Facies Transition Research Articles

The Shanxi Formation responses to astronomically forced climate changes during the Upper Paleozoic in the Eastern Ordos Basin, NW China

A clearly defined geochronology and stratigraphic interpretation will enhance our understanding of the evolution of paleo water depth and transitional facies, contributing to hydrocarbon exploration. Transitional deposition is extensively observed in the upper Paleozoic of the Ordos Basin. Nevertheless, the correlation between the variations of paleo water depth and global sea-level changes remains uncertain. In this study, we employed gamma-ray logging (GR) data to analyze the cyclostratigraphy of the upper Paleozoic Shanxi Formation in the Ordos Basin. We characterized the elemental geochemistry of the Shanxi Formation using X-ray fluorescence. We reconstructed changes in Paleo Water Depth in the Ordos Basin during the upper Paleozoic by employing sedimentary noise modeling of the tuned GR logging data. Time series analysis showed evidence of the existence of 405 kyr long eccentricity, 125 kyr short eccentricity, and 35.5 kyr obliquity in sedimentary records, which was supported by a sediment accumulation rate model. Using the tuned GR data, we constructed a ∼3.07 Myr astronomical time scale in the Shanxi Formation of the sampling well. Moreover, we used the volcanic ash age (298.18 ± 0.32) Ma at the boundary of the Shanxi Formation and Taiyuan Formation as the anchor point to provide an absolute astronomical time scale from 295.11 Ma to 298.18 Ma for the study area. The variation of Paleo Water Depth in the upper Paleozoic transitional facies was confirmed by both the sedimentary noise model and the X-ray fluorescence (XRF) data, thus supporting each other. Additionally, we discovered a long-period obliquity of approximately 1.2 million years in this model, which is consistent with the global sea-level fluctuations observed during the same period. These results reveal a novel approach for evaluating the Paleo Water Depth variations in transitional facies. Furthermore, our study offers a detailed explanation of the relationship between the evolution of the sedimentary environment and the climate change resulting from astronomical orbit during the glacial period.

Fine-Grained Lithofacies Types and Sedimentary Model of the Upper Permian Longtan Formation Coal Measures in the Western Guizhou Region, South China.

The upper Permian Longtan Formation is widely distributed in southwestern China and is well known for multilayer coal and high organic shale, with significant shale gas potential that has yet to be fully explored and developed. The Longtan coal-bearing strata are composed of complex lithological assemblages of fine-grained sedimentary rocks such as sandstone, coal, shale, and limestone, which exhibit significant differences from marine shale. To better understand the organic-rich lithofacies, their distribution, and their controlling factors, this study carried out a detailed survey of the outcrop and drill cores in the western Guizhou region and examined the fine-grained lithofacies, their assemblages, and their geochemical characteristics. The results showed that (1) the total organic carbon of the Longtan Formation shale in western Guizhou ranged from 1.44 to 14.79%, with an average of 6.41%, and the organic matter was mainly composed of vitrinite. The mineral composition was mainly clay minerals and brittle minerals; the clay minerals were mainly composed of kaolinite (average 11.13%) and illite/smectite mixed layers (average 26.69%) and the brittle minerals were mainly composed of quartz (average 31.63%) and feldspar (average 12.88%). (2) Eight types of lithofacies were identified, including silty mudstone, muddy siltstone, carbonaceous mudstone, carbonaceous shale, bioclastic-bearing mudstone, bioclastic-bearing sandstone, fine sandstone, and coal seam. (3) The six typical lithofacies assemblages were developed in the Longtan Formation, which represented different sedimentary environments of the marine-continental transitional facies in the study area. The lithofacies assemblages A and C represent sedimentation in the lagoon environment. The lithofacies assemblage B represents peat swamp facies. The lithofacies assemblage D represents a tidal flat facies peat flat-mixed flat-sand flat sedimentary environment. The lithofacies assemblage E and F represent the delta sedimentary environment. (4) The sedimentary model of the Longtan Formation in western Guizhou was predominantly deltaic and tidal flat sedimentary systems. Lithological and lithofacies studies of Longtan fine-grained rocks were used to provide a geological framework for examining the fine grain deposition distribution and shale gas resource evaluation. This study is highly important for understanding the sedimentology and oil and gas exploration in the region, providing a basis for identifying and exploring coal-bearing shale gas potential and a reference for the analysis of shale in the world's continental transitional areas.

