Skeletal Grainstones Research Articles

Diagenetic development of rhizoliths in the Ironshore Formation (Pleistocene) of the Cayman Islands

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ13C and δ18O, which vary in accord with the internal zones are more variable than the δ13C and δ18O of the host sediments. For the rhizoliths there is a high correlation between the δ13C and δ18O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ13C values to determine if the rhizoliths developed in association with C3 or C4 vegetation failed because the δ13C values correlate with the calcite content that developed largely as diagenetic products.

Geologic characterization of the Lower Cretaceous upper Travis Peak and Pettet Formations in the Wright Mountain field, Smith County, East Texas basin: Potential reservoir analog for other Travis Peak and Pettet strata

The Wright Mountain field in Smith County, Texas, is a tight carbonate reservoir that produces from the Lower Cretaceous Pettet Formation. The Pettet Formation is dominantly made up of cyclic shallow-water shelfal carbonates associated with skeletal oolitic shoal complexes. These shoals are widespread hydrocarbon-reservoir targets, have poor to fair matrix reservoir quality, and are commonly sealed by transgressive argillaceous lime mudstones and wackestones. Complex regional paleotopography has significant impact on the regional distribution of these shoal bodies and overall depositional patterns. This investigation, therefore, aims to characterize the Pettet Formation and underlying fluviodeltaic Travis Peak Formation in the Wright Mountain field, located on the western flank of the Strickland High near the northeastern margin of the East Texas Basin, to assess reservoir parameters and to compare depositional patterns to existing shoal-complex studies centered on the top of the paleohigh in Rusk County. The Pettet section in Smith County consists of three fourth-order sequences (Pettet A–C), each of which shoals upwards into a shallow-water, high-energy environment near or within shoal complexes. The best reservoir lithofacies are the mixed skeletal-ooid and skeletal grainstones, as observed elsewhere in the Pettet Formation in Rusk County. However, texture and biota vary in the Wright Mountain field area as compared to the Rusk County area as there is abundant evidence of reworking and resedimentation in the Wright Mountain field area, suggesting a different depositional setting perhaps related to off-shoal deposition or greater depositional dip. Therefore, this field characterization study is important for constructing a regional depositional model and can be applied as an analog for equivalent systems with similar lithofacies and pore networks elsewhere in the northern Gulf of Mexico.

Regional stratigraphy and proximal-to-distal variation of lithology and porosity within a mixed carbonate-siliciclastic system, Mississippian strata of northern and central Oklahoma

Mississippian carbonate and silica-rich reservoirs of northern and central Oklahoma formed along a regionally extensive carbonate ramp to basin transect. The stratigraphy, lithology, and porosity characteristics of the Mississippian Meramec and Osage series significantly vary as older ramp carbonates prograde southward and transition into younger calcareous and quartz-rich silt deposits of the Anadarko Basin. Lithofacies identified within the northern carbonate-dominated portion of the system commonly include altered chert, skeletal grainstones, peloidal packstones-grainstones, bioturbated wackestones-packstones, bioturbated mudstones-wackestones, glauconitic sandstones, and siliceous shale. Lithofacies within the southern siliciclastic-dominated portion of the system include structureless to bioturbated sandstones, siltstones, and laminated mudstones, each with varying degrees of carbonate content. We have grouped these core-based lithofacies into dominant lithologies/rock types, which tie to well-log properties. Electrofacies classification methods including the artificial neural network (ANN) and k-means clustering predict lithologies in noncored wells. ANN yielded the highest overall prediction accuracy of 85% for lithologies. Core, well log, and lithology log data establish the regional stratigraphic framework. The Mississippian interval of interest subdivides into 16 stratigraphic zones. A depositional-dip-oriented cross section and the associated reservoir models illustrate the proximal to distal and stratigraphic variability of the lithology and porosity. Lithology trends moving from north to south, from older to younger strata, reveal a carbonate-dominated succession capped by diagenetically altered chert northward shifting into a siliciclastic-dominated interval, which increases in clay content southward. Northward, prospective conventional reservoirs developed near cycle tops within diagenetically replaced cherts and cherty limestones associated with subaerial exposure and sea-level fluctuations. Southward, higher total porosity associates with increased clay content linked to the suppression of calcite cement, forming prospective unconventional targets near the bases of depositional cycles.

