Sable Subbasin Research Articles

Self-supervised learning for efficient seismic facies classification

Seismic facies classification is an important task in seismic interpretation that allows the identification of rock bodies with similar physical characteristics. Manual labeling of seismic data is immensely time consuming, given the recent surge in data volumes. Self-supervised learning (SSL) enables models to learn powerful representations from unlabeled data, thereby improving performance in downstream tasks using limited labeled data. We investigate the effectiveness of SSL for efficient facies classification by evaluating various convolutional and vision transformer-based models. We pretrain the models on image reconstruction and fine-tune them on facies segmentation. Results on the southern North Sea F3 seismic block in the Netherlands and the Penobscot seismic volume in the Sable Subbasin, offshore Nova Scotia, Canada, show that SSL has comparable performance to supervised learning using only 5%–10% labeled data. Further, SSL exhibits stable domain adaptation on the Penobscot data set even with 5% labeled data, indicating an improved generalization compared with the supervised learning setup. The findings demonstrate that SSL significantly enhances model accuracy and data efficiency for seismic facies classification.

Chemostratigraphic Analysis of the Cretaceous Logan Canyon Formation, Sable Sub-Basin, Offshore Nova Scotia: Implications for Reservoir Characterization and Fluid Flow

Chemostratigraphic Analysis of the Cretaceous Logan Canyon Formation, Sable Sub-Basin, Offshore Nova Scotia: Implications for Reservoir Characterization and Fluid Flow

Chemostratigraphic Analysis of the Cretaceous Logan Canyon Formation, Sable Sub-Basin, Offshore Nova Scotia: Implications for Reservoir Characterization and Fluid Flow

Chemostratigraphic Analysis of the Cretaceous Logan Canyon Formation, Sable Sub-Basin, Offshore Nova Scotia: Implications for Reservoir Characterization and Fluid Flow

IDENTIFCATION OF HIGH VELOCITY ANOMALIES, IMPERCEPTIBLE TO SEISMIC RESOLUTION, BY INTEGRATION OF SEISMIC ATTRIBUTES, IN PENOBSCOT FIELD, CANADA

Penobscot Field is located in the Sable Sub-basin, in the Nova Scotia Basin, belonging to the Canadian territory, and for the year of 1991, a 3D seismic acquisition was carried out in this region. Also, two exploratory wells were previously drilled (L-30 and B-41). Then, with the subsequent interpretation of the area, a discrepancy was found around 10ms to 15ms (approximately 25m) in the travel times of the seismic waves in the vicinity of well B-41, resulting in a false structural high in the Mississauga Formation and Naskapi Member. It is possibly caused by changes in geological facies and thickness of the Wyandot formation, consequently, B-41 well became dry. For these reasons, to identify the possible problem which caused everything mentioned above, the area between the wells was delimited by a seismic subvolume (by reducing the seismic cube), making a seismic interpretation of the top of the formations of interest, as well as an analysis of certain petrophysical properties in the wells, in order to delimit the thickness of the anomaly and, consecutively, the use of the seismic attributes for lateral identification of the anomaly in the Wyandot Formation.

SOLUTION OF HIGH VELOCITY ANOMALIES IMPERCEPTIBLE TO SEISMIC RESOLUTION, BY MEANS OF SYNTHETIC MODELS, PENOBSCOT FIELD, CANADA

Penobscot Field is located in the Sable Sub-basin in Nova Scotia, Canada, where a 3D seismic acquisition campaign was carried out in 1991 and also two oil wells were previously drilled (L-30 and B-41). In the interpreted seismic data, a discrepancy was found in the travel times of the seismic waves near well B-41, causing a false structural height in the Naskapi Member, Mississauga Formation (Early Cretaceous or Lower Cretaceous) and deeper formations (10 to 15 ms, i.e. approximately 25 m). It was decided to find a solution of this problem using synthetic models. First, making a delimitation of the study area by means of a seismic subcube, which were later interpreted surfaces of interest, through the well data. The average velocity and density properties were found for each interpreted strata. Subsequently, a 2D seismic acquisition was simulated, choosing a line that crossed both wells and through the use of ray traces, synthetic shot gathers were obtained and processed through the use of different seismic migration tests, and the final solution was found to be Kirchhoff pre-stacking migration in depth.