Flow transformations, mud partitioning and the variable stratigraphic architecture of basin-floor fan fringes

Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.

New sponge spicules from the Ediacaran-Cambrian transition in deep-water facies of South China

Sponges have great significance for understanding the origin and evolution of metazoans. Biomarker fossil and molecular clock studies suggest a Precambrian origin of sponges, but no unambiguous fossil of sponge spicules have been recovered from units older than the early Cambrian to date. Here, we report abundant siliceous spicules from Ediacaran-Cambrian transition strata, representing a deep-slope setting on the southeastern margin of the Yangtze Block in South China. The first appearance of monaxon spicules in the Sifangjing section is in the lower Liuchapo Formation (uppermost Ediacaran), from below the Basal Cambrian Carbon isotope Excursion (BACE), which is consistent with the timing of their initial appearance in shallow-platform settings (e.g., the Three Gorges area). These spicules are monaxons that are isolated or aggregated as spiculate bundles, the latter having a structural arrangement consistent with construction by demosponges. Polyaxial spicules first appear in phosphatic nodules of the basal Jiumenchong Formation (early Cambrian Stage 2). Relative to shallow-water settings, the appearance of polyaxial spicules in deep-water slope settings was delayed (∼10 Ma). Oxygenation of anoxic deep watermasses and increasing concentrations of dissolved Si in seawater during Cambrian Stage 2 may have facilitated the expansion of the Hexactinellida into deep-water settings. The triaene spicules from the basal Jiumenchong Formation are the oldest (∼522 to 525 Ma) Tetractinellida spicule fossils identified to date.

Dynamic development of the Athabasca Valles outflow system from volcanic facies and 15 m scale roughness

Surface roughness properties provide context for geologic terrains, and are commonly partnered with field-based mapping on Earth and satellite-based photogeologic mapping in planetary sciences. In this study, we combine morphology-based facies mapping and perform quantitative roughness analysis and characterization of facies for the Athabasca Valles lava flow-field on Mars to identify flow features, provide insight into eruption conditions, and link roughness patterns throughout the flow to emplacement conditions. The root-mean-square (RMS) height and effective slope were acquired at 15 m wavelength for 14 unique lava facies using statistically derived components from the Shallow Radar (SHARAD) surface echo strength. Quantitative RMS height surface roughness of Athabasca lava features range from 1.09 m to 1.76 m. We show that the RMS height response is generally consistent with facies transitions, confirming the linkage between surficial morphologies and lava flow roughness, including the ability to constrain the relative spatial and temporal evolution of emplacement processes. Roughness patterns and facies localities suggest that the emplacement of Athabasca lava experienced a dynamic progression of local discharge surges and substrate influence on morphology. Given a more complete view of the surface statistical properties, our analyses of nadir-looking radar tracks prove to be a useful tool to distinguish between transitional lava facies.

Geochemical characteristics of coal measure source rocks of Upper Carboniferous Benxi Formation in Daning-Jixian area, eastern margin of Ordos Basin

The Carboniferous-Permian in Daning-Jixian area of Ordos Basin is a typical marine-continental transitional facies deposit, with many sets of organic-rich shales, and abundant shale gas resources. Through unremitting exploration, the shale gas exploration of Shanxi Formation has achieved a major breakthrough, but the related research of Benxi Formation remains to be further studied. Therefore, based on the test and analysis of total organic carbon (TOC) content, rock pyrolysis, kerogen stable carbon isotope (δ13CPDB) and vitrinite reflectance (Ro), this paper systematically discusses the geochemical characteristics and hydrocarbon generation potential of Benxi Formation coal-bearing source rocks, in order to grasp their exploration prospect. The results show that the shale of Benxi Formation has high TOC content (averaging 5.13%) and high Ro (averaging 2.75%), showing good hydrocarbon generation potential. In addition, shale, coal and sandstone are interbedded and superimposed, which can realize the combined production of three gases, improve the yield and benefit, and further promote the exploration process of marine-continental transitional shale gas.