Controls on microbial and oolitic carbonate sedimentation and stratigraphic cyclicity within a mixed carbonate‐siliciclastic system: Upper Cambrian Wilberns Formation, Llano Uplift, Mason County, Texas, USA

AbstractThe upper Cambrian Wilberns Formation in central Texas records deposition on a low‐gradient shelf within a mixed carbonate–siliciclastic tidal‐flat system that changes offshore to subtidal shelf and open‐marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE–SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low‐frequency and intermediate‐frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea‐level fluctuation. Random facies stacking and the lack of metre‐scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms.

Tectonic and eustatic impact on depositional features in the upper Cretaceous Austin Chalk Group of south-central Texas, USA

The upper Cretaceous Austin Chalk Group in south-central Texas, USA, displays vertical and lateral variations in thickness and facies resulting from an irregular paleotopography of the seafloor and variable preservation of 3rd-order sea-level cycles. During the Coniacian to early Campanian, the tectonic evolution of the San Marcos Arch, a prominent basement feature in the study area, impacted facies and sequence architecture within the Austin Chalk Group. Outcropping sections of the Austin Chalk Group along the margin of the Edwards Plateau in south-central Texas stratigraphically transition between inner and outer ramp environments. The cyclic stacking pattern of facies is recorded in vertical stratigraphic succession as 3rd-order cycles separated by discontinuity surfaces and condensed deposits. Inner ramp facies consist of Thalassinoides-burrowed bivalve wackestone to packstone with minor occurrences of skeletal grainstone. Outer ramp facies comprise Planolites-burrowed foraminiferal chalk and marl with fine skeletal debris. Condensed facies associated with transgressive re-working are characterized by glauconitic packstone with high concentrations of coarse skeletal debris and lithoclasts. Gamma-ray profiles of outcrops investigated in the study area are correlated to subsurface wireline logs of wells drilled along the outcrop and subsurface trend, in order to construct a stratigraphic framework across the San Marcos Arch. This stratigraphic framework is biostratigraphically well constrained and correlated to the type section of the Austin Chalk Group farther north in the vicinity of Austin. The integration of sedimentological and stratigraphic information enables the identification of four regional 3rd-order depositional sequences within the Austin Chalk Group. Thickness maps of each depositional sequence illustrate the tectonic evolution of the San Marcos Arch and its influence on the deposition of the Austin Chalk Group, which is judged as negligible during most of the lower Austin Chalk Group (Coniacian), and started to trigger lateral thickness variations and pinch-outs in the Santonian and early Campanian. Syndepositional tectonic movement played an active role in the geometry of Austin Chalk depositional sequences and should be accounted for in sequence stratigraphic models developed in chalk depositional settings to explain the occurrence of coarser and cleaner facies.

Electrofacies analysis of the Asmari reservoir, Marun oil field, SW Iran

This article integrates core and well log data to determine reservoir electrofacies of the Oligo-Miocene Asmari Formation in the western Dezfol Embayment, SW Iran. At the start, an unsupervised neural network was employed based on the selforganizing map (SOM) technique to identify and extract electrofacies groups of Asmari Formation in the Marun oilfield, which is a mixed siliciclastic and carbonate reservoir. Using inexpensive and accessible petrophysical wireline logs, such as gamma ray, sonic, density and neutron, along with calculated reservoir data (water saturation and effective porosity) from 12 reference wells and their correlation with reservoir core data, led to recognition of seven electrofacies including first four class as limestone rocks (EF1–EF4) and others three classes as sandstone rocks (EF5–EF7). Based on the porosity and permeability maps and water saturation at different depth levels, EF4 and EF7 with low porosity and permeability, high water saturation are considered as non-reservoir relating mostly to sedimentary textures of mudstone and argillaceous sandstones, respectively. By contrast, EF1 and EF5 with high values of porosity, permeability and low percent of water saturations characterize the best reservoir quality rocks, and EF2, EF3 and EF6 have medium reservoir quality. EF1 consists of dolomitic skeletal grainstone, in which biomoldic, vuggy and intercrystalline porosity is the dominant pore type, while the intergranular porosity is the major pore type in EF4 as an unconsolidated sand electrofacies. In general, the results obtained in this study indicate a satisfactory agreement between core data and log facies. This correlation allows rockfacies to be classified in the cored wells and predict those facies in the uncored wells. Using the methodology outlined here, it is possible to track reservoir electrofacies from conventional well log data.

Decoding depositional and diagenetic conditions of the mid-Cenozoic Puesto del Museo Formation, southern Golfo San Jorge Basin, Patagonia, Argentina

This paper describes an integrated field, petrographic and geochemical study of the Puesto del Museo Formation, a poorly known unit that evidences a Cenozoic transgression at the southernmost Golfo San Jorge Basin. The most striking features of the Puesto del Museo Formation is its occurrence as restricted ~20 m-thick-outcrops composed exclusively of friable cross-stratified grainstones.We analyzed the sedimentary facies and facies associations in order to reconstruct the depositional setting. The petrographic characterization allows assessing the composition as a very homogeneous lithological succession dominated by skeletal grainstones. Geochemistry of trace elements (Mn, Sr, Rb, rare earth elements plus yttrium = REY), and stable isotopes (C and O) were used to characterize the grainstones compositionally, and to constrain environmental and climate conditions that controlled its deposition. The diagenetic environmental comprehension helped to the interpretations of the syn- and post-depositional conditions related to the grainstones. Finally, the stratigraphic position of the Puesto del Museo grainstones into the context of the basin is discussed. This succession can be correlated with the Patagonian marine event (i.e., Golfo San Jorge and Austral basins) that took place during the late Oligocene–early Miocene, related to an elevated global sea level controlled by glacio-eustatic conditions.

Llandovery microfossils and microfacies of the Hýskov section, Prague Basin

Abstract The lower Silurian strata of the Prague Basin, as well as in many other regions, are primarily represented by black shales, with limited to no record of benthic life. In this paper, we describe an exceptional late Aeronian volcanic-carbonate succession from the Hýskov locality near Beroun, which contains a diverse and well-preserved fossil assemblage and specific microfacies features. The studied section is mostly represented by well-washed, poorly sorted skeletal grainstones, rudstones and floatstones along with volcaniclastic material. The environment is interpreted as agitated, above storm wave base, and with rapid sedimentation and burial. The collection of acid-resistant microfossils includes abundant scolecodonts, conodonts and chitinozoans, of which only the chitinozoans have previously been studied. The scolecodonts recovered represent at least 14 polychaete genera and 20 species. The fauna bears a great similarity to the early Silurian assemblages previously reported from Baltica, and some environmentally sensitive species allow constraining the assemblage to shallow-shelf settings. The recovered conodonts similarly show close affinities to Baltic faunas, and suggest a Rhuddanian – Aeronian age for the studied section. The chitinozoan assemblage, containing at least eight species, is dominated by Conochitina, and most likely belongs to the C. alargada Biozone.

Global geoheritage significance of Ordovician stratigraphy and sedimentology in the Cliefden Caves area, central western New South Wales

Globally significant geoheritage features of the Cliefden Caves area, in the Belubula River Valley between Orange and Cowra in central western New South Wales, comprise a richly fossiliferous shallow-water limestone succession of Late Ordovician age (the Cliefden Caves Limestone Subgroup) overlain by deep-water laminites and allochthonous limestones of the Upper Ordovician Malongulli Formation. Key features of the Ordovician geology of the Cliefden Caves area that have been identified using the Geoheritage Toolkit as being of international significance are the abundance of unique and exceptionally diverse fossils in the Fossil Hill Limestone (forming the lower part of the Cliefden Caves Limestone Subgroup), which supplement detailed interpretation of carbonate-dominated deposition within an Ordovician volcanic island setting. The fossiliferous limestones preserve biostromes and local small bioherms of stromatoporoids and corals, and recurrent in situ and disarticulated/imbricated Eodinobolus shell beds formed in shallow, quiet-water, dominantly muddy carbonate sediments that passed up-sequence to clay-free carbonate environments. These mud-dominated carbonate sediments are interspersed with higher-energy conditions, represented by skeletal, lithoclastic and calcrete-ooid grainstones overlying disconformities, leading to the identification of subaerial disconformities and associated diagenesis in the Fossil Hill Limestone. The Fossil Hill Limestone is succeeded by massive limestones in the middle part of the Cliefden Caves Limestone Subgroup and then, in turn by the Vandon Limestone and the deeper-water graptolitic laminites of the Malongulli Formation—this completes a succession that is rarely preserved in the geological record, further enhancing the geoheritage significance of the Cliefden Caves area.

A Permian-Triassic boundary microbialite deposit from the eastern Yangtze Platform (Jiangxi Province, South China): Geobiologic features, ecosystem composition and redox conditions

A Permian-Triassic (P-Tr) boundary microbialite (PTB-microbialite) deposit occurs near Xiushui on the southern margin of the eastern Yangtze Platform, South China. This 2-m-thick microbialite overlies uppermost Permian bioclastic limestone with a 5-cm-thick oolite-like grainstone layer at the contact. This PTB-microbialite consists of lower thrombolite (1.5-m thick) and an upper dendrolite (0.5-m thick). The thrombolite interval is characterized by a clotted texture of reddish sparitic patches embedded in a gray micritic matrix, whereas the dendrolite texture consists of brownish sparitic patches embedded within a yellow micritic matrix. Abundant calcified microbes occur in both the thrombolite and the dendrolite, including 1) columnar and clustered fabrics, which are especially rich in the thrombolite, assigned as products of Gakhumella or related microorganisms and 2) microspheroids, 40–60μm in diameter, with different interior structures. These calcimicrobes are considered to have played an important role in constructing the Xiushui PTB-microbialite. The demise of the microbialite was likely due to a sea-level fall during the early Griesbachian (lowermost Triassic), which resulted in a lowering of the wave base thereby destroying the microbialite frameworks and facilitating the deposition of winnowed skeletal grainstone. Metazoan community structures within the microbialite changed sharply across the end-Permian extinction horizon, which is placed at a sharp negative shift in δ13Ccarb. Quantitative analyses show that the Xiushui microbialite community is similar to that of Chongyang and Cili microbialites. This is probably because all of these microbialites grew along the margins of the Lower Yangtze region during the P-Tr transition. However, unlike other PTB-microbialites, the Xiushui ecosystem contains fairly abundant metazoans, which are dominated by ostracods with minor occurrences of foraminifers, microgastropods and microconchids. Both the ostracod assemblage and the pyrite framboid analyses indicate an upper dysoxic zone to a well-oxygenated condition for the Xiushui microbialite. This implies that some PTB-microbialite ecosystems developed in conditions that were not always harsh; instead, some were hospitable for some metazoans to survive immediately after the end-Permian crisis. Considering that the redox conditions indicated by PTB-microbialites are quite variable, we infer that oxygen levels probably were not the crucial factor affecting the growth of the PTB-microbialites.

Impact of strong easterly trade winds on carbonate petroleum exploration - Relationships developed from Caicos Platform, southeastern Bahamas

Brisk, persistent easterly trade wind influences define Holocene patterns of carbonate sedimentation across the Caicos Platform (southeastern Bahamas). Resultant predictive sedimentary facies models based on trade wind influences are more widely applicable to the exploration for subsurface carbonate plays than are existing models based on the northern Bahamas facies patterns, which are characterized by gentle trade winds and strong platform-margin-related oceanic processes (swells and tidal currents). The Caicos Platform relationships may be more applicable because many ancient shallow carbonate depositional environments were within the trade wind belts and commonly within broad intracratonic seas that were little influenced by oceanic processes.The grainstone-dominated Caicos Platform exhibits reservoir potential over much of its surface, in contrast to northern Bahamian platforms, where oceanic tidal currents or swells and gentle easterly trade winds confine higher energy environments with reservoir potential to platform margins. Strong easterly trade winds across Caicos Platform promote widespread Holocene platform-interior oolitic, skeletal and grapestone grainstone bodies on this platform. Orientations of ooid sand bodies vary depending on preexisting topography, water depth and bottom energy. Shallow subtidal ooid sand shoals orient parallel to these winds. Ooid sands developed along older shorelines orient parallel to the shorelines but prograde perpendicular into these winds. Deeper platform-interior oolitic sands exist as widespread, sheet-like deposits. These trade winds allow reefs and ooids to coexist in many settings, permit isolated linear reefs to flourish on certain leeward platform margins, and promote effective off-bank transport of carbonate sands that create onlapping grainstone wedges. These relationships are very applicable to the rock record.Strong trade wind influences, such as seen on Caicos Platform, better explain the occurrence of Cretaceous reef and/or oolitic grainstone reservoirs developed well in from platform margins (Fairway Field in East Texas; Black Lake Field in Louisiana) than do existing northern Bahamian models. Depositional models based on conditions in the northern Bahamian models would predict low-energy facies in these platform-interior settings. A trade-wind-driven depositional model, characterized by strong persistent easterlies, also better explains the origin of the onlapping wedge of skeletal grainstones at Poza Rica oilfield in Mexico.The Caicos Platform easterly trade-wind-driven depositional model should generally apply to any ancient shallow carbonate environment that developed 5–22° north or south of the paleoequator, whether or not a platform margin was near by. Future carbonate exploration or exploitation should always factor in not only geological age, but also physiographic, latitudinal and climatic setting at a global and local scale.