Interpretation of the Penobscot 3D seismic volume using constrained sparse spike inversion, Sable sub-Basin, offshore Nova Scotia

In this study we investigate the utility of constrained sparse spike inversion (CSSI) applied to a small (87 km2), public domain, 3D seismic volume at Penobscot in the Sable sub-Basin, offshore Nova Scotia. The objective is to establish the effectiveness of this technique at Sable, in mapping absolute acoustic impedance that can be directly and quantitatively related to lithology. The intent is to subsequently apply the workflow to large 3D seismic volumes sub-regionally, with the goal of identifying and mapping, source, seal, reservoir, and overpressured intervals either not known, or partially known, from well control. Absolute acoustic impedance is an incremental parameter that is not explicit in the reflection amplitude cube and is the primary driver for undertaking this inversion. It has quantitative advantages over relative acoustic impedance that can be derived using ‘fast-track’ inversion techniques. CSSI at Penobscot has been completed and provides a valid geological model in the upper Jurassic to lower Cenozoic interval that is constrained by wireline logs at two well penetrations (one of which has 12 m net pay). This acoustic impedance volume facilitates interpretation of: (1) low impedance Cretaceous reservoir sandstones, in both complex confined channel systems and extensive unconstrained marginal marine systems; (2) polygonal fault systems (PFS) in a high impedance, late Cretaceous, chalk; (3) interfingering of low impedance shales and high impedance carbonates at the margin of the Jurassic Abenaki Carbonate Bank.Three other inversion studies have been reported at Penobscot: Ahmad, 2013; Sayers 2013; Qayyum et al., 2014. Relative acoustic impedance is derived in each study, using ‘recursive’, ‘spectral’ and ‘coloured’ inversion algorithms respectively. None of these studies documented the PFS and Cretaceous channel systems described here and there are significant interpretation differences in the Jurassic section, which is only partly penetrated by wells at Penobscot.Based on this work at Penobscot, CSSI is the optimum technique for progressively building large sub-regional scale inversions at Sable where we need to balance work-effort, signal-to-noise ratio (spectral inversion results are very broadband but appear very noisy), reliable quantitative mapping of absolute acoustic impedance, and the identification of stratigraphic features on horizon-slices (for which ‘fast track’ inversions are also be suitable).

Detrital quartz sources in the Scotian Basin, eastern Canada, using hot-cathode cathodoluminescence: Availability of coarse-grained sand for reservoirs

Quartz is the principal framework mineral in clastic sediment reservoirs. In a frontier basin with sparse wells, the source of quartz in sandstones may be a predictor of the availability of medium- to coarse-grained quartz sand from plutonic sources, likely to provide good reservoirs. The Scotian Basin, offshore eastern Canada, was used to test this hypothesis because of its well-understood provenance history and geographic variability in known medium- to coarse-grained reservoir sandstones. The sources of detrital quartz in fine-grained sandstones were determined using hot-cathode cathodoluminescence (CL), supplemented by other petrographic techniques. The CL color shift for different quartz types was calibrated against the CL properties of representative source rocks in the hinterland, because generalizations in the literature do not precisely match our basin-specific observations. Grain size of sandstone exerts a strong control over quartz type, with plutonic-hypabyssal quartz and high-grade metamorphic quartz more abundant in coarse-grained sandstones and low-grade metamorphic quartz more abundant in fine-grained sandstones. Nevertheless, the analysis of fine-grained sandstones shows that plutonic-hypabyssal quartz is more abundant in fine-grained sandstones of the Sable subbasin than in those of the Abenaki subbasin. The abundance of plutonic-hypabyssal quartz correlates with the abundance of medium- to coarse-grained sandstone reservoirs in the Sable subbasin. This study suggests that, in frontier basins, the abundance of plutonic-hypabyssal quartz in fine-grained sandstones can be used as an indicator of available medium- to coarse-grained sandstone reservoir.

Detrital zircon geochronology and polycyclic sediment sources, Upper Jurassic – Lower Cretaceous of the Scotian Basin, southeastern Canada 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin.

Sources of Tithonian–Albian sediment in the Scotian Basin are interpreted from detrital zircon geochronology to test previous hypotheses about the sources and pathways of sediment to thick deltaic successions that are important hydrocarbon reservoirs. Sediment provenance influences reservoir quality, but also provides information on tectonism during rifting of the North Atlantic Ocean. More than 760 zircons were dated by laser ablation U–Pb methods from nine offshore wells and one borehole on land and were characterized by external morphology, internal zoning, and Th/U ratio. A Meguma terrane source to the LaHave Platform was confirmed by peaks in detrital zircon abundance at 550–650 Ma, 1.0–1.2 Ga, and ∼2.1 Ga. Samples from the Sable Subbasin show a large peak in detrital zircon abundance at ∼1050 Ma, with lower peaks from 400–650, ∼1480, ∼1650, ∼1860 Ma and 2.7 Ga, characteristic of inboard Appalachian terranes of Laurentide affinity. Many late Paleozoic to Neoproterozoic zircons are euhedral or subhedral, and apparently first cycle, as are a few older zircons that indicate transport from the rising rift shoulder in southern Labrador as far north as the Makkovik Province (∼1860 Ma). About half the zircons are rounded and polycyclic. Samples from the Abenaki Subbasin are similar, but late Paleozoic to Neoproterozoic zircons are rare and ∼40% of the Mesoproterozoic zircons are subhedral, implying a different Laurentide source through the Humber valley. Euhedral–subhedral unzoned zircons yielded two groups of Cretaceous dates: ∼105 Ma from the Cree Member, and ∼120 Ma from the Missisauga Formation.

