Distal Facies Research Articles

The offshore Mancos play in the San Juan Basin as a component of the Mancos total petroleum system

Historically, the offshore Mancos play of the San Juan Basin has produced oil from noncommercial to marginally commercial reservoirs consisting of dark-gray marine shale with thin beds and laminae of fine-grained sandstone of limited permeability. During the last decade, horizontal drilling with horizontal wells has resulted in substantially increased production. However, placement of the offshore Mancos play within the Mancos total petroleum system has been poorly understood. Here, the offshore Mancos play is interpreted as a carrier bed play within the Mancos total petroleum system. Unconventional reservoirs of the offshore Mancos play are stratigraphically equivalent to updip conventional reservoirs deposited nearer to the shoreline and to downdip source rocks matured to peak oil generation. Offshore Mancos facies include a proximal facies to the southwest, a medial facies, and a distal facies to the northeast. Oil production has been obtained from the proximal and medial facies. Mancos shales contain oil-generative kerogens that are within the upper oil window near conventional reservoirs in the south and have matured to peak oil generation downdip of and to the northeast of the offshore Mancos reservoirs. Uniform API gravities of light sweet produced oils that transcend thermal maturity variations of Mancos shales indicate that oils generated in downdip mature source rocks migrated updip through the carrier bed sandstones of the offshore play and into the conventional reservoirs. Residual oil saturations are consistent with the concept of migrating oils moving updip through only a small number of interconnected pathways within a carrier bed.

Multiproxy analyses of the Triassic Agua de la Zorra Formation, Cuyana Basin, Argentina: Palynofacies, geochemistry, and biotic record

The Triassic Agua de la Zorra Formation crops out in the Paramillos de Uspallata area (NW of Mendoza) and represents a deltaic-lacustrine system with volcanic activity as well as episodic incursions of peperitic basalts. The succession is characterized by distal lacustrine facies at base and top, with proximal shallow facies in the middle section. The paleontological record includes invertebrates (spinicaudatans, insects), vertebrates (mainly fishs), tetrapod tracks and invertebrates traces, and plant remains (mainly referred to Umkomasiales). Kerogen in the distal facies is mostly terrigenous, with scarce amorphous and palynomorphs (disacates and microalgae). It shows a variable percentage of the total organic carbon content (between 0.2 and 4.6) with hydrogen and oxygen indices indicating a type III kerogen. In the shallow facies, kerogen is practically absent and the trace fossils reflect sporadic subaerial exposure. The palynological evidence indicates an oligotrophic lake and the low productivity recorded was likely caused by dilution of the lake water by strong fluvial influence in an overfilled lake basin type. The organic geochemistry data also suggests an overfilled lake basin and the inorganic geochemistry data support the development of anoxic conditions at the bottom of the paleolake. The spinicaudata valves occur in densely packed beds that could have been driven by changes in the water physicochemical parameters as a result of volcanic activity in the area. The volcanic activity in the area is a good explanation for variability in the biota diversity. However, the continuous record of plant remains through out the section seems to indicates that the paleoenvironmental conditions in the surrounding terrestrial areas were stables.

Groundwater Recharge Area Based on Hydrochemical and Environmental Isotopes Analysis in the South Bandung Volcanic Area

The determination of recharge areas needs to support the groundwater conservation in the southern volcanic Bandung area. This study aims to determine the recharge area based on environmental isotopes and hydrochemical. A sampling of 26 groundwater was carried out at springs, dug wells, and drilling wells. The variation in groundwater chemistry principally is controlled by a combination of ion exchange, silicate weathering, calcite, and dolomite dissolution of minerals. The hydrochemical facies were CaCl, CaMgCl, CaMgHCO3, CaHCO3, and NaKHCO3. The CaHCO3 facies describe moderate groundwater flows. The NaKHCO3 facies shows the mixing of shallow and deep groundwater. The recharge area in the central, proximal, and medial facies zone consists of 3 groups. Group I is considered water originating from local rainwater infiltration; Group II is considered the infiltration elevation which ranges from 980–1230 m asl; Group III estimated to be derived from the recharge elevation between 750–970 m asl, Group IV are more likely to show symptoms of evaporation or interaction with surface water. The discharge area is characterized by less active groundwater circulation, with dominant HCO3– and TDS value in the distal facies zone. Hydrochemical variation helped the identification of recharge areas in the volcanic facies.

