with displacement apparently dying out on the more southerly strand such that a tip line probably exists at its northeastern extremity. Consequently, the structural relationship is more consistent with that of a relay ramp than a transfer fault zone (Fig. 1) as suggested by Thomson & Underhill (1993). That interpretation would also be in accordance with offshore observations in the Inner Moray Firth where there is a singular absence of linking transfer faults and a close association between the source of linear sand-prone mounds and overlapping normal faults (Underhill 1991a, b). This interpretation

Mid-Cretaceous crustal extension recorded in deep-marine half-graben fill, Cedros Island, Mexico

Marine, mid-Cretaceous fore-arc strata on Cedros Island (Vargas and Pinos Formations) record the onset of extensional brittle deformation of the upper crust concomitant with initial uplift of blueschist-grade rock from great depth. The pre-extensional Vargas Formation is chiefly fine grained, and it contains facies interpreted as basin-plain, sand-rich-fan, and slope deposits that onlapped a Jurassic unconformity toward the north. The syn-extensional Pinos Formation is coarser grained than is the Vargas and records the initiation and filling of a northeast-southwest-trending, deep-marine, half-graben structure that formed by reactivation of a Jurassic fault zone located along the northern margin of the basin. The axis of the half graben acted as a submarine canyon that funneled gravelly sediment-gravity flows toward the south-west, whereas the shoulder of the half graben was draped by sandy turbidites and mud. A second episode of faulting on a fault parallel to, and synthetic with, the older half-graben fault produced a stress-transfer zone that steepened the intervening block, producing a catastrophic slope failure in the basin. Several points of evidence suggest that the canyon-bounding fault zone mapped at the northern margin of the basin was syndepositionally active. Basin-floor scarps are indicated by the presence of large, angular blocks derived from the Jurassic strata; these blocks occur within Cretaceous conglomerate and olistostromes, and as clast-supported breccia. Asymmetric basin subsidence is recorded in monotonic lithofacies trends that occur laterally away from the inferred fault zone. These trends include a decrease in average grain size and bed thickness, and an increase in interbedded mudstone and in lateral continuity of individual beds. Abundant slope-failure features, including slide blocks as much as 100 m wide by 40 m thick, also point to frequently recurring intrabasinal seismicity. The facies distribution and basin structures exposed on Cedros Island are analogous to aspects of the southern Viking Graben and the Ebro turbidite complex. Modern tectonic and physical analogues for the half graben are found at the Kuril margin and in the Aleutian are, where oblique convergence has produced fault-bounded submarine canyons of the same lateral and vertical scale as inferred for the Cretaceous basin on Cedros Island. The temporal coincidence between brittle deformation in the upper plate with peak blueschist metamorphism in the lower plate of the convergent margin suggests that initial blueschist uplift began soon after deepest burial.

Palaeoecology and sedimentology across a Jurassic fault scarp, NE Scotland

A detailed palaeoecological and sedimentological analysis of the Kimmeridgian coastal outcrops around Helmsdale (northeast Scotland) has been related, for the first time, to a detailed biostratigraphical scheme allowing the depositional history of this important area (adjacent to some North Sea oilfields) to be elucidated. A broad, shallow marine, sub-tropical, shelf was disrupted by the onset of normal faulting along the Helmsdale fault in earliest Kimmeridgian times. Initially, water depths remained shallow on the hanging wall, reflected by a normal benthic fauna, but a rapidly steepening southeast-directed palaeoslope produced frequent sediment sliding. Shoreface sands developed immediately to the west of the fault and were transferred to the hanging wall via a fault-transfer zone where they now constitute the Allt na Cuile Formation. Continued rapid subsidence during the remainder of the Kimmeridgian and the Portlandian produced deep, poorly oxygenated to anoxic benthic conditions on the hanging wall in which a combination of rock slide/debris flow/turbidity current deposition alternated with hemipelagite deposition to produce the Helmsdale Boulder Bed Formation. The Eathie locality (Cromarty), previously regarded as a deeper, more distal setting than Helmsdale, contains a slightly more diverse benthic fauna implying shallower water. The deepest waters were immediately adjacent to the fault scarp as predicted by the standard half-graben depositional model.

