Lacustrine Sandstones Research Articles

Linking environmental changes and organic matter enrichment in the middle part of the Yanchang Formation (Ordos Basin, China) to the rollback of an oceanic slab in the eastern Paleo-Tethys

The Yanchang Formation, which records the whole evolutionary history of the Middle–Late Triassic lacustrine basin in the western part of the North China Block, provides an opportunity to understand the relationship between the tectono-sedimentary evolution and massive organic matter burial in lacustrine systems. The present study investigates the changes in the lithofacies association, provenance, regional tectonism, depositional environment, and organic matter accumulation in the lower-middle part of the Yanchang Formation, aiming to unravel their links. As a result of the rapid deepening of the southern Ordos Basin, the deposits in the depocenter of the basin shifted dramatically from shallow lacustrine sandstones of the lower part of the Yanchang Formation (i.e., Chang 8 Member) to deep lacustrine shales and gravity flow deposits of the middle part of the Yanchang Formation (i.e., Chang 7 Member). The provenance of the overlying Chang 7 Member is distinct from that of the underlying Chang 8 Member. It is proposed that crustal extension caused by the southwestward rollback of an oceanic slab in the eastern Paleo-Tethys may have triggered the rapid extensional subsidence in the southern Ordos Basin during the early Ladinian. In response to this deep geodynamic process, the depositional environment of the contemporaneous Ordos Basin changed significantly, as reflected by improved primary productivity, enhanced bottom-water anoxia, as well as reduced terrigenous clastic supply in the Ordos Lake. The coupling of these processes further led to the enrichment of organic matter in the Chang 7 Member.

The Occurrence of Adsorbed Tight Oil and Its Effect on Porosity and Permeability Reduction of Triassic Lacustrine Sandstone Reservoir

The adsorption of crude oil in the tight sandstone reservoir is significant. Occurrence states of adsorbed tight oil and its pore and throat reduction effect are two significant directions in tight oil exploration and development. In this paper, the occurrence state of adsorbed tight oil and its porosity and permeability reduction effect is systematically analyzed based on a detailed description and discussion of occurrence space, states, changes of porosity and permeability, and its controlling factors. Five occurrence states are recognized based on the differences in spatial location and morphology of adsorbed tight oil. Adsorbed oil is distributed in all kinds of reservoir spaces and primarily concentrated in the pore − throat radius < 0.25 μm. The distribution characteristics of reservoir spaces control the occurrence states of adsorbed tight oil. The emulsion form, cluster form, throat form, thin film form, and isolation form are mainly stored in intergranular pores, feldspar dissolved pores, throats, the surface of minerals, and intercrystalline pores, respectively. The massive development of quartz, feldspar, and illite is conducive to the distribution of emulsion form, cluster form, and the throat form of tight oil, while the abundance of clay, especially chlorite, controls the distribution of the thin film form and isolated form of tight oil. The adsorption of tight oil causes clogging in the pore network, resulting in reservoir damage. The adsorption of tight oil in the reservoir leads to increased reservoir density by 0.290 g/cm3 and the reduction of porosity and permeability by 3.14% and 0.321 × 10 − 3 μm2, respectively. And the damage of adsorbed oil to permeability (30.79%) is more severe than that of porosity (19.96%). The effect of pore and throat reduction is more evident in reservoirs with higher content of quartz and feldspar. However, the strong adsorption of clay makes it difficult to separate the tight oil adsorbed on its surface. The nature of adsorbed oil in different occurrence states also can determine the separation efficiency in the extraction process. The adsorbed tight oil with emulsion, cluster, and throat forms is easier to desorb than the isolation and thin film forms. The research results can provide a basis for analyzing the occurrence characteristics and reasonably formulating the development method of tight oil. In addition, it can even provide a new understanding and basis for the densification process of reservoir permeability under different conditions.