Laser ablation-ICPMS analysis of trace elements in pyrite from the Tharsis massive sulphide deposit, Iberian Pyrite Belt (Spain)

High sensitivity laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) has been utilized for the determination of trace elements in pyrite from the Tharsis VMS deposit, one of the most significant volcanogenic massive sulphide deposits in the southern Iberian Pyrite Belt. The study tracks the content and distribution of trace elements within the different facies of massive sulphides. Widespread and variable enrichment in elements such as Cu, Zn, and Pb in pyrite is interpreted to be due to the presence of nanoinclusions of chalcopyrite, sphalerite, and galena, respectively. A second group of elements, including As, Au, Tl, Mn and Mo, shows significant chemical variations according to the type of mineralization and the textural evolution of the pyrite. This behaviour is thought to be due to the fact that these metals are included in the mineral lattice and its incorporation is controlled by the degree of crystallinity, the temperature, and the redox conditions during crystallization. Early, spongiform and colloform pyrite in the stockwork is enriched in As and Au and interpreted to have formed from a high temperature and low fS2–fO2 fluid. Thallium, Mn, and Mo are enriched in pyrite precipitated at lower temperature and higher oxidation state, i.e., in the exhalative part of the system or in distal facies to the hydrothermal vents. In general, both Co and Ni show an irregular distribution and do not have the sympathetic distribution observed in equivalent studies.

Progradation of a mid‐Cretaceous distributive fluvial system: The upper member of the Bima Formation, Northern Benue Trough, Nigeria

AbstractBasin‐wide analysis of an ancient distributive fluvial system (the upper Bima Formation) was carried out using field‐based observation/measurement and ‘photo‐realistic’ virtual outcrop data from 19 locations with good exposure of the upper Bima Formation in the Northern Benue Trough. The goal of this study is to provide an assessment of the spatial distribution of both vertical and lateral trends within the upper Bima distributive fluvial system. Vertical and lateral variations of the different facies of the formation were assessed quantitatively; this includes the analysis of the vertical and lateral distribution of proximal, medial and distal facies, the thickness and widths of channel fill deposits, grain‐size variation and the proportion of floodplain to channel fill deposits across the 19 locations in the basin. Trends observed across the sections indicate a progradation of the mega‐fan. The facies tracts preserved within the different portions of the distributive fluvial system vary. In the proximal part, in Lamurde, Tashan Alaji, Ture and Tula, the sections show only medial passing up to proximal facies. The distal lacustrine facies at the base of the section are absent. In the medial part of the system at Hinna, the successions preserve distal passing upward into medial facies. The gradual lateral transition from proximal to medial to distal facies, upward decrease in floodplain to channel fill deposits, upward increase in channel fill thicknesses and widths, all suggest that the Bima system is a prograding distributive fluvial system.

Paleocene storm-related event beds in the Gaoyou Sag of the Subei Basin, eastern China: A new interpretation for these deep lacustrine sandstones

Lacustrine storm deposits in hydrocarbon-rich lake basins can be excellent reservoirs; therefore, the identification and interpretation of storm deposits have great significance for future petroleum exploration. Recent progress in facies analysis helps to discriminate the sedimentary effects of combined flow and oscillatory flow in the depositional processes. Through core and well log data and thin-section analysis, the sandy deposits of Paleocene E1F3 member in the middle part of the Gaoyou Sag, Subei Basin, are interpreted as storm-induced deposits. Petrologic analysis shows that the coarser sediments are mostly fine-grained to silt-sized feldspathic litharenite. Five lithofacies were identified based on core observation, including massive sandstone, cross-bedded sandstone, hummocky cross-stratification (HCS) sandstone, wave rippled sandstone and siltstone, and massive mudstone. Lithofacies associations revealed a storm “sequence” beginning with an erosional surface and then fining upward, indicating hydrodynamic conditions that ranged from gravity flow, unidirectional flow, combined flow to oscillatory flow, which are attributed to storm-related processes. According to the different lithofacies associations and depositional processes, a depositional model including four facies tracts of storm-related deposits are recognized in plane view: (1) storm-induced turbidity channel facies characterized by coarser particles and formed by storm-triggered turbidity current, (2) proximal facies dominated by gravity, unidirectional, and combined flows, (3) transitional facies influenced by all the flow regimes and exhibiting a relatively complete storm “sequence”, (4) distal facies formed by low-energy deep-lake turbidity flow. Because storm deposits have petroleum significance in an oil-bearing basin such as Subei Basin, this study provides practical examples and guidance for future hydrocarbon exploration in other ancient lacustrine basins.

Plant Taphonomy and Paleoenvironment of the Bahía Laura Complex, Middle–Late Jurassic, at the Laguna Flecha Negra Locality (Santa Cruz Province, Argentina)

Taphonomic studies were carried out at Laguna Flecha Negra locality (Bahía Laura Complex, Middle–Late Jurassic), Santa Cruz Province, Argentina. Sedimentary facies and preservational styles were defined to recognize plant taphofacies in the studied sequence. Eleven taphofacies were identified and plant sources within a volcanic and geothermal system are proposed. Plant remains are of autochthonous to para-autochthonous origin and best preservation was found in distal facies of siliceous hot spring (sinter) systems. Lateral and vertical taphonomic differences were found in the studied sequence. These are due to changes in the sedimentary input and distance to the geothermal fluids. Results enable the reconstruction of the depositional history of this region of the Deseado Massif geological Province. We infer formation of a hot-spring (sinter) system that was subsequently destroyed by a phreatic eruption process at the margin of an andesitic effusive dome in partially reworked fall pyroclastic subfacies. After the destruction of the geothermal system, a fluvial and lacustrine epiclastic subfacies developed preserving a plant community typical of the Middle-Late Jurassic of Gondwana. Later, volcanic activity produced pyroclastic subfacies, with thick ash-fall and flow deposits from different sources and separated by a time gap that promoted fossilization of an in situ forest. Taphonomic studies of these plant communities allowed reconstruction of a chain of geological events and how these processes have influenced the preservation of a Jurassic flora from Patagonia, thus contributing to an understanding of the paleoecology of the Deseado Massif geological province.

The role of shelf morphology on storm‐bed variability and stratigraphic architecture, Lower Cretaceous, Svalbard

AbstractThe dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.

Identification characteristics of volcanic deposits in Cimarga, Southeast Banten

Volcanic eruption(s) in the past occurred around rawa danau volcanic complex is marked by the presence of a caldera and widespread volcanic deposits in the Banten Province as Tuff Banten. This study aims to determine the possible mechanism of the eruption based on volcanic deposits. Moreover, this study will also present the prolonged research process to understand the dynamics of volcanic deposits in Banten. Field study and sampling of volcanic deposits were completed followed by thorough petrographic analysis. There are three outcrops all with different characteristics. There are 6 layers in outcrop 1. Out of six layers, layer 1 (P1) until layer 5 (P5) appear similar, consisting of pumice rich (as fragment) and lapilli layer, while layer 6 (P6) consists of massive ash. In outcrop 2, there are 5 layers, which show cross and parallel laminations in each layer. Outcrop 3 contains 3 layers with the top and bottom layers consisting of lithic and pumice lapilli that are medium in size. Based on grain size from each outcrop, grain size from outcrop 1 until outcrop 2 progress into finer grains. From these characteristics outcrops 1 indicated flow deposits, outcrop 2 indicated surge deposits, and outcrop 3 indicated flow deposits. Therefore the facies from all three outcrops lean towards medial to distal with outcrop 3 as a distal facies.

Quaternary volcanostratigraphy setting in Rawa Danau Caldera, Banten Province

The volcano complex in the Rawa Danau of Banten Province or called the Dano Volcano has an association with the eruption of the Krakatoa volcano, observed from the direction of the ridge lineament. The Dano volcanic eruptions occurred during the Holocene period, causing the accumulation of volcanic material deposits into the northern part of Banten Province. This research area is covering five mountains which are Mount Parakasak, Mount Marikangen, Mount Tukung-Gede, and Mount Karang (sorted from the oldest to the youngest). Field mapping method we use in accordance with previous research and literature studies, as well as the results of the analysis in the form of the senses and geomorphology, stratigraphy, geological structure, and petrography to conclude volcanic facies in the Rawa Danau and surrounding areas. The results of the study showed that there were 4 volcanic facies around the Dano Volcano area. The central facies comprise of dome of Mount Parakasak with steep morphology while the proximal facies with andesitic lava. The structural geology feature formed from sheeting joints in the Southwest-Northeast direction with azimuth of N15° E/44° and tectonic fractures with Northwest-Southeast direction with azimuth N135° E/70° with steep morphology. The medial facies is tuffs and tuff breccias with gentle slope morphology, and the distal facies only have mud deposit, clay lithology with flat morphology. Rawa Danau in this study is located in the distal zone which consists of swamp deposits resulting from the destruction of the surrounding volcanic material.

Quaternary ring plain- and valley-confined pyroclastic deposits of Aragats stratovolcano (Lesser Caucasus): Lithofacies, geochronology and eruption history

Aragats stratovolcano, located on the South Armenian block, a micro-plate in the Arabian-Eurasian collision zone, has a complex and long-lasting Quaternary eruption history. Cataclysmic eruptions formed widespread pyroclastic deposits, which are interrupted by numerous basaltic to trachytic lava flows as well as lacustrine and lacustrine-alluvial sediments. Contrary to previous concepts, detailed mapping of lithofacies variation combined with stratigraphic documentation and new high-precision 40Ar/39Ar ages revealed four eruption cycles, each started with mafic lavas and generally finalized with pyroclastic deposits. These data contributed to a fundamental reinterpretation of the development of the Aragats Volcanic Province (AVP). Within an estimated activity of one million years, Aragats itself experienced three eruption stages, two of which culminated in pyroclastic deposits of remarkable lithofacies variability with ages of 0.97 to 0.90 Ma and 0.75 to 0.65 Ma, respectively. Vertical and lateral changes in proximal and distal facies distribution of pyroclastic flow deposits indicate temporal variations in eruption intensity and periods of relative volcanic quiescence.Aragats' explosive activities were dominated by Plinian eruptions with columns as high as 25 km, while others are characterized by low eruption columns followed by instantaneous collapse, in one case associated with vent widening, possible caldera collapse and breccia formation. Furthermore, one AVP unit features juvenile lapilli and blocks including banded and armored pumice, indicating magma mingling and their involvement in the erupting mixture before magma fragmentation.

Mixed carbonate–siliciclastic sedimentation in the Upper Cretaceous Nilkanth Formation, Garhwal Himalaya, India

The Upper Cretaceous Nilkanth Formation awaits a process-based depositional model despite being a topic of discussion between stratigraphers, palaeobiologists and structural geologists over the last few decades. Sedimentary facies analysis of a ~50 m thick section along a ~2.8 km long section along Rishikesh–Tal Bidhashini in Pauri Garhwal district of Uttarakhand allowed documentation of mixed carbonate–silicicalstic facies types, dominantly consisting of sand- and pebble-sized carbonate debris mixed with siliciclastics in a proximal to distal facies tract. Ten different facies types that include matrix-rich and matrix-poor shelly conglomerate, mixed clastic-carbonate wackestone, packstone, impure calcirudite and calcarenite, biomicrite and ferruginous sandstone are documented. Delineation of facies association and documentation of facies stacking pattern provide a post-Santonian mixed carbonate–siliciclastic sedimentation history of the Nilkanth Formation, deposited in the form of mass flows of varied rheology on a barred low- to moderate-gradient carbonate ramp, formed at the leading edge of the India plate before its collision with the Kohistan–Ladakh arc. Carbonate clasts comprising bivalves, crinoids, algae, bryozoan, etc., were produced in a narrow high-energy transgressive coastline and supplied across shelf along with reworked siliciclastics from clastic shoreface bar. It is argued that the reworked fossils, including the bryozoa Ceriocava Nilkanthi, present within massflows may not justify fixing of an absolute age for the formation but may definitely help in providing an age range.