Structural controls on the Late Jurassic age shelf system, Ula Trend, Norwegian North Sea

The Ula Trend of the Norwegian North Sea is an important producing area sited on the eastern margin of the Central Trough. Two fields, Ula and Gyda (combined reserves of 635 MMBBL), are on stream, and several other petroleum pools have been discovered. They key to success in the area is to predict the presence of Kimmeridgian to Portlandian age shallow-marine shelf sandstone reservoirs. Well and seismic data have shown the reservoir sequences to have a complex distribution controlled largely by the structural topography at the time of deposition. Structural interpretation of the 3D seismic datasets over the Ula and Gyda field areas and dense 2D data over the Ula Trend as a whole have revealed new insights into the geometry of the Jurassic rift margin. A relationship has been established between the Jurassic faults and the underlying salt wall topography to a deeper Early Permian age extensional system. During Late Jurassic extension, the deeper faults cut up-section and involved the Upper Permian, Triassic and Jurassic cover. Original Early Permian dip-slip faults maintained their sense of motion, while a linked oblique-slip set acted in sinistral transtension. The cover section accommodated, the differing styles with both simple extensional structures and more complex transtensional and contractional geometries, respectively. The structural pattern has been further complicated by episodic hangingwall inversion in Cretaceous and Tertiary times and the inversion geometries provide important petroleum traps (e.g. Ula Field). The Jurassic rift margin fault system and the geometry of its hangingwall governed the location of the Kimmeridgian shelf and shelf-break, as well as the entry and dispersal of sediment on the shelf. The structural interpretation has thus allowed a predictive model for shelf geometry and hence reservoir distribution to be established, and this forms the basis for continued exploration in this complex and increasingly mature play fairway.

REGIONAL SIGNIFICANCE OF THE ARQUEBUS- I WELL, BROWSE BASIN, NORTH WEST SHELF, AUSTRALIA

The Arquebus-1 well was drilled in 1991 on exploration permit WA-206-P located in the southern portion of the Browse Basin. The Browse Basin is one of the least explored offshore basins in Australia and although two major gas discoveries have been made, no significant oil accumulations have been found. The Arquebus-1 well tested Middle to Upper Jurassic sandstones in a large three-way dip closed structure along the main Jurassic basin margin fault system, which has subsequently been inverted by Tertiary wrench faulting. The well was plugged and abandoned despite the presence of numerous shows and pay indicated by wireline logs. Five formation tests were performed and despite a long 12-hour test, only mud filtrate was recovered, suggesting that significant fluid invasion had occurred. The pressure data indicate a good water gradient with a 51 m gross column of light oil and gas. This is supported by detailed analysis of fluid inclusions, capillary pressure analysis, wireline logs and sidewall cores. The total gross hydrocarbon column may be as great as 105 m. The extremely low clay content, the uniform pore throat size, the slow drilling rate and the overbalanced drill mud made the sandstones prone to the nearly complete flushing of formation fluids and associated formation damage. The presence of an inferred oil column in the Jurassic sandstones at Arquebus-1, indicates that oil has been generated in the Browse Basin and that three-way dip closed structures are potentially viable traps in the area.

Shongaloo Field: A Recent Smackover (Jurassic) Discovery in the Arkansas-Louisiana State-Line Graben