Ichnology of a Middle Devonian regression: Environment, terrestrialization and true substrate controls on trace fossils of the Hangman Sandstone Formation, SW England

The Eifelian-aged Hangman Sandstone Formation of southwest England is a unit of the ‘Old Red Sandstone’ that has traditionally been considered relatively barren of trace fossils. Following recent investigations, we here show it to contain the most diverse Middle Devonian non-marine ichnofauna known globally. The fluvial-lacustrine facies of its constituent Trentishoe Member contain representatives of 21 ichnogenera including Archaeonassa, Arenicolites, Beaconites, Bifungites, Circulichnis, Cruziana, Diplichnites,?Gluckstadella, Gordia,?Halimededes,?Lockeia, Merostomichnites, Palmichnium, Petalichnus, Planolites, Polarichnus, Rusophycus, Siskemia, Spirophyton, Steinsfjordichnus, and Taenidium, as well as several microbially induced sedimentary structures. The transitional marine-influenced facies of the underlying Hollowbrook Member are less extensively exposed but contain four additional unique ichnogenera in the form of Cochlichnus, Halopoa, Phycodes, and Teichichnus, as well as further examples of Cruziana, Planolites and Taenidium. A revised ichnological assessment of the unit reveals several notable features, including the youngest known example of the arthropod refugia trace, Polarichnus, as well as a transitional trace fossil form of Diplichnites-Beaconites, demonstrating that the latter was produced by arthropods. As the Hangman Sandstone Formation records the culmination of a marine regression, its ichnofauna can be compared with that of the conformably underlying Lynton Formation, which was deposited in a sandy marine shelf setting. The ichnological differences between these units are acute, with 88% of the Hangman Sandstone Formation trace fossils being exclusive to non-marine or transitional facies. This observation sheds light on the terrestrialization process and indicates that the segregation of marine and non-marine trace fossil communities had accelerated by the Middle Devonian. Internal variability in ichnological signatures within the Hangman Sandstone Formation is also pronounced, with greater ichnodiversity and ichnodisparity in distal fluvio-lacustrine facies than in proximal fluvial facies. By viewing the depositional environment of the unit as a regionally extensive distributive fluvial system, this variability can be partly explained by a bias arising from the distribution of true substrates (bedding planes that demonstrably preserve an ancient sediment-air interface) in the unit, with such phenomena being more readily preserved in the lower energy outer reaches of the depositional system.

Hydrogeochemistryof groundwater in Es-Sbikhat El-Mahmel basin, Khenchela (NE Algeria)

Es-Sbikhat El-Mahmel basin, located at the southeastern piedmont of Aures massif (Northeastern Algeria), is characterized by a semi arid condition . The sampling process was realized within 39 boreholes and wells during May 2015. Results of analysis data projected on the PCA (Principal Componet Analysis) plan which express 52.70% of variance has shown a chemical facies evolution from calcium bicarbonate water into calcium chloride water, going through a transition facies marked by calcium-sulfate to magnesium-sulfate water.Thermodynamic analysis corroborated that water rock interaction is the main process that control mineralization (dissolution precipitation, cationic exchange and anthropogenic pollution).

Early Oligocene gravity-flow deposits at the base of the Meiji Reef system in the South China Sea, and their palaeoclimate significance

In this paper, we describe gravity flow facies that form the base of the Meiji Reef system in the South China Sea, and discuss their palaeoclimate significance. These Early Oligocene (26.1–23.5 Myr) deposits occur in Well NK1 and comprise a 110.6 m thick succession developed on top of volcanic basement. The deposits can be divided into two intervals, both of which are dominated by debris flow facies. The lower 17.6 m thick interval has a gravel content of 50–65% with parent rocks including dacite, basalt, tuff, and coral reef fragments. From base to top, these units coarsen upward and show increasing gravel contents. Gravel clasts are well-rounded but poorly sorted (diameter 10–350 mm), which, together with their complex composition, indicate transportation by the gravity flows from the near-shore volcanic rock basement to the slope. The upper 93 m thick interval comprises abundant bioclastic materials, including benthic foraminifera, crustal red algae, and red algae detritus, with a few volcanic gravel clasts. The content of volcanic rock gravels decreases towards the top, which may be attributed to the reduction or disappearance of volcanic rocks in the provenance area due to the establishment of coral reef facies that cap the succession and dominate the overlying interval from a core depth of 885.6 m to the seafloor. Based on U-Pb dating, the cap-shaped red algae laminae at the facies transition were formed at 24.6 ± 1.1 Ma, dating the earliest initiation of reef building to end-Oligocene times. The emergence of gravity flow sediments during the early Oligocene may be attributed to the onset of the Antarctic glaciation and concurrent regional tectonic activities that led to a shallowing of the sea. The origin of debris with high roundness characteristics is believed to be associated with transportation along the slope triggered by tectonic and climatic events, resulting in the formation of debris flow deposits. Studying the gravity flow facies at the Meiji Reef system is crucial for determining the onset of reef-building and helps establish the relationship between gravity flow deposition, paleoclimate, and regional tectonic evolution in the South China Sea.

Molecular dynamics simulation of the diffusion crystallization mechanism of the binary alkane mixture nC5H12–nC24H50 under the water-wetting condition of a pipe wall

Wax deposition on the inner wall of oil pipes, sucker rods, oil- and gas-gathering pipelines and equipment caused by wax crystals has been a challenging problem in the petroleum industry. At present, most of the marine terrestrial transitional facies and continental shale oil and gas being explored and developed in China have high-wax-content volatile oil or condensate, and many shale oil and gas wells have been seriously blocked by wax deposition. In this work, the wax deposition behavior of the binary alkane mixture nC5H12–nC24H50 at different temperatures under the condition of water wetting of a pipe wall was studied through molecular dynamics simulation. When the temperature is 130–70 °C, the wax molecules are loosely distributed without obvious aggregation, and the pipe wall is mainly occupied by water molecules, exhibiting a water-wetting state. With the decrease in temperature, the distance between nC24H50 molecules decreases correspondingly, which enhances the interaction between wax molecules and increases the probability of mutual aggregation. When the temperature is 50 °C, a small amount of nC24H50 molecules pass through the water molecular layer and gather on the pipe wall. When the temperature is <50 °C, the binding energy between the crystal surface of the pipe wall and the molecule nC24H50 exhibits a first-order phase transition, and the molecule nC24H50 can more easily bind to the pipe wall than the molecule H2O. When the temperature is 40–20 °C, the aggregation amount of nC24H50 molecules on the pipe wall further increases, exhibiting an obvious crystallization tendency, and a small amount of nC5H12 molecules begin to aggregate with nC24H50 molecules through the water molecular layer on the pipe wall. At the same temperature, the diffusion speed of nC24H50 molecules in nC5H12 is faster than that in H2O.

Alluvial fan and fan delta facies architecture recording initial marine flooding in the Mio‐Pliocene syn‐rift sequence of the Fish Creek‐Vallecito Basin, southern California

AbstractThe timing and character of the initial marine flooding of extensional basins has implications for their tectonic history. Yet, the recognition of such flooding is difficult along rift basin margins due to the dominance of coarse‐grained systems and the lack of marine fauna. This study conducts detailed facies and stratigraphic analysis of a Mio‐Pliocene alluvial fan and fan‐delta succession in the Fish Creek Vallecito Basin in southern California. Our goal is to characterize the marine flooding surface, determine the paleogeographic position of the shoreline and estimate the magnitude of relative base‐level rise that occurred during the marine incursion associated with the opening of the Gulf of California. Our results show that the flooding of the Elephant Trees alluvial fans is often marked by an abrupt lithologic and facies change from meter‐scale boulder‐rich subaerial debrites (proximal alluvial fan facies association) to centimetre‐scale granule‐rich subaqueous debrites, ripple‐laminated sandstones and mudstones (prodelta facies association). By delineating the zone of transition between the subaerial and subaqueous facies, we place the initial flooding paleo‐shoreline 4 km up the alluvial fan's paleo‐depositional slope. Considering alluvial fan slope gradients between 1° and 5°, this 4 km transgression would require an estimated 70–350 m of water depth during the initial marine incursion. Interfingering of fan‐delta deposits with subaqueous marine and planktonic‐rich evaporites suggests that the basin was below sea‐level after, and perhaps even before, the marine flooding. Subaerial subsea‐level basins exist in Death Valley and the Salton Trough today within similar extensional and transtentional tectonic regimes. This subaerial subsea‐level interpretation might explain the high magnitude and abrupt relative base‐level rise recorded by the facies transitions in the Fish Creek Vallecito Basin. These results suggest that the Fish Creek Vallecito Basin underwent significant extension during its early and nonmarine depositional phase, allowing it to reach subsea‐level elevations. The tectonic history of the Fish Creek Vallecito Basin maybe similar to other extensional basins where rapid subsidence allows the accumulation of nonmarine strata below sea‐level prior to the marine flooding, then restricted and deep marine strata immediately after flooding.

Relationship between coal composition, coal facies, and desorbed methane gas content: Thermal histories and exhumation processes in the middle Magdalena Valley basin, Colombia

We analyzed a total of 25 coal samples from high volatile bituminous to medium volatile bituminous range from 2 wells drilled in the Landázuri area of the Middle Magdalena Valley Basin at the Eastern Colombia Cordillera. This study reveals the relationship between the measured gas concentration, petrographic characteristics, and coal facies. We identify that gas desorption in coal seams is related to maceral composition.Three coal seam groups were defined in Landázuri 1 based on petrographic and statistical evaluation. Two groups were identified in Landázuri 2 based on gas content.The evaluation of maceral content and preservation of reflectance vitrinite found that the highest desorbed gas values correlate with the abundance of inertinite and structured inertinite macerals, followed by the percentage of vitrinite. Structured inertinite lumens may serve as macro/mesopores which allow for higher gas desorption because they are free of mineral matter or other macerals.Using existing coal facies diagrams, two types of coal facies were differentiated: lower delta plain (fen, marsh) and back-barrier low moor developed in a humid environment, where reotrophic conditions prevail and herbaceous and aquatic/marginal vegetation predominate.The relationship of these transitional facies with gas content shows are not directly related to the higher gas values measured at Landázuri. Instead, the relationship between the high degree of destruction of organic matter in these facies (marine incursions and other flows) and the measured gas content shows the importance of quantifying the structured macerals as potential gas retainers.Using low-temperature thermochronometers apatite fission-track, and (U–Th)/He in zircon, combined with vitrinite reflectance data, we reconstructed time-Temperature histories for areas near the wells. Differences in gas content can be explained by an asynchronous behaviour of the burial-cooling processes in this part of the Colombian Eastern Cordillera. Our work shows it is possible to distinguish gas desorption in coal seams based on the maceral composition.

Discussion on Transitional Shale Gas Accumulation Conditions from the Perspective of Source-Reservoir-Caprock Controlling Hydrocarbon: Examples from Permian Shanxi Formation and Taiyuan Formation in the Eastern Margin of Ordos Basin, NW China

Transitional shale gas, rich in resources, is expected to be a practical contributor to the increase in shale gas reserves and production in China. Its exploration prospect has been demonstrated by several wells in the Daning-Jixian block on the eastern margin of the Ordos Basin. In this paper, the Lower Permian Shanxi Formation (P1s) and Taiyuan Formation (P1t) in the eastern margin of Ordos Basin were compared for organic geochemical parameters, revealing that the overflow fan + lagoon combination (OLC) of the third sub-member of the second member of Shanxi Formation (P1s23) and the marine + lagoon combination (MLC) of the first member of Taiyuan Formation (P1t1) are the most favorable shale gas intervals. The two intervals were comparatively analyzed with respect to mineral composition, brittleness, caprocks, and preservation conditions. It is found that the OLC of P1s23 has a similar porosity to and much lower permeability than the MLC of P1t1 (or MLC1) and a BET surface area of 10–15 m2/g, which is smaller than the MLC1 (15–20 m2/g). Moreover, OLC has a brittle mineral content equivalent to MLC1 but a brittleness index of 33.73–62.36 (avg. 49.86), smaller than MLC1 (53.34–58.27, or avg. 55.85). OLC contains sandstones at both the roof and floor, with a higher permeability than shale in the interval, which cannot serve as good physical seals. In contrast, MLC1 contains limestones with lower permeability at the roof and floor, which, together with the overlying coal seams, have hydrocarbon generation capacity and can physically seal the MLC1 shale but also fill it with hydrocarbons, making MLC1 have higher gas content and superior for shale gas exploration than the OLC shale. Due to the multi-lithologies developed in transitional facies, besides the organic matter enrichment, and reservoir characteristics, it is necessary to find a suitable lithological combination to ensure the gas in shale can be better preserved and retained.

Reservoir characteristics of marine–continental transitional shale and gas-bearing mechanism:Understanding based on comparison with marine shale reservoir

Transitional shale gas layers are widely distributed in China, but no significant exploration breakthrough has been made so far. This paper investigates and analyzes the characteristics and gas-bearing mechanisms of transitional shale gas reservoirs in detail, aiming to clarify the shale gas accumulation mechanism of transitional shale and provide theoretical support for the selection of favorable intervals. The transitional shale gas layer is characterized by a thin single-layer thickness, rapid lithological change, low brittle mineral content, and poor kerogen type. The lack of organic matter (OM) sponge pore development process from the oil-generation stage results in limited numbers of OM nanometer-scale pores. Shale pore space is dominated by pores and fractures related to clay minerals. The measured gas content is well consistent with the theoretically calculated gas content for marine organic-rich shales. However, the actual measured gas content is far lower than the theoretically calculated gas content for the transitional shale gas reservoir. The main mechanisms are summarized to be (1) the high hydrocarbon expulsion efficiency of the “sandwich” space structure of the sandstone–shale–coal association, which gives rise to most natural gas migrating into nearby sandstone, and (2) high water saturation resulting in insufficient storage space for free gas in the shale reservoir. Unlike marine shale gas, natural gas in the transitional shale reservoir is primarily dominated by adsorbed gas in kerogen, and free gas is relatively low. The favorable lithofacies types are organic-rich siliceous/calcareous shales. Multiple layers of siderite-bearing shales/siderites are developed vertically and continuously distributed horizontally in transitional strata, particularly in flat-lagoon facies. It is easy to form a “micro-trap” to store gas in siderite-bearing shale, and siderite-bearing shale has strong sealing properties due to low porosity, low permeability and high breakthrough pressure. This property can form overpressure and trap shale gas inside the shale, providing a new research perspective for the optimization of vertical favorable intervals, as well as exploration breakthrough in transitional shale gas. Further research should strengthen the systematic sedimentological study of transitional facies, reveal shale gas occurrence state and dynamic transformation, and optimize favorable interval evaluation systems to clarify the feasibility of coal-measure gas commingled production.

Lateral and temporal variations of a multi-phase coarse-grained submarine slope channel system, Upper Cretaceous Cerro Toro Formation, southern Chile

ABSTRACTUnderstanding variations in the sedimentary processes and resulting stratigraphic architecture in submarine channel systems is essential for characterizing sediment bypass and sedimentary facies distribution on submarine slopes. In the Santonian to Campanian Cerro Toro Formation, southern Chile, a coarse-grained slope system, informally known as the Lago Sofia Member, developed in a structurally controlled environment, with complex and poorly established relationships with the surrounding mud-rich heterolithic deposits.A detailed architectural analysis of the most continuous and best-exposed channel system in the Lago Sofia Member, the Paine C channel system, provides insights on lateral facies transitions from channel axis to margin, stacked in a multi-phase sequence of events marked by abrupt changes in facies, facies associations, and architecture.The Paine C channel system is incised into siltstones and claystones interbedded with thin-bedded very fine sandstones, interpreted to be either channel-related overbank or unrelated background deposits. The coarse-grained deposits are divided into a lower conglomeratic unit and an upper sand-rich unit. The lower conglomeratic unit can be further subdivided into three phases: 1) highly depositional and/or aggradational, dominated by thick and laterally continuous beds of clast- to matrix-supported conglomerate, herein named transitional event deposits; 2) an intermediate phase, including deposits similar to those dominant in phase 1 but also containing abundant clast-supported conglomerates and lenticular sandstones; and 3) a bypass-dominated phase, which records an architectural change into a highly amalgamated ca. 45-m-thick package composed purely of lenticular clast-supported conglomerates with local lenticular sandstones. Between the conglomeratic phases, a meter-scale package composed of interbedded thin- to medium-bedded sandstone and mudstone deposits is interpreted to drape the entire channel, indicating periods of weaker gravity flows running down the channel, with no evidence of bedload transport.The upper sand-rich unit is divided into lower amalgamated and upper non-amalgamated phases, and represents a rapid architectural change interpreted to record an overall waning of the system. The sandstone unit laps out onto a mass-transport complex which is interpreted to have been triggered initially at the same time as major architectural change from conglomerates to sandstones.While mindful of the fact that each system is a complete analogue only for itself, we propose a new depositional model for coarse-grained submarine channel systems, in which particular characteristics can provide significant insights into architectural heterogeneity and facies transitions in channelized systems, allowing substantial improvement in subsurface facies prediction for fluid reservoirs.

Geological characteristics and gas-bearing evaluation of coal-measure gas reservoirs in the Huanghebei coalfield

Bystudying the source reservoir geochemical characteristics, reservoir cap physical properties, gas-bearing characteristics and reservoir-forming types of the coal-measure gas (CMG) accumulation system, the potential of CMG resources in the Huanghebei Coalfield was determined, and the sedimentary reservoir control mechanism was analyzed, which is of great significance for the orderly development of CMG reservoirs. CMG in the Huanghebei Coalfield mainly includes abundant shale gas (SG; 2100.45 × 108 m3), coalbed methane (CBM; 137.89 × 108 m3) and a small amount of tight sandstone gas, limestone gas and magmatic rock gas. CMG rocks mainly include shale and coal seams of the marine–terrestrial transitional facies Taiyuan Formation and continental facies Shanxi Formation, providing a favorable material basis for CMG enrichment. The coal seams are largely mature to highly mature, with satisfactory hydrocarbon generation potential and reservoir connectivity. They are low-porosity, low-permeability, underpressurized and undersaturated reservoirs. The shale organic matter is primarily Type II, with a moderate organic matter abundance. The material is at the middle-to high-maturity stage, provides a suitable hydrocarbon generation potential, belongs to low-porosity and low-permeability reservoirs, and exhibits favorable fracturing properties. There are several stable and independent gas-bearing systems among the coal measures. The CMG accumulation combination types are mostly self-sourced gas reservoirs and self-sourced+other-source near-source gas reservoirs, followed by other-source gas reservoirs. CBM and SG play the role of vertical regulation and horizontal regulation, respectively.

Shale gas accumulation patterns in China

Although significant progress has been made in the geological theory and exploration and development of shale gas in China, the research on the geological particularity and enrichment regularity associated with shale gas accumulation remains weak. With this in mind, we analyzed the shale gas accumulation patterns in China. The findings show that: (1) A variety of organic-rich shales were deposited in the continental margin-type marine and transitional environments during the Pre-Mesozoic period and the basin-type terrestrial environment during the Mesozoic and Cenozoic eras. (2) The basins dominated by Mesozoic and Cenozoic terrestrial sediments superimpose on the Pre-Mesozoic continental margin-type marine–or transitional-facies sediments. The former developed the terrestrial shales with large thickness, high organic matter abundance, new age, and low thermal maturity; meanwhile, the deep parts of the basins are favorable for shale gas enrichment. However, the latter preserves the shale with old age, high thermal maturity, and undergoing multiphase tectonic movements; and the tectonically stable regions are favorable for shale gas enrichment. (3) The aulacogens mainly formed in the Meso-Neoproterozoic, Sinian-Ordovician, and Devon-Triassic periods are favorable for marine organic-rich shale deposition; moreover, this type of shale and other types including transitional facies shales with favorable reservoir-forming assemblages, coal-measure shales in small and medium-sized basins, and low-medium mature shales affected by magma intrusion, have jointly constituted the various shale gas accumulation patterns in China. (4) Based on the drilling discoveries in China, shale gas accumulation patterns can be categorized into two types: tectonic-controlled and stratigraphic-controlled. The former is mainly related to structural features, whereas the latter is mainly related to the variation of lithology and strata. (5) Shale development and gas content are mainly affected and controlled by geological factors including sedimentation, diagenesis, and later tectonic movements. Favorable sedimentary environments, appropriate thermal maturity, good sealing capacity of caprock, and excellent structural preservation are the necessary conditions for shale gas enrichment.

Formation conditions and the main controlling factors for the enrichment of shale gas of Shanxi Formation in the southeast of Ordos Basin, China

Based on the dissection of the geological characteristics of typical shale gas wells, the relationship between reservoir-forming factors and shale gas enrichment is deeply analyzed, and the controlling factors of shale gas enrichment are summarized. An analysis of data from drilling, logging, testing, and shale sample has revealed that the factors contributing to the formation of shale gas reservoirs in marine–continental transitional facies of Shanxi Formation in the southeast of Ordos Basin are highly heterogeneous. This is mainly reflected in the rapid spatial change of shale organic matter, physical properties, and other parameters of the shale in this area. The shale found in the Shanxi Formation has a high content of organic carbon (TOC), with an average of 2.24%. The kerogen type is mainly type Ⅲ, and the average value of RO is 2.48%, indicating that it is in the high mature-overmature stage and is a high-quality source rock. The rock composition is rich in clay (40.5%–88.5%), mainly composed of illite-mongolian mixed layer and kaolinite, and its relatively high specific surface area is beneficial for improving the gas adsorption capacity of the shale. Thin sandstone interbeds, measuring less than 3 m are widely distributed in the Shanxi Formation and have good physical conditions. With an average porosity 3.37% and an average permeability of 0.1 mD. The natural gas generated by the shale can be transported to these sandstone interlayers (laminae) in a short distance, which is beneficial for improving the gas-bearing property of the shale. The gas-bearing property of the Shanxi Formation in the study area is affected by different lithologic assemblages. The gas-bearing property of the thick mudstone-coal assemblage of the second member of the Shanxi Formation is the best, with more than 80% of the samples having a gas content of over 1 m3/t. This study suggests that the high thermal evolution degree of shale, special marine–continental transitional facies reservoir conditions, and good preservation conditions are the main factors affecting the enrichment of shale gas in the study area. This makes the shale gas of the Shanxi Formation in the southeast of the Ordos Basin have certain potential for exploration and development, and delineates the enrichment targets of shale gas in the Zichang-Yan'an-Fuxian and Yichuan areas.

