Stratigraphic Play Research Articles

Sequence Stratigraphic Model for Simpson Group of the Southern Mid-Continent: The Key to A New Stratigraphic Play: ABSTRACT

Sequence Stratigraphic Model for Simpson Group of the Southern Mid-Continent: The Key to A New Stratigraphic Play: ABSTRACT

THE GULF OF CARPENTARIA—A NEW BASIN AND NEW EXPLORATION TARGETS

The sparsely explored Gulf of Carpentaria is a shallow water frontier area of stacked basins. The petroleum potential was not tested by the one offshore well drilled in the Gulf in 1984.Recent re-interpretation of offshore seismic in Queensland waters delineated the Bamaga Basin, a new infrabasin below the Carpentaria Basin. This new basin is a northerly trending asymmetrical sag basin that continues north of the international boundary. The Bamaga Basin, containing up to 1.8 seconds of gently folded and faulted sediments, is untested and offers a new exploration objective. Apparent high velocities make the age of the basin uncertain, but Paleozoic reservoir and source rocks, similar to sedimentary rocks in nearby basins, are inferred, although analogue basins are not readily identifiable.Bamaga Basin source rock burial is sufficient to generate hydrocarbons and could source reservoirs in the Bamaga and Carpentaria Basins via migration along faults. Possible direct hydrocarbon indicators increase support for the presence of hydrocarbons in the Gulf.Structural and stratigraphic plays in the Carpentaria Basin that provide new exploration targets include: basal sandstones onlapping areas of higher relief or filling basin floor depressions, sandstone layers within the Wallumbilla Formation draping highs and possible carbonate zones appearing as high amplitude chaotic reflectors. Within the Bamaga Basin, horst, fault structures and anticlinal features are potential structural plays, and termination of units against the main unconformity are possible stratigraphic play targets.

Tectonics Vs. Eustasy: North African Mesozoic and Cenozoic Facies Patterns and Hydrocarbon Exploration

North-northeast-south-southwest [open quotes]Atlantic[close quotes] trending, east-northeast-west-southwest to east-west [open quotes]Mediterranean[close quotes] trending as well as northwest-southeast-trending fault systems of variscan or even pre-variscan age controlled the facies distribution between the Atlantic shoreline of Morocco in the west and the eastern margin of the Pelagian block east of the Maltese Islands. This region incorporates large parts of Algeria and Tunisia. Multiple reactivation of these fault zones as normal faults and as reverse faults with significant strike-slip components occurred during a period beginning with Triassic red bed sedimentation until recent times. Fault movements reflect the westward propagation of the Tethys in combination with the evolution of the Atlantic Ocean. The region corresponds to the western part of the southern passive margin of the Tethys until the end of Paleogene marine deposition. Though overprinted by Alpine collisions in younger Tertiary times, the role of such fault trends can be demonstrated all over the region. Comparable fault directions as well as comparable shape and size of isolated fault blocks have been observed in an area ranging from Morocco to the Maltese Islands. Vertical and horizontal movements including small-scale rotations and their relationship to different stages of plate reorganization will be discussed using examples from more » Morocco and Tunisia. The interaction between local fault movements and global sea level changes caused a characteristic facies pattern for different Mesozoic to Cenozoic stages. The distribution of continental red beds, marine shallow water, slope and/or deep water deposits can be used for the prediction of reservoirs, sources, and seals. Stratigraphic play concepts will be introduced for Jurassic, Cretaceous, and Paleogene rocks. « less

Carbonate ramp depositional systems

The classification, tectonic settings, stratigraphy and early diagenesis of carbonate ramp systems are reviewed. Carbonate ramps are common in all geological periods, but were dominant at times when reef-constructing organisms were absent or inhibited. Ramps can be subdivided into inner-, mid-, and outer-ramp environments. The mid-ramp zone extends from fair-weather wave base to normal storm wave base, although the water depths which these boundaries represent vary. An additional outer-slope environment occurs on distally steepened ramps. As with siliciclastic shelves, a range of wave-, storm-, and tide-dominated ramps can be recognised and this forms the most convenient basis for ramp classification. The carbonate productivity profile of ramps differs from that of rimmed shelves, with the inner ramp showing lower production rates than comparable shallow-water facies on rimmed shelves. The zone of greatest organic carbonate sediment production appears to have shifted from the mid-ramp to the inner ramp since the Late Jurassic. Carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas, as in foreland and cratonic-interior basins and along passive margins. The featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines. Some show low-angle sigmoidal or shingled clinoforms, suggesting that ramps may seldom be “homoclinal”, but possess subtle slope geometries which reflect depositional environments. Because of their low-angle slopes, ramps respond differently to rimmed shelves during relative sea-level changes although results seem to be strongly dependent on the rate of relative sea-level change. During a minor fall, shallow-ramp facies belts will simply shift basinwards in a “forced regression”. In contrast, the whole surface of a steep-sloped, flat-topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced. During a major fall, shallow ramp-bounded basins may empty completely. Conversely, ramps also flood gradually, whereas rimmed shelves do so more rapidly. Homoclinal ramps develop no resedimented lowstand deposits; rimmed shelves and distally steepened ramps, in contrast, may develop lowstand talus or turbiditic wedges. Distally steepened ramps may behave more like homoclinal ramps during minor base-level falls and like rimmed shelves during major base-level falls. Many ramps consist of layered successions of several ramp sequences stacked one upon the other. Ramp “stacks” of this sort may show gross vertical accretion, but individual ramp sequences seldom appear to develop in a “keep-up” style, apart from minor organic buildups, as with many rimmed shelves. Steepening of the outer-ramp margin due to tectonism, slope inheritance, or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf. A wide variety of organisms have constructed buildups in mid- and outer-ramp environments. Isolated buildups may seed early in ramp development, accrete to wave base or sea level, and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp. The location of isolated buildups on ramps is governed by tectonism, halokinesis, antecedent topography, or by the subtle slope geometry of the previous ramp sequence. Diagenesis on ramps shows some major variations compared with diagenesis on steep-sloped, flat-topped carbonate platforms. Ramp-bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution. Isolated buildups in the mid- and outer-ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts. Grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs. Grainstone sandbodies, occur in both highstand systems tracts and in prograding lowstand wedges.

Sequence Stratigraphic Play Analogs from the Gulf of Mexico Applied to the Niger Delta

Sequence Stratigraphic Play Analogs from the Gulf of Mexico Applied to the Niger Delta

A seismic analysis of Black Creek and Wabumun salt collapse features, western Canadian sedimentary basin

Two Devonian salts of western Canada, those of the Black Creek member (Upper Elk Point subgroup) in northwest Alberta and those of the Wabamun group in southeastern Alberta, were widely distributed and uniformly deposited within their respective basins. Both of these salts are interbedded within predominantly carbonate sequences and both have been extensively leached. They are now preserved as discontinuous remnants of variable thickness and areal extent. These salt remnants and their associated collapse features are often associated with structural or stratigraphic traps. Structural traps typically form where reservoir facies are closed across remnant salts, stratigraphic traps often develop where reservoir facies were either preferentially deposited and/or preserved in salt collapse lows. As a result of these relationships between dissolution and hydrocarbon entrapment, the distribution (areal extent and thickness) of these salt remnants is of significant interest to the explorationist. Both the Black Creek and Wabamun salts have relatively abrupt contacts with the encasing higher velocity, higher density carbonates. Where these salts are sufficiently thick, their top and base typically generate high amplitude reflections, and lateral variations in the salt isopach can be directly determined from the seismic data. Relative salt thicknesses can also be indirectly estimated through analyses of lateral variations in the thicknesses of the encompassing carbonates, time structural drape and velocity pullup. Such seismic information about the thickness and the extent of these salts should be used together with well log control to generate subsurface distribution maps. These maps will facilitate both the delineation of prospective structural and stratigraphic play fairways and the determination of the timing of salt dissolution. In addition, an appreciation of regional salt distribution will decrease the likelihood that remnant salts will be misinterpreted as either reefs and/or faulted structures.