ABSTRACT The new North Shongaloo/Red Rock/Haynesville/East Haynesville field (herein called Shongaloo) is the largest discovery in the mature Gulf Coast Jurassic Smackover play in the past 20 years. Field reserves are 20-30 MMBOE, developed from over 50 producing wells. The field is located within the Arkansas-Louisiana state line graben. The previously under-explored graben bisects the mature state line Smackover play, where exploration dates back to the 1940's. Shongaloo field is an upthrown fault trap within the graben. Vertical separation on the trapping fault ranges from 500 ft. to less than 200 ft. The trapping fault can only be resolved on modern, post-1982 seismic data. Shongaloo field pay is from the Smackover C sand and B carbonate. The C sand is overlain by a key transgressive marker horizon, the marine shale. The marine shale is overlain by prograding carbonate cycles including porous ooid grainstones of the B carbonate. The C sand is a tight gas-condensate bearing sandstone which requires hydraulic fracture stimulation for economic completion. B carbonate grainstone reservoirs have average porosity of 10% and average permeability of 60 md, ranging up to 1,800 md. The majority of the field reserves are from the B carbonate. Two technical methods were advanced in the discovery of the field. First, sub-surface correlations and core descriptions of Smackover facies documented that reservoir rocks follow paleo-shorelines across the graben in the Shongaloo area. Regional stratigraphic study indicates that the vast majority of porous grainstone facies within a mappable cycle of the Smackover B are parallel to and immediately down-dip (< 1.5 mi) of paleo-shorelines. The Smackover B facies in the state line area are associated with paleo-shorelines and are not greatly affected by Jurassic fault movements. Second, the critical trapping fault was resolved by shooting and interpreting modern proprietary seismic data. The discovery of Shongaloo Field demonstrates that significant reserves can be found in mature domestic plays by the application of modern technology and stratigraphic concepts.

The structural evolution of the Snorre Field and surrounding areas

The Snorre Field is located on the Tampen Spur, Northern Viking Graben between 61°N and 62°N. Main structural elements are large, westerly tilted fault blocks. The Snorre Structure, one of the largest fault blocks in the East Shetland Basin, extends across Block 34/7 into Block 34/4. The Snorre Fault Block includes the Snorre Field, Tordis Field and the Vigdis Field oil discoveries. Major oil discoveries have so far been made in the Brent Group, the Statfjord Formation and the Lunde Formation. Discoveries have also been made in the Upper Jurassic and Paleocene. Upper Jurassic stratigraphic traps are the main target for further exploration in the area. The westerly rotated fault blocks on the Tampen Spur are delineated by NNE–SSW- to NE–SW-trending faults. From west to east, these are the Murchison Fault, the Outer Snorre Fault, the Southern Snorre Fault and the Inner Snorre Fault. Other faults recognized are the E–W- and SE–NW-striking cross-faults dividing the elongated fault blocks into minor compartments. The N–S-striking faults are common in the Snorre area. The number of these faults decreases towards the north. The Snorre Escarpment is the surface expression of the major Inner Snorre Fault representing the eastern border fault of the Snorre Fault Block. Two major rifting episodes are recognized in the area. In the thermal subsidence phase following the late Permian to early Triassic rifting, large amounts of sediments of the Teist-, Lomvi-, Lunde-, Statfjord Formation, Dunlin- and Lower Brent Group were accumulated. A second major rifting episode resulted in intense fault activity from the early Bathonian to the late Kimmeridgian. Two major pulses of increased tectonic activity are recognized in the second rift episode. In the north, in the Zeta area, there is indication of late Jurassic lateral movements. Late Jurassic faulting, block rotation and footwall uplift, combined with Jurassic to Cretaceous subsidence, make the Snorre Fault Block of the Tampen Spur a very pronounced structural high in the area.

Models of burial diagenesis for deep exploration plays in Jurassic fault traps of the Central and Northern North Sea

