Mixed Sedimentary Rocks Research Articles

Pore structure of the mixed sedimentary reservoir of Permian Fengcheng Formation in the Hashan area, Junggar Basin

The Permian Fengcheng Formation (P1f.) in the Hashan area, situated on the southwestern margin of the Junggar Basin, has witnessed a remarkable breakthrough in shale oil exploration in recent years with nearly 789 million tons of shale oil resources. As a unique set of mixed sedimentary shales, the Fengcheng Formation in the Hashan area is characterized by mixed sedimentation of terrigenous siliciclastic sediments, authigenic minerals, and tuffaceous materials. However, the understanding of pore characteristics in the mixed sedimentary reservoir still remains limited, prohibiting accurate estimation of the oil content and insights into oil mobility. Scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), X-Ray Computer Tomography (X-CT), and geochemical analysis were performed to investigate the pore size distribution and main controlling factors of the mixed sedimentary reservoir. Results showed that the main pore types in the mixed sedimentary reservoir are intergranular pores and dissolution pores. The pores of the P1f. mixed shales in the Hashan area were classified into II-micropores (< 25 nm), I-micropores (25–100 nm), mesopores (100–1000 nm) and macropores (> 1000 nm). In general, the mixed sedimentary rocks of P1f. formation feature few macropores but a large number of micropores and mesopores. The CS exhibits the most favourable physical properties among all lithofacies. It is concluded that the abundance and maturity of organic matter, mineral composition, sedimentary structure, and diagenesis of reservoir together impact the pore structure in the mixed sedimentary reservoirs. The maturity of organic matter and the content of tuffaceous minerals are the most significant in influencing the pore structure of P1f. shales. Overall, the pore structure of complex lithologic reservoir formed by mixed deposition and its influence on physical properties are studied, and the characteristics of the microscopic pore-throat system of the dominant lithofacies in the Hashan area are clarified, which is of great significance as a guide for the exploration and development of mixed sedimentary reservoirs in continental shale oil in China.

Influences of lithofacies on fluid mobility in mixed sedimentary rocks: Insights from NMR analysis of the middle Permian Lucaogou Formation, Junggar Basin

The multi-source mixed sedimentation resulted in a unique series of mixed fine-grained sedimentary rocks evolved within the Permian Lucaogou Formation in the Jimusar Sag, located in the southeastern Junggar Basin, China. The variety of lithofacies within this series resulted in pronounced heterogeneity of pore structures, complicating the analysis of fluid occurrence space and state within reservoirs. As a result, the impact of lithofacies on fluid mobility remains ambiguous. In this study, we employed qualitative methods, such as field emission scanning electron microscopy (FE-SEM) and thin section observation, and quantitative analyses, including X-ray diffraction (XRD), total organic carbon (TOC), vitrinite reflectance (Ro), high-pressure mercury intrusion (HPMI) porosimetry, and nuclear magnetic resonance (NMR), along with linear and grey correlation analyses. This approach helped delineate the effective pore characteristics and principal factors influencing movable fluids in the fine-grained mixed rocks of the Lucaogou Formation in the Jimusar Sag, Junggar Basin. The findings indicate the development of three fundamental lithologies within the Lucaogou Formation: fine sandstone, siltstone, and mudstone. Siltstones exhibit the highest movable fluid saturation (MFS), followed by fine sandstones and mudstones sequentially. Fluid mobility is predominantly governed by the content of brittle minerals, the sorting coefficient (Sc), effective pore connectivity (EPC), and the fractal dimension (D2). High content of brittle minerals favors the preservation of intergranular pores and the generation of microcracks, thus offering more occurrence space for movable fluids. A moderate Sc indicates the presence of larger connecting throats between pores, enhancing fluid mobility. Elevated EPC suggests more interconnected pore throat spaces, facilitating fluid movement. A higher D2 implies a more intricate effective pore structure, increasing the surface area of the rough pores and thereby impeding fluid mobility. Ultimately, this study developed a conceptual model that illustrates fluid distribution patterns across different reservoirs in the Lucaogou Formation, incorporating sedimentary contexts. This model also serves as a theoretical framework for assessing fluid mobility and devising engineering strategies for hydrocarbon exploitation in mixed fine-grained sedimentary rocks.

Formation mechanism of organic-rich mixed sedimentary rocks in saline lacustrine basin, Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, Northwest China

The organic-rich mixed sedimentary rocks deposited in a saline lacustrine environment are widespread in the Lucaogou Formation (P2l), Jimsar Sag, Junggar Basin. These rocks have complex lithology (mainly including mudstone, dolomitic mudstone and argillaceous dolomite) and significant changes in geochemical characteristics. Presently, the formation mechanism of organic-rich mixed sedimentary rocks in the P2l of the Jimsar Sag is obscure. To address these issues, the formation mechanism of mudstone, dolomitic mudstone and argillaceous dolomite is revealed by evaluating their sedimentary environment, primary productivity and sources of organic matter using organic petrology, organic geochemistry and inorganic geochemistry. The results show that the mixed sedimentary rocks of the P2l exhibit excellent hydrocarbon generation potential. The mixed sedimentary strata were mainly deposited in saline and suboxic to anoxic water column, and the main source of organic matter was aquatic organisms. During the changes from mudstone, dolomite mudstone to argillaceous dolomite deposition, the paleoclimatic condition varied from warm-humid to arid-hot with more frequent hydrothermal activity, which enhanced the salinity of the water column, leading to a higher proportion of halotolerant algae (cyanobacteria and green algae). The improvement in the paleoproductivity caused by volcanic activity was the most critical factor to promote the enrichment of organic matter in the mixed sedimentary rocks. In addition, the decomposition of algae and stratified water column were conducive to the formation of a stable and suboxic to anoxic bottom water environment, which improves the preservation conditions of organic matter, and also favored the organic matter enrichment in mixed sedimentary rocks. Therefore, the frequent changes in paleoclimatic conditions and periodic hydrothermal activity controlled the formation of different lithologies in the P2l. High primary productivity and good preservation conditions promoted the accumulation of organic matter in these mixed sedimentary rocks.

Silicate and carbonate mixed shelf formation and its controlling factors, a case study from the Cambrian Canglangpu formation in Sichuan basin, China

Abstract Mixed sedimentation is mainly a phenomenon by which siliciclastic and carbonate are mixed under a similar sedimentary environment. The study of the composition of mixed rock can reflect the sedimentary environment. The mixed sediments of the Canglangpu formation in the eastern Sichuan basin are widely developed and exhibit well exploration prospects. In this study, a classification diagram of mixed sediments suitable for the Canglangpu formation was proposed by combining lithologic characteristics with logging data. Based on the observations of typical outcrop profiles, the identification of thin sections, and drilling and geological logging data, a model was established to identify the electrical responses of the mixed sedimentary rocks in the study area. The results show that (1) the Canglangpu formation can be vertically divided into two complete transgression-regression cycles with the lower part mainly consisting of sedimentary carbonate diamictite and the upper part principally composed of clastic diamictite; (2) mixed sediments in the Canglangpu formation can be divided into six classes, namely carbonate sandstone, argillaceous sandstone, sandy carbonate rock, argillaceous carbonate rock, sandy mudstone, and Carbonate mudstone; (3) the sedimentary environment of the Canglangpu formation be divided into (a) mixed sediments in the shore, which include mixed lagoon and mixed tidal flat, (b) mixed sediments in the shallow marine shelf, which includes mixed beach bar, terrigenous detrital mixed shelf, and carbonate mixed shelf; (4) a mixed shore-mixed shelf sedimentary model has been established; it says that the mixed sediments show obvious characteristics of ring-like development around underwater paleo-heights. The formation mechanism of mixed sedimentation in the eastern Sichuan basin is mainly controlled by the change of sedimentary facies belt, and the development intensity is mainly influenced by early tectonic movement and sea-level change.

Formation mechanisms and geological significance of the Lower Triassic fine-grained mixed sedimentary rocks in the Central Uplift of the South Yellow Sea Basin, eastern China

Formation mechanisms and geological significance of the Lower Triassic fine-grained mixed sedimentary rocks in the Central Uplift of the South Yellow Sea Basin, eastern China

Lithofacies and Shale Oil Potential of Fine-Grained Sedimentary Rocks in Lacustrine Basin (Upper Cretaceous Qingshankou Formation, Songliao Basin, Northeast China)

Shale oil has become a global hotspot of unconventional exploration and development. In this study, the latest drill core and experiment analyses of the Qingshankou Formation in the northern Songliao Basin were used to evaluate its lithofacies classification, sedimentary environment, pore types, pore-throat structure characterization, and shale oil potential. Lithofacies classification was determined according to the total organic carbon (TOC) content, sedimentary structure, and rock mineral content. Laminae genesis and micro-sedimentary structures indicate the deposition of fine-grained sedimentary rocks (FGSRs) in a semi-deep to deep lacustrine environment; however, evidence also suggests partial reworking by storm events and bottom current flows. FGSRs mostly comprise type I kerogen, with small amounts of type II1. The average vitrinite reflectance of the FGSRs was 1.37%, indicating middle to high stages of thermal maturation within the oil generation window. The N2 adsorption experiment indicated that silty mudstone (SM), silty fine mixed sedimentary rock (SFMR), and argillaceous fine mixed sedimentary rock (AFMR) had ink-bottle-shaped and slit-shaped pores, and the lithofacies were dominated by mesopores, accounting for 77.4%, 71.9%, and 80.8% of the total pore volume, respectively. Mercury injection capillary pressure analysis indicated that SM and SFMR had an average pore-throat radius of 0.01–0.04 μm, whereas AFMR and CM were dominated by nanopores, mainly distributed in the range of 0.004–0.0063 μm. Based on the comprehensive studies of TOC content, pore development, and brittleness, we concluded that organic-rich laminated SM and SFMR should be the focus of shale oil exploration of the Qingshankou Formation in the northern Songliao Basin, followed by organic-rich or organic-moderate laminated and layered AFMR, as well as calcareous fine mixed sedimentary rocks.

Multi-scale Pore Structure Characterization of Lacustrine Fine-Grained Mixed Sedimentary Rocks and Its Controlling Factors: A Case Study of Lucaogou Formation in Jimusar Sag

The microscopic pore structure of fine-grained mixed sedimentary rocks (FMSRs) is of great significance for the occurrence mechanism and sweet spot optimization of mixed shale oil. At present, there is no clear understanding of the microscopic pore system of FMSRs and their coupling relationship with rock composition, which restricts the fine understanding of the reservoir forming mechanism of this type of reservoir. Taking the FMSR samples of the Permian Lucaogou Formation in the Jimusar Sag, the southeastern margin of the Junggar Basin, China, as an example, this paper quantitatively characterizes the full-scale pore structure of lacustrine FMSRs by casting thin sections, scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), high-pressure mercury injection, and nuclear magnetic resonance (NMR); analyzes the coupling relationship between the full-scale pore structure and rock composition; and then summarizes the controlling factors. The results showed that the full-scale pore size distribution (PSD) can be revealed by combining the LTNA and NMR methods. The PSDs of FMSRs are evaluated, mostly corresponding to small pores (30–200 nm) and mesopores (200–1500 nm). The sediment grain size, mineral composition, sedimentary environment, and diagenesis jointly controlled the development of FMSR reservoirs. The development of coarser grain components (such as fine sand and arenaceous dolomite), high content of feldspar, low content of clay and calcite, and appropriate content of dolomite (less than 20%) were conducive to high-quality reservoirs. A near-provenance sedimentary pore throat is better. Siltstones, dolomitic siltstones, and dolarenites were mainly developed in shoreland sandy bar, distal sand bar, and dolomitic mud flat microfacies; type I pore throats with residual intergranular pores and dissolved pores are the most favorable for the Lucaogou Formation.

Evaluation of land resilience against natural disasters using ecosystem services approach in Kendari City, Southeast Sulawesi, Indonesia

Kendari City has multiple disasters hazard, especially floods and landslides. Consequently, the city requires systematic preventive efforts to minimize the potential disaster risk. This study aims to create a spatial model to determine the resilience of the land against floods and landslides with an ecosystem services approach. We used Sentinel-2 and DEMNAS (National Digital Elevation Model of Indonesia) as our primary data collection tools. Sentinel-2 was used to compile land use maps, and DEMNAS was the basis for compiling landform maps. The integration of the two was carried out by the analytic hierarchy process (AHP). The results showed that 8,259.98 acres (30.01 percent) of land in Kendari had low resilience to disasters. Those areas are located in a residential area on the TWH (Rocky hill over mixed sedimentary rocks) and KHY (Coalescent estuarine/riverine plain) landforms. The dominant disaster hazard in the area is inundation flooding that occurs almost every year in Kendari. Poor soil infiltration capacity is one factor affecting the area become vulnerable to flooding.

Reservoir Characteristics of Fine‐grained Sedimentary Rocks in Saline Lakes in the Cenozoic Upper Ganchaigou Formation, Southwestern Qaidam Basin, NW China

AbstractCenozoic fine‐grained sedimentary rocks in the southwestern Qaidam Basin, as typical salinized lacustrine sediments, are significantly different from those in other freshwater lakes under the impact of the high salinity in the lake. Many fine‐grained sedimentary rock samples were selected from the lower member of Neogene (N1) in the Gaskule Oilfield, Qaidam Basin, northwest China, with the aim to analyze and test their petrology, pore structure, and organic geochemical characteristics and determine its genesis and geological significance. Based on previous results, a new genesis of mixed sedimentary rocks is proposed. The Cenozoic lacustrine fine‐grained sedimentary rocks in SW Qaidam Basin belong to mixed sedimentary rocks, which can be divided into three types, including carbonate and terrigenous clastic‐bearing mudstone, carbonate‐bearing to terrigenous clastic mixed sedimentary rocks, and carbonate‐terrigenous clastic mixed sedimentary rocks. The geneses of mixing are quite different from that of mixed marine rocks. According to the sedimentary characteristics of the salinized lakes, three new geneses of mixed deposits are proposed: syn‐sedimentary mixing, varve interbedded mixing, and biogenic mixing. The reservoir spaces of the Cenozoic lacustrine fine‐grained sedimentary rocks in SW Qaidam Basin include residual intergranular pores, intercrystalline pores, microfractures, and minor dissolved pores. The reservoirs have super‐low porosity and ultra‐low permeability, with a porosity of 1.82%–21.04%, averaging 7.71%, and permeability of (0.0028–254.86) × 10–3 μm2, averaging 2.82 × 10–3 μm2. The test results of total organic carbon (TOC) content show that the highest content of organic carbon reached 1.41%, and those of vitrinite reflectance show that Ro ranges from 0.525%–0.824%, with an average of 0.806%. The organic geochemical characteristics show that the fine‐grained mixed sedimentary rocks of SW Qaidam Basin have the potential for self‐generation and self‐storage. As an unconventional reservoir, it has potential industrial value. This is the first study of fine‐grained sedimentary rocks in the Qaidam Basin, which offers certain theoretical and practical values.

Microbial properties in tropical montane forest soils developed from contrasting parent material—An incubation experiment

AbstractBackgroundSoil microbes are key drivers of carbon (C) and nutrient cycling in terrestrial ecosystems, and their properties are influenced by the relationship between resource demand and availability.AimsOur objective was to investigate patterns of microbial properties and their controls to understand whether they differ between soils derived from geochemically contrasting parent material in tropical montane forests.MethodsWe measured microbial biomass C (MBCSoil), potential extracellular enzyme activity (pEEA), and assessed microbial investments in C and nutrient acquisition at the beginning and end of a 120‐day laboratory incubation experiment using soils developed from three geochemically contrasting parent material (i.e., mafic, mixed sediment, and felsic) and three soil depths (0–70 cm).ResultsWe found that MBCSoil and pEEA were highest in soils developed from the mafic parent material. Microbial investment in C acquisition was highest in soils developed from mixed sedimentary rocks and lowest in soils developed from the felsic parent material. We propose that our findings are related to the strength of contrasting mineral‐related C stabilization mechanisms and varying C quality. No predominant microbial investment in nitrogen (N) acquisition was observed, whereas investment in phosphorus (P) acquisition was highest in subsoils. We found lower microbial investment in C acquisition in subsoils indicating relatively high C availability, and that microbes in subsoils can substantially participate in C cycling and limit C storage if moisture and oxygen conditions are suitable. Geochemical soil properties and substrate quality were important controls on MBCSoil per unit soil organic C (MBCSOC), particularly after the exhaustion of labile and fast cycling C, that is, at the end of the incubation.ConclusionAlthough a laboratory incubation experiment cannot reflect real‐world conditions, it allowed us to understand how soil properties affect microbial properties. We conclude that parent material is an important driver of microbial properties in tropical montane forests despite the advanced weathering degree of soils.

A Comparison of Geological Characteristics of the Main Continental Shale Oil in China and the U.S.

Abstract Shale oil is an important strategic resource in the future. The development and utilization of shale oil is a pivotal revolution in the field of fossil energy. The successful development in North America brings new thinking to the world. In recent years, there have been many definitions of shale oil, but no agreement has been reached in the petroleum industry. Marine shale oil is dominant in the United States (the U.S.), while continental shale oil is the counterpart in China. This study takes Uinta Basin, Ordos Basin, and Junggar Basin, for example, to make a comparison of the concept and geological characteristics of shale oil in continental basins between the U.S. and China. Different kinds of shale oil are generated due to the distinctions of sedimentary environments, hydrocarbon supply conditions, and sandy sediment development in different regions. High-quality source rocks have been developed in freshwater and salty water environments of continental lacustrine basins. The lithologies of the Uteland Butte Member, Chang 7 Member, and Lucaogou Formation are lacustrine carbonate rocks, clastic rocks, and mixed sedimentary rocks, respectively. Results indicate that the lower pressure can be identified in continental basins in China. Furthermore, the comparison shows that the Chang 7 Member characterizes lighter crude oil and larger scale of “sweet spot” and the Lucaogou Formation developed a thicker target layer, while the Chang 7 Member is at a disadvantage of insufficient stratum energy and the Lucaogou Formation is difficult in heavier crude oil and higher oil viscosity.

Micro/Nanopore Systems in Lacustrine Tight Oil Reservoirs, China.

A nanoscale pore throat system develops extensively in rocks of unconventional reservoirs serving as both source and reservoir rock. The nanoscale pores provide the main storage spaces, accounting for 70% to 80% of the total unconventional tight reservoirs in China. As one of most important unconventional petroleum accumulations, tight oil has accumulated in more than 20 lacustrine strata since the Permian in China. Three types of tight oil reservoirs were identified based on the lithology and provenance in the lacustrine basins, including terrigenous sandstone, endogenous carbonate rocks and mixed sedimentