Paleo-sedimentary Environment Research Articles

Lithofacies characteristics and genetic mechanism of hybrid sedimentary rocks in Da'anzhai member, Sichuan Basin

The Jurassic Da'anzhai Member in the Sichuan Basin is a stratigraphic unit characterized by widespread mudstone and hybrid sedimentation. It comprises various lithofacies types and sedimentary structures, including massive structure, laminated structure, mud rip-up clasts and erosion surfaces. By means of detailed analysis of core, logging, thin sections, scanning electron microscope, XRD and XRF data, six lithofacies types are identified in the Da'anzhai Member, and the genetic mechanisms of these lithofacies such as sandy debris flow, turbidity current, bioclastic flow and still water sedimentation are analyzed based on the characteristics of lithofacies. A complete third-order cycle was identified in the LA 1 and RA 1 well based on the variation in the value of gamma ray, bioclastic content as well as lithofacies characteristics. The paleo-sedimentary environment of Da'anzhai Member of Sichuan Basin is reconstructed on the basis of geochemical proxies such as Sr/Cu, Ca/(Ca + Fe), V/(V + Ni), V/Cr, Ti, Al, Al/(Al + Fe) and Fe/Mn. Different sedimentary environments in the Da'anzhai Member controlled the characteristics of lithofacies distribution and the lake level (water depth) primarily controlled the scale and frequency of the development of hybrid sedimentation and mudstone. During the early and late sedimentary periods of the Da'anzhai Member, the climate was arid, water salinity was high, reducing conditions were weak, and the water body was deep. Sediments such as sandy and bioclastic materials in shallow areas were transported to deeper areas through gravity flows under the influence of sudden events like earthquakes, floods or storms and interbedded with still water mudstones. In the middle period of the Da'anzhai Member, the climate was humid, water salinity was low, reducing conditions were strong, and the water body remained deep. A large amount of deep-water mudstone developed in this period, with gravity flow deposits appearing as thin layers. Finally, the comprehensive sedimentary model of the study area is completed based on this characteristics.

Geological Features, Paleosedimentary Environment, and Organic Matter Accumulation Mechanisms of the Lacustrine Shale Oil System: A Case Study of the Jurassic Dongyuemiao Member in the Sichuan Basin

Lacustrine shale has garnered significant attention due to its significant resource potential. The Jurassic shale in the Sichuan Basin is an important target for lacustrine shale exploration in China. However, previous studies have predominantly focused on the Da’anzhai member of the Ziliujing Formation, and little attention has been paid to the shale of other strata. This paper aims to address this gap by investigating the Jurassic Dongyuemiao member in the Sichuan Basin. The study systematically characterizes the geological properties of the Dongyuemiao shale system, reconstructs the paleosedimentary environment, identifies the key factors influencing organic matter (OM) enrichment and physical properties, and assesses its resource potential through comparative analysis. Our results show that the Dongyuemiao shale was deposited in an oxic and semi-humid sedimentary environment characterized by intense weathering conditions. The enrichment of OM in the shale is primarily controlled by redox conditions and salinity, with redox conditions playing the most crucial role in OM accumulation. In terms of pore system characterization, clay mineral-associated pores dominate the pore types in the Dongyuemiao shale, while two types of organic matter-associated pores are also widely observed. The development of framework grain-associated pores is limited and only present in certain siliceous and carbonate minerals. The shales of the Dongyuemiao member and the Da’anzhai member exhibit slight differences in TOC content. However, the kerogen in the Dongyuemiao member displays higher hydrocarbon generation potential, and the Dongyuemiao shale exhibits more favorable pore structure parameters. Overall, the Dongyuemiao shale does not exhibit any significant disadvantages compared to the Da’anzhai shale. Therefore, it deserves greater attention in future exploration endeavors. The research findings presented in this paper provide a solid theoretical foundation for expanding the exploration scope of lacustrine shale in the Sichuan Basin.