Depositional Sequence Stratigraphy and Play Fairway Analysis in the Paleogene of the North Sea

The Paleogene is a major hydrocarbon play in the North Sea with 18 producing fields and more than 30 further hydrocarbon discoveries. Recent exploration activity has emphasized the potential for stratigraphic trapping to contribute significant reserves. Notwithstanding these successes, these plays remain poorly understood and immature. Conventional regional analysis is based largely on seismic stratigraphic mapping, which without careful well control is likely to generate significant misties. The resultant lack of precision and the potential for misinterpretation means that this approach on its own is of limited practical application in the North Sea which is a well data-rich area. The generation of a new lithostratigraphic framework, using all available data, is based on the recognition of a series of depositional sequences using wire-line log, biostratigraphic, and seismic criteria. Depositional sequence boundaries correlate with high gamma shale log markers often coinciding with condensed intervals with a strong biostratigraphic fingerprint. These are interpreted as maximum flooding surfaces. These synchronous events can be correlated from basinal to shelfal areas and provide the key for typing the lithostratigraphic and chronostratigraphic framework. The placing of individual reservoir/topseal couples in a chronostratigraphic framework is critical in developing stratigraphic play concepts and producing play fairway maps thatmore » show the areas of maximum hydrocarbon prospectivity where all the regional play parameters are favorably combined. Using this approach in basin analysis has generated new plays that can be evaluated at the local or prospect level. Three plays are illustrated and their future exploration potential highlighted.« less

A TECTONOSTRATIGRAPHIC FRAMEWORK FOR THE VULCAN GRABEN, TIMOR SEA REGION

The Vulcan Graben is a northeast trending intracratonic rift system developed in response to Late Jurassic-Early Cretaceous break-up of the Australian northwest continental margin. A depositional sequence study was undertaken of the Vulcan Graben and the surrounding area, incorporating regional well and seismic data. From this a tectonostratigraphic framework has been established. Three megasequences are defined in relation to the Callovian rift episode; Pre-rift (pre-Callovian), Syn-rift (Callovian to Valanginian) and Post-rift (Valanginian to Present Day), each consisting of a number of discrete depositional sequences. This study has clarified the structural and stratigraphic evolution of the region, enabling development of dynamic depositional models. These models constrain the vertical and lateral facies distribution across the region, thus providing a powerful basis for petroleum exploration, including prediction of reservoir, seal and source rocks.Significantly, while Late Jurassic rifting was initiated in the Late Callovian, imprinting a NE-SW oriented grain, a second major tectonic episode occurred in the Kimmeridgian trending ENE-WSW, which generated the dominant regional structural architecture. Recognition of this Kimmeridgian event has considerable impact on the successful delineation of structural plays within the region and provided significant control on syn-rift facies distribution and consequently stratigraphic play potential within the Vulcan Graben. Stratigraphic and structural relationships clearly indicate that rifting ceased in the middle Valanginian, followed by post- rift thermal subsidence and consequent transgression and inundation of the Vulcan Graben. The intra-Valanginian event represents a regional disconformity rather than a major tectonic unconformity and defines the boundary between the syn- and post-rift megasequences. This is consistent with the revised Valanginian date for the onset of sea-floor spreading in the Argo Abyssal Plain.A revised lithostratigraphic nomenclature is proposed for the region which complements the resolution achieved by depositional sequence mapping.

Use of Sequence Stratigraphy to Define Stratigraphic Plays in Upper Pennsylvanian and Lower Permian Lowstand Systems Tracts, North-Central Texas: ABSTRACT

Use of Sequence Stratigraphy to Define Stratigraphic Plays in Upper Pennsylvanian and Lower Permian Lowstand Systems Tracts, North-Central Texas: ABSTRACT

Stratigraphic Play Types in Syn-Rift Sequences: Examples from the Gulf of Corinth Asymmetric Grabens, Greece: ABSTRACT

Since the late Miocene, the Gulf of Corinth system is central Greece has extended approximately north-south within a back-arc environment. Major east-southeast-west-northwest-trending normal faults dissect prerift sequences, creating asymmetric grabens on kilometers to tens of kilometers scales. Episodic activity on discrete fault segments controls rates of footwall uplift and basinal subsidence. These together with regional uplift across north Peloponnesus generate a complex sedimentary response. Syn-rift sequences range from continental to deep marine facies. Facies architecture reflects competing lateral and axial supply contributions, each creating distinct reservoir geometry and stratigraphic trap styles.

