The Gulf of Mexico (GOM) is the 9th largest body of water on earth, covering an area of approximately 1.6 million km2 with water depths reaching 4,400 m (14,300’). The basin formed as a result of crustal extension during the early Mesozoic breakup of Pangaea. Rifting occurred from the Late Triassic to early Middle Jurassic. Continued extension through the Middle Jurassic combined with counter-clockwise rotation of crustal blocks away from North America produced highly extended continental crust in the subsiding basin center. Subsidence eventually allowed oceanic water to enter from the west leading to thick, widespread, evaporite deposition. Seafloor spreading initiated in the Late Jurassic eventually splitting the evaporite deposits into northern (USA) and southern (Mexican) basins. Recent work suggests that this may have been accomplished by asymmetric extension, crustal delamination, and exposure of the lower crust or upper mantle rather than true sea floor spreading (or it could be some combination of the two). By 135 Ma almost all extension had ceased and the basic configuration of the GOM basin seen today was established. The Laramide Orogeny was the last major tectonic event impacting the GOM. It caused uplift and erosion for the NW margin from the Late Cretaceous to early Eocene.
Criteria for recognizing stratigraphic sequences are well established on continental margins but more challenging to apply in basinal settings. We report an investigation of the Upper Devonian Woodford Shale, Permian Basin, west Texas based on a set of four long cores, identifying sea level cycles and stratigraphic sequences in an organic-rich shale.
The Woodford Shale is dominated by organic-rich mudstone, sharply overlain by a bioturbated organic-poor mudstone that is consistent with a second-order eustatic sea level fall. Interbedded with the organic-rich mudstone are carbonate beds, chert beds, and radiolarian laminae, all interpreted as sediment gravity-flow deposits. Bundles of interbedded mudstone and carbonate beds alternate with intervals of organic-rich mudstone and thin radiolaria-rich laminae, defining a 5–10 m (16–33 ft)-thick third-order cyclicity. The former are interpreted to represent highstand systems tracts, whereas the latter are interpreted as representing falling stage, lowstand, and transgressive systems tracts. Carbonate beds predominate in the lower Woodford section, associated with highstand shedding at a second-order scale; chert beds predominate in the upper Woodford section, responding to the second-order lowstand.
Additional variability is introduced by geographic position. Wells nearest the western margin of the basin have the greatest concentration of carbonate beds caused by proximity to a carbonate platform. A well near the southern margin has the greatest concentration of chert beds, resulting from shedding of biogenic silica from a southern source. A well in the basin center has little chert and carbonate; here, third-order sea level cycles were primarily reflected in the stratigraphic distribution of radiolarian-rich laminae.
This article introduces a new set of seismic attributes that play an important role in extracting detailed stratigraphic information from seismic data.
Sequence stratigraphy and coal cycles based on accommodation trends were investigated in the coal-bearing Lower Cretaceous Mannville Group in the Lloydminster heavy oil field, eastern Alberta. The study area is in a low accommodation setting on the cratonic margin of the Western Canada sedimentary basin. Geophysical log correlation of coal seams, shoreface facies, and the identification of incised valleys has produced a sequence-stratigraphic framework for petrographic data from 3 cored and 115 geophysical-logged wells. Maceral analysis, telovitrinite reflectance, and fluorescence measurements were taken from a total of 206 samples. Three terrestrial depositional environments were interpreted from the petrographic data: ombrotrophic mire coal, limnotelmatic mire coal, and carbonaceous shale horizons. Accommodation-based coal (wetting- and drying-upward) cycles represent trends in depositional environment shifts, and these cycles were used to investigate the development and preservation of the coal seams across the study area.
The low-accommodation strata are characterized by a high-frequency occurrence of significant surfaces, coal seam splitting, paleosol, and incised-valley development. Three sequence boundary unconformities are identified in only 20 m (66 ft) of strata. Coal cycle correlations illustrate that each coal seam in this study area was not produced by a single peat-accumulation episode but as an amalgamation of a series of depositional events. Complex relations between the Cummings and Lloydminster coal seams are caused by the lateral fragmentation of strata resulting from the removal of sediment by subaerial erosion or periods of nondeposition. Syndepositional faulting of the underlying basement rock changed local accommodation space and increased the complexity of the coal cycle development.