Paleo-Sedimentary Environments and Controlling Factors for Enrichment of Organic Matter in Alkaline Lake Sediments: A Case Study of the Lower Permian Fengcheng Formation in Well F7 at the Western Slope of Mahu Sag, Junggar Basin

The Lower Permian Fengcheng formation is a significant source of rocks and a reservoir for the Mahu Sag in the Junggar Basin. Recently, the paleo-environment deposition factors of the P1f formation have become a popular research topic. This research was conducted using data from the F7 well of Mahu Sag, based on the geochemical analysis results of TOC (total organic carbon), REE (rare earth elements), and major and trace elements of 53 samples from P1f (Lower Permian Fengcheng formation), and some deposition factors are discussed. The P1f deposition process was classified into four stages based on paleo-environment elemental indicators. This research describes the deposition process of the evolution of alkaline lakes. The early and preliminary stages of alkali lake evolution are considered as late P1f1 to middle P1f2; the paleoclimate of this process was dry, the reduction conditions increased, and the paleo-productivity and lake salinity were enhanced. The terminal stage of alkali lake evolution is considered as late P1f2 to middle P1f3; in this period, the paleoclimate changed with seasonal cycles, resulting in a decrease in water salinity and an increase in oxidation; the paleo-productivity of the alkaline lake was at a medium level. Until the end of P1f3, the salinity of the lake decreased, and the water body became anoxic and weakly alkaline. Furthermore, the research on TOC and sedimentary parameters confirmed that the deposition of P1f organic matter is affected by multiple types of factors. A relatively warm climate, lack of oxygen, fresh water–brackish water, suitable debris flow, and high primary productivity conditions promoted organic matter deposition.

Evolution of the Pristane/Phytane Ratio with Maturity of Lacustrine Source Rocks: Implications of a Thermal Simulation Experiment

The pristane/phytane ratio (Pr/Ph) of source rocks has been extensively used for the identification of paleosedimentary environments and oil–source rock correlations. However, the influences of maturity on the Pr/Ph ratio are not fully understood, which largely limits the applicability of this parameter under geological conditions. Based on a thermal simulation experiment performed on typical lacustrine source rock samples and systematic research on geological source rock samples, the evolution of the Pr/Ph ratio with increasing maturity was investigated in this study. The results indicate that the Pr/Ph ratios of the two source rock extracts progressively decrease with increasing maturity, and the decrease in this parameter is more significant for shallow lacustrine samples than for deep lacustrine samples. The Pr/Ph ratios of the two types of source rocks are different at the oil generation stage with EasyRo less than 1.02% but are similar at the oil cracking stage with EasyRo larger than 1.02%. The Pr and Ph of source rocks are contributed by both biogenic and thermal origins, which are largely determined by the sedimentary environment and maturity, respectively, and the contributions of the two origins are diverse at different maturity stages. The Pr/Ph ratio is mainly controlled by the sedimentary environment at the diagenetic stage, by both sedimentary and thermal maturity at the oil generation stage, and by thermal maturity at the oil cracking stage. The Pr/Ph ratio of source rocks can be used to identify sedimentary environments and oil–source rock correlations at diagenetic and oil generation stages.

Paleo-sedimentary environment and lithofacies of Jurassic Da’anzhai Member in the Central Sichuan Basin-A case study of Well Ren’an 1

The Jurassic Da’anzhai Member in Sichuan Basin is an important target for shale oil exploration. Whole rock X-ray diffraction analysis, thin section, SEM and TOC testing, as well as major and trace element analysis are used to analyze the relationship of lithofacies and pore types with the sedimentary environment of the Da’anzhai Member in the Central Sichuan Basin. The results show the Da’anzhai Member in the Well Ren’an 1 (RA1) is divided into six types of lithofacies, based on a three-level division method of mineral composition-TOC-mineral structure— namely, massive mud-bearing shell limestone lithofacies (F), thick argillaceous shell limestone lithofacies (E), laminated clay silt shale lithofacies (D), thin shell calcareous shale lithofacies (C), laminated shell-bearing shale lithofacies (B), and thick clay shale lithofacies (A). The pore spaces include intergranular pores, intergranular dissolution pores, authigenic calcite intergranular pores, authigenic quartz intercrystalline pores, intergranular pores of clay minerals or pyrite nodules, as well as organic pores. The Da’anzhai Member was deposited in a semi-deep to shallow lake sedimentary environment, with relatively low salinity. This strata experienced moderate-to-strong chemical weathering as a result of the semi-arid, warm and humid paleoclimate environmental conditions that persisted during its deposition; low weathering and dry paleoclimate conditions occurred periodically. Paleoenvironment is an important influence on the lithofacies and pore types of the Da’anzhai Member. The persistence of arid climate conditions contributed to a decline in water depth, relatively high salinity, low organic productivity, and the deposition of carbonate rocks that characterize lithofacies E and F; mostly intercrystalline pores formed in these lithofacies in response to the prevailing arid climate conditions. Conversely, warm, and humid climate conditions increased the inflow of fresh water, terrigenous debris, and nutrients into the lacustrine basin that led to the deposition of fine-grained sedimentary rocks of lithofacies A, B, and C. These lithofacies are characterized by abundant nano pores, such as clay intercrystalline pores and organic pores.