Early Cretaceous volcanism in the Scotian Basin 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin.

Early Cretaceous volcanism is widespread in the eastern Scotian Basin. The stratigraphic position of volcanic rocks within wells was re-evaluated and the volcanological character of the rocks was refined by study of cuttings and well logs. Hauterivian–Barremian volcanic rocks on the SW Grand Banks and Aptian–Albian volcanic rocks in the Orpheus Graben and SE Scotian Shelf resulted from Strombolian type eruptions. More extensive Hawaiian type flows were mapped from seismic profiles near the Mallard and Brant wells on the SW Grand Banks and they appear to have been derived from local basement highs with a positive magnetic anomaly interpreted as volcanic centres. Igneous rocks in the Hesper well on the SE Scotian Shelf are the erosional remnant of basaltic flows that terminated at the paleoshoreline. They correlate with basalt flows both in extensive outcrop on Scatarie Ridge and in several Orpheus Graben wells. The interpretation of the Hesper basalts as an erosional remnant of more extensive basalt flows is consistent with detrital petrographic evidence for substantial uplift of the inboard part of the Scotian Basin in the Hauterivian–Aptian. Widespread volcanic activity indicates a regional and long-lived magma source, which resulted in elevated regional heat flow. Effects of this heat flow are seen in sedimentary rocks of the Sable Subbasin and it had a discernable impact on hydrocarbon maturation.

Controls on diagenesis of Lower Cretaceous reservoir sandstones in the western Sable Subbasin, offshore Nova Scotia

Lower Cretaceous deltaic sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. The influence of several factors on diagenesis has been investigated: depositional lithofacies, sea level changes, chemistry of basinal sediments and basinal fluid flux during burial. The distribution and chemistry of diagenetic minerals was determined from nine wells located along a dip section of the Sable Subbasin. Mineral type and paragenesis were characterized using a combination of optical petrography, back-scattered electron images, and electron microprobe analyses. Siderite is unusually abundant in marine sediments of the Scotian Basin and has thus been studied in detail. Siderite occurs in several generations. Early and late siderites are similar in chemical composition, suggesting buffering by pre-existing siderite, but rare low-Mg siderite is related to a greater contribution of meteoric water. Siderite has locally dissolved to create microporosity and has suppressed quartz overgrowths. Siderite is most common in those muddy prodeltaic lithofacies where there is the highest availability of detrital ilmenite. Reactive Fe released by breakdown of this ilmenite is responsible for the unusual presence of early siderite in marine sediments, with the Ca and Mg content of the siderite indicating fully marine waters. Lithofacies have a strong influence on early diagenetic mineral assemblages. Lithofacies deposited in the transgressive system tract have abundant early Fe-calcite and siderite. Early kaolinite occurs principally in proximal (fluvial and river mouth) lithofacies, where meteoric water was most likely available during the deposition. Contrary to other studies, we find little impact of sequence stratigraphy on diagenetic minerals except in the transgressive system tract. Mesogenetic minerals are related to flux of formation water and maturing hydrocarbon products, resulting first in pyrite and siderite and later in ankerite and ferroan calcite. The principal controls are interpreted to be high Fe 2+ and low Ca 2+ of formation waters and vertical movement of the waters along faults. Overall, the bulk chemical composition of terrigenous sediments and the depositional lithofacies are the most important factors controlling diagenetic minerals in the Lower Cretaceous of the Scotian Basin.