Ship Shoal Block 91 field, offshore Louisiana: Discovery and development of a stratigraphic field

The Ship Shoal 91 field is a subtle stratigraphic play which consists of a seismic amplitude whose updip limits do not fit structure. The amplitude corresponds to both gas and oil in the Upper Miocene sand. To attempt to predict pay sand distribution accurately, several geologic models were defined as development drilling progressed. The models utilized self‐potential curve shape, core, and seismic character analyses, combined with computer modeling studies, engineering data, and scanning electron microscope studies. Four geologic models evolved, ranging from a structural trap formed by small faults, to a stratigraphic pinchout of the pay sand, to a channel sand, and finally to a complex channel sand, resulting in the current model: a faulted stratigraphic trap. The development strategy for determining well locations also evolved, due to unexpected well results. The initial strategy was based on the presence of seismic amplitudes, until it became apparent that the amplitude alone is not a reliable hydrocarbon indicator. The revised strategy is based on complex attribute analysis, including the amplitude envelope and the weighted frequency. The discovery and development of this stratigraphic field required thorough integration of geology, geophysics, and engineering disciplines. The Ship Shoal 91 field demonstrates that complex stratigraphic fields are still of exploration interest and can be successfully developed by using an integrated approach.

Late Jurassic Channels and Associated fill, Eromanga Basin (ATP 259P), Queensland: A Seismic Stratigraphic Study

A seismic stratigraphic study of the southern Alkina Block (ATP 259P) in SW Queensland was undertaken in order to develop a stratigraphic framework in which the exploration potential of the area could be assessed. The area had been recognized (Moore and Pitt, 1984) as comprising a seismically complex series of channels, and contained fill that could provide new stratigraphic play opportunities. A model is herein proposed to explain the origin and occurrence of stratigraphic features of Late Jurassic-Early Cretaceous age.

MURILLA CREEK — AN UNTESTED STRATIGRAPHIC PLAY IN THE SURAT BASIN

In 1978 Shell farmed into Western Mining Corporation's (WMC) permit ATP-241P in the central Surat Basin. A major objective of the venture was to explore for Lower Jurassic stratigraphic traps in the Murilla Creek area on the western flank of the Mimosa Syncline.The Murilla Creek stratigraphic play comprises a series of sand-filled valleys incised into the base Jurassic unconformity surface, forming a dendritic drainage pattern which supplied sediment to the braided stream deposits of the Lower Precipice Sandstone to the east. At their western depositional limit these potential reservoirs are likely to be sealed vertically by a regionally extensive lacustrine shale, and laterally by siltstones and shales within the sub-cropping Middle Triassic Moolayember Formation. Oil charge may either be directly from underlying Triassic source rocks or indirectly from the Permian Blackwater and Back Creek Groups. Exploration of the Murilla Creek area involved the acquisition of 776 km of experimental, reconnaissance and infill seismic and one well (Coalbah 1) which enabled seismic calibration and delineation of the Lower Precipice Sandstone to its seismically resolvable limits.Seismic interpretation confirmed the potential for several stratigraphic traps in the Murilla Creek area. However trap geometries and seal constraints suggest that any stratigraphically trapped oil accumulation would be small (less than 3 MMBBL). Coupled with the considerable geological risks associated with the stratigraphic play, the low potential rewards rendered the play economically unattractive to Shell and WMC and the acreage was subsequently relinquished. However the Murilla Creek stratigraphic play remains untested and, in view of the potential for numerous small oil accumulations, may prove to be an attractive exploration target under more favourable economic circumstances.

Hydrocarbon Potential and Exploration Methods Within Cretaceous of Utah-Wyoming Overthrust Belt: A New Frontier in a Not-So-Old Area: ABSTRACT

A largely untested stratigraphic play exists within the Cretaceous section of the Utah-Wyoming Overthrust belt. Coarse Cretaceous sediments on the western Absaroka plate merge gradationally eastward into a predominantly shale section, which sets up the potential for Cretaceous sands to pinch out within Cretaceous shale in the Overthrust belt. The discovery well in this play is the Sam Gary Jr. 4-6X Lazeart with a reported initial potential of 87 BOPD from the Upper Cretaceous Frontier Formation. The anticipated source of hydrocarbons is Cretaceous shale - the documented source of most hydrocarbons found to date in the Utah-Wyoming Overthrust belt. Published research indicates that Cretaceous shales have reached adequate maturation levels under the Darby and Absaroka plates and probably on the back of the Absaroka plate. In addition, hydrocarbons may have moved updip from the generation area to shallower reservoirs as a result of secondary migration. Therefore, the geographic area of the play is extensive. State-of-the-art seismic data are an essential prerequisite for systematic exploration of the Cretaceous play. After acquiring a regional seismic grid and tying in well control and surface geology, the following procedure should be implemented: (1) use seismic reflections to establish a regional chronostratigraphic framework, (2) interpretmore » seismic facies to determine geographic distribution of depositional environments, and (3) perform detailed interpretation and forward modeling to locate specific prospects. A reprocessed seismic line from the Overthrust belt confirms that data quality is high enough to use this procedure. An aggressive exploration program in this area is likely to result in significant reserve additions; a commitment to the search for the subtle trap in the Overthrust belt is now needed.« less

CRETACEOUS OF THE EROMANGA BASIN — IMPLICATIONS FOR HYDROCARBON EXPLORATION

The Cretaceous sequence in the Eromanga Basin covers nearly one-fifth of the Australian mainland and has a maximum thickness of about 2000 m. The lowermost prospective unit is the Neocomian Cadna-owie Formation, which commonly contains hydrocarbon shows and recently yielded oil in Merrimelia 15. Likely source rocks include organic-rich sediments at the base of the overlying Bulldog Shale and Wallumbilla Formation. Future commercial production from the Cadna-owie Formation will be hampered by fine grain size and carbonate cements, resulting in low porosity and permeability. In southwestern Queensland, the Cadna-owie Formation appears to have been locally dissected by prominent channels. If sand-infilled, these channels will represent an interesting stratigraphic play, although they are likely to be mudstone-and diamictite-infilled submarine canyons, similar to those in the Tertiary Latrobe Group, Gippsland Basin. As such, they represent one component of a complex hydrocarbon play that has yet to be evaluated.A third, stratigraphically higher exploration target is the middle Albian Coorikiana Sandstone. Although the marine mudstones which encase the Coorikiana Sandstone are only poor to fair source rocks, hydrocarbon shows are common in the unit, which yielded 9900 m3 of gas per day when tested in Strzelecki 8. The potential of the unit is limited by poor reservoir quality and marginal oil maturity.The Toolebuc Formation, of middle-late Albian age, is the fourth promising target for significant hydrocarbons. The formation consists of organic-rich mudstone which is oil-shale bearing over large parts of Queensland. Minor oil and gas shows, attributed to early generation from a very rich source rock, suggest that oil production from the Toolebuc Formation may be possible if naturally fractured shale reservoirs can be located. Overall, the Cretaceous sequence in the southwestern Eromanga Basin is considered to have a modest oil and gas potential which should be evaluated in the course of drilling for deeper targets.It is emphasised that knowledge of Cretaceous and Cainozoic stratigraphy is essential in understanding the evolution of the Eromanga Basin. Rapid deposition of up to 2000 m of Cretaceous sediments enabled generation of hydrocarbons in the Jurassic sequence, beginning in the latest Cretaceous and continuing today. Although early, syn-sedimentary structuring and mild epeirogeny are evident in Cretaceous sediments, of far greater significance is a major phase of Early Tertiary folding, due probably to the effects of continental breakup and collision. This greatly enhanced pre-existing structures, produced some very large anticlines, and caused up to 800 m of the Cenomanian Winton Formation to be eroded from elevated areas. A complex interplay between timing of structural growth and maturation of the sequence is thus considered to be a major control on the present distribution of hydrocarbons in the Eromanga Basin.