Barrier Sandstones Research Articles

Controls on Gas-Reservoir Formation in the Benxi Formation in the Ganquan–Fuxian Area of the Ordos Basin, China

The Benxi Formation is one of the most important gas-producing layers in the Ganquan–Fuxian area, but the complex gas–water distribution and lack of sandstone have severely constrained natural gas exploration and development in this area. This study analyzed the structure, paleogeomorphology, sedimentary facies, reservoir closures, and gas–water distribution of the Benxi Formation in the study area through drilling, coring, logging, seismic surveying, and experimental testing. The results show that the gas reservoirs in the Benxi Formation are mainly lithologic traps distributed along NW-trending barrier sandstones, with a small portion of updip pinchout closures. The water layers are mainly composed of thin sandstones with a single-layer thickness of less than 2 m, which are tidal-channel or barrier-margin microfacies sandstones. The water saturation in some thick sandstones is related to the activity and destruction of large individual faults. The dry layers are tight sandstones with porosity of less than 3.2%, mainly associated with high amounts of volcaniclastic matrix and lithic fragments, as well as compaction. The charging of the underlying high-quality Ordovician limestone reservoirs by carboniferous source rocks in the Benxi Formation reduces the probability of gas accumulation in Benxi sandstone. Based on the control of sedimentary facies and physical properties on gas accumulation, favorable reservoir distributions were predicted using seismic attributes and gas detection methods, providing the basis for the next phase of natural gas exploration and development in this area.

Environmental significance of Ophiomorpha in a transgressive–regressive sequence of the Spitsbergen Paleocene

The study focuses on occurrences of Ophiomorpha burrows in a transgressive–regressive succession composing the Early Paleocene Firkanten Formation deposited in paralic, prodelta to delta front conditions in the Central Basin of Spitsbergen. The burrows colonize sandstones of the Todalen Member at four sites and belong to two ichnospecies: Ophiomorpha cf. nodosa , forming dominantly vertical shafts; and O. cf. irregulaire , consisting of horizontal sinuous tunnels ending in subconical shafts. Both species are observed in shoreface sandstones deposited as a barrier bar. Lithological features and stratigraphic positions suggest that the trace-makers preferentially colonized high-energy sand environments. Foraminiferal faunas occurring below and above the barrier sandstones indicate brackish water conditions for the Ophiomorpha levels, which accordingly are of restricted, monospecific nature. It is inferred that the trace-makers had a dominantly suspension feeding habit as a modern analogue Calianassa major . Portrayed in a sequence stratigraphic framework, the Ophiomorpha -bearing sandstones in middle reaches of the Central Basin were deposited in the final stage of the transgressive systems tract, which drowned the underlying coal-bearing paralic facies. Moreover, in the northern, coal-rich parts of the basin, occurrence of Ophiomorpha signals marine ingression into the paralic system. Keywords: Trace fossils; Paleocene; shoreface sandstones; brackish water; transgression signal. (Published: 6 April 2016) Citation: Polar Research 2016, 35 , 24192, http://dx.doi.org/10.3402/polar.v35.24192

The Curlew Field, Block 29/7, UK North Sea

abstract In 1964, Block 29/7 was awarded as part of the first licencing round to Shell and Esso (UK). The Curlew area comprises a series of terraces that step down from the Western Platform into the West Central Graben. Exploration drilling started in 1977 but it wasn't until 1990 that well 29/7-4 successfully tested a 100 ft column of undersaturated volatile oil in the Upper Jurassic Fulmar Formation sandstones. Further exploration drillng, aided by 3D seismic data (acquired in 1991), yielded additional discoveries: Curlew C (1993, oil) and Curlew D (1994, condensate-rich gas). In Curlew D new technologies were succesfully applied to described and evaluate the isolated hanging wall compartment. Oil was discovered in what is now called Curlew D South in February 2000. Hydrocarbon accumulations in the Curlew area occur in dip-closed faulted anticlinal structures. The producing Upper Jurassic Fulmar Formation is a heterogeneous mix of bioturbated shallow marine sandstones. barrier sandstones, and lagoonal/swamp deposits. The range in reservoir quality is reflected in the field porosity (5-27%) and permeability (0.001-800 mD). Reservoir compartments in the field (curlew B, D, and D South) are controlled by complex reservoir architecture, structure and fault geometry. Each reservoir compartment is characterized by a different fluid contact and a distinct fluid type. The reservoir heterogencity provieds potential for bypassed hydrocarbons in undrilled compartments, as was proven by the Curlew D South development. Curlew B, D, and D South have since been developed. Evacuation of hydrocarbons from the Curlew area is via the Maersk Curlew FPSO, to which the producing wells have been tied back. Gas is fed nto the Fulmar Gas Line and oil is taken onshore by shuttle tankers. The initially-in-place volume in Curlew B at standard conditions is 24 MMSTB of oil and 25 BSCF of solution gas. Curlew B was developed via horizontal well 29/7-10y and came on stream in November 1997 at a rate of about 23 000 STB/D. The initially-in-place volume in Curlew D is 340 BSCF of gas and 100 MMSTB of condensate. Curlew D was developed via two vertical wells, 29/7-8z and 29/7-9, and started production in February 1998 with production rates up to 115 MMSCF/D of gas and 34 000 STB/D. Gas production from the field is constrained. however, by the facilities. Curlew D South is developed by the vertical well 29/7-11 which came on stream in March 2000 after a North Sea record-time tie back of the original appraisal well.