Lateral variability of shallow-water facies and high-frequency cycles in foreland basin carbonate platforms (Pennsylvanian, NW Spain)

The kilometer-sized and 100-meter-thick carbonate platforms of the Escalada Fm. I and II (Middle Pennsylvanian) accumulated in the foredeep of a marine foreland basin during the transgressive phases of 3rd-order sequences and were buried by prograding siliciclastic deltaic systems in the course of the subsequent highstand. The carbonate successions show a general upward trend from grain- to mud-supported carbonates, interfingering landwards with siliciclastic deposits of a mixed siliciclastic-carbonate shelf (Fito Fm.) adjacent to deltaic systems. The spatial variability of the carbonate facies and the high-frequency (4th–5th order) cycles, from the platform margin-outer platform to the deltaic systems, has been interpreted from basin reconstruction. Carbonate facies include skeletal grainstone to packstone, ooidal grainstone, burrowed skeletal wackestone, microbial and algal boundstone to wackestone forming mounds, various algal bafflestone and coral biostromes in areas with siliciclastic input. These high-frequency transgressive–regressive cycles are interpreted to record allocyclic forcing of high-amplitude glacioeustasy because they show characteristic features of icehouse cycles: thickness >5 m, absence of peritidal facies, and in some cases, subaerial exposure surfaces capping the cycles. In the mixed cycles, siliciclastics are interpreted as late highstand to lowstand regressive deposits, whereas carbonates as transgressive-early highstand deposition. The lateral and vertical variability of the facies in the glacioeustatic cycles was a response to deposition in a rapidly subsiding, active foreland basin subjected to siliciclastic input, conditions that might be detrimental to the growth of high-relief carbonate systems.

Climatically Versus Eustatically Controlled, Sediment-Supply-Driven Cycles: Carbonate–Siliciclastic, High-Frequency Sequences In the Valanginian of the Neuquén Basin (Argentina)

Carbonate–siliciclastic high-frequency sequences are formed by reciprocal sedimentation. In siliciclastic-dominated successions, the carbonate-rich interval is typically associated with transgressive conditions and relatively low terrigenous influx, particularly in distal parts of the marine basin. However, key questions for each individual case are: What sort of depositional system developed during transgressions, and what controls sediment-supply fluctuations? This paper integrates outcrop and subsurface data to present a high-resolution sequence-stratigraphic study of small-scale (2–18 m), carbonate–siliciclastic cycles deposited during the Valanginian in a proximal–distal transect of the Neuquen Basin (western Argentina) and discusses those key issues. Carbonate hemicycles in this study are composed of cross-bedded oolitic–skeletal grainstones and packstones to massive skeletal floatstones, which accumulated in carbonate ramps with high biogenic production in their distal sector (bivalves, serpulids, echinoids), and significant contribution of non-biogenic (ooids) material in inner-ramp settings. Siliciclastic material is present only at the base of the hemicycles, but rapidly diminishes upwards. These carbonate hemicycles have retrogradational staking patterns in proximal sectors and less obvious trends in distal areas. The siliciclastic hemicycles comprise shallowing-upward successions that include mudstones, bioturbated muddy sandstones, sandstones with HCS and SCS, and bioturbated sandstones. These facies were collectively deposited in a storm- and wave-influenced offshore-shoreface depositional system during normal regressions. These shoreface systems were likely fed by littoral drift from riverine inputs located tens of kilometers to the south. The duration of the investigated small-scale, transgressive–regressive cycles can be estimated in 50 to 150 kyr, and they are bounded by marine transgressive surfaces placed at the bases of carbonate hemicycles. The bounding discontinuities together with the sequence architecture do not suggest periods of sea-level falls between cycles, nor high-amplitude relative sea-level changes during a cycle formation. Additionally, the low proportion of mixed sediments suggests rapid changes in the rate between terrigenous supply and carbonate productivity. The resulting sequence architecture cannot be reasonably explained by autocyclic controls or eustatic changes alone. It is argued here that high-frequency changes in sediment supply, driven by orbitally induced climate fluctuations, could be a better explanation for these sequences, particularly in the context of a long-term relative sea-level rise. In this scenario, periods of low siliciclastic influx during extreme arid conditions would develop a negative balance with accommodation and prompt carbonate systems and transgression. On the other hand, during more humid conditions relatively high sediment flux would temporarily overcome long-term accommodation, producing the regressive siliciclastic hemicycles. As this scenario does not require high-frequency sea-level falls, it would satisfactorily explain the observed sequence architecture. The distal Mulichinco carbonate hemicycles would be hard to distinguish from transgressive, fossiliferous-rich deposits that characterize transgressive–regressive cycles in clastic systems. In the Mulichinco case, however, these carbonate hemicycles are recording the onset of carbonate ramps, not the trapping of siliciclastic sediments in transgressive coasts (e.g., filling estuaries). This alternative scenario, ultimately controlled by reciprocal sedimentation, could be common in epicontinental basins developed under arid to semiarid conditions, and its identification could provide additional templates for basin-scale correlations and hydrocarbon exploration within carbonate–siliciclastic small-scale cycles.