Indicators for identification of lacustrine sandstones of sandy debris‐flow origin: A case study of Es1 Member, Palaeogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

The first member of Shahejie Formation of Eocene–Early Oligocene (Es1, Nanpu Sag) consists of fluvio-deltaic to deep lacustrine shale-sandstone accumulations, and the deep lacustrine deposits include sandy debris flow (SDF), turbiditic and mudrock facies. In the process of hydrocarbon exploration, it was found that that SDF sandstones in the deep lacustrine deposits can constitute excellent hydrocarbon reservoirs. Therefore, the identification of these lithofacies, especially SDF sandstones, is crucial for hydrocarbon exploration. We document here parameters that discriminate lacustrine SDF sandstones from turbiditic and mudrock deposits. Data from cores, wireline logs, and seismic sections are integrated and interpreted. Core-based identification indicators of the SDF sandstones include massive structures with upper and lower sharp contacts with interbedded mudrocks, lateral pinch-outs, locally imbricated to floating pebble-size grains, and elongated (teared) mudrock clasts within the middle and upper parts of normally- or inversely-grading sandstone and poor to high matrix content. These properties are attributed to deposition of non-Newtonian flows with sediment-support mechanism of dispersive pressure, matrix strength and buoyancy, sediment-transportation mechanism, and sediment-settling properties (hindered settling and freezing of the SDF). Log-facies identification indicators of the SDF sandstones are characterized by higher RLLD/RLLS values (40–80 Ω m), higher SP values (35–70 mV), lower Gamma ray (GR) values (70–120 API), and strongly serrated curve motifs. Seismic facies identification indicators of the SDF sandstones have mound-like, lenticular, or wedge-shaped external geometries, and interior worm-like and fusiform chaotic reflections with medium to strong amplitude, medium to high frequency, and medium to low continuity. The results and interpretation of the integrated core-log-seismic data as identification indicators are useful tools for identifying SDF sandstones and effectively distinguishing them from other deep lacustrine deposits.

The occurrence of tight oil in the Chang 8 lacustrine sandstone of the Huaqing area, Ordos Basin, China: Insights into the content of adsorbed oil and its controlling factors

Tight oil is a great potential resource for unconventional oil exploration and development. Determining the content of adsorbed tight oil in sandstone reservoirs and the factors controlling its content is crucial for the evaluation and development of tight oil resources. Using fluorescence petrography, environmental scanning electron microscopy, porosity as well as permeability analyses, high-pressure mercury intrusion, and nuclear magnetic resonance imaging, the present study analyzed the occurrence form, location, and relative content of free and adsorbed tight oils and effect of controlling factors on the content of adsorbed tight oil in sandstone reservoirs of the Chang 8 member in the Huaqing area of the Ordos Basin. The free tight oil mainly exists in residual intergranular, intergranular dissolved, intragranular dissolved, and dissolution-enlarged pores as well as some throats in the form of flakes with various areas and slender strips. The adsorbed tight oil mainly occurs on the surfaces of the mineral particles and mica fragments as well as in fractures, some throats or the intragranular as well as intergranular dissolved pores in the form of films with variable thicknesses and long strips. The lower limit of the pore radius at which free tight oil occurs in sandstone is approximately 0.1 μm and the adsorbed tight oil occurs on the surface of mineral particles and in spaces with pore radii of 0.02–0.3 μm. The contribution of free tight oil to total tight oil in the present study has highest and lowest content of 60.95% and 32.48%, respectively, with an average content of 50.14%. The contribution of adsorbed tight oil to total tight oil has highest and lowest content of 67.52% and 39.05%, respectively, with an average content of 49.86%. The content of adsorbed tight oil is generally proportional to the median pressure and inversely proportional to the porosity as well as permeability of the reservoir; particularly, porosity is relevant. The content of adsorbed tight oil is inversely proportional to the quartz content and directly proportional to the feldspar, carbonate cement, and clay mineral content of the sandstone.

Unusual intraclast conglomerates in a stormy, hot‐house lake: The Early Triassic North China Basin

AbstractEarly Triassic temperatures were some of the hottest of the Phanerozoic, sea‐surface temperatures approached 40°C, with profound consequences for both the sedimentology and faunal distributions in the oceans. However, the impact of these temperatures in terrestrial settings is unclear. This study examines shallow lacustrine sediments from the Lower Triassic succession of North China. These consist of diverse fluvial to shallow lacustrine sandstones and also spectacular, coarse conglomerates composed of diverse, intraformational clasts reworked from the interbedded sediments. The conglomerate beds can show inverse grading and high angle, flat‐pebble imbrication in their lower part and vertically orientated flat pebbles in their upper part. The cobbles include cemented and reworked conglomerate intraclasts and sandstone concentrically‐laminated concretions that record multi‐step histories of growth and reworking, pointing to rapid cementation of the sandy lake bed (likely facilitated by high temperatures). The conglomerates record frequent, high‐energy events that were capable of brecciating a lithified lake bed and transporting cobbles in wave‐influenced sediment‐gravity flows. Initially, powerful oscillatory flows brecciated and deflated the lake bed and subsequently helped to sustain turbulence during short‐distance lateral flow. It is possible that hurricanes, originating from the adjacent hyper‐warm, Palaeo‐Tethyan Ocean travelled into the major lakes of the North China continent during the Early Triassic.

