Sandstone Lithofacies Research Articles

THE SEDIMENTOLOGY OF THE LOWER PERMIAN DANDOT FORMATION: A COMPONENT OF THE GONDWANA DEGLACIATION SEQUENCE OF THE SALT RANGE, PAKISTAN

The Dandot Formation is a part of the Lower Permian, dominantly continental, Gondwanan Nilawahan Group in the Salt Range, Pakistan. The formation conformably overlies the glacio-fluvial Tobra Formation and has a sharp conformable contact with the overlying fluvio-continental Warchha Sandstone. Sedimentary analyses show that the Dandot Formation consists of: 1. bioturbated sandstone lithofacies (L1), 2. dark green mudstone/shale lithofacies (L2), 3. flaser bedded sandstone lithofacies (L3), 4. rippled sandstone lithofacies (L4), 5. cross-bedded sandstone lithofacies (L5), and 6. planar sandstone lithofacies (L6). These can be grouped into shoreface, inner shelf, and tidal flat and estuarine facies associations, deposited in shallow marine to intertidal environments. The upper part of the Tobra Formation at the Choa-Khewra road section, where it conformably underlies the Dandot Formation, contains palynomorphs assignable to the earliest Permian 2141B Biozone. In south Oman, the 2141B Biozone is closely associated with the Rahab Shale Member, a widespread shale unit which is considered to represent part of a Permian deglaciation sequence which culminates in the marine beds of the Lower Gharif Member, interpreted as due to post glacial marine transgression. Thus, the Tobra Formation and the overlying marine Dandot Formation may form part of a similar deglaciation sequence.

Sedimentary petrology and provenance interpretation of the sandstone lithofacies of the Paleogene strata, south-eastern Nigeria

The Paleogene strata in the south-eastern Nigeria include the Imo Formation, the Ameki Group and the Ogwashi Formation. Petrographic studies suggest quartz arenite to arkosic arenite and sub-arkose for sandstones of the Imo Formation. Sandstones of the tide-dominated estuarine system of Ameki Group are classified as quartz arenites, sub-arkoses, sub-litharenites and litharenites. Sandstones of the Ogwashi Formation are classified as quartz arenites and sub-litharenites. These Paleogene sandstones are derived from mixed provenances based on the petrographic interpretation. The provenances include cratonic interiors, transitional continental blocks as well as recycled orogenic and transitional continental, transitional recycled and quartzose recycled. Paleoclimatic condition for the Paleogene region is semi-humid to humid climate. The sandstone composition suggests detritus from igneous rocks, metamorphic rocks and recycled sedimentary rocks. Heavy mineral analysis confirms the possibility of mixed provenance for the Paleogene sediments. The source areas include the Oban Massif, the West African Massif and the recycled sedimentary rocks of the older sedimentary rocks of probably the Anambra Basin and Afikpo Syncline.

The effect of interbedding on shale reservoir properties

North Sea oil is overwhelmingly generated in shales of the Upper Jurassic – basal Cretaceous Kimmeridge Clay Formation. Once generated, the oil is expelled and ultimately migrates to accumulate in sandstone or carbonate reservoirs. The source rock shales, however, still contain the portion of the oil that was not expelled. As a consequence such shales and juxtaposed non-source lithofacies can form the targets for the exploration of ‘unconventional oil’.In this paper, we examine part of the Kimmeridge Clay Formation as a hybrid shale resource system within which ‘Hot Shale’ and organic-lean sandstone and siltstone intervals are intimately interbedded. This hybrid system can contain a greater volume of oil because of the increased storage capacity due to larger matrix porosities of the sand-silt interbeds, together with a lower adsorptive affinity in the interbedded sandstone. The relationship between the estimated volume percentages of sand and mudstone and free oil determined from Rock-Eval® S1 yields is used to place limits on the drainage of oil from source mudstone to reservoir sand at the decimeter scale. These data are used to determine oil saturations in interbedded sand-mudstone sequences at peak oil maturity. Higher values of free hydrocarbon (as evidenced by the S1 value in mudstone) suggest that more oil is being retained in the mudstone, while higher S1 values in the interbedded sands suggest the oil is being drained to saturate the larger pore spaces. High silica content in the interbeds confirms the brittleness in this mudstone–sandstone lithofacies – an important factor to be considered for fracture stimulation to successfully work in a hybrid system. The key points of this hybrid unconventional system are the thickness, storage capacity and the possibility to capture a portion of the expelled, as well as retained oil.