Revisiting the Eland Field Lodgepole Mound Complex (Stark County, North Dakota) Twenty Years after its Discovery

Eland Field, the most prolific Lower Mississippian Lodgepole mound complex found to date in the Williston Basin, covers an area of about 6 mi2 and has produced more than 29 MMBO from 16 wells in the 20 years since the field was discovered. Three of the field’s updip wells have each produced more than 4 MMBO from mounds more than 250 ft thick although generally only the top 20 to 30 ft of the mound is perforated. The lower Lodgepole reservoir rocks are commonly called Waulsortian mounds, but they are Waulsortian in age only and not the micrite-rich mudmounds found in the type area of Waulsort, Belgium. Instead of being mudmounds, they are composed mainly of marine-cemented microbial boundstones and skeletal grainstones with local stromatactis structures. The edges of the mounds dip as steeply as 40 to 60 degrees. Such steep dips would be impossible in a typical micrite-rich Waulsortian mound. In addition to the abundant microbial and marine cementation that lithified the Eland Field mound complex penecontemporaneously, organisms such as stalked crinoids, fenestrate bryozoans, and articulated brachiopods and ostracods thrived on the hard substrate provided by the mounds. However, these organisms were themselves incapable of forming a true reef framework. Unusual inward dip of the Upper Bakken black shale and significant thickening of the “Extra Bakken Shale” from zero up to about 40 ft immediately beneath the oil-producing lower Lodgepole mounds both support the idea that something structurally significant such as salt dissolution helped localize mound development. We conclude that dissolution of the Lower Devonian Prairie salt created a fracture network during and immediately after deposition of the upper Bakken black shale that allowed compaction-water expulsion-vents and/or warm springs to initiate formation of carbonate “towers” that localized formation of the Lodgepole mounds. The discovery well for Eland Field, the Knopik #1-11 (NW NW Sec. 11, T139N, R97W), was completed in January 1995 for 2707 BOPD and 1550 MCFGPD with no water. The field was developed over the next few years with up to 16 producing wells and 7 water injection wells. Waterflooding of Eland Field began in 1997 and climbed to over 500,000 barrels per month in just two months. Through September 2015, more than 95 MMBW had been injected into the field, but it has produced over 29.6 MMBO, doubling the 1996 estimated ultimate recovery of 12 to 15 MMBO. The field continues to produce with about a 5.5% oil cut. In terms of total cumulative oil production in the U.S. portion of the Williston Basin, Eland Field contains 3 of the top 10 wells, and other nearby Lodgepole producing fields contain 4 additional top 10 wells. This excellent oil production leads to the question: Is it possible that the only productive Lodgepole mound reservoirs in the Williston Basin, eight of which have been discovered to date (with all but one discovered in the mid-1990s) are limited to a small atoll-like area about 7 miles in diameter in northern Stark County, North Dakota? Certainly the presence of similar Lodgepole mounds in outcrops in central Montana (e.g., Bridger Range and Big Snowy and Little Belt mountains) and in the subsurface of the western Williston Basin suggests that other Lodgepole mound-type reservoirs could occur across much of the basin. The most promising areas in which to search for other Lodgepole mound reservoirs will be those where the Upper Bakken black shale is well developed and thermally mature because it is the source for the oil in the Lodgepole reservoirs in the Eland Field area.

Depositional architecture of a mixed travertine-terrigenous system in a fault-controlled continental extensional basin (Messinian, Southern Tuscany, Central Italy)