This study represents a low-accommodation example from a spectrum of stratigraphic studies that have been used to establish a terrestrial sequence-stratigraphic model. The frequency of changes in coal seam quality is an important control on methane distribution within coalbed methane reservoirs and resource calculations in coal mining. A depositional model based on the coal cycle correlations, as shown by this study, can provide coal quality prediction for coalbed methane exploration, reservoir completions, and coal mining.
New dimensions: Geoscientists study how 3-D views of Eagle Ford outcrops are a great tool.
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.
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.
This article addresses the controls exerted by sedimentologic and diagenetic factors on the preservation and modification of pore-network characteristics (porosity, pore types, sizes, shapes, and distribution) of carbonates belonging to the Bolognano Formation. This formation, exposed at the Majella Mountain, Italy, is composed of Oligocene–Miocene carbonates deposited in middle- to outer-ramp settings. The carbonates consist of (1) grainstones predominantly composed of either larger benthic foraminifera, especially Lepidocyclina, or bryozoans; (2) grainstones to packstones with abundant echinoid plates and spines; and (3) marly wackestones to mudstones with planktonic foraminifera.
The results of this field- and laboratory-based study are consistent with skeletal grain assemblages, grain sizes, sorting, and shapes, all representing the sedimentologic factors responsible for high values of connected primary macroporosity in grainstones deposited on the high-energy, middle to proximal outer ramp. Cementation, responsible for porosity reduction and overall macropore shape and distribution in grainstones to packstones deposited on the intermediate outer ramp, was mainly dependent on the following factors: (1) amount of echinoid plates and spines, (2) grain size, (3) grain sorting and shapes, and (4) clay amount. Differently, in the wackestones to mudstones, laid down on the low-energy, distal outer ramp, matrix is the key sedimentologic factor responsible for low values of scattered macroporosity and dominance of microporosity. The aforementioned results may be useful to improve the prediction of reservoir quality by means of mapping, simulating, and assessing individual carbonate facies with peculiar pore-network characteristics.
This course will alternate between lectures and practical exercises involving cores, logs and seismic data.
A succession of exercises and complementary
lectures will expose the participants to deep-water depositional systems, facies analysis, chronostratigraphic framework, comparison of local to global depositional patterns, and application of an integrated approach to stratigraphic analyses using multiple data sets.
This short course is designed to provide information to facilitate exploration for microbial carbonate buildups and associated reservoir facies and to assist with the formulation of development plans for fields producing from microbial carbonates. The course consists of a series of seven lectures supplemented by core samples.
The overall goal of this course is to provide tools for efficient and effective re-exploration and development. It uses a two-part approach. First it uses petrophysical analysis to understand all that can be derived from examination of standard open-hole logs. This is followed by integrated approaches to discover key factors controlling oil and gas distribution in carbonate reservoirs in the greater Midcontinent USA. Methodologies and workflows reviewed include geosteering and evaluation of horizontal wells and optimizing carbon storage utilization and management.
This course presents the concepts and practical applications of sequence stratigraphy for petroleum exploration.
This field seminar will give participants an overview about the geology, reservoir engineering and operation aspects of the Lodgepole-Bakken-Three Forks Petroleum System. Excellent outcrops illustrate how facies, reservoir properties and rock strength can vary along a lateral well bore. Engineers, geologists and operators will find this especially interesting.
The Hay River region in the Northwest Territories is one of the best locations in North America for the examination of Devonian carbonates, and the Pine Point mine site is one of the best localities for viewing the fabrics and geometries associated with hydrothermal dolomitization.
Participants will learn through the use of spectacular outcrops, subsurface datasets, and stratigraphic modeling how these systems tracts and key surfaces (flooding surfaces and sequence boundaries) may be recognized.
Recognition and Correlation of the Eagle Ford, Austin Formations in South Texas can be enhanced with High Resolution Biostratigraphy, fossil abundance peaks and Maximum Flooding Surfaces correlated to Upper Cretaceous sequence stratigraphic cycle chart after Gradstein, 2010.
This presentation is designed for exploration/production geologists and geological managers or reservoir engineers.
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