Lahar facies of the Latest Paleozoic Arbasay Formation: Geomorphological characters and paleoenvironment reconstruction of Northern Tian Shan, NW China

Texturally well-preserved lahars are exposed in the Latest Paleozoic Arbasay Formation, Northern Tian Shan. The lahars consist primarily of two lithofacies: massive, poorly lithified diamictites and stratified, moderately lithified gravelly sandstones. The diamictites can be generally divided into two subfacies, i.e., the matrix-supported and the clast-supported diamictites. Most diamictites are structureless and nongraded, however some crudely stratified beds with a certain grading have also been found. They are thick in beds and contain large clasts up to 3m in dimension. The gravelly sandstones display much finer particle size and have wedge or lenticular geometries. Large clasts are absent within them and the sorting characters are much better than the diamictites. Despite the different size grading, the matrix and the clasts of the two lithofacies appear to be homogeneous. The matrix is generally sandy mudstone. The clasts comprise rhyolites, dacites, andesites, andesitic basalts and basalts, same to the co-existing volcanic rocks, suggesting they originate from the cognate volcanics.The disorganized diamictites are supposed to deposit from a turbulent flood or pyroclastic surge. The gravelly sandstone lithofacies are interpreted as sand-rich flood flows or hyperconcentrated flood flows during the waning stage of a mass-flow event. The overall characteristics of the deposits suggest a mass-flow dominated alluvial fan environment. They are rapidly accumulated within small, normal faults bounded depocenters.The characteristics of the Arbasay Formation lahars, together with the geochemistry characters of the co-existing volcanic rocks, clearly prove that they were deposited in a post-collisional extensional regime. LA-ICP-MS zircon dating of the intercalated fallout tuff sample provided an age of 314.4±3.4Ma (MSWD=1.6), suggesting they were deposited at Late Carboniferous. Therefore, the North Tian Shan Ocean must have closed before 314Ma, and the initial basin type in Northern Tian Shan should be extensional faulted basins rather foreland basins.

Analysis of sedimentary facies and paleo-depositional environments of the DC70X reservoir in Mbakan field of the central swamp, Niger Delta

The reservoir quality has been investigated in DC70X sandstone interval from Mbakan-6 well, Mbakan field, Niger Delta. This study involves the lithologic description and classification of 155 ft (47 m) interval of sandstone core recovered from the well. The sandstone lithofacies with their corresponding reservoir quality is strongly controlled by the paleo-depositional system. Two depositional settings are recognized in the sand-rich facies associations separated by a mudstone layer. The lower sequence consists of deposits of a prograding wave-dominated shoreface, while the upper sand consists of transgressive estuarine deposits. The deposits comprise good-quality reservoir sandstones interbedded with poor-quality reservoir sandstones, shales, and mudstones.

Constraints on the Statherian evolution of the intraplate rifting in a Paleo-Mesoproterozoic paleocontinent: New stratigraphic and geochronology record from the eastern São Francisco craton

An integrated approach of stratigraphic analysis and U–Pb age dating reveals some information on the tectono-sedimentary evolution of the Statherian cover of the São Francisco craton in the so-called Espinhaço basin (Atlantic shield in eastern Brazil). Here, continental sedimentation patterns, such as alluvial fan, braided-plain and lacustrine facies associations, with associated volcanic rocks are documented in two superposed basin fill-successions, which are defined as the Algodão and Sapiranga Synthems and grouped in the Botuporã Supersynthem. Both studied units consist mainly of conglomerates and cross-bedded sandstones and minor amounts of mudstones, sedimentary breccias, volcanic lava beds and volcaniclastic rocks, which were deposited in a rift basin – the Botuporã rift – during two syn-rifting phases. The Algodão Synthem represents the first rifting phase. The basal synsedimentary conglomerates of this unit were deposited mainly by subaerial debris flows, most likely along and near a rift border fault. The framework of this rock consists of only crystalline rock clasts from the basement and no fragments of volcanic rocks. Detrital zircon grains that were extracted from this facies show ages older than 2.05Ga. The remainder of the section is dominated by fluvial sandy lithofacies with minor conglomerate lenses and sandstone–mudstone heterolithic lithofacies, which represent distal, waning-flood deposits in a lacustrine environment. The upper section also contains hummocky cross-stratified sandstone lithofacies, which are related to a storm-influenced deposition. On top of the Algodão succession, the volcanic rocks were dated at 1775±7Ma, which was interpreted as the near final age of the first rift-phase. Representing the second rift-phase, the Sapiranga Synthem shows similar sedimentation patterns to the Algodão Synthem. The Sapiranga Synthem rests directly on the volcanic rocks of the Algodão Synthem, and its basal conglomerates (which are most likely also related to a master fault) contain voluminous clasts of volcanic rocks, sandstones and crystalline rocks. The detrital zircon grains that were extracted from this facies show ages of 1741±14 and 1766Ma as well older than 2.05 Ga. The volcanic rocks on the upper succession of the Sapiranga Synthem record ages of 1740±10Ma, which finalized the Botuporã rift evolution. A preliminary geochemical study of volcanic rocks from the Botuporã Supersynthem showed that these rocks are alkaline rocks with high K2O/Na2O ratios, which belong to an ultrapotassic suite. The low concentrations of MgO wt.% suggest a felsic ultrapotassic character. The Botuporã Supersynthem is unconformably covered by a volcano-sedimentary rift-succession of EoCalymmian age — the Pajeú Synthem, which represents the second rifting stage of the Espinhaço basin. Several Statherian-related volcano-sedimentary sequences and anorogenic granitoids occur dispersed in the São Francisco block, which requires a regional geologic model to explain the extensional and magmatism process during this time. We used the last Columbia paleocontinental reconstruction to constrain these processes by relating them to far-field continental extensional and magmatic record as part of a silicic LIP, which was triggered by the convection-driven tectonic-plate motion on the western border of the Atlantica block inside of the Columbia supercontinent.

