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Bulletin Article

A transition from carbonate shelf to pelagic basin environments of deposition: Rifting and depositional systems in the Jurassic of northeastern Tunisia

Jurassic deposition in the Maghrebian tethys was governed by eustasy and rifting. Two periods were delineated: (1) a carbonate shelf (Rhaetian–early Pliensbachian) and (2) a platform-basin complex (early Pliensbachian–Callovian). The carbonate shelf evolved in four stages, generating three sedimentary sequences, J1 to J3, separated by boundary sea level falls, drawdown, exposure, and local erosion. Sediment facies bear evidence of sea level rises and falls. Lateral changes in lithofacies indicate shoaling and deepening upward during the Sinemurian. A major pulse of rifting with an abrupt transition from carbonate shelf to pelagic basin environments of deposition marks the upper boundary of the lower Pliensbachian carbonate shelf deposits. This rifting episode with brittle fractures broke up the Rhaetian–early Pliensbachian carbonate shelf and has created a network of grabens, half grabens, horsts, and stacked ramps. Following this episode, a relative sea level rise led to pelagic sedimentation in the rift basins with local anoxic environments that also received debris shed from uplifted ramp crests. Another major episode spanning the whole early Pliensbachian–Bajocian is suggested by early brecciation, mass flows, slumps, olistolites, erosion, pinch-outs, and sedimentary prisms. A later increase in the rates of drifting marked a progress toward rift cessation during the Late Jurassic. These Jurassic carbonates with detrital deposits and black shales as the source rocks in northeastern Tunisia may define interesting petroleum plays (pinch-out flanking ramps, onlaps, and structurally upraised blocks sealed inside grabens). Source rock maturation and hydrocarbon migration began early in the Cretaceous and reached a maximum during the late Tortonian–Pliocene Atlassic orogeny.

Added on 30 June, 2013

Explorer Emphasis Article By Louise Durham

Diverse Dynamics Impact Bakken Productivity

Complex considerations: Mention the Bakken Formation and most people think of unlimited potential – but several dynamics have a huge impact on productivity.

Added on 01 June, 2013

Explorer Emphasis Article By Louise Durham

Unconventional Uteland Butte Sparks New Utah Activity

Who’s got the last laugh now? The Uteland Butte once was a sandstone that operators quickly passed through – and often ignored – on their way to other targets. But things are changing in Utah.

Added on 01 June, 2013

Bulletin Article

Permian–Holocene tectonostratigraphic evolution of the Mandal High, Central Graben, North Sea

A three-dimensional seismic data set and published data from exploration wells were used to reconstruct the tectonostratigraphic evolution of the Mandal High area, southern North Sea, Norway. The Mandal High is an elongated southeast-northwest–trending horst. Three fault families in the Lower Permian sequence, inherited from the basement structural grain of Caledonian origin, are interpreted: (1) a north-northwest–south-southeast–striking fault family, (2) a northeast-southwest–striking fault family, and (3) a near east-west–striking fault family. In addition, an east-southeast–west-northwest–striking fault family (4) that formed during Late Jurassic rifting and was reverse reactivated in the Late Cretaceous is interpreted. We suggest that inversion occurred because of small dextral motion along fault family 1. A final fault family (5) displays various strike orientations and is associated with salt movements. Seven chronostratigraphic sequences defined by well data and recognized on three-dimensional seismic data are interpreted and mapped: Early Permian rifting in a continental environment; Late Permian deposition of the Zechstein salt and flooding; Triassic continental rifting; uplift and erosion in the Middle Jurassic with deposition of shallow-marine and deltaic sediments; rifting and transgression in a deep-marine environment during the Late Jurassic; a post-rift phase in a marine environment during the Early Cretaceous; and flooding and deposition of the Chalk Group in the Late Cretaceous. An eighth sequence was interpreted—Paleogene–Neogene—but has not been studied in detail. This sequence is dominated by progradation from the east and basin subsidence. Well and seismic data over the Mandal High reveal that large parts of the high were subaerially exposed from Late Permian to Late Jurassic or Early Cretaceous, providing a local source of sediments for adjacent basins. Similar to the Utsira High, where several large hydrocarbon discoveries have been recently seen, the Mandal High might consist of a set of petroleum plays, including fractured crystalline basement and shallow-marine systems along the flanks of the high, thereby opening up future exploration opportunities.

Added on 31 May, 2013

Bulletin Article

Controls on the deposition and preservation of the Cretaceous Mowry Shale and Frontier Formation and equivalents, Rocky Mountain region, Colorado, Utah, and Wyoming

Regional variations in thickness and facies of clastic sediments are controlled by geographic location within a foreland basin. Preservation of facies is dependent on the original accommodation space available during deposition and ultimately by tectonic modification of the foreland in its postthrusting stages. The preservation of facies within the foreland basin and during the modification stage affects the kinds of hydrocarbon reservoirs that are present. This is the case for the Cretaceous Mowry Shale and Frontier Formation and equivalent strata in the Rocky Mountain region of Colorado, Utah, and Wyoming. Biostratigraphically constrained isopach maps of three intervals within these formations provide a control on eustatic variations in sea level, which allow depositional patterns across dip and along strike to be interpreted in terms of relationship to thrust progression and depositional topography. The most highly subsiding parts of the Rocky Mountain foreland basin, near the fold and thrust belt to the west, typically contain a low number of coarse-grained sandstone channels but limited sandstone reservoirs. However, where subsidence is greater than sediment supply, the foredeep contains stacked deltaic sandstones, coal, and preserved transgressive marine shales in mainly conformable successions. The main exploration play in this area is currently coalbed gas, but the enhanced coal thickness combined with a Mowry marine shale source rock indicates that a low-permeability, basin-centered play may exist somewhere along strike in a deep part of the basin. In the slower subsiding parts of the foreland basin, marginal marine and fluvial sandstones are amalgamated and compartmentalized by unconformities, providing conditions for the development of stratigraphic and combination traps, especially in areas of repeated reactivation. Areas of medium accommodation in the most distal parts of the foreland contain isolated marginal marine shoreface and deltaic sandstones that were deposited at or near sea level lowstand and were reworked landward by ravinement and longshore currents by storms creating stratigraphic or combination traps enclosed with marine shale seals. Paleogeographic reconstructions are used to show exploration fairways of the different play types present in the Laramide-modified, Cretaceous foreland basin. Existing oil and gas fields from these plays show a relatively consistent volume of hydrocarbons, which results from the partitioning of facies within the different parts of the foreland basin.

Added on 31 May, 2013

Explorer Historical Highlights By Richard Vincelette

Lucas and the Michigan Basin Reef Project

This story is an illustration of how a single geologist working for a major company can develop new exploration concepts in a “mature basin” and turn it into one of that company’s most successful plays in the onshore United States.

Added on 01 May, 2013

Explorer Emphasis Article By Louise Durham

Mississippi Lime – A 'Thoughtful' Challenge

Across the board: The Mississippi Lime play proves that hydraulic fracturing and horizontal drilling techniques can be used for more than just shale production.

Added on 01 May, 2013

Bulletin Article

Outcrop analog for an oolitic carbonate ramp reservoir: A scale-dependent geologic modeling approach based on stratigraphic hierarchy

Considerable effort has been devoted to the development of simulation algorithms for facies modeling, whereas a discussion of how to combine those techniques has not existed. The integration of multiple geologic data into a three-dimensional model, which requires the combination of simulation techniques, is yet a current challenge for reservoir modeling. This article presents a thought process that guides the acquisition and modeling of geologic data at various scales. Our work is based on outcrop data collected from a Jurassic carbonate ramp located in the High Atlas mountain range of Morocco. The study window is 1 km (0.6 mi) wide and 100 m (328.1 ft) thick. We describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest: (1) stacking pattern of high-frequency depositional sequences, (2) facies association, and (3) lithofacies. Five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data. The surface-based model shows a low-angle profile with modest paleotopographic relief at the inner-to-middle ramp transition. Facies associations were populated using truncated Gaussian simulation to preserve ordered trends between the inner, middle, and outer ramps. At the lithofacies scale, field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation. This study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks. The selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings. This study demonstrates that a scale-dependent modeling approach should be a common procedure when building subsurface and outcrop models.

Added on 30 April, 2013

Bulletin Article

Modern internal waves and internal tides along oceanic pycnoclines: Challenges and implications for ancient deep-marine baroclinic sands

Thus far, the subject of deep-marine sands emplaced by baroclinic currents associated with internal waves and internal tides as potential reservoirs has remained an alien topic in petroleum exploration. Internal waves are gravity waves that oscillate along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100–200 m [328–656 ft] in bathymetry) and in the deep ocean over areas of sea-floor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations in the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans reveal that internal solitary waves travel in packets. Internal waves commonly exhibit (1) higher wave amplitudes (5–50 m [16–164 ft]) than surface waves (2 m [6.56 ft]), (2) longer wavelengths (0.5–15 km [0.31–9 mi]) than surface waves (100 m [328 ft]), (3) longer wave periods (5–50 min) than surface waves (9–10 s), and (4) higher wave speeds (0.5–2 m s–1 [1.64–6.56 ft s–1]) than surface waves (25 cm s–1 [10 in. s–1]). Maximum speeds of 48 cm s–1 (19 in. s–1) for baroclinic currents were measured on guyots. However, core-based sedimentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are lacking. No cogent sedimentologic or seismic criteria exist for distinguishing ancient counterparts. Outcrop-based facies models of these deposits are untenable. Therefore, potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, basin-floor fans, and others. Economic risks associated with such misinterpretations could be real.

Added on 30 April, 2013

Bulletin Article

Three-dimensional paleomorphologic reconstruction and turbidite distribution prediction revealing a Pleistocene confined basin system in the northeast Nankai Trough area

Integrated three-dimensional (3-D) paleomorphologic and sedimentary modeling was used to predict the basin architecture and depositional pattern of Pleistocene forearc basin turbidites in a gas hydrate field along the northeast Nankai Trough, off central Japan. Structural unfolding and stratigraphic decompaction of the targeted stratigraphic unit resulted in successful modeling of the paleobathymetry at the time of deposition. This paleobathymetry was characterized by a simple U-shaped paleominibasin. Subsequent turbidity current modeling on the reconstructed paleobathymetric surface demonstrated morphologically controlled turbidity current behavior and selective turbidite sand distribution within the minibasin, which strongly suggests the development of a confined turbidite system. Among three candidate inflow patterns, a northeasterly inflow pattern was determined as most likely. In this scenario, flow reflection and deflection caused ponding and a concentration of sandy turbidite accumulation in the basin center, which facilitated filling of the minibasin. Such a sedimentary character is undetected by seismic data in the studied gas hydrate reservoir formation because of hydrate-cementation–induced seismic anomalies. Our model suggests that 3-D horizon surfaces mapped from 3-D seismic data along with well-log data can be used to predict paleobasin characteristics and depositional processes in deep-water turbidite systems even if seismic profiles cannot be determined because of the presence of gas hydrates.

Added on 30 April, 2013

DL Abstract By Lesli Wood

The Shelf is a Dangerous Place: Surfing the Cretaceous Seaways

Engineering of wind farms, development of carbon sequestration projects in shelfal waters, the proliferation of communication cables that connect the world, all of these things suggest that it is time to re-examine what we know about shelf processes both updip-to-downdip and along shoreline, and the influence of shelf processes on erosion and transport of sediments.

Request a visit from Lesli Wood!

Added on 19 August, 2024

DL Abstract By Tao Sun

Integrated Reservoir Characterization and Modeling with Computational Stratigraphy

As oil and gas exploration and production occur in deeper basins and more complex geologic settings, accurate characterization and modeling of reservoirs to improve estimated ultimate recovery (EUR) prediction, optimize well placement and maximize recovery become paramount. Existing technologies for reservoir characterization and modeling have proven inadequate for delivering detailed 3D predictions of reservoir architecture, connectivity and rock quality at scales that impact subsurface flow patterns and reservoir performance. Because of the gap between the geophysical and geologic data available (seismic, well logs, cores) and the data needed to model rock heterogeneities at the reservoir scale, constraints from external analog systems are needed. Existing stratigraphic concepts and deposition models are mostly empirical and seldom provide quantitative constraints on fine-scale reservoir heterogeneity. Current reservoir modeling tools are challenged to accurately replicate complex, nonstationary, rock heterogeneity patterns that control connectivity, such as shale layers that serve as flow baffles and barriers.

Request a visit from Tao Sun!

Added on 20 July, 2022

VG Abstract By Keith Gerdes

Tethyan Carbonates - Linking Tropical Seas and Crashing Plates to Some of the Largest Oil and Gas Fields in the World

The carbonate sequences that were deposited in the now exhumed Tethyan Ocean influence many aspects of our lives today, either by supplying the energy that warms our homes and the fuel that powers our cars or providing the stunning landscapes for both winter and summer vacations. They also represent some of the most intensely studied rock formations in the world and have provided geoscientists with a fascinating insight into the turbulent nature of 250 Million years of Earth’s history. By combining studies from the full range of geoscience disciplines this presentation will trace the development of these carbonate sequences from their initial formation on the margins of large ancient continental masses to their present day locations in and around the Greater Mediterranean and Near East region. The first order control on growth patterns and carbonate platform development by the regional plate-tectonic setting, underlying basin architecture and fluctuations in sea level will be illustrated. The organisms that contribute to sequence development will be revealed to be treasure troves of forensic information. Finally, these rock sequences will be shown to contain all the ingredients necessary to form and retain hydrocarbons and the manner in which major post-depositional tectonic events led to the formation of some of the largest hydrocarbon accumulations in the world will be demonstrated.

Request a visit from Keith Gerdes!

Added on 24 August, 2015

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