Factors controlling spatial distribution of complex lithology in transitional shale strata: Implications from logging and 3D seismic data, Shan-2 Lower Sub-member, Eastern Ordos Basin

The Shanxi Formation Shan-2 Lower Sub-member transitional shale in the eastern Ordos Basin is characterized by high total organic carbon value, wide distribution, and large single-layer/cumulative thickness; it is the key target interval for transitional shale gas exploration. Based on drilling, logging, 3D seismic, and natural energy spectrum data, this study discusses the spatial distribution and controlling factors of complex lithology in the transitional shale stratum. Using the multi-lithological eigenvalue method, the lithologic differential distribution characteristics of the Shan-2 Lower Sub-member were identified through seismic attribute analysis and post-stack seismic parameter inversion. On this basis, the controlling factors of lithology spatial difference distribution are revealed by paleogeomorphology restoration, the sedimentary environmental indicator characteristics of shale, and seismic facies division. The results indicate that the lithology distribution of transitional shale strata in the Shan-2 Lower Sub-member is significantly different, and the geomorphological pattern of interjacent depressions and highlands lays a foundation for the formation of the sedimentary system. The environmental and energy differences of paleo-sedimentary water form a sedimentary environment with frequent facies transitions, which controls the distribution of sediments. Th/U and Th/K radios, which indicate redox conditions and sedimentary water energy during the early sedimentary stage of the Shan-2 Lower Sub-member, show that the geomorphic uplift area is characterized by a high-energy transitional environment of mainly developed sandstone and silty mudstone, while the depression is relatively low-energy under a brackish water reduction environment, mainly composed of both dark and carbonaceous shale. During the late sedimentary stage of the Shan-2 Lower Sub-member, when geomorphic control was weakened, it presented a shallow-water, high-energy, and oxidized transitional environment, with a lithology dominated by the complex interbedding of fine-grained sandstone, silty shale, shale, carbonaceous shale, and coal seam. Therefore, it is inferred that the Shan-2 Lower Sub-member may have developed as a barrier island-lagoon sedimentary model. The early depositional period was dominated by barrier island-lagoon-tidal flat depositional combination, thence the sedimentary environment gradually evolved into a tidal flat-swamp environment.

Pliocene bivalves (Pteriomorphia) of Algiers Sahel (Algeria): Systematics and palaeoecology.

A hundred specimens of pteriomorph bivalves were collected from the marine deposits of the marly-sandy formations of Algiers Sahel, which correspond to transitional facies between the Piacenzian deep marly deposits and the Astian molassic deposits. They are herein analysed from a systematic, taphonomic, palaeoecological and paleoenvironmental point of view. The preliminary inventory list consists of 27 species belonging to eight families: Arcidae, Nuculidae, Glycymeridae, Spondylidae, Pectinidae, Plicatulidae, Gryphaeidae, and Ostreidae. Two sedimentary units with different bivalve biodiversity are recognized: the first: a shallow infralittoral unit characterised by large Flabellipecten alessii, Aequipecten angelonii, Ostrea lamellosa and O. edulis; the second: A deeper unit of circalittoral environment mainly includes Amusium cristatum. These bivalve shells display sclerobiont traces. Bioerosion traces are mostly assignable to clionid sponges (Entobia isp.), polychaete worms (Maeandropolydora isp. and Caulostrepsis isp.), bivalves (Gastrochaenolites isp.) and predatory gastropods (Oichnus isp.). Identified encrusting organisms are juvenile oyster shells, cirripedes, polychaete worms (serpulids), and indeterminate bryozoans. Analysis of boring and encrusting traces indicate a progressive environmental change from a relative high energy setting to a lower energy and deeper water setting, and correspond to a shallow sea with well-oxygenated waters.