Lithofacies and depositional environment of the Amasiri Sandstone, southern Benue Trough, Nigeria

Eight lithofacies typical of tidally-influenced shelf, mass flow and turbidity current processes characterize the Amasiri Sandstone (Cenomanian – Turonian) in the southern Benue Trough, Nigeria. The cross bedded sandstone lithofacies (Sxm) in Afikpo area were deposited in tidally influenced, shallow sandy shoreline environment while similar lithofacies associated with the conglomeratic lithofacies (Sfc) in Akpoha are proximal canyon–fill deposits. The conglomeratic lithofacies with rip-up clasts together with the massive, horizontal-bedded lithofacies (Smm) and parallel-laminated sandstone lithofacies (Sfl) in Akpoha were deposited in confined channels in proximal submarine canyon setting. The wavy/ripple-laminated sandstone lithofacies (Sfw) and very fine grained bioturbated sandstones lithofacies (Sfb) represent weakly confined distributary splay and unconfined associations in proximal to distal submarine canyon settings. The bioturbated mudstone lithofacies (Msb) and parallel-laminated mudstones lithofacies (Msl) comprise the bypass/levees association in the inner to outer shelf and in the distal canyon settings. Overall, these lithofacies indicate deposition in shelf to deep water depositional environments.

Late Cretaceous Sub-Marine Fan System in Batain Mélange Zone, the Fayah Formation in Northeastern Oman

The Batain coast along the northeastern margin of Oman between Ra’s Al-Hadd and Ra’s Jibsch, is comprised of Permian to Late Cretaceous complex stratigraphy in a tectonically deformed area recording Permian rifting to late Cretaceous Tethys closure events. These rocks are thrust over Mesozoic and older autochthonous sedimentary cover in the form of a major nappe structure known as the Batain Nappe. The uppermost part of the Batain nappe is comprised of isolated outcrops of early Maastrichtian siliciclastic Fayah Formation dominated by gravity flow deposits. The Fayah Formation in the Jabal Fayah area is over four hundred meters thick and comprised of five distinct facies associations; namely, i) coarsening-up sandstone, ii) conglomerate, iii) debris- flow, iv) turbidite, and v) inter-bedded sandstone and shale lithofacies. These lithofacies associations are repeated many times in the section. The sandstone lithofacies association exhibits a coarsening-upward trend making sequences tens of meters thick in various parts of the formation. Waterscape structures are common along with occasional sandstone dykes and convolute bedding, reflecting fluidized conditions of deposition. The conglomerate lithofacies association is comprised of a series of interbedded coarsening-upward pebble to gravel size conglomerates containing chert, limestone, granite and volcanic clasts ranging a few mm to cm in diameter. Occasionally these are interbedded with sandstone lithofacies. The conglomerate lithofacies was deposited by a high-energy channelized flow in a sub-aqueous setting. The debris-flow lithofacies association is a matrix supported chaotic mixture of clay and boulders of granite, limestone and volcanic rocks, some of which are meter-sized in diameter, and possibly derived from the nearby basement rocks such as the Jabal Ja’alan basement rocks. It constitutes the most dominant part of the formation. These sediments were deposited along a slope setting, possibly as olistostrome formed due to submarine slumping and sliding. The turbidite lithofacies association is comprised of monotonous grayish-green to brown coloured clays tens of meters thick interbedded with thin, clean, well-sorted sandstone. The Interbedded sandstone and shale lithofacies association is comprised of a half to one meter thick cross-bedded, burrowed arkosic sandstone and plane laminated shale. The sandstone constitutes about 25% of the association with ripple lamination in the upper part of the unit indicating a fining-upward trend. Dewatering structures are common. This association constitutes the upper 100m of the formation. These sediments were deposited in shallow water conditions by channelized flows. Based on the lithofacies associations described above, especially the dominance of debris-flow units and turbidites, the greater part of the Fayah Formation are interpreted as having been deposited under a sub-marine fan setting. Only the upper part of the formation was deposited in a shallow water setting before the onset of overlying carbonate deposits. The sub-marine fan system was active during the last stages of the Tethys Ocean closure at the time of onset of the Batain nappe.

Channel morphology, hydrology and geomorphic positioning of a Middle Miocene river system of the Siwalik foreland basin, India

AbstractSystematic lithofacies, palaeocurrent, palaeomorphological and palaeohydrological analyses have provided detailed information about a hitherto unstudied river system of the Siwalik foreland basin of the Himalaya. Three distinct lithofacies associations, each representing a specific depositional setting, have been identified and named as ‘Facies Association A’, ‘Facies Association B’ and ‘Facies Association C’. The ‘Facies Association A’ comprises pebbly sandstone, cross-bedded sandstone, ripple-laminated sandy siltstone and bioturbated mudstone lithofacies and represents deposits of a braided channel. The ‘Facies Association B’ comprises cross-bedded sandstone, bioturbated mudstone, fine sandstone–mudstone alternation and lensoid to prismatic sandstone lithofacies and represents deposits of a channel, natural levee, crevasse-splay and flood plain of a meandering stream. The ‘Facies Association C’ comprises mottled siltstone–mudstone heterolith and fine sandstone lithofacies and represents deposits of the upland interfluve region. The braided stream had a maximum depth of 4.15 m, maximum width of 305 m and maximum discharge of 7045 cumec, whereas the meandering stream had a sinuosity of 1.26, maximum depth of 3.71 m, maximum width of 180 m and maximum discharge of 4070 cumec. The area had a regional radial outward flow pattern, but dominantly towards the SSW. However, the braided river had a bimodal flow pattern due to an active basement-high-induced bend along its course. A comparison of the sediment characters and morphological and hydrological parameters of these streams with those of the modern rivers of the Ganga (Gangetic) basin has enabled us to infer that this river system was located in the medial-distal megafan-interfan setting of the basin.

Lithofacies architecture and depositional environment of Late Cretaceous Lameta Formation, central India

Establishment of lithofacies and interpretation of depositional environment of the Lameta Formation have been carried out for a new area having three nearby exposures. Overall, the successions are broadly differentiable into three major units, i.e. arenaceous in the lower part, argillaceous in middle and calcareous at the top, which more or less serve as three lithofacies associations. Each association consists of various lithofacies which are identified and established on the basis of field observations, added with significant petrological details. The arenaceous association includes five lithofacies, viz. massive sandstone lithofacies; green sandstone lithofacies; thinly bedded, yellowish orange and greyish brown sandstone lithofacies; coarse-grained sandstone lithofacies and dark brown bioturbated sandstone lithofacies. The argillaceous lithofacies association consists of yellowish-brownish-greenish clay–siltstone lithofacies and light grey silty–clay with concretions lithofacies, whereas the calcareous lithofacies association is represented by calcrete lithofacies, nodular limestone lithofacies, chertified limestone lithofacies and intraformational breccia lithofacies. The facies analysis reveals that the deposition of Late Cretaceous Lameta sediments took place in fluvial–lacustrine environments in semi arid condition.

Marginal marine depositional setting and correlation of the Devonian Sperm Bluff Formation (Taylor Group), southern Victoria Land, Antarctica

AbstractAnalysis of conglomerates and sandstones of the Sperm Bluff Formation at the base of the Taylor Group (Devonian) between the Mackay Glacier and Bull Pass provides new insights into the nature of initial coarse-grained deposition on basement along the northern side of the McMurdo sedimentary basin. Six lithofacies are recognized in the Sperm Bluff Formation: conglomerate lithofacies, pebbly sandstone lithofacies, cross-bedded sandstone lithofacies, low-angle cross-stratified sandstone lithofacies, bioturbated sandstone lithofacies and interbedded siltstone/sandstone lithofacies. Sedimentary environments ranged from wave-dominated delta, estuary or lagoon to shoreface and inner shelf. The assemblage is thought to reflect changes in sea level. Rhyolite is the most abundant clast type in the coarse lithofacies, but is unknown in outcrop in southern Victoria Land. The rhyolites correlate in age and geochemistry with Cambrian granites in the basement. Coarse beds also contain numerous quartzite clasts, probably derived from the late Precambrian Skelton Group. Palaeocurrents on Mount Suess indicate a strong unimodal flow to the west, but other sites show polymodal palaeoflow. The Sperm Bluff Formation is correlated with Terra Cotta Siltstone, New Mountain Sandstone and Altar Mountain formations based on the lithology of sandstones and their ichnology. A northward onlap during the Early Devonian is indicated.

Stratigraphic Correlation of Devonian Sequences of Northwestern Darling Basin, New South Wales, Australia

The study is focused on the Northwestern part of the Darling Basin, and deals with a Stratigraphic correlation. This study involves correlation of the regional stratigraphy in the Darling Basin of western New South Wales that results from deformation of the Devonian sequences. The interpretation based on well logs and lihological data. Integrating well and lithological data to correlate the sequence stratigraphic units, using sequence stratigraphy approach are the main goals of this study. Analyses of lithostratigraphic units from well-logs and lithology data were used to develop a stratigraphic correlation of the northwest Darling Basin. The stratigraphic boundaries of the intervals were defined at marked changes in well-log characteristics and lithology information. Correlation of three wells in the northwestern part of Darling Basin allowed correlation and chronostratigraphic subdivision of Devonian sediments. The lithofacies of three wells Bancannia North-1, Bancanna South-1 and Jupiter-1 have been identified from northwest to southeast. The lithofacies dominated by sandstone and shale sediments mostly in Lower to Middle Devonian age, red sandstone, porous sandstone and shale lithofacies (red beds) of Middle to Upper Devonian sediments. Three stratigraphic units were identified based on well logs characteristics and lithofacies namely A, B and C.

Lateral variation in sandstone lithofacies from conventional core, Scotian Basin: implications for reservoir quality and connectivity 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin. 2Geological Survey of Canada Contribution 20120021.

Facies analysis in outcrops on land is strongly dependent on the lateral variability of lithofacies. Interpretation of conventional core in wells relies principally on the vertical succession of lithofacies. To better understand depositional environments and reservoir sandstone connectivity, the lithofacies of reservoir sandstones sampled in conventional core were correlated laterally through two sets of closely spaced wells in the Scotian Basin: in the Barremian–Albian succession around the Panuke–Cohasset field and in the Late Jurassic succession west of the Venture field. Regional correlation by gamma logs is confirmed by lithologically similar transgressive units including shelly mudstones or coals. A standard scheme of lithofacies and recognition of three types of parasequences were used for comparisons between wells. Some transgressive surfaces are of limited extent and may represent delta distributary switching and subsidence rather than regional changes in sea level. Major sandstone packets extend at least 10–30 km laterally, but commonly show lateral changes in lithofacies, and some are bounded by the margins of incised valleys. Such packages show poor correlation of lithofacies with porosity and permeability, probably because of the variable effects of diagenesis. Lateral transitions from tidal estuary sandstones in Panuke B-90 to thick-bedded river-mouth turbidites in Lawrence D-14, over a distance of 15 km, demonstrates the scale of delta lobes and confirms that sharp-based sandstone beds are turbidites related to river floods, not storm deposits. Similar lateral transitions in the Venture field are on a similar scale and pass distally into prodeltaic muddy landslide deposits.

Lithofacies attributes, depositional system and diagenetic properties of the Permian Gharif Formation from Haushi–Huqf area, Central Oman

Over 70 m thick interbedded sandstone, siltstone and claystone of the upper member of the Gharif Formation are exposed in western Huqf area in Oman Interior Sedimentary Basin. The Gharif Formation, particularly its upper member hosts major hydrocarbon reservoir in the subsurface of the Oman Interior Sedimentary Basin. The upper member of the Gharif Formation is comprised of interbedded thick sandstone, siltstone, carbonaceous clays and intraformational conglomerates. The sandstone lithofacies, on average, constitute 10 m thick multistoreyed sequences, which are composed internally of 2–3 m thick and 100 s of metres across vertically and laterally amalgamated sandstone bodies. Two major types of sandstones (types 1 and 2) are identified on the basis of their lithofacies association and internal architecture. The type 1 sandstone constitutes the lower part of the member and is comprised of pebbly to coarse-grained, planar and trough cross-bedded sandstone, plane bedded sandstone and pebble lags at the base of major sandstone bodies. The cross-beds are, on average, 30 cm thick exhibiting a dominant paleoflow direction towards NW (280–300° N). It is interpreted to be deposited by low sinuosity braided streams. The type 2 sandstone constitutes the upper part of the member and is comprised of medium-grained sandstone, trough to low angle plane bedding associated with lateral accretion surfaces. It is commonly interbedded with carbonaceous clays. Silicified plant fragments are commonly distributed in the upper part of the sandstone. Interbedded clays and siltstones are red, mottled and extensively bioturbated due to root burrows. It is interpreted to be deposited by high sinuosity meandering streams. In the uppermost part of the section, several dark grey to black carbonaceous clay/coal beds with plant matter are interbedded with sandstone and red clay indicating development of swampy conditions during onset of the coastal setting in the uppermost part of the formation. About 30 cm thick bioclastic sandstone deposited by the marine coastal bars mark transition from the Gharif Formation to carbonate dominated Khuff Formation. The sandstone of the Gharif Formation is arkosic in composition. Very small amount of cement and negligible compaction of constituent grains in sandstone indicates shallow burial before uplift.

Siliciclastic deposit of the Nahr Umr Formation, sedimentological and depositional environment studies, central and southern Iraq

The depositional facies and environments were unraveling by studying 21 subsurface sections from ten oilfields in the central and southern Iraq and a large number of thin sections of the Nahr Umr (siliciclastic deposit) Formation (Albian). This formation is mainly composed of sandstone interlaminated with minor siltstone and shale, with occurrence of thin limestone beds. Nahr Umr Formation is subdivided into three lithostratigraphic units of variable thicknesses on the basis of lithological variations and log characters. Mineralogically and texturally, mature quartz arenite and sandstones are the common type of the Nahr Umr Formation. The sandstones are cemented by silica and calcite material and have had a complex digenetic history. Compaction, dissolution, and replacements are the main diagenetic processes. Prodelta, distal bar, distributary mouth bar, distributary channel, over bank, and tidal channel are the main depositional environments recognized for the Nahr Umr Formation, within the studied wells. This formation was deposited in shallow marine and fluvial–deltaic environments and exhibit progradational succession of facies. Eight sedimentary facies that have been identified in the Nahr Umr Formation include claystone lithofacies, claystone siltstone lithofacies, lenticular-bedded sandstone–mudstone lithofacies, wavy-bedded sandstone–mudstone lithofacies, flaser-bedded sandstone–mudstone lithofacies, parallel and cross lamination sandstone lithofacies, trough cross-bedded sandstone lithofacies, and planar cross-bedded sandstone lithofacies. The depositional model of the Nahr Umr Formation environment was built based on the lithofacies association concepts.

Sedimentological controls on the fracture distribution and network development in Mesaverde Group sandstone lithofacies, Uinta Basin, Utah, USA

Abstract The Mesaverde Group, Uinta Basin, Utah is the source of current significant natural gas production and contains several trillion cubic feet of undiscovered natural gas resources. To evaluate and model the potential connectivity of hydraulically induced fractures to natural fractures in the subsurface, the natural fracture network was examined using scanline sampling, image and well logs, core and microstructural analyses. Regional fracture sets include subvertical fractures with dominant orientations of: north–south (006–015°), NE (045–059°), NNW (326–342°) and a WNW (271–286°). Sedimentologic and diagenetic characteristics of seven sandstone lithofacies control the fracture development and distribution in the group. Key sedimentologic and diagenetic influences on fracture distribution include bed thickness, stratigraphic architecture, the degree of cementation and the type of cement. From these sedimentologic controls on the character of natural fractures, lithofacies can potentially predict fracture distribution within the Mesaverde Group based on environments of deposition. The presence of NW-trending discontinuous sandstone reservoirs deposited in meandering fluvial environments that are highly fractured by a pervasive WNW-striking fracture set helps to explain fairways of prolific natural gas production within the basin.

The significance of deep-water cryptic bioturbation in slope-channel massive sand deposits of the lower Rio Dell Formation, Eel River basin, California

Deep-water massive (featureless) sands are commonly interpreted as indicators of flow conditions during or shortly after deposition and are almost never interpreted to reflect post-depositional homogenization due to bioturbation. Deep-water deposits of the lower Rio Dell Formation (Pliocene), Eel River basin (NW California), however, reveal previously unrecognized cryptically bioturbated fabrics in massive sandstone and mudstone. This study represents the first detailed documentation of cryptically bioturbated massive-appearing sandstone in deep-water channelized environments. Although fluid escape contributed to some of the massive-appearing textures in the Rio Dell Formation, we present criteria for recognizing cryptic bioturbation fabrics: (1) tubes with lithologically distinctive concentric linings or infillings, (2) meniscate backfill in burrows, (3) lithologically distinctive spreite, (4) concentric grain orientations, and (5) grains realigned adjacent to tubular structures. Using biostratigraphic data, detailed stratigraphic descriptions, core photos, and photomicrographs, we document ichnofabrics associated with two main lithofacies and their bounding surfaces: (1) diffuse muddy siltstone lithofacies (e.g. Planolites, Phycosiphon, Schaubcylindrichnus), (2) massive sandstone lithofacies (e.g. cryptic bioturbation , Planolites), (3) lithofacies crossers, (4) burrows that prefer the top-of-sand/base-of-mud interface (e.g. Planolites, Skolithos), and (5) a Glossifungites-demarcated discontinuity at the base of the massive sand lithofacies. We also establish two main lithofacies associations with characteristic sedimentologic traits. We use relative percentages of the lithofacies associations to interpret slope-channel depositional facies: distal channel margin, proximal channel margin, and channel axis. By comparing ichnologic characteristics to depositional facies, we conclude that proximity to the channel axis appear to control the degree and type of bioturbation observed in the massive-appearing deposits. Distal channel margin deposits preserve the smallest burrows, highly developed cryptic bioturbation fabrics, and the most complete sequence of background bioturbation. Proximal channel margin and channel axis deposits record larger burrows but incomplete cryptic bioturbated fabrics and an overall smaller proportion of preserved background bioturbation.

Tidally influenced clastic epeiric sea at a Mesoproterozoic continental margin, Rjukan Rift Basin, southern Norway

Precambrian epeiric sea paleoenvironments record information about ancient tidal conditions and nearshore sedimentation patterns. The Mesoproterozoic Brattefjell Formation, part of the Rjukan Rift Basin in southern Norway, presents diagnostic criteria of an ancient tidal influence in a shallow clastic epeiric sea environment during the post-rift stage of the sedimentary basin. Eight different sedimentary lithofacies/subfacies and two genetically related lithofacies associations were recognized from the formation. The conglomerate lithofacies records a transgressive ravinement surface due to wave scouring. The sandstone lithofacies consists of six subfacies: ripple-marked, planar cross-bedded, trough cross-bedded, low-angle cross-bedded, and horizontal laminated sandstones. The sandstone lithofacies records sedimentation by shallow current flows or oscillatory waves of lateral migrating straight- or slightly sinuous-crested sandbars and channel-fill deposits. The associated bi-polar cross-bedding, slump-folds, ladder-back type ripples, shrinkage cracks, double-mud drapes, iron ooids, load casts, and flame structures indicate variation of the flood-ebb cycles in tidal sandbars, rapid sedimentation, and sediment compaction. Tidal bundle measurements suggest that 16–18 bundles were formed during the neap-spring cycle, indicating mixed semidiurnal tides. A mudstone lithofacies (two subfacies) records waning current strength deposits, channel abandonment, and either vertical aggradation of the channels or coastal plain deposits. The lithofacies associations reflect fluctuating energy conditions in a shallow clastic shoreline influenced by tides and waves. At least 23 shallowing upward cycles caused by autocyclic tidal water level changes were recognized from the lower part of the formation.Based on the lithofacies assemblages, the Brattefjell Formation can be subdivided into sandy subtidal, lagoon-estuary, and shallow marine shoreface-offshore environments deposited above the fairwater wave base. The Brattefjell Formation records millions of years of fluctuating water levels with an overall transgressive trend, indicating allocyclic patterns caused by regional subsidence. The formation is interpreted to represent a tidally influenced epeiric sea in which shallow water levels extended over part of a continent and it was associated with marine transgression. The shoreface was fed sediment by alongshore and coastal current drifts during shoreface transgressive erosion, in a low shoreline gradient with reduced sediment input and lack of major braided-fluvial channel(s).

Sediment waves and depositional implications for fine-grained rocks in the Cerro Toro Formation (upper Cretaceous), Silla Syncline, Chile

The Cerro Toro Formation contains a variety of lithofacies including pebble and cobble conglomerate, coarse-grained sandstone, thin-bedded mudstone and sandstone, and slumped to chaotic mudstone. Coarse-grained rocks in the Cerro Toro have been described and well documented, whereas characterization and description of mudstone-prone lithofacies are not well documented. These mudstone-prone rocks exhibit two architectural patterns: 1) broad (>200 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap) and downlap patterns, and 2) laterally persistent (>400 m), horizontal, thin-bedded mudstone and sandstone. Adjacent to Channel Complex 3 (Paine C Member), these facies exhibit a stratigraphic transition from horizontal to wavy and curved beds concurrent with pronounced aggradation of laterally equivalent, and possibly coeval, channel facies. Sandstone and mudstone beds within the wavy-bedded facies exhibit turbidite lithofacies that include current-ripple lamination (Tc), planar lamination (Tb) massive, graded intervals (Ta), and laminated to structureless silt- and clay-rich beds (Tde). Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1 m thick. These thick sandstone bedsets display inclined bedding or lamination associated with mudstone rip-up clasts and are confined to troughs or swales within large-scale wavy-bedded units. Erosion surfaces within this thin-bedded fine-grained lithofacies are spaced vertically at 10–15 m, commonly associated with the crest of curved bedding and display at least 5 m of relief. Based on map patterns, channel-form architectural elements, and stratal-stacking relationships, the coarse-grained rocks in the Cerro Toro have been interpreted as the fill within deep-water channels. In contrast, the thin-bedded mudstone and sandstone lithofacies located adjacent to and eroded into by the channel facies are interpreted as coeval levee facies. Planar, tabular bedsets located at the base of this fine-grained lithofacies are interpreted as initial overbank deposition associated with the upper part of Channel Complex 3, whereas the curved, wavy beds in the fine-grained lithofacies are interpreted as sediment waves developed on the backside of a levee. The curved bedding, lenticular, medium-grained sandstone bedsets, scattered erosional surfaces, and onlap-downlap stratal geometries in the Cerro Toro bear resemblance to sediment waves associated with Quaternary coarse-grained, channel-levee systems. In contrast, sediment waves generated in other deep-water settings, such as upper slope, continental rise or basinal settings, usually are dominated by silt- and very fine-grained sand, very continuous layering (over 1 km), relatively long wavelength (>1 km) and relatively high amplitudes (>10 m). A possible analog for the coarse-grained and fine-grained rocks in the Cerro Toro is the Quaternary Var Ridge and associated fan valley located in the Mediterranean Sea. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size clasts, and thin-bedded mud and sand in levees with documented sand bed thickness up to 40 cm and up to medium-grained sand. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap downlap, and toplap, similar to the Cerro Toro. The fine-grained material can be linked to deposition from unconfined turbidity currents that were generated via overbank flows from the Var fan channel. The development of these architectural elements requires out of channel turbidity currents and relatively thick flows. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation.

Finite Markov chain model in lithofacies analysis: an example from the bida sandstone, Bida Basin, Nigeria

The Markov Chain Stochastic Process has been used both to analyze the vertical lithofacies of the Bida Sandstone (Campanian – Maastrichtian) in Bida area and interprete the corresponding depositional systems. Six lithofacies, comprising of F1: conglomerates (breccias); F2 : medium – very coarse, pebbly sandstone with dominant trough beddings ; F3: coarse-grained pebbly sandstone with small scale trough or planar cross-beds; F4: mediumgrained planar cross- bedded or massive sandstone lithofacies with abundant scattered pebbles ; F5: very fine to fine -grained, planar cross-bedded or parallel laminated sandstones; and F6: siltstone and claystone, variegated and mottled in places - ripple/parallel laminations and rootlets were recognised. A high frequency fining-upward (FU) transition pattern (or cycles) with a “memory function” was distilled from the sequence using the Markov Chain Stochastic Process. The fining- upward (FU) motif is interpreted as a product of braided river depositional system with well- developed flood plains and lakes behind non-cohesive river banks. The cycles indicate channel migration within a broad topographically variable alluvial plain, fed by basin-margin fans.