Tectonostratigraphic evolution, palaeogeography and main petroleum plays of the Nuussuaq Basin: An outcrop analogue for the Cretaceous–Palaeogene rift basins offshore West Greenland

The onshore Nuussuaq Basin in central West Greenland comprises a complete Albian–Paleocene succession and as such represents the only complete rift succession outcrop analogue for the Cretaceous–Palaeogene offshore frontier basins in West Greenland. The basin fill reflects five tectonostratigraphic phases: 1) Pre-rift, 2) Albian – Early Cenomanian Early Rift, 3) Early Cenomanian – Early Campanian Thermal Subsidence, 4) Early Campanian – Early Paleocene Late Rift, and 5) Early Paleocene – Late Eocene Break-up and Drift. In addition, the succession is divided into nine tectonostratigraphic sequences (TSSs), each representing a specific configuration of depositional elements resulting mainly from tectonic events, which caused major palaeogeographic reconstruction of the basin and therefore critical to the petroleum systems. The sequences are bounded by unconformities or, in one case, by a major flooding phase. The Early Rift Phase in the Nuussuaq Basin was initiated in the Albian with development of half-grabens along N–S directed extensional faults. This phase was characterised by continued growth along extensional faults and embraces three TSSs (TSS1–3). They are characterised by alluvial fan, fan-delta and lacustrine sedimentation, wave- and tidal-dominated deltaic deposition, followed by a major fall in relative sea level and canyon incision. Oil seeps indicate presence of a lacustrine/brackish-water source rock at depth. During the following Late Cenomanian – Early Campanian Thermal Subsidence Phase a regional marine drowning of the basin took place with deposition of organic-rich mudstones, followed by re-establishment of deltaic deposition in the Coniacian–Santonian (TSS4). The Early Campanian – Early Paleocene Late Rift Phase was associated with a change in stress regime, formation of NW–SE directed extensional faults, uplift of highs and a major change in depositional environments from deltaic deposition to deposition from gravity flows in a confined system of slope channels and canyons. During the initial phase, structurally-controlled turbidite channels were established in a NW–SE trending graben-like structure that acted as conduits for sediment transport into the offshore areas (TSS5). This was followed by several phases of major uplift resulting from the rise of the Proto-Icelandic Mantle Plume prior to continental break-up eventually leading to the deposition of two structurally controlled TSSs (TSS6–7) characterised by the formation of submarine and subaerial canyons that acted as conduits for huge amounts of sediments transported into the offshore areas. The Nuussuaq Basin experienced significant subsidence during the initial phase of break-up volcanism with deposition of organic-rich Lower Paleocene marine mudstones blanketing the basin followed by a thick succession of Paleocene–Eocene volcanic rocks referred to as the West Greenland Basalt Province (TSS8–9). Volcanism gradually spread eastwards eventually blocking the connection to the sea stemming up a large lake between the prograding volcanic front and the cratonic mainland to the east. Finally, when the volcanic rocks covered the entire basin, rivers were redirected, and sediments were deposited in the offshore Sisimiut Basin to the south and the Melville Bay Basin to the north. During this latest rifting and break-up phase, the Nuussuaq Basin was aborted and terminated as a failed rift basin.Four main plays have been defined based on the tectonostratigraphic subdivision: An Early Rift Play, a Thermal Subsidence Play, a Late Rift Play and a Break-up and Drift Play. The Early Rift Play has alluvial and estuarine sandstones as reservoir, Albian lacustrine/brackish-water mudstones as source rock and Cenomanian–Turonian mudstones as seal. The Thermal Subsidence Play has deltaic and shallow marine sandstones as reservoirs, Cenomanian–Turonian mudstones as source and Campanian mudstones as seal. The Late Rift Play has incised valley, submarine canyon and turbidite sandstones as reservoirs, and Campanian and Paleocene mudstones as seal and source. The Break-up and Drift Play is subdivided into three sub-plays, including: a Lacustrine Sub-play, an Intra-basaltic Siliciclastic Sub-play and a Fractured Volcanic Sub-play. The Lacustrine Sub-play has fluvio-deltaic sandstones as reservoir and intra-formational lacustrine and delta-plain mudstones as seal. The Intra-basaltic Siliciclastic Sub-play comprise fluvial and lacustrine sandstones as reservoir and volcaniclastic or siliciclastic mudstones as seal. The Fractured Volcanic Sub-play comprises fractured, porous volcanic rocks as reservoir and intra-basaltic mudstones or tight fine-grained volcanic rocks as seal. All three sub-plays require vertical or long-distance migration of hydrocarbons from deeper-seated Cretaceous – Lower Paleocene kitchens. Because the tectonostratigraphic phases recognised in the Nuussuaq Basin can also be applied to the regional seismic mapping offshore Greenland the implications of the Nuussuaq Basin plays can be applied to the frontier offshore basins.