Reconstruction of Paleoenvironment and Paleoclimate of the Neogene Guantao Formation in the Liaodong Sub-Uplift of Bohai Bay Basin in China by Sedimentary Geochemistry Methods

The paleosedimentary environment and paleoclimate of the Neogene Guantao Formation in the Liaodong sub-uplift of Bohai Bay Basin in China, which is an important oil and gas exploration horizon, are not clear owing to the lack of quantitative analysis. The paleosedimentary environment and paleoclimate can be qualitatively or semi-quantitatively reconstructed by sedimentary geochemical indicators sensitive to the environment and climate. Based on the evaluation of whether the elements and isotopes can effectively record paleosedimentary environment and paleoclimate information or not, the paleoclimate (temperature and humidity), paleoenvironment (salinity, water depth, redox conditions), and evolution of the paleoenvironment and paleoclimate are studied by analyzing the trace elements, carbon and oxygen isotopes, strontium isotopes, whole-rock mineral compositions, and clay minerals of mudstones of the Guantao Formation in the Liaodong sub-uplift, Bohai Bay Basin. The study results show that (1) according to the trace element distribution patterns, high concentrations of continental elements (Ti, Zr, Th), clay minerals, and detrital content, the Guantao Formation is a product of proximal deposits, and the provenance mainly originates from the intermediate–acidic magmatic rocks near surrounding uplifts. (2) The paleoclimate during the sedimentation of the Guantao Formation was mainly semi-arid and semi-humid, as revealed by the Sr/Cu ratio. However, the high Rb/Sr and 87Sr/86Sr ratios demonstrate that the paleoclimate is relatively warm and humid. (3) The paleosedimentary temperature calculated by the Sr content and δ18O is roughly 30 °C, indicating that the Guantao Formation was deposited in a subtropical to tropical climate. (4) The Guantao Formation was deposited in continental freshwater according to the Li, Ni, and Sr content, Sr/Ba ratios, 87Sr/86Sr ratios, and salinity index Z. (5) The water depth of the paleoenvironment of the Guantao Formation is mainly in nearshore shallow water, reflected by the Mn/Ti ratio and calculated by the Co content, and the oxidizing condition of the paleoenvironment of the Guantao Formation, reflected by the U/Th, V/Cr, and Ni/Co ratios, is in a good coupling relationship with this environment. (6) As revealed by the variations in the geochemical data in the section, the temperature, humidity, and water depth of the Guantao Formation gradually increased from the early to late period, while the salinity gradually decreased, and the chemical weathering effect gradually increased.

Influence of Paleosedimentary Environment on Shale Oil Enrichment in the Raoyang Sag, Bohai Bay Basin

Influence of Paleosedimentary Environment on Shale Oil Enrichment in the Raoyang Sag, Bohai Bay Basin

Lithofacies analysis and paleosedimentary evolution of Taiyuan Formation in Southern North China Basin

The Permian Taiyuan Formation contains alternate shale and coal lithologies which are vital for the exploration of shale gas reservoirs. However, previous studies have neglected the influence of paleosedimentary environment on the formation and distribution of marine-continental shale. A better understanding of the evolution characteristics of the paleosedimentary environment of the marine-continental strata is particularly critical to the accurate prediction of high-quality shale reservoirs. In this study, the geochemical and mineralogical characteristics of Taiyuan Formation, marine-continental strata in the Southern North China Basin, were carried out using lithology identification, element analysis, and X-ray diffraction analysis. These interpretations were helpful to develop a sedimentary depositional model by principles of sedimentology and sequence stratigraphy. The evolution of the paleosedimentary environment and the control of the paleosedimentary environment on the organic matter abundance, mineral composition, lithofacies, and gas enrichment of shale were analyzed. The results show that the vertical variation of the paleosedimentary environment had obvious stages during the Taiyuan Stage of the Early Permian. Based on sequence identification and division, a multi-stage superimposed marine-continental sedimentary model of transgressive and regressive assemblages was established. From tidal flat facies to lagoon facies and then to restricted platform facies, the contents of felsic and clay minerals decreased, while the contents of TOC and carbonate minerals increased sequentially. The lithofacies types of limited platform shale and tidal flat shale were different and relatively single, while the lagoon contained the shale lithofacies types of the above two sedimentary microfacies due to its sedimentary microfacies development location. The lagoon facies shale of highstand system tract had high-quality paleo-productivity, and was distributed in a stretch on the plane and superimposed in multiple stages vertically of the basin, which could be selected as a favorable facies belt of marine-continental facies shale gas.