With increasing burial depth, intergranular porosity declines in a predictable manner according to theoretical and empirically established mathematical relationships. In basins with simple structural and thermal histories, average porosity can often be represented as an exponential function of depth. There are, however, common significant deviations from such a simple model of porosity evolution. Four mechanisms are recognized by which enhanced porosity may be generated or maintained during burial: development of overpressures due to restricted pore fluid escape; inhibition of mechanical compaction by selective cementation; inhibition of cementation as a result of hydrocarbon accumulation; or generation of secondary porosity due to either grain or cement dissolution together with simultaneous removal of reaction products. Secondary porosity may be generated either near the surface by the ingress of dilute meteoric-derived waters or at depth by the generation of ‘aggressive’ pore fluids. Deep burial diagnesis may encompass both closed and open system porosity generating reactions. In closed systems, increase in temperature concomitant with burial overcomes kinetic barriers enabling dissolution of metastable detrital minerals. By contrast, in more open systems, movement of fluids from either mudstones or evaporites to either overlying or adjacent sandstones introduces chemical disequilibrium that may cause mineral dissolution. Rapid, intermittent movement of fluids along active faults may additionally introduce temperature disequilibrium to further facilitate mineral dissolution in the proximity of faults. Over geological periods of time, however, faults are barriers to fluid flow and provide a mechanism for generating overpressure. In the Central and Northern North Sea basins, an upper diagenetic regime is separated from a lower diagenetic regime by a regional cover of Cretaceous mudstones and chalks. The Cretaceous sediments provide a regional seal which often coincides with the development of overpressure above deep graben centres reflecting restricted pore water escape. Fluid and solute movement between these two regimes is only possible where faults or gas chimneys penetrate the Cretaceous seal. In the upper regime, porosity loss in Tertiary sediments broadly conforms to the predicted decline of porosity with depth. In the lower regime, Jurassic sandstones on structural highs are characterized by extensive secondary porosity resulting from feldspar grain and carbonate cement dissolution, best preserved where oil accumulation rapidly follows porosity generation. Enhanced porosity can be expected either where sandstones interdigitate with mudstones or in graben centre plays adjacent to faults. This contrasts with graben margin plays where faults are the locus of pronounced cementation and complex diagenetic sequences are developed. In the absence of either oil accumulation or overpressure, secondary porosity either compacts or is occluded by a characteristic deep burial authigenic mineral assemblage. The present heat flow in the North Sea can be interpreted in terms of a residual thermobaric fluid flow from basin centres to basin margins which was probably initiated during the early Tertiary, coincident with the onset of rapid thermal relaxation subsidence.

Triassic to Neogene geologic evolution of the Queen Charlotte region

A wealth of new geological and geophysical data from recent studies of the Queen Charlotte region are integrated into a coherent model. In this paper, we summarize these new studies and discuss possible correlations with other areas. Four tectonostratigraphic divisions are distinguished by stratigraphic, structural, and magmatic character, and each is separated by a major unconformity. The oldest division comprises widely distributed, upper Paleozoic through Middle Jurassic strata of Wrangellia that accumulated in volcanic-arc and stable shelf and basinal settings. No significant deformation occurred in the Queen Charlotte Islands region during the accumulation of these rocks. A Middle and Upper Jurassic assemblage comprises two plutonic suites and volcanic and epiclastic rocks. The unconformity below the Middle and Upper Jurassic assemblage marks a regional, southwest-vergent contractional deformation that is the most significant Mesozoic or Cenozoic deformation in the region. Jurassic plutons in the Queen Charlotte Islands are the oldest and most primitive members of an eastwardly migrating and evolving Jura-Cretaceous magmatic front recognized by other workers in the Coast Plutonic Complex. Widespread Late Jurassic block faulting led to differential uplift and erosion of northwest-trending fault blocks. A third assemblage consists of Cretaceous marine sedimentary rocks derived principally from subjacent Jurassic volcanic rocks as well as older strata. The present distribution of Cretaceous strata reflects a gradual eastward transgression, briefly interrupted in the Coniacian by progradation of conglomerate fans from the east. A second regional contractional deformation event in latest Cretaceous time was concentrated along a northwest-trending zone coinciding with Jurassic block faults. The early Tertiary marked another distinct shift in sedimentation style, with the inception of local nonmarine deposition on the present islands and widespread volcanism and plutonism on the southern islands. Syntectonic deposition in offshore extensional basins (Hecate Strait and Queen Charlotte Sound) may have commenced at this time. Later in the Tertiary, extensive deposition occurred in offshore regions, coeval with northward migration of plutonism and volcanism on the islands. Contractional structures in Pliocene sediments in Hecate Strait are the youngest deformational features observed.

The influence of tectonics on early Cretaceous sedimentation in Bedfordshire, England

The Lower Cretaceous sediments of the Bedfordshire area are Aptian and Albian in age. During this time the area was situated within an extensional tectonic regime, and the pattern of sedimentation, in terms of lateral variation both in sediment type and thickness, reflects the influence of normal faulting. Jurassic topography, created by block faulting and erosion, controlled the distribution of later facies largely by continued tectonic processes, especially the reactivation of the Jurassic fault block system, which produced syn-sedimentary faulting of the overlying sequence. Thus the ‘traditional’ view of the infill of an erosional basin by the Lower Greensand is challenged.

Geological evidence for intra-Jurassic faulting in the Wessex Basin and its margins

Geological observations on Jurassic outcrops close to major faults in the Wessex Basin–Mendip area reveal the local presence of ammonite- and brachiopod-bearing sediments penetrating underlying strata. Toarcian and Bajocian neptunian dykes and particularly sills are associated with the Eypemouth Fault and Bajocian sills with the Bride Fault and Mere Fault. In the Mendip area numerous neptunian dykes of Hettangian, Sinemurian, Pliensbachian and Bajocian ages, cross-cutting Carboniferous Limestone, are recorded, typically also associated with major basement faults (e.g. Cranmore and Leighton Faults). These periods of assumed sediment injection are taken as indicating times of displacement along the faults in question. Variations in facies (Hettangian–Sinemurian, Toarcian, Bajocian, uppermost Oxfordian, Kimmeridgian) spatially linked to faults are documented from some areas, and boreholes reveal considerable fault-controlled thickness changes in Hettangian–Sinemurian, Bajocian and Kimmeridgian sediments. The timing of Jurassic faulting in the Wessex Basin–Mendip area thus polarizes into two intervals: Hettangian–Bajocian and latest Oxfordian onwards, correlating with the early rifting phases of the Central and North Atlantic respectively.

The Exploration Potential of Offshore Northwest Korea (P.D.R.K.)

Recent political changes have demonstrated that previously taboo countries are now becoming fair game for western explorationists. Numerous areas or basins that have not been the focus of high technology - Technologically Attenuated Basins (TABs) - offer a new challenge for the new venture groups of E and P companies. Most recently the USSR together with other Eastern European countries continue to be a source of technical interest and frustration. The People's Democratic Republic of Korea, North Korea, possibly the most isolated of the Communist block, contains several TABs where there has been minimal exploration. One such TAB is West Korea Bay, which covers an area of 25,000 km{sup 2} containing at least one major Tertiary basin. The tectonic evolution of the Tertiary basin is similar to the intracratonic Chinese basins with significant differences, notably the Songnim and Daebo orogenies (Middle Triassic to Upper Jurassic and Jurassic to middle Cretaceous) that resulted in extensive igneous activity, folding, and thrust faulting, followed by an extensional stress regime during the Mesozoic and Cainozoic. Very few wells have been drilled in West Korea Bay in the past decade (one per 2,500 km{sup 2}). Though commercially unsuccessful, the wells have proven the existence ofmore » oil, mature source rocks, and reservoirs (Jurassic, Cretaceous, and Oligo-Miocene). Structural plays such as rotated Jurassic and Cretaceous fault blocks predominate, but there is also potential for higher risk stratigraphic potential in the Jurassic and Tertiary, with expected field size distributions in the 20-180 MMBOR range.« less

The Clyde Field, Block 30/17b, UK North Sea

Abstract The Clyde Field, which was discovered in 1978, is located on the SW edge of the North Sea Central Graben. The reservoir is developed with Late Jurassic shallow marine sands of the Fulmar Sand Formation. An estimated 408 MMBBL of oil is present (Annex B), of which 154 MMBBL is considered recoverable. The structure of the Clyde Field takes the form of a rotated Jurassic fault block, truncated at its crest by a major unconformity. Oil is retained within a combination trap, sourced from Late Jurassic Kimmeridge Clay thermally matured in the highly productive basinal lows, adjacent to the field. Reservoir sand quality is highly variable, ranging from excellent with permeabilities in excess of Id, to poor with permeabilities of less than 1 md. The principal control on reservoir quality appears to be original depositional texture, although strong diagenetic effects are also present. Production is from a single, centrally located, platform provided with thirty slots. Aquifer support is insufficient to maintain reservoir pressure at the current plateau production rate of 50 000 BOPD and so a programme of water injection has been implemented.