The extensional Neogene Albegna Basin (Southern Tuscany, Italy) includes several thermogene travertine units dating from the Miocene to Holocene time. During the late Miocene (Messinian), a continental fault-controlled basin (of nearly 500-km2 width) was filled by precipitated travertine and detrital terrigenous strata, characterized by a wedge-shaped geometry that thinned northward, with a maximum thickness of nearly 70m. This mixed travertine–terrigenous succession was investigated in terms of lithofacies types, depositional environment and architecture and the variety of precipitated travertine fabrics.Deposited as beds with thickness ranging from centimetres to a few decimetres, carbonates include nine travertine facies types: F1) clotted peloidal micrite and microsparite boundstone, F2) raft rudstone/floatstone, F3) sub-rounded radial coated grain grainstone, F4) coated gas bubble boundstone, F5) crystalline dendrite cementstone, F6) laminated boundstone, F7) coated reed boundstone and rudstone, F8) peloidal skeletal grainstone and F9) calci-mudstone and microsparstone. Beds of terrigenous deposits with thickness varying from a decimetre to >10m include five lithofacies: F10) breccia, F11) conglomerate, F12) massive sandstone, F13) laminated sandstone and F14) claystone.The succession recorded the following three phases of evolution of the depositional setting: 1) At the base, a northward-thinning thermogene travertine terraced slope (Phase I, travertine slope lithofacies association, F1–F6) developed close to the extensional fault system, placed southward with respect to the travertine deposition. 2) In Phase II, the accumulation of travertines was interrupted by the deposition of colluvial fan deposits with a thickness of several metres (colluvial fan lithofacies association, F10 and F12), which consisted of massive breccias, adjacent to the alluvial plain lithofacies association (F11–F14) including massive claystone and sandstone and channelized conglomerates. Travertine lenses, of 2–3-m thickness, appeared intermittently alternating with the colluvial fan breccias. 3) In the third phase, the filled fault-controlled basin evolved into an alluvial plain with ponds rich in coated reed travertines, which record the influence of freshwater (travertine flat lithofacies association, F7–F9).This study shows the stratigraphic architecture and sedimentary evolution of a continental succession, wherein the hydrothermal activity and consequent travertine precipitation were driven by the extensional tectonic regime, with faults acting as fluid paths for the thermal water. Fault activity created the accommodation space for travertine and colluvial fan accumulation. Erosion of the uplifted footwall blocks provided the source of sediments for the colluvial fan breccias, which alternated with the thermogene travertine precipitation. Climatic oscillations might have led to the recharge of the aquifer that fed the hydrothermal vents.The studied continental succession in an extensional basin provides valuable information about the interplay between thermogene travertine and alluvial/colluvial deposition, which in turn might improve the understanding of similar fault-controlled continental depositional systems in outcrops and the subsurface.

Lower Carboniferous ramp sedimentation of the Central Alborz Basin, northern Iran: integrated sedimentological and rock–magnetic studies

Abstract The Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility (χ in ) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between χ in and the facies; the average χ in values were higher for the distal facies than for the proximal facies. The χ in profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average χ in values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between χ in and facies indicates at least a partly preserved primary χ in signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the χ in , we performed hysteresis measurements on selected samples. It appears that the χ in signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).

High-resolution sequence stratigraphy of the Upper Ordovician Montoya Group, southern New Mexico and western Texas: Outcrop analog of an unconventional chert and carbonate reservoir

The Upper Ordovician Montoya Group crops out in southern New Mexico and westernmost Texas and records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently almost entirely dolomitized. The Montoya Group is a third-order composite sequence composed of six regionally correlative, shallowing-upward, third-order depositional sequences (M0–M5). Sequence M0 has sandstone at its base that is overlain by skeletal packstone-grainstone. Sequence M0 occurs only locally and was likely deposited in a topographic low formed during regional development of the unconformity following El Paso Group deposition. Sequence M1, marking the initial widespread transgression over the Ellenburger unconformity, consists of sandstone updip that passes downramp into skeletal packstone. The highstand systems tract (HST) of M1 consists of a prograding skeletal grainstone that was subaerially exposed upramp. Sequence M2, which contains the second-order maximum flooding surface, has abundant subtidal cherty carbonate at its base, which shallows upward into a widespread, prograding coral packstone-grainstone in the HST. Sequence M3 also contains abundant downramp chert that passes upramp into an aggrading crinoidal shoal and farther upramp into peritidal mudstone. Sequence M4 records an extensive basinward shift in facies as peritidal burrowed and cryptalgalaminated mudstone prograded over subtidal carbonate. Sequence M5 is only locally developed downramp and consists of crinoidal grainstone with abundant evidence of subaerial exposure. A regional unconformity separates the Montoya Group from the Silurian Fusselman Dolostone or younger units. Parasequences (meter-scale cycles) recording low- to moderate-amplitude relative sea level fluctuations are ubiquituous features at individual outcrops but are difficult to correlate regionally. The abundance of syn- or early depositional chert in the subtidal facies indicates that the Montoya Group was deposited within a region of strong regional upwelling along southern Laurentia. This early formed chert was the reservoir facies in a successful Upper Ordovician gas play in Ward and Reeves Counties, Texas.