Characteristics of the Tectonic Background and Sedimentary Paleoenvironment of Coal-Bearing Strata from the Yan’an Formation in the Ordos Basin: Evidence from the Geochemistry of Major and Trace Elements

The coal-bearing strata of the Jurassic Yan’an Formation in the Ordos Basin, China, contain various energy sources, including coal, oil, and gas. However, due to the complex geological conditions of the coexistence of multiple energy sources, it is difficult to explore coal, oil, and gas. Geochemical features record important geological information about rock formations at that time. Therefore, it is of great significance to study the characteristics of major and trace elements and the paleoenvironment of the coal-bearing strata of the Jurassic Yan’an Formation in the Huangling area for the subsequent smooth mining of coal mines and other energy sources. An X-ray fluorescence spectrometer and inductively coupled plasma mass spectrometer were used to determine the major and trace elements of the Yan’an Formation, respectively, and the coal-bearing strata of the Yan’an Formation were comprehensively analyzed from the vertical and different lithology aspects: the paleoweathering characteristics, provenance environment, sediment source, and tectonic background. At the same time, the paleosedimentary environment characteristics of coal-bearing strata were recovered through a series of environmental discrimination indicators, including Sr/Cu, Sr/ppm, Sr/Ba, Th/U, U/Th, V/Cr, V/(V + Ni), V/Sc, and V/Ni. The results show that the sedimentary mode of the Yan’an Formation in the Ordos Basin is mainly fine-grained deposition, and tight sand-mud interbeds and mudstone layers are developed in fine-grained sandstone deposits. The sedimentary rocks are dominated by granite (obvious negative Eu anomaly), mixed with some basement volcanic rocks and ancient sedimentary rocks, and the parent rocks have many geneses (high SiO2, low MgO, and low CaO). The degree of weathering is generally stable but has not experienced complete weathering. Among them, the lower clastic particles have undergone moderate weathering from the source area to the sedimentary basin, and the sandstone is slightly higher than the mudstone. The paleoclimate of the Yan’an Formation was warm and humid in the early stage and tended to be dry and hot. The overall paleowater depth was less than 10 m, and the local area showed a shallow lake subfacies. It experienced a transition from a saline lake facies to a freshwater environment. It is typical brackish–brackish water with a partially oxidized environment (i.e., a sub-rich oxygen environment).

Depositional Environment Variation and Organic Matter Accumulation Mechanism of Marine–Continental Transitional Shale in the Upper Permian Longtan Formation, Sichuan Basin, SW China

Marine–continental transitional shale gas exploration is one of the hottest topics in the academic and industrial community. Frequent sedimentary environment changes lead to substantial heterogeneity and complex organic matter enrichment mechanisms, and it is of great practical significance to clarify the paleoenvironment of marine–continental transitional shale and reveal the enrichment mechanism of organic matter for subsequent shale gas exploration. However, strong heterogeneity creates divergence for our understanding of organic matter enrichment mechanisms and resource potential. This paper selected the marine–continental transitional shale of the Permian Longtan Formation in the southeastern Sichuan Basin as an example. Through TOC analysis, XRD measurements, and major and trace element analyses, we identify the depositional cycle and lithofacies, characterize the paleosedimentary environments of different lithofacies, establish depositional models, and clarify the organic matter enrichment mechanism. Four lithofacies can be identified in the Longtan Formation in the study area: organic–lean carbonate shale (LC), organic–lean mixed shale (LM), organic–lean argillaceous shale (LCM), and organic–rich argillaceous shale (RCM). The terrestrial detrital influx of the LCM and RCM shales is obviously strong, while the RCM shale was deposited in a relatively dysoxic and humid environment. For the Longtan shale, the terrestrial detrital influx and redox environment determine the enrichment of organic matter. In the lagoon sediment system, due to the high terrestrial detrital influx, the argillaceous shale in the transitional shale gas system should be given more attention in future exploration. For different kinds of shale gas systems (marine, marine–continental transitional, and lacustrine), the effect of the terrestrial detrital influx is the main difference. In marine–continental and lacustrine shale gas systems, shales with high TOC values can still exist in the context of oxic sedimentary environments. The research results of this paper can provide a theoretical basis for future marine–continental transitional shale gas sweet spot prediction and exploration from geochemical perspectives.

Heterogeneous characteristics and controlling factors of oil bearing of the Chang 7 shale in the Ordos Basin

Oil-bearing evaluation is the key issue of shale oil geologic exploration, which affects the mobility and effective development of the remaining oil in the shale system. To study the oil content and its controlling factors of shale in Chang 7 member of the Yanchang Formation in the Ordos Basin, we take the core of well H317 as the main research object, analyze the oil-bearing characteristics of shale by conventional rock pyrolysis and multistep pyrolysis experimental methods, and analyze the change characteristics of the elements by handheld X-ray fluorescence. The results indicate that the organic heterogeneity of Chang 7 shale in well H317 is obvious. It can be divided into upper shale and lower shale with 2467 m as the boundary. The upper shale has low total organic carbon (TOC), poor kerogen type, and slightly low oil content, but strong mobility. The lower shale has high TOC, good kerogen type, and good oil content, but poor mobility. The content and type of organic carbon in shale have obvious control effect on oil bearing and mobility. Differences in paleosedimentary environment control the formation of original sedimentary organic matter. During the lower shale deposition period, the climate was humid and the terrigenous input was relatively weak, high paleoproductivity and strong reducing water effectively preserved the original sedimentary organic matter, and finally formed an organic-rich interval with high TOC. Early diagenesis further altered the abundance and type of original organic matter.

Paleoenvironmental modes and organic matter enrichment mechanisms of lacustrine shale in the Paleogene Shahejie Formation, Qikou Sag, Bohai Bay Basin

The Paleogene Shahejie Formation (Shahejie Fm) of Bohai Bay Basin has ignited unprecedented interest among scholars owing to another breakthrough achieved in shale exploration besides Kongdian Fm. However, this breakthrough, particularly high output submember—the third Member of the Paleogene Shahejie Formation (Mbr 3) lacks elaborate study on paleoenvironment and its covariations with organic matter (OM) concentration for shales, which impedes the further acquaintances of OM distribution and shale oil–gas possibility. The present study adopts advanced approaches to gain insights into the deposition process of shales in the Mbr 3, Qikou Sag, Bohai Basin. Petrological characteristics, organic geochemistry, and an abundant element database offer a solid foundation for performing master controlling factor analysis and establishing deposition modes for OM accumulation and preservation. Our results indicate that the entire Mbr 3 has potential. Shales in the Mbr 3 of the Shahejie Fm were deposited under a temperate environment with weak weathering and sediment influx surrounded by suboxic freshwater. However, the organic-enriched shale in the upper Mbr 3 is a more desirable exploration layer. Disparate paleoproductivity support and clastic detritus sediment afforded a diminished consequence of OM accumulation between the upper Mbr 3 (L1–L2 in this study) and middle Mbr 3 (L3 in this study). Our investigation suggests that the OM concentration in the shales was dominated primarily via paleoproductivity and secondarily via redox reactions. Clastic sediment influx sets the primarily passive effect, while effect of salinity was minimal. Two sedimentary modes were established to interpret the variability between L1–L2 and L3 during the two depositional stages. The lake expansion period of L3 resulted in shales being deposited mainly on a gentle slope with a shallow water column containing insufficient OM susceptible to redox condition compared to the optimal state in the semideep lake in L1–L2 stage. The results provide an integrated reconstruction of paleosedimentary environment for the target layer and offer a solid basis for further exploration through coupling relationship between sedimentary environment and OM enrichment in the study area.