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.

Porosity Development Controlled by Deep-Burial Diagenetic Process in Lacustrine Sandstones Deposited in a Back-Arc Basin (Makó Trough, Pannonian Basin, Hungary)

Deeply buried Pannonian (Upper Miocene) siliciclastic deposits show evidence of secondary porosity development via dissolution processes at a late stage of diagenesis. This is demonstrated by detailed petrographic (optical, cathodoluminescence, fluorescence, and scanning electron microscopy) as well as elemental and stable isotope geochemical investigations of lacustrine deposits from the Makó Trough, the deepest depression within the extensional Pannonian back-arc basin. The analyses were carried out on core samples from six wells located in various positions from centre to margins of the trough. The paragenetic sequence of three formations was reconstructed with special emphasis on sandstone beds in a depth interval between ca 2700 and 5500 m. The three formations consist, from bottom to top, of (1) open-water marls of the Endrőd Formation, which is a hydrocarbon source rock with locally derived coarse clastics and (2) a confined and (3) an unconfined turbidite system (respectively, the Szolnok and the Algyő Formation). In the sandstones, detrital grains consist of quartz, feldspar, and mica, as well as sedimentary and metamorphic rock fragments. The quartz content is high in the upper, unconfined turbidite formation (Algyő), whereas feldspars and rock fragments are more widespread in the lower formations (Szolnok and Endrőd). Eogenetic minerals are framboidal pyrite, calcite, and clay minerals. Mesogenetic minerals are ankerite, ferroan calcite, albite, quartz, illite, chlorite, and solid bituminous organic matter. Eogenetic finely crystalline calcite yielded δ13 C V − PDB values from 1.4 to 0.7‰ and δ18 O V − PDB values from –6.0 to –7.4‰, respectively. Mesogenetic ferroan calcite yielded δ13 C V − PDB values from 2.6 to –1.2‰ and δ18 O V − PDB values from –8.3 to –14.0‰, respectively. In the upper part of the turbidite systems, remnants of the migrated organic matter are preserved along pressure dissolution surfaces. All these features indicate that compaction and mineral precipitations resulted in tightly cemented sandstones prior to hydrocarbon migration. Interconnected, secondary, open porosity is associated with pyrite, kaolinite/dickite, and postdates of the late-stage calcite cement. This indicates that dissolution processes took place in the deep burial realm in an extraformational fluid-dominated diagenetic system. The findings of this study add a unique insight to the previously proposed hydrological model of the Pannonian Basin and describe the complex interactions between the basinal deposits and the basement blocks.

Manchuriophycus-like elliptical cracks in thin mudstones intercalated with lacustrine sandstone: Intrastratal crack formation in water-saturated sediments

Manchuriophycus-like elliptical cracks are found in thin mudstone layers and lenses intercalated with wave-rippled lacustrine sandstone beds of the Lower Cretaceous Jinju Formation, South Korea. The cracks are interpreted as intrastratal shrinkage cracks formed by liquidization and injection of sand, hydroplastic deformation of mud, and concomitant dewatering of the sediments in water-saturated conditions. The cracks are considered to have been formed by earthquake and/or wave-induced loading and initiated where smectite was presumed to be locally concentrated in the mud. It is hypothesized that the cracks were developed into the elliptical outline, reflecting that of an interference ripple trough, due to permeability differences between two thin mud layers deposited over the rippled sand layer.