Modern and Pleistocene large‐sized benthic foraminifers from Tahiti, French Polynesia, collected during IODP Expedition 310

AbstractThis paper reports the fauna and distribution of large‐sized benthic foraminifers (LsBFs; defined as benthic foraminifers >0.5 mm in size) from surface sediments collected around Tahiti, French Polynesia, as well as from the Pleistocene deposits drilled off the Tiarei area, Tahiti, during Integrated Ocean Drilling Program (IODP) Expedition 310. At least 6 and 22 genera of living and dead tests of LsBFs, respectively, were identified from modern surface sediments. Foraminiferal abundance in surface sediments depends mainly on the water depth and substratum type. The taxonomic composition of dead LsBF assemblages varies with depositional environments and water depths. Back‐reef lagoon assemblages are dominated by Schlumbergerina alveoliniformis, whereas fore‐reef slope and shelf assemblages are dominated by Amphistegina spp. The relative abundance of Amphistegina lessonii decreases with increasing water depth. On the shelf, A. lessonii is replaced by Amphistegina bicirculata and Planostegina sp. Pleistocene samples consisting mainly of volcaniclastic sandstone/siltstone and skeletal grainstone contained very small numbers and low species diversity of fossil LsBFs. Amphistegina lobifera, Heterostegina depressa, and Eponides repandus were commonly found in the Pleistocene samples. The taxonomic compositions showed little variation within holes. The Pleistocene assemblages were compositionally similar to those found in modern fore‐reef slope environments of <30 m depth. Combined results of coral U/Th ages and paleoenvironmental interpretations from in situ corals and LsBFs indicate that the Pleistocene sequence in the Tiarei area was deposited in a fore‐reef slope environment during Marine Isotope Stage 3.

Spacing and distribution of bed-perpendicular joints throughout layered, shallow-marine carbonates (Granada Basin, southern Spain)

This contribution focuses on the controls exerted by sedimentological, petrographic and mechanical rock properties on the distribution of bed-perpendicular joints throughout layered carbonates. The study was conducted on Tortonian, shallow-marine skeletal grainstones and rudstones cropping out in the Granada Basin, southern Spain. The results of combined field and laboratory analyses are consistent with the rock grain size of the studied carbonates exerting a key control on distribution of the bed-perpendicular joints. A positive correlation between joint spacing and mechanical unit thickness is computed for the grainstones, whereas joints are almost absent in rudstones. The rock grain size affected the diagenetic processes, such as cementation and dissolution, and therefore the resulting porosity and uniaxial compressive strength (UCS) values. Quantitative data show that higher UCS values are commonly associated with greater calcite amounts, lower values of porosity and finer grain sizes and, hence, denser bed-perpendicular joint sets. In conclusion, this study documents that it is possible to infer the density of bed-perpendicular joints in layered carbonates based on the sedimentological, petrographic and mechanical parameters. Considering the impact that this type of joints has on subsurface fluid flow, the acquired knowledge can help the management of geofluids as well as the overall prediction of carbonate reservoir quality.

The Miocene Nullarbor Limestone, southern Australia; deposition on a vast subtropical epeiric platform

The early to middle Miocene Nullarbor Limestone forms the vast, karsted Nullarbor Plain in southern Australia, and may be the most extensive Miocene carbonate deposit described to date. These carbonates were deposited at southern paleolatitudes of ~40°S and are interpreted to be subtropical to warm-temperate in character because of the presence of certain genera of tropical coralline algae (rhodoliths and articulated types), large benthic foraminifera, tropical molluscs, zooxanthellate corals, and micrite envelopes. Facies are dominated by skeletal grainstones and floatstones that accumulated in three interpreted paleoenvironments: (1) seagrass banks (upper photic zone), (2) rhodolith pavements (lower photic zone), and (3) open seafloors (lower photic to subphotic zone). A decrease of tropical components from west to east across the platform implies that warm oceanic currents (possibly related to a proto-Leeuwin Current), as well as a period of warm climate (Miocene Climatic Optimum), resulted in subtropical deposition at southern latitudes. The Southern Ocean extended inboard ~450km from the shelf edge during Nullarbor Limestone deposition, but interpreted paleodepths did not extend much below the base of the photic zone. A small slope angle (~0.02°) over a wide shelf (~300,000km2) implies deposition on an epeiric platform or epeiric ramp. A Miocene barrier reef was likely coeval with Nullarbor Limestone deposition. Therefore, the inboard portion of the Nullarbor Limestone can be considered part of an extensive back-reef lagoon system on a rimmed epeiric platform, perhaps attaining a size similar to the modern Great Barrier Reef system.