River‐dominated and tide‐influenced shelf‐edge delta systems: Coarse‐grained deltas straddling the Early–Middle Jurassic shelf–slope break and transforming downslope, Lajas–Los Molles formations, Neuquén Basin, Argentina

AbstractThe three‐dimensional facies and architecture variability of shelf‐edge deltaic units cropping out at the transition between the Lower–Middle Jurassic Lajas and Los Molles formations of southern Neuquén Basin, Argentina, is presented here, as well as their stratigraphic relationship to uppermost deep‐water slope channel systems. Deep‐water, slope mudstones with thin turbidite beds merge upward with prodelta mudstones and thin sandstones, which are truncated by delta‐front to mouth‐bar sandstones. The latter sandstones are then downcut by large‐scale, trough cross‐stratified coarse‐grained sandstones and conglomerates of distributary channel systems and along‐strike, amalgamated with cross‐bedded sandy units showing evidence of tidal reworking. Proximal–distal facies and architecture variability within a shelf‐edge deltaic succession demonstrates that distributary channel‐complexes become wider and deeper basinward, forming channelized river‐dominated distributary fairways separated by tidally reworked inter‐distributary sand belts at the shelf edge. Evidence from depositional‐dip oriented outcrops shows a lack of collapsed and slumped strata at the shelf edge, and that the coarse shelf‐edge distributary channel fills continue far down the deepwater slope, and conglomerates transform to become high‐density turbidites to mainly thick‐bedded, sand‐matrix‐supported debrites. The interplay between flood tides and river currents is interpreted to have primarily modulated the focusing of river drainages, and consequently coarse‐grained sediment transport, along preferential routes on the outer‐shelf to shelf‐edge and down onto the slope. This contribution documents a unique example of coarse‐grained (mostly conglomeratic) shelf‐edge delta systems, tying bed‐scale facies and architecture data to a seismic‐scale shelf‐margin morphology, thus providing outcrop analogue data for the characterization of shelf‐edge delta systems in the subsurface.

Palaeoenvironmental interpretation of the Pliocene fan-delta system of the Vera Basin (SE Spain): Fossil assemblages, ichnology and taphonomy

The marine fossil assemblages of the Pliocene of south Spain constitute the record of the marine fauna that colonised the western part of the Mediterranean after the Messinian Salinity Crisis. This work focuses on the analysis of lithofacies and fossil assemblages including trace fossils, invertebrates, and vertebrates with special attention to taphonomic features, for interpreting palaeoenvironmental conditions in the Vera Basin (SE Spain). The sedimentary sequences of the northern region of the Vera Basin display diverse stratigraphical, sedimentological, and palaeontological features that correspond to the evolution of a fan-delta in a narrow basin. The Vera Basin was characterised by shallow-marine shelf conditions during the early-mid Pliocene (Cuevas Formation). The basin emergence with the development of Gilbert-type fan deltas (Vera Member), and a protected, partially-enclosed marine embayment (Almanzora Member) occurred during the mid-late Pliocene due to regional uplift and movements of the Palomares strike-slip Fault along the eastern basin margin. The progradation of the central fan-delta lobes and the interaction with marginal fan-delta resulted in the partitioning of the basin that formed a small sub-basin with restricted stagnant conditions that favoured a Konservat Fossil-Lagerstätte. The high input of siliciclasts due to the uplift context of the margins of the basin favoured a high sedimentation rate and the fast burial of vertebrate remains. Fossil marine mammals occurs from shallow shelf deposits (Cuevas Formation) to bottomset (Cuevas Formation-Vera Member transition) and lower part of the clinoforms in the foreset (Vera Member). Cetacean remains are usually recorded not only in the relatively deep-water silty marls and sandy marls of the outer shelf and distal facies of the fan-delta (Espiritu Santo Formation), but also in the shallower coarse sands and conglomerates (Cuevas Formation and Espiritu Santo Formation). Sirenian remains, in contrast, are only recorded in coarse sand facies (Cuevas Formation and Espiritu Santo Formation) associated to charcoal wood fragments deposited in shallow waters near the shoreline. This narrow and relatively protected basin is interpreted as an area of reproduction and nursery of juveniles on the basis of the presence of cetaceans.