Diversity of silica bodies in Mourera Aublet (Podostemaceae) and its phylogenetic, paleobotanical, and taxonomic applications

We tested the phylogenetic, paleobotanical, and taxonomic (i.e., presence, location, and morphology) relevance of silica bodies in leaves of Mourera (M. aspera, M. elegans, M. fluviatilis, M. glazioviana, M. schwackeana, and M. weddelliana). Leaf samples were macerated and examined using light and scanning electron microscopy. The location of silica bodies was determined through cross- and longitudinal–anatomical sections. Silica bodies were found in all analyzed species of Mourera, occurring in the subepidermal cells and/or in the perivascular cells. These structures presented different shapes in each analyzed species, being noteworthy the presence of polygonal and trapeziform shapes in M. fluviatilis, bifid shape in M. aspera and M. weddelliana, and fusiform shape in M. elegans and M. schwackeana. Three ornamentation types were recognized: verrucate, verrucate–granulate, and verrucate–nodulate. The width/length ratio ranged from 34 (M. elegans) to 60.8% (M. fluviatilis). The location and morphology of silica bodies allowed the identification at the species level. Our results emphasize the relevance of these character states on a forthcoming phylogenetic analysis and taxonomic revision of Mourera, as well as in its recognition in paleo-sedimentary environments.

Influence of a Paleosedimentary Environment on Shale Oil Enrichment: A Case Study on the Shahejie Formation of Raoyang Sag, Bohai Bay Basin, China

The characteristics of paleosedimentary environments are of great significance for the enrichment of organic matter (OM) and hydrocarbons in lacustrine shale. This study analyzed mineralogy, well logging data, organic geochemical parameters (total organic carbon and pyrolyzed hydrocarbon), inorganic geochemical parameters (major and trace elements), and multiple geochemical proxies based on inorganic geochemical parameters. These were used to reconstruct the paleosedimentary environment of the lower 1st Member of the Shahejie Formation (Es1L) to reveal OM and shale oil enrichment mechanisms and establish a shale oil enrichment model. The (Fe2O3+Al2O3)/(CaO + MgO), Sr/Ba, Rb/Sr, Cu/Al, and Th/U parameters indicate that the Es1L in Raoyang Sag was deposited in a paleoenvironment dominated by arid paleoclimate, reducing conditions, and saltwater. Paleoclimate, clastic influx intensity, preservation conditions, paleoproductivity, and paleosalinity all affect OM abundance. The OM accumulation in the shale of Es1L was mainly controlled by the high primary productivity of surface water due to algal blooms and moderate salinities, which was achieved using stratified water columns with low oxygen conditions in bottom water. As the main valuable sites for shale oil storage, carbonate mineral depositions are of great significance for oil enrichment. As the dominant lithofacies for oil enrichment, carbonate-rich shale and calcareous shale lithofacies were deposited under a drier paleoclimate, low clastic influx intensity, strong reducing conditions, high paleoproductivity, and moderate salinity paleoenvironment. Additionally, the profile of the shale oil sweet spot was determined through the combination of lithofacies, logging, and paleosedimentary environment data.

Geochemical characteristics and geologic significance of rare earth elements in oil shale of the Yan’an Formation in the Tanshan area, in the Liupanshan Basin, China

In the Tanshan area, in the Liupanshui Basin, abundant oil shale resources are associated with coals. We analyzed the cores, geochemistry of rare earth elements (REE), and trace element of oil shale with inductively coupled plasma mass spectrometry technology to define the paleo-sedimentary environment, material source, and geologic significance of oil shale in this area. The results of the summed compositions of REE, and the total REE contents (ΣREE), in the Yan’an Formation oil shale are slightly higher than the global average of the composition of the upper continental crustal and are lower than those of North American shales. The REE distribution pattern is characterized by right-inclined enrichment of light REEs (LREE) and relative loss of heavy REEs (HREE), which reflects the characteristics of crustal source deposition. There is a moderate degree of differentiation among LREE, whereas the differences among HREE are not obvious. The [Formula: see text] values indicate a weak negative anomaly, and the [Formula: see text] values indicate no anomaly, which are generally consistent with the distribution of REE in the upper crust. The characteristics of REE and trace elements indicate that the oil shale formed in an oxygen-poor reducing environment and that the paleoclimatic conditions were relatively warm and humid. The degree of differentiation of REE indicates that the sedimentation rate in the study area was low, which reflected the characteristics of relatively deep sedimentary water bodies and distant source areas. The results also proved that the source rock mainly consisted of calcareous mudstone, and a small amount of granite was also mixed in.