Fractionation of alkylated carbazoles in petroleum during subsurface migration: Evidence from molecular simulation and application in sandstone reservoirs

Carbazole (CA) and alkylated carbazoles are common nitrogen-containing heterocyclic aromatic compounds widely found in crude oils and sedimentary rocks extracts. Their geochemical significances in petroleum exploration have been widely investigated in previous studies. Using density functional theory (DFT) and molecular dynamics (MD) simulation, we calculated the polarity of alkylated carbazole isomers/homologues and their adsorption energy with water and α-quartz, respectively. The calculated results show that there are significant differences in migration fractionation effects for alkylated carbazole isomers/homologues. This study confirmed the validity of previously proposed carbazole migration indicators, i.e. 4-/1-MCA (the relative abundance of 4-methylcarbazole to 1-methylcarbazole), 2,5-/1,8-DMCA (2,5-dimethylcarbazole to 1,8-dimethylcarbazole) and 2,4-/1,8-DMCA (2,4-dimethylcarbazole to 1,8-dimethylcarbazole) on the basis of quantum chemistry and molecular simulation. All these three parameters have been successfully applied in a lacustrine sandstone reservoir in the Fushan Depression, Beibuwan Basin (South China Sea). The calculations of Gibbs free energy show that the differences in thermal stability between the isomers of alkylcarbazoles can be negligible. The source input and depositional environment have no significant effect on these indicators for oils from a same oil family. Hence, the values of these parameters are mainly controlled by subsurface petroleum migration processes. Therefore, the molecular parameters relative to alkylated carbazoles can be used as practically effective geochemical markers to trace the oil filling orientation and pathways for subsurface oil migration.

Multi-approach provenance in stratigraphy: Implications for the Upper Mesozoic evolution of the Parnaíba Basin, NE Brazil

Provenance analyses are highly affected by transport, depositional and diagenetic processes, as well as the grain size chosen for evaluation. In order to avoid misleading data, provenance research requires multi-approach techniques. In this work, a set of widespread methods were applied to investigate a debatable stratigraphic interval in the Parnaíba Basin, northeastern Brazil. Discontinuous exposures and fault displacements result in enigmatic stratigraphic relations in the Mesozoic portion of this basin. This succession consists of the lacustrine sandstones and shales of the Pastos Bons Formation and aeolian sandstones of the Corda Formation, composing the Mearim Group, and fluvial conglomerates and sandstones of the Grajaú Formation. The provenance was studied through sandstone petrography, quartz petrography, hot cathodoluminescence and heavy minerals analyses. Sandstones from Mearim Group plotted in recycled orogenic and craton interior fields, whereas quartz petrography and cathodoluminescence constrained predominantly volcanic sources. The heavy minerals assemblages of the lacustrine and aeolian deposits are very similar, whereas the fluvial succession presents texturally immature grains and anomalous high hornblende content. Additionally, ZTR, GZi and RZi indexes are higher for lacustrine and aeolian deposits, and lower for fluvial beds. These data indicate polycyclic minerals and mixed sources for Mesozoic sandstones of the Parnaíba Basin. The Mearim Group presents both volcanic contribution, probably supplied by basalts from the Central Atlantic Magmatic Province (CAMP), and low to medium-grade metapelitic heavy minerals assemblage. Heavy minerals were probably supplied by Neoproterozoic metapelites from the Ceará Central Domain (Ceará Group?), Borborema Province. Conversely, heavy minerals of the Grajaú Formation were derived from I-type Brazilian granites, incompatible with the Mearim Group. These data provided new stratigraphic relations for the Mesozoic deposits of the Parnaíba Basin and implications for the evolution of this basin.

Lithostratigraphy, geology and geochemistry of the volcanic rocks of the Maligât Formation and associated intrusions on Disko and Nuussuaq, Paleocene of West Greenland