The Orestes Melt Zone, McMurdo Dry Valleys, Antarctica: Spatially Distributed Melting Regimes in a Contact Melt Zone, with Implications for the Formation of Rapakivi and Albite Granites

Abstract The Orestes Melt Zone (OMZ) is a massive contact melt zone (∼20 m thick by several kilometers long), located in the McMurdo Dry Valleys of Antarctica. The OMZ formed at shallow crustal depths by melting of the A-type Orestes Granite owing to intrusion of the underlying, doleritic Basement Sill. The OMZ can be divided broadly into two melting facies. The upper melting facies is distal from the contact and formed by melting at low temperature and water-saturated, or near water-saturated, conditions. The lower melting facies is proximal to the contact and formed by melting at high temperature and water-undersaturated conditions. Separate melting reactions occurred in both of the melting facies, resulting in distinct textures and melt compositions. Melting in the distal facies generated melts with compositions that plot near a predicted eutectic composition. Melting in the proximal facies was accomplished in part by replacement reactions in restitic feldspars. These reactions resulted in the development of plagioclase mantles on both restitic plagioclase and K-feldspar, and melt compositions that diverged from predicted minimum melt along an unexpected path, towards enrichment in orthoclase component. Thermal modeling indicates that this melt zone was active for a minimum of ∼150 years, with a contact temperature of ∼900 °C. Upon cooling, recrystallization generated ocellar textures around restitic quartz, as well as faceted albite as a late-stage product. Observations of the OMZ, combined with thermal modeling, provide new insights into the origin of rapakivi and albite granites. This study has implications for the origin of these two associated granite types in other geological settings.

Effective mapping of large ignimbrites by using a GIS-based methodology; case of the Xáltipan ignimbrite from Los Humeros caldera, Mexico

The Xáltipan ignimbrite (XI) records the largest single-eruption of the Trans-Mexican Volcanic Belt up to date (≥344 km3 uncompacted volume). As most large ignimbrites, the XI presents common challenges when mapping. These are abrupt welding variations, burial by younger units near other volcanic sources, and highly eroded deposits in active-water valleys. In this work, we deal with these issues by integrating classical cartographic techniques such as field measurements, geometric projections of energy lines and construction of geologic sections, with a GIS-based methodology that quantifies the eroded bulk. As a result, we obtained a map with the main XI features that is presented in an interactive format. Furthermore, we discuss the resources used for the map construction and how the results contributed to the interpretation of the eruptive history. These contributions include: 1) the energy lines point to a low-height pyroclastic density currents source, typical of caldera-forming, boiling-over eruptions; 2) the landscape of the XI, coupled with the geologic sections, indicate that in the proximal facies the ignimbrite filled a volcanic field, leaving a radially shaped distribution with a flat surface, while in the distal facies it channelized into a steep mountainous ground; and 3) the geologic sections in the valley areas, where the active rivers highly eroded the original XI deposits, revealed the preferential development of welded zones along channel centers.

Stratigraphic evolution of the nearshore to fluvial plain of the Upper Cuyo Group, Neuquén, Argentina

AbstractThe upper portion of the Cuyo Group in the Zapala region, south‐eastern Neuquén Basin (Western Argentina), encompasses marine and transitional deposits (Lajas Formation) overlain by alluvial rocks (Challacó Formation). The Challacó Formation is covered by the Mendoza Group above a second‐order sequence boundary. The present study presents the stratigraphic framework and palaeophysiographic evolution of this Bajocian to Eo‐Calovian interval. The studied succession comprises the following genetic facies associations: (i) offshore and lower shoreface–offshore transition; (ii) lower shoreface; (iii) upper shoreface; iv) intertidal–subtidal; (v) supratidal–intertidal; (vi) braided fluvial to delta plain; (vii) meandering river; and (viii) braided river. The stratigraphic framework embraces four third‐order depositional sequences (C1 to C4) whose boundaries are characterized by the abrupt superposition of proximal over distal facies associations. Sequences C1 to C3 comprise mostly littoral deposits and display well‐defined, small‐scale transgressive–regressive cycles associated with fourth‐order depositional sequences. Such high‐frequency cycles are usually bounded by ravinement surfaces associated with transgressive lags. At last, the depositional sequence C4 delineates an important tectonic reorganization probably associated with an uplift of the Huincul Ridge. This is suggested by an inversion of the transport trend, north‐westward during the deposition of C1 to C3 depositional sequences (Lajas Formation) to a south‐west trend during the deposition of the braided fluvial strata related to the C4 depositional sequence (Challacó Formation).