Formation Mechanism and Prediction Method for the Permian Fused Breccia Tuff Reservoir, Wuxia Region, Junggar Basin

Fused breccia tuff occurs globally, but its formation mechanism is very controversial. Volcanic reservoirs have developed at the bottom of the Permian Fengcheng Formation in the Wuxia region of the Junggar Basin, and here, the lithology is fused breccia tuff. The reservoir porosity is mainly vesicles, but the development and relative filling of the vesicles vary spatially, resulting in strong reservoir heterogeneity. Through core and thin section observations and structural analysis, and combined with reconstructions of the paleosedimentary environment, we discussed in detail the formation mechanism of the fused breccia tuff reservoir. Our conclusions are as follows. In the high-temperature and high-pressure environment of the deep crust, intermediate acidic lava containing volatile components rapidly rose to the earth’s surface along a fault. The volatile components in the lava foamed strongly and then exploded due to the sharp decline of pressure and temperature. A small part of the volcanic dust and pyroclastic material was erupted into the upper atmosphere. Most of the magma became magmatic pyroclast, vitric pyroclast, rock debris, dust, and other matter. This material was in a semimolten state and overflowed into a nearby low-lying lake. The extremely high-temperature pyroclastic flow quickly vaporized the water into high-pressure water vapor, which was squeezed into the pyroclastic flow and became mixed with other volatiles in the foam. On cooling, the pyroclastic material solidified into rock, and the vesicles were preserved. In a later period, due to strong tectonic movement, faults and fractures developed, surface water penetrated into the vesicles along the faults and fractures, and silica and other substances were deposited, filling the primary vesicles. To quantify the development and relative filling of vesicles, drilling parameters were used to establish different geologic models, and wave equation forward modeling was used to obtain a relationship between the development and filling of vesicles, and the seismic amplitude. The 3D seismic amplitude attributes were then extracted to predict the extent of the reservoir, yielding prediction results consistent with the drilling observations.

Key factors controlling the occurrence of shale oil and gas in the Eagle Ford Shale, the Gulf Coast Basin: Models for sweet spot identification

The key geological parameters controlling the enrichment and productivity of shale oil and gas (O&G) are studied systematically using 991 shale core analyses, and assay data from 72 systematic coring wells and 1317 rigs from the lower section of the Eagle Ford Formation in the Gulf Coast Basin of the United States. The key parameters include both source and derived parameters. Source parameters are controlled mainly by the paleo-sedimentary environment, whereas derived parameters combine source parameters, thermal evolution, and tectonic evolution. Here we examine the evolution of, and relationships among, the source and derived parameters with respect to thermal and tectonic evolution. Derived parameters for shale, including total porosity (φt) and matrix permeability (Ke), are related to the source parameters of kerogen type, total organic carbon (TOC), clay volume content (Vclay), and effective shale thickness (Heshale), and can be represented by the source parameters and vitrinite reflectance (Ro), which directly reflects the thermal evolution of shale O&G. Shale O&G-derived parameters, including oil (So), gas (Sg), and water (Sw) saturation, are related to Ro, Vclay, and TOC, and can be represented by the source parameters and Ro. Shale O&G-derived parameters that control the “sweet spots” and enrichment of shale O&G include oil density (ρo), gas relative density (ρg), gas–oil ratio (GOR), water–gas ratio (WGS), oil volume coefficient (Boi), and gas compression factor (Z), and can be represented by Ro. Under the same conditions as the roof and floor in the effective shale layers, the formation pressure gradient (Pindex) can be represented by Ro, and shale compressibility can be represented by Vclay. Five parameters [TOC, Vclay, Heshale (effective shale thickness), Ro, and Pindex], combined with microfractures and sealing conditions, are the key parameters for assessing the enrichment, productivity, and “sweet spots” of shale O&G.

The relationship between paleoclimate change and paleosedimentary environment identification in coastal areas