Cyclic emersion surfaces and channels within Dinantian limestones hosting the giant Navan Zn-Pb deposit, Ireland

Abstract The Carboniferous succession in the Navan Mine area, which lies near the northern margin of the Dublin Basin, rests unconformably on folded Lower Palaeozoic rocks. The informal stratigraphic nomenclature of the mine is used throughout. Sedimentation began in the late Devonian-early Courceyan with Red Beds reflecting deposition in braided streams and alluvial fans. The overlying Laminated Beds consist of shallow-marine barrier sandstones, tidal-flat/lagoonal mudstones, and sabkha evaporites. The Muddy Limestone is a mud-dominated carbonate sequence reflecting deposition in an open clastic-influenced lagoon. The Pale Beds that follow, and on which attention is focused here, contain at least 44 peritidal and shallow-shelf depositional cycles. The overlying Shaley Pale and Argillaceous Bioclastic Limestones reflect deeper water open-sea conditions. This sequence is truncated by a conspicuous erosion surface overlain by Chadian submarine debris-flows and limestone turbidites. The succession reflects a progressive deepening of the waters as subsidence outpaced sediment accumulation, but emersion surfaces capping cycles indicate that neither deposition nor relative sea-level rise were continuous. Cyclicity and emergence are believed to reflect the interaction of regional subsidence, glacio-eustatic sea-level oscillation, and sediment supply. Emersion surfaces occur in the Laminated Beds and throughout the Pale Beds, and include palaeosols, in situ breccias, pinnacled and hummocky surfaces, and karst-modified topography. Deeply incised channels at four intervals point to larger-scale incision. The lithoclast-bearing conglomerates which they contain indicate extensive emergence nearby. The distribution of these features may be related to the margins of outcrops of Old Red Sandstone and Lower Palaeozoic rocks. The Dinantian limestones in this region host several Zn-Pb ore deposits including Europe’s largest at Navan. The results of this study suggest a relationship to the margins of former emergent carbonate terrains.

Stratigraphic Setting of Transgressive Barrier-Island Reservoirs with an Example from the Triassic Halfway Formation, Wembley Field, Alberta, Canada

Established models for landward barrier-island migration focus primarily on the preservation pattern of transgressive facies in the shoreface, which are typically thin and buried beneath finer grained marine lower shoreface-to-offshore sediments. In contrast, transgressive barrier-island sandstones in the Triassic Halfway Formation of Wembley field in Alberta are preserved interbedded with, and overlain by, backbarrier and nonmarine sediments. These transgressive barrier sandstones formed from coalescing washover fans during shoreface retreat and were subsequently as the shoreline stabilized and resumed progradation. The abandoned transgressive barrier sandstones were subsequently blanketed by backbarrier and nonmarine sediments as the coastline continued to p ograde. Abandoned transgressive barrier island sandstones in the Halfway Formation are 2-6 m thick, up to 2 km wide, and form paleocoastline-parallel trends tens of kilometers in length. The trends define the paleolandward limit of transgressive events. The updip pinch-out of these sandstones in backbarrier mudstones forms a stratigraphic trap for hydrocarbons in Wembley field. Top seal is provided by nonmarine mudstones and evaporites which buried the abandoned transgressive barrier island. The sandstone has porosities and permeabilities averaging 11% and 63 md, respectively. By using well logs and cores to correlate individual parasequences in the Halfway Formation to their updip termination, it is possible to define the extent of associated marine flooding events and therefore identify exploration targets for abandoned transgressive barrier-island sandstones.