The coprolite Lumbricaria Münster in the Early Tithonian of the Neuquén Basin, Argentina: new evidence for a holothurian producer

Lumbricaria Munster is described for the first time outside the Tethys domain from distal facies of the Tithonian Vaca Muerta Formation, Neuquen Basin, Argentina. The studied material consists of elongate, convolute, cylindrical coprolites, showing an overlapping pattern, and commonly exhibiting constrictions. Internally, coprolites show a densely packed fabric, where the particles show the distinctive morphology of saccocomid ossicles. These specimens are assigned to Lumbricaria intestinum Munster and three morphotypes are defined: (a) morphotype 1 shows a disordered convolute structure; (b) morphotype 2 is shorter and has a non-convolute arrangement; and (c) morphotype 3 has a spiral arrangement. Coprolites are preserved within microbial laminites, as well as concretions in calcareous mudstones. In this paper we discuss the possible producers of these coprolites, where evidence from the studied material would support a holothurian producer.

Shale oil resource potential and oil mobility characteristics of the Eocene-Oligocene Shahejie Formation, Jiyang Super-Depression, Bohai Bay Basin of China

Oil mobility in liquid-rich shale is crucial for economic success of resource development. The characteristics of the host rock and the properties of oil fluids are two primary factors controlling fluid mobility. Organic geochemistry data allows examination of both resource potential and oil mobility. We propose data-driven empirical models in this paper to describe hydrocarbon resource potential with respect to oil mobility. The proposed methods were applied to a dataset collected from the Eocene-Oligocene Es3 and Es4 source rock units in four recent shale oil exploration wells in the Jiyang Super-Depression of the Bohai Bay Basin. The dataset, results from standard programmed pyrolysis of whole rock samples as well as solvent extracted replicates of 139 core samples, was studied to examine the source rock characteristics and shale oil resource potential. Bulk geochemical characteristics and thermal decomposition trends allow for sub-division of the samples into two groups; a) samples with high initial hydrogen index (>800 mg HC/g TOC) and average present day TOC of 4.07% from LY1 and NY1 wells in a distal facies; and b) samples with slightly lower initial hydrogen index (about 700 mg HC/g TOC) and average TOC of 3.11% from FY1 and L69 wells in a transitional facies. Three different categories of oil resource with respect to oil mobility can be defined: adsorbed, restricted and movable based on organic geochemical characteristics and their variations in the stratigraphic intervals represented by the samples in the four wells. We show that expulsion efficiency is a good indicator of hydrocarbon retention in source rock reservoir and that the two wells in the distal facies have higher total resource potentials with abundant movable oil resource than the two wells in the transitional facies. The total oil potential is separated into three types of oil resource to show oil mobility, thus providing more information for business decision.

Relation of lineaments and volcano-stratigraphy of tertiary volcanic rocks in Kulon Progo Mountains Area, Yogyakarta-Indonesia

The structural lineaments developed in the Kulon Progo Mountains, which is mainly composed of Tertiary volcanic rocks. The direction and distribution of these lineaments density differs from one area to another. This research was conducted to reveal the geological factor of controlling the variation of existing lineaments density. Lineaments delineation is done on the whole area on the satellite image of Kulon Progo Mountain area. From the lineaments map is then made a lineaments density variation map. Lineaments density variation map that are overlaid by volcanic rock distribution, based on the results of field studies, will result in a relationship between the geological structure density and the existing rock types and facies of volcanic rocks. The result of alignments density analysis shows high density values 3-6 km/km2 especially located at proximal volcanic rock facies. Low density values of 1-3 km/km2 are mainly located in medial and distal volcanic rock facies.