The Paleocene volcanic rocks in the Nuussuaq Basin on Disko and Nuussuaq comprise the picritic Vaigat Formation (c. 62–61 Ma) and the overlying basaltic Maligât Formation (c. 60 Ma). The Maligât Formation is up to 2000 m thick on western Disko where the top of the formation is least eroded. The formation is divided into four members, the Rinks Dal, Nordfjord, Niaqussat and Sapernuvik members, which are formally defined here. On central and eastern Disko and Nuussuaq the Maligât Formation lavas are interbedded with fluvial and lacustrine sandstones and mudstones of the Atanikerluk Formation.The Rinks Dal Member is the lowest member and originally constituted around 61% by volume of the formation. It is divided into 12 informal units based on chemically recognisable oscillations in the fractionation state of the basalts. The oldest units are present on central and south Disko close to the Disko Gneiss Ridge. The younger lavas spread farther to the east, north and west, filled the Assoq Lake basin east of the ridge and gradually onlapped the shield of the earlier Vaigat Formation that rose to the north. Only the lavas of the upper Rinks Dal Member reached far into Nuussuaq. The lavas are generally not crustally contaminated and comprise evolved basalts with 4.4–9.2 wt% MgO and a few picrites. The most evolved basalts with 3.2–4.8 wt% TiO2 occur in the middle part of the member where they form the Akuarut unit.
 The Nordfjord Member originally constituted around 6% by volume of the formation. It is not subdivided because the lithological variability is local. The member is widespread but has its depocentre on north-western Disko where thicknesses reach 350 m and eruption sites, intermediate lavas and acid tuffs are present. Over most of the area the member consists of just a few lava flows with combined thicknesses of 30–100 m. The member has a very diverse lithology with rock types ranging from silicic basalt with 5.3–10.0 wt% MgO through magnesian basaltic andesite and andesite with 2.4–10.6 wt% MgO to dacite with 1.2–2.2 wt% MgO. Rhyolite with 0.2–1.2 wt% MgO and up to 77 wt% SiO2 occur in tuffs and conglomerate clasts. All rocks are crustally contaminated and some are native-iron-bearing.
 The Niaqussat Member originally constituted around 33% by volume of the formation. It is subdivided into three informal units. The member is widespread, but much of it has been removed by erosion. Lithologies in the lower unit range from silicic picrite with up to 15 wt% MgO to basalt with 6–12 wt% MgO and a few basaltic andesite flows. The middle and upper parts of the Niaqussat Member comprise more evolved basalts with respectively 6.1–7.2 wt% MgO and 4.9–6.4 wt% MgO. All rocks are crustally contaminated and a few lava flows are native-iron-bearing.
 The Sapernuvik Member comprises three uncontaminated basalt flows with 7.5–10.7 wt% MgO. It is only preserved in a small area on western Disko.
 Dyke systems with up to 80 km long dykes and subvolcanic intrusions associated with the Nordfjord and Niaqussat members occur on western and north-eastern Disko. The rocks are crustally contaminated and range from silicic basalt with 4–13 wt% MgO to magnesian andesite with 3–10 wt% MgO. They commonly form composite intrusions, some of which contain accumulations of native iron and sulfides.
 The contaminants are carbon- and sulfur-bearing sediments of the Nuussuaq Group. Major contamination mechanisms were mixing with partial melts from the sediment sidewall and xenoliths and selective exchange of some elements, including carbon and sulfur, between magma and sediment. Degrees of contamination vary from 2−5% in the basalts to 10−50% in the more silicic rocks. No rocks more evolved than basalt were produced by ordinary fractional crystallisation.

Pore-throat characteristics of tight sandstone reservoirs composed of gravity flow sediments: Yingcheng Formation, Longfengshan sag, China

The Lower Cretaceous Yingcheng Formation in the Longfengshan sag (Songliao Basin, China) represents a typical tight sandstone reservoir that hosts large accumulations of hydrocarbons. The lacustrine sandstones mainly reflect gravity flow deposits making up various lithofacies displaying varying degrees of heterogeneity. This study reports on the links among gravity flow type, diagenetic overprinting and pore-throat characteristics that control reservoir quality. Core observations, especially grain size variations and related sedimentary structures are used to differentiate types of gravity flows. Microscopic observations, cathodoluminescence, scanning electron microscopy, pressure-controlled mercury injection tests and rate-controlled mercury injection tests are used to determine the mineralogy, sedimentary composition and pore-throat characteristics of the tight sandstones.In the study area, the gravity flows are divided into muddy debris flows, sandy debris flows, high-density turbidity flows, low-density turbidity flows and mud density flows. The petrophysical properties of these different gravity flow sediments reflect a range of permeability and porosity characteristics. Because of their mud-dominated textures and related weak compressive strength, mud density flows and muddy debris flows become ineffective as reservoirs during burial. In contrast, for approximately similar burial depths, turbidites and sandy debris flows show better reservoir quality. However, reservoir units with high porosities still possess considerable heterogeneity; this is especially true of some sandy debris flows that possess large amounts of secondary pores but which are not charged with hydrocarbons. In particular, in some poorly sorted sandy debris flows that are characterized by high contents of fine-grained matrix, some of the pore throats are blocked by matrix material. Moreover, the dissolution of feldspars and volcanic debris led to the formation of authigenic clays and quartz, which preferentially developed in pore throats with small radii. This process significantly reduced permeability.Overall, the permeabilities of the sandy debris flows are generally lower than those of the turbidites. Due to their relatively large-radius pore throats, turbidites can be categorized as potential effective hydrocarbon reservoirs in the study area. This research provides significant insights into the characteristics of pore throats within tight reservoirs consisting of gravity flow sediments in lacustrine basinal settings.

Fractionation of dibenzofurans during subsurface petroleum migration: Based on molecular dynamics simulation and reservoir geochemistry

Dibenzofurans, oxygen-containing heterocyclic aromatics, are ubiquitous in crude oils. Their geochemical implications in petroleum geochemistry have been studied in numerous previous publications. Here, we present the distribution of dibenzofuran and its methylated homologues, and calculate the polarity and solubility of methyldibenzofuran isomers based on the Connolly surfaces and molecular dynamics simulation. The results show that the geo-chromatographic fractionation effect is evident due to absorption onto the water/silica matrix carrier bed during the oil migration process. Two potential molecular indicators relative to dibenzofurans, i.e. the total content of dibenzofurans and the relative abundances of 1-methyldibenzofuran to 4-methyldibenzofuran, are proposed for tracing the oil migration orientation and filling pathways. These two indicators were applied successfully in Paleozoic marine carbonate and Tertiary lacustrine sandstone reservoirs. The variations of source input, depositional environment, and maturity for oils from same source bed/kitchen appear to have no significant effect on these two indicators. It suggested that these two dibenzofuran indicators are potentially effective molecular tracers for orientation and filling pathways in oil migration.

Radialimbricatus, a new ichnogenus from the lacustrine sandstone facies of the Lower Cretaceous Jinju Formation, South Korea

Lee, D.-C. February 2017. Radialimbricatus, a new ichnogenus from the lacustrine sandstone facies of the Lower Cretaceous Jinju Formation, South Korea. Alcheringa XX, XXX–XXX.The trace fossil Radialimbricatus igen. nov. with type species of R. bitoensis isp. nov. occurs in river-mouth-bar sandstone beds of a lacustrine sequence of the Lower Cretaceous Jinju Formation in South Korea. Radialimbricatus is a horseshoe-shaped or elliptical, convex hyporelief structure characterized by imbrication of traces with radial ridges and a median furrow. These features differentiate Radialimbricatus from arthropod ichnofossils such as Rusophycus and Cruziana, possibly cnidarian ichnofossils such as Astropolichnus and vermiform organism ichnofossils such as Oldhamia. Cross-sectional features of R. bitoensis, including upward mud protrusions at lateral margins, demonstrate that R. bitoensis can be formed as an open epigenic furrow at the water/mud interface or an endogenic burrow along the sand/mud interface at the same time. Analysis of discrete and imbricating horseshoe-shaped traces indicates that the tracemaker maintained an obligate inclined posture even when forming trails. The radial ridges and pit along the median furrow are interpreted to reflect feeding behaviour, and the imbrication to result from unidirectional and more or less regular movements to next feeding sites. The tracemaker is thought to have been an organism with an elliptical body outline, radially arranged body parts at the base of or around the body (probably for collecting and transporting foods and moving to feeding sites) and a mouth near the centre of the body, which is consistent with body plan features of arthropods, cnidarians and annelids.Dong-Chan Lee [dclee@chungbuk.ac.kr], Department of Earth Sciences Education, Chungbuk National University, Cheongju, 28644, South Korea.

Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation (Late Triassic), Binchang area, Ordos Basin, China

Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accumulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sandstone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. However, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.

The Liuyuan Volcanic Belt in NW China revisited: evidence for Permian rifting associated with the assembly of continental blocks in the Central Asian Orogenic Belt