Development of a major polygonal fault system in Upper Cretaceous chalk and Cenozoic mudrocks of the Sable Subbasin, Canadian Atlantic margin

Detailed interpretation of 2D and 3D seismic data from the Sable Subbasin (continental margin of eastern Canada) has revealed an extensive polygonal fault system affecting Upper Cretaceous chalk of the Wyandot Formation (the Wyandot Chalk) and Cenozoic mudrocks of the Banquereau Formation. This paper offers the first detailed study of a basin-scale polygonal fault system affecting a chalk-dominated unit. The Wyandot Chalk is up to 300 m thick and pervasively deformed by small-scale faults that are morphologically and quantitatively similar to those previously described from polygonal fault systems observed in mudrocks of the North Sea, and elsewhere. This suggests that they may have formed due to a common mechanism. The data presented in this paper point towards a lithological control upon the development of the polygonal fault system and argue that the syneresis hypothesis posed by Cartwright and Dewhurst [Geol. Soc. Am. Bull. 110 (1998) 1242] best explains the distribution and initiation of the analysed fault system. 3D seismic attribute maps reveal an orthogonal intersection relationship between polygonal faults and tectonic faults in the study area. This relationship is interpreted to indicate that local stress perturbations around the tectonic faults influenced the propagation of the polygonal faults.

Early Cretaceous sediment failure in the southwestern Sable Subbasin, offshore Nova Scotia

Conventional cores from Lower Cretaceous outer-shelf and upper-slope prodelta facies in the Alma and Glenelg fields of the Scotian Basin show a wide range of synsedimentary deformation. Storm-dominated prodelta sandstone and mudstone beds have common load casts and structureless sandstone beds overlain by deformed sediment and sandstone dykes that are caused by storm- or earthquake-induced local liquefaction. Blocks of sediment 5–15 m (15–50 ft) thick, principally on the outer shelf, have foliated mudstone at their base, common internal shear zones in mudstone, shear deformation of sandstone, and are capped by intraclast conglomerates that are interpreted as debris-flow deposits. These are all features observed in shallow slides on the modern Mississippi prodelta. At the paleoshelf edge and upper slope, larger slide blocks are recognized with zones of intense internal deformation and high-angle thrusts near their base. The lack of a basal foliated mudstone may be the result of more rapid slide motion than on the shelf, resulting in hydroplaning. In addition, there is early postdepositional interstratal deformation as a result of loading by sandy delta distributaries and by slide blocks on the upper slope. The scale of many deformation structures is below the limits of three-dimensional seismic vertical resolution, yet they are likely to substantially influence reservoir properties.

Influence of poroelastic behavior on the magnitude of minimum horizontal stress, Sh in overpressured parts of sedimentary basins

In many sedimentary basins of the world the minimum hori- zontal stress, S h , is greater in overpressured zones than in normally pressured zones at equivalent depths. A common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress, S v and S h within certain bounds, and the difference is smaller under lower effective stress (i.e., higher pore pressure, P p ). However, in the overpressured parts of the central North Sea graben, United Kingdom, and the Sable subbasin of the Scotian Shelf, Canada, conventional friction envelopes underestimate the magnitude of S h . These data instead indicate that S h increases at a rate proportional to but less than the rate of increase of P p , a condition consistent with a P p -induced deformation of the rock called poroelastic behavior. This paper argues that, whereas friction may govern S h in normally pressured basins, poroelastic behavior is responsible for the unusually high S h in the overpres- sured parts of these same basins. Data on the P p and S h gradients from these basins suggest that Δ S h /Δ P p ∼ 0.7.

Petroleum Geochemistry of the Verrill Canyon Formation: A Source for Scotian Shelf Hydrocarbons

ABSTRACT A variety of maturation indicators from 55 wells have been used to construct a maturation facies map for the base of the Cretaceous sediments on the Scotian Shelf. The Verrill Canyon Formation is largely mature in the Sable subbasin and contains terrestrially derived (Type III) organic matter. This organic matter is largely gas-prone except for an area in the northern part of the Sable subbasin where it has some oil potential. This oil potential is minor, however, because of the relatively low content of organic matter (C% = 1.20 ± 0.29%) in the Verrill Canyon sediments. On the basis of analysis of gasoline-range and aromatic hydrocarbons from sediment and oil/condensate samples, the Verrill Canyon Formation is considered to be the source of hydrocarbon reservoirs in the Logan Canyon, Verrill Canyon, Mic Mac and Missisauga Formations. The composition of the hydrocarbon product is related to the degree of maturation of the source rock. Oils and condensates occurring in the Logan Canyon Formation in the West Sable and Cohasset structures are more mature than those in stratigraphically older and deeper reservoirs. This maturity reflects vertical migration of those hydrocarbons from the deep Verrill Canyon source. Variations in maturation of the source have been found to have a profound effect on the composition of gasoline-range and aromatic fractions of the generated hydrocarbons.