Based on the close relationship between paleoclimate change and paleosedimentary environment, the relationship and influence between paleoclimate change and paleosedimentary environment identification in coastal areas were further studied. Taking the coastal areas of Zhejiang Province as the study area, the soil samples of the coastal areas of Zhejiang Province were collected by BZK1 borehole, and the samples were preliminarily treated and identified by optical microscope. The foraminifera, Ostracoda, and grain size were identified, and the paleosedimentary environment was analyzed by mapping. The results show that the coastal strata in Zhejiang can be divided into four stages according to the changes of sporopollen assemblage, species distribution, abundance, and diversity of foraminifera and Ostracoda. Since the late Pleistocene, the general trend of climate change in the study area is warm-wet, warm-wet and warm-wet, and gradually turns to cold-dry, which is consistent with the change of sea level fluctuation. According to the general trend of paleoclimate change in the study area, the paleosedimentary environment of the study area is further analyzed by the characteristics of grain size parameters and grain size composition. According to the depth, the sediments can be divided into seven stages. Since late Pleistocene, the sediments are mainly silt. The changing process of hydrodynamic conditions during deposition is medium energy-medium low energy-medium energy-medium low energy-medium energy. The changes of sedimentary facies are as follows: shore shallow lake, flood plain, shallow lake beach bar, lake swamp, flood plain, and shallow sea swamp.

Element geochemical characteristics, provenance attributes, and paleosedimentary environment of the Paleogene strata in the Lenghu area, northwestern Qaidam Basin

The Lenghu area is a key exploration block in the northwestern Qaidam Basin. Due to the low exploration level, the provenance and paleosedimentary environment of the Paleogene strata is not clear. Based on the major and trace element contents and the carbon and oxygen isotopes of 547 samples collected from the Paleogene strata (E32, E31, and E1+2), this study investigated the provenance attributes and paleosedimentary environment of the entire Paleogene period. The samples from the Paleogene strata have relatively low contents of SiO2 (39.75%–61.81%) and Al2O3 (6.23%–20.48%) and high contents of La, Sc, and Co (223.07 ppm, 273.84 ppm, and 325.45, respectively). Moreover, the La–Th-Sc, Ti/Zr–La/Sc and SiO2–K2O/Na2O discrimination diagrams suggest that the tectonic environment of the provenance is an oceanic island arc. The Co/Th–La/Sc and Zr/TiO2-(V + Ni + Cr)/Al2O3 diagrams indicate that the provenance lithology of the study area is mafic rocks with a high degree of weathering. These results imply that the source rocks of the Paleogene strata are the mafic submarine volcanic rocks of the Tanjianshan group in the Saishiteng Mountains. The Sr/Cu, Sr/Rb, and Mg/Ca indexes indicate that the paleosedimentary environment had a humid climate with a trend of slight aridity. The Sr/Ba and Rb/K ratios and the isotope coefficient Z (calculated by carbon and oxygen isotopes) indicate that the paleosalinity of the water body was predominantly fresh water with a salty trend from the Early to the Late Paleogene. The paleoredox environment during the Paleogene sedimentation was relatively oxidized with a reduction trend based on the V/(V + Ni) and V/Cr values. This combined with the trend of increasing paleosalinity suggests that there was a relatively large lake transgression in the Late Paleogene.

Effects of paleosedimentary environment on organic matter enrichment in a saline lacustrine rift basin - A case study of Paleogene source rock in the Dongpu Depression, Bohai Bay Basin

Organic-rich shale, the third member of the PaleogeneShahejie Formation (Es3), is considered as an excellent lacustrine source rock in the Bohai Bay Basin. However, studies on how paleosedimentary environments affect organic matter enrichment are still lacking. In this study, cores of the Es3 shale in the Dongpu Depression (DD) nearly 200 m in length were collected at well Pu 115 to investigate organic matter enrichment mechanism. Results suggest that the Es3 shale was deposited under reductive conditions based on the redox index (V/(V + Ni)) and the large amount of pyrite. The Mg/Sr and Fe/Mn ratios indicate a generally arid climate with a frequently changing paleowater depth. The paleosalinity analyses indicate the Es3 shale developed under fresh-brackish conditions. The Ba content ranges from 75 to 435 ppm (avg. 284 ppm), showing a moderate paleoproductivity. Correlation analyses among the total organic carbon (TOC) and major and trace element ratios show that the TOC rises with the increasing water reduction degree. With increasing water paleosalinity, the TOC increases initially and then decreases, reaching a maximum TOC at a paleosalinity of 11‰. In addition, the TOC increases initially and then decreases with increasing detrital input, indicating that a moderate detrital input would be beneficial for organic matter enrichment. The redox conditions, paleosalinity and quantity of detrital input are the key factors controlling the organic matter enrichment. Further analyses show that the redox conditions were controlled by the paleoclimate, and the paleosalinity and terrestrial detrital input quantity were affected by the paleowater depth. The root factors controlling the organic matter enrichment are paleoclimate and paleowater depth in the saline lacustrine rift basin.