AbstractThe basaltic pillow lavas in the Liuyuan region of NW China, considered to be part of an ophiolitic suite, have been central to the models on tectonic setting, evolution and timing of the final closure of the Palaeo-Asian Ocean. New field evidence on the sedimentary units associated with the basalts reveals comparable sequences in the northern and southern flanks of the Liuyuan Volcanic Belt with coarse to fine sediments from periphery to the centre. The dacites and rhyolites formed coevally with the pillow basalts. The pillow basalts are interlayered with lacustrine sandstone, claystone and clayey lake deposits. Detrital zircons from these sediments yield zircon U–Pb ages of 291–285 Ma. Andesites, dacites and rhyolites from the basaltic sequence yield U–Pb ages of 280–277 Ma, similar to the 282–280 Ma ages of gabbros that intrude the pillow lavas. All these rocks cover the 460–440 Ma granite and greenschist basement and have been intruded by gabbros ofc.272 Ma age, with subsequent (230–227 Ma) north–south contractional thrusting and folding. The data from our study are incompatible with the existing models that consider the basalts as part of an ophiolitic suite. Along the northern continental margin of China from west to east, the Tarim, Dunhuang-Alxa and North China cratonic areas all show evidence for regional extension through rifting during early–middle Permian time. These rift features and basaltic eruptions occurred coevally with the assembly of various microcontinental blocks against the Siberian craton atc.300–250 Ma, synchronous with amalgamation of the Central Asian Orogenic Belt (CAOB) on the northern side of the Liuyuan Rift. These events were also broadly synchronous with formation of the global supercontinent Pangea.

Carbonate Cemented Zone in Deep-water Lacustrine Sandstones of Huaqing Area, Ordos Basin: Origin, Distribution and Effect on Reservoir Properties

Carbonate Cemented Zone in Deep-water Lacustrine Sandstones of Huaqing Area, Ordos Basin: Origin, Distribution and Effect on Reservoir Properties

Prediction of ultrathin lacustrine sandstones by joint investigation of tectonic geomorphology and sedimentary geomorphology using seismic data

Weak seismic signals from ultrathin (1–7 m) sandstones increase uncertainties of thickness estimation, imposing a significant challenge for accurate prediction of subtle hydrocarbon reservoirs. These types of uncertainties, however, can be reduced by a combined use of kinematic (travel time) and dynamic (amplitude) information extracted from 3D seismic data. In this case study in the Kuqa Depression of the Tarim Basin, China, we conducted an investigation of seismic-based tectonic geomorphology and then sedimentary geomorphology in four steps: (1) reconstruction of basin-floor paleogeomorphology using a residual structure map of a closely underlain erosional unconformity, (2) study of seismic lithology by seismic modeling and 90° phase adjustment of seismic traces, (3) mapping of geomorphologic patterns of depositional systems on stratal slices, and (4) combined analysis of tectonic geomorphology and sedimentary geomorphology for a better understanding of depositional history and reservoir distribution. Results show a good match between valley and lowland systems on the basin floor, sandstone thickness trend, and lobate sedimentary geomorphologic patterns, indicating a widespread, yet laterally highly variable, ultrathin sandy fan-delta system.

Diagenetic contrast of sandstones in hydrocarbon prospective Mesozoic rift basins (Ethiopia, UK, USA)

Diagenetic studied in hydrocarbon-prospective Mesozoic rift basins were carried out in the Blue Nile Basin (Ethiopia), Ulster Basin (United Kingdom) and Hartford Basin (United States of America). Alluvial fan, single and amalgamated multistorey meandering and braided river, deep and shallow perennial lake, shallow ephemeral lake, aeolian and playa mud-flat are the prominent depositional environments.The studied sandstones exhibit red bed diagenesis. Source area geology, depositional environments, pore-water chemistry and circulation, tectonic setting and burial history controlled the diagenetic evolution. The diagenetic minerals include: facies-related minerals (calcrete and dolocrete), grain-coating clay minerals and/or hematite, quartz and feldspar overgrowths, carbonate cements, hematite, kaolinite, illite–smectite, smectite, illite, chlorite, actinolite, laumontite, pyrite and apatite.Diversity of diagenetic minerals and sequence of diagenetic alteration can be directly related to depositional environment and burial history of the basins. Variation in infiltrated clays, carbonate cements and clay minerals observed in the studied sandstones. The alluvial fan and fluviatile sandstones are dominated by kaolinite, illite calcite and ferroan calcite, whereas the playa and lacustrine sandstones are dominated by illite–smectite, smectite–chlorite, smectite, chlorite, dolomite ferroan dolomite and ankerite. Albite, pyrite and apatite are predominantly precipitated in lacustrine sandstones. Basaltic eruption in the basins modified mechanically infiltrated clays to authigenic clays.In all the studied sandstones, secondary porosity predominates over primary porosity. The oil emplacement inhabited clay authigenesis and generation of secondary porosity, whereas authigenesis of quartz, pyrite and apatite continued after oil emplacement.