Sequence Stratigraphic Patterns and Reservoir Implications in the Oligocene and Cretaceous, Northern Monagas, Eastern Venezuela

A sedimentological study based on 876 ft of core, electric logs, and cuttings from the well MUC-20 in Mulata field, Northern Monagas, has shown that the Cretaceous to Oligocene section exhibits a variety of facies, ranging from coarse-grained cross-stratified sands, fine-grained bioturbated sands, marine mudstones with varying proportions of silt and thin coal seams. These sediments accumulated in environments varying from fluvial to deltaic to marine shelf. The study of stacking patterns and facies successions indicates that six type I sequence boundaries are visible in the Cretaceous to Oligocene section. The lowest sequence boundary is Maestrichtian and is an incised valley complex filled with lowstand fluvial sandstones. The overlying sequence boundary is interpreted to be at the Cretaceous-Paleocene boundary (68 m.y. ) and is overlain by estuarine transgressive tract deposits. The third sequence boundary is thought to represent the 30 my eustatic sea level fall and incorporates a major stratigraphic hiatus from the lower Paleocene to Upper Oligocene. This sequence boundary is overlain by estuarine transgressive tract deposits. The other three sequence boundaries were recognized within the Upper Oligocene and are capped by nearshore and coastal sediments comprising coastal barrier and tidal inlet sandstones, and marsh and lagoonal mudstones. Themore » sediments overlying the fourth and fifth sequence boundaries are interpreted as lowstand shoreline deposits. The uppermost sequence boundary is represented by barrier sandstone transgressive tract deposits. The identification of these sequence boundaries and the parasequences comprising the systems tracts formed the basis for fieldwide reservoir correlations which enabled detailed mapping of the individual reservoir units. The Maestrichtian fluvial incised valley complex forms a widespread sand sheet, whereas the transgressive barrier sandstones are more restricted, and tend to shale out laterally.« less

Gondwana coal basins of Australia and South Africa: tectonic setting, depositional systems and resources

Abstract Upper Carboniferous and Permian coals of eastern Australia (Bowen, Sydney, Cooper, Galilee and satellite basins) and southern Africa (Karoo Basin) resemble one another in their general composition and associated clastic facies. The Australian examples, however, show more petrographic and geometric variability than their southern African Gondwana counterparts. Critical controls on the distribution and quality of coal in both regions appear to have been (1) a near-polar location with accompanying periglacial conditions, (2) Permian through early Triassic warming independent of any significant change in palaeolatitude, (3) associated floral changes from tundra vegetation to swamps and deciduous Glossopteris forests, (4) variable subsidence and sedimentation rates in foredeep, rift, and epicratonic basin settings, (5) local palaeotopographic effects, (6) eustatically induced changes in base level and marine transgression, and (7) a range of depositional systems from proximal conglomeratic alluvial fans through fluvial, delta-plain, lake-margin, back-barrier, and blanket peat mires in sediment-starved basins. In most cases, palaeotopography and depositional environment were the most important factors in determining coal seam geometry, and had considerable influence on coal quality, specifically its maceral content. Most of the commercial seams are contained in fluvial and delta-plain sequences, with thick, dip-elongate interchannel seams and really extensive coals typically overlying abandoned delta lobes. However, some important seams are juxtaposed with conglomerates of wet alluvial-fan origin or are associated with quartzose barrier sandstones; others originated in clastic-starved systems bordering shallow lakes or in blanket swamps that received moisture from groundwater discharge or direct rainfall. Some seams display characteristics that were primarily controlled by other more regional factors such as rapid tectonic subsidence, changes in groundwater regime, or gradual marine transgression. While influencing the thickness and areal extent of seams, these factors were even more important in determining microlithotypes, maceral content, and inorganic composition of the coals. Not only is coal mining crucial to the economies of both Australia and S Africa, with further expansion likely in the coming decades, but coal and dispersed coal particles have generated billions of barrels of commercial oil reserves along with large volumes of gas. Australian petroleum reserves are predominantly of land-plant origin, and source-reservoir relationships provide important analogues for exploring other non-marine basins.

Mississippian-Pennsylvanian Unconformity near Somerset, South-Central Kentucky: ABSTRACT

The Mississippian-Pennsylvanian systemic boundary near Somerset, south-central Kentucky, is unconformable. Contact relationships in eastern Kentucky are controversial because of unclear genetic associations displayed by Chesterian(?)-Morrowan Lee Sandstone lobes, which have been interpreted as being of either barrier-beach or fluvial-deltaic origin. The barrier shoreline model stipulates that Meramecian, Chesterian, and Morrowan rocks represent carbonate sediment barriers and carbonate mud islands, offshore clays, quartzarenite barriers, and lagoonal-tidal flat sediments that were penecontemporaneously deposited during north-westerly progradational episodes. The tabular erosion model stipulates that Meramecian, Chesterian, and Morrowan(?) predominantly marine sediments we e deposited and lithified as tabular units before and penecontemporaneous with deposition of Chesterian(?)-Morrowan fluvial-deltaic sediments. Field study of exposures near Somerset indicates that the tabular erosion model satisfactorily explains contact relationships. Tidal flat, tidal channel, and lagoonal lithofacies without barrier sandstones are unconformably overlain by southwesterly progradational fluvial-dominated deltaic lithofacies. Elongate, fining-upward sandstone bodies typically above coal seams, which are oriented parallel with the southwesterly paleoslope, display lag concentrates of carbonized plant debris, epsilon cross-stratification (or apparently lack of stratification), and low-energy ichnofacies, and are enclosed in deltaic lithofacies. End_of_Article - Last_Page 1434------------

Depositional Systems of a Tight Gas-Productive Barrier-Strandplain Sequence: Corcoran and Cozzette Sandstones, Northwest Colorado: ABSTRACT

The Corcoran and Cozzette sandstones (Campanian) are members of the Price River Formation within the Mesaverde Group of the Piceance Creek basin, and consist of marginal-marine facies between the underlying Mancos Shale and the overlying continental Mesaverde. Log-interpreted facies within the Corcoran-Cozzette vary from shale with thin siltstone to sandstone interbeds (shelf to lower shoreface), through upward-coarsening sequences (upper shoreface to foreshore), to interbedded thin coal, sandstone, and shale (alluvial plain to bay or lagoon). Plateau and Shire Gulch are the largest fields that produce gas from low-permeability (0.002-0.08 md), fine-grained to very fine grained sandstone of the Corcoran and Cozzette (over 8.4 bcf produced through 1982). Within these fields, 7 distinct northeast-trending strandplain and barrier units have been identified. Strandplain sandstones form the lower half to two-thirds of each member, and barrier sandstones form the upper depositional units. Net sandstone thickness of the strandplain units increases from less than 20 ft southeast of the fields to more than 80 ft within Plateau field. Along the southeastern Plateau field margin, upper units in each member contain 30-55 ft thick sandstones that repeat the shoreface sequence. Most completions are in the lower (39%) and upper (33%) Corcoran, whereas the lower and upper Cozzette each account for 14% of perforated intervals. The underlying Mancos Shale and overlying coal may account for the greater gas production of the Corcoran. Gas productivity of individual depositional units cannot be defined due to commingled production from multiple intervals in most wells. End_of_Article - Last_Page 255------------

Uranium Distribution and Sandstone Depositional Environments--Oligocene and Upper Cretaceous Sediments, Cheyenne Basin, Colorado: ABSTRACT

Wyoming-type roll-front uranium deposits occur in the Upper Cretaceous Laramie and Fox Hills sandstones in the Cheyenne basin of northeastern Colorado. The location, geometry, and trend of specific depositional environments of the Oligocene White River and the Upper Cretaceous Laramie and Fox Hills formations are important factors that control the distribution of uranium in these sandstones. The Fox Hills Sandstone consists of up to 450 ft (140 m) of nearshore marine wave-dominated delta and barrier island-tidal channel sandstones which overlie offshore deposits of the Pierre Shale and which are overlain by delta-plain and fluvial deposits of the Laramie Formation. Uranium, which probably originated from volcanic ash in the White River Formation, was transported by groundwater through the fluvial-channel deposits of the White River into the sandstones of the Laramie and Fox Hills formations where it was precipitated. Two favorable depositional settings for uranium mineralization in the Fox Hills Sandstone are: (1) the landward side of barrier-island deposits where barrier sandstones thin and interfinger with back-barrier organic mudstones, and (2) the intersection of barrier-island and tidal channel sandstones. In both settings, sandstones were probably reduced during early burial by diagenesis of contained and adjacent organic matter. The change in permeability trends between the depositional strike-oriented barrier sandstones and the dip-oriented tidal-channel sandstones provided sites for dispersed groundwater flow and, as demonstrated in similar settings in other depositional systems, sites for uranium mineralization. End_of_Article - Last_Page 512------------

A prograding coastal sequence of wave-built structures of Messinian age, Sorbas, Almeria, Spain

A well-preserved and virtually non-burrowed coastal sequence of Messinian age is described from the Sorbas basin, southeastern Spain. The sequence was formed as an out-building coast under the dominant influence of waves. Four main units are distinguished (Figs. 4 and 19), from base to top: 1. (1) A muddy unit with storm layers (original water depth greater than normal wave base, here ca. 10 m). 2. (2) Lower shore face, characterized by wave-rippled sandstone and parallel-laminated sheet sandstone. 3. (3) The upper shore face and beach with mega-cross-bedded conglomeratic sandstones, swash lamination and beach-rock breccia. The original depth of the sheet sandstone is estimated to be less than 9 m and of the mega-cross-bedded sandstones, less than 6 m. The mega-cross-bedded unit often shows herringbone structures, very similar to those of tidal regimes but here generated by waves and comparable to the internal structures of lunate mega-ripples of Clifton et al. (1971). The Sorbas sequence points to high-energy conditions on this coast. 4. (4) Units 1 to 3 are overlain by a fourth unit consisting of shallow lagoonal sediments with bird's foot imprints and chenier-like barrier sandstones with associated washover fans. A special feature of the Sorbas sequence is the transition from small-scale wave ripples to plane bed to mega-ripples by increasing wave energy. This sequence is comparable to that described by Davidson-ArnottDavidson-Arnott and Greenwood (1976) from the east Canadian coast, and differs from the Oregon coast sequence, described by Clifton et al. (1971).

Estuarine-Coastal Plain Coal Deposition in Southern West Virginia; Pennyslvanian Beckley Seam: ABSTRACT

Current studies of the Beckley (Pennsylvanian) seam in an area 60 by 30 km in southern West Virginia indicate that the Beckley was formed in a back-barrier depositional setting. Examination of about 1,800 core records as well as underground workings shows that the Beckley stratigraphic position is characterized by linear northeast-southwest-trending orthoquartzitic sandstone bodies about 1,500 m wide representing stranded barriers on a prograding coastal plain. Areas between the barrier sandstones are about 15 km wide and are occupied by coal and shale of estuarine and tidal-creek origin. The thick bodies of coal, which are relatively small (4.8 by 9.6 km or less), are located on the flanks of the barrier and thin toward the shaly central part of the interbarrier area. Ad acent to the barriers, the coal is split by small linear tongues of sandstone produced by erosion of the barrier. Where the coal adjoins estuarine and tidal-creek sediments, it interfingers and thins into shale and sandy shale. Within the interbarrier areas, the thickest coal is near the headward parts of the tidal creeks; closer to the major estuary the coal bodies are thinner and smaller in areal extent. Knowledge gained from exploration and mining of the Beckley seam should aid in searching for and developing coals in similar depositional settings. End_of_Article - Last_Page 533------------

Sedimentary environments and palaeogeography of the late Llandovery and earliest Wenlock of North Connemara, Ireland

The upper Llandovery rocks of North Connemara record a gradual marine transgression at the northern margin of the Lower Palaeozoic geosyncline. The lowest rocks are red sandstones deposited from mainly southward flowing rivers • These are overlain by lagoon and coastal barrier sandstones and siltstones, and deeper water shelf siltstones. Red marine siltstones and mudstones, deposited in 100 to 200 m depth of water, are found near the top of the Llandovery sequence. Deeper water turbidite conglomerates and sandstones, with some thick mudstones, both overlie and laterally interfinger with the sequence of non-marine and shelf sediments. Basement faults controlled rates of subsidence during shelf sedimentation, and topographically defined the margins of turbidite accumulating basins. A palaeogeographic synthesis of the area is presented.

Recognition of Barrier Environments

The vertical succession of sedimentary structures and textures at Galveston Barrier Island, Texas, is identical with vertical successions in two ancient barrier complexes, one in the Lower Cretaceous of Montana and the other in the Lower Jurassic of England. Within both Holocene and ancient examples, there is a gradation upward from (1) irregular interlaminations of siltstone and claystone at the base, through (2) burrowed and generally structureless sandstone, to (3) low-angle and microtrough cross-laminated sandstone, terminating in two of the examples in (4) structureless and rooted sandstone. This sequence represents deposition in (1) lower shoreface, (2) middle shoreface, (3) upper shoreface-beach, and (4) eolian environments, respectively. Analyses of quartz size and content of the Holocene and ancient barriers yield textural and compositional parameters that are environmentally sensitive. Plots of these parameters demonstrate that each of the environments may be distinguished on the basis of thin-section analyses. Consequently, full diameter cores, which show sedimentary structures, may not be necessary for precise environmental interpretation in the subsurface. Indeed, thin sections of sidewall cores may yield significant and reliable environmental interpretations in barrier sandstones. Textural and sedimentary structural similarities between Galveston Island and the ancient examples permit a general model of barrier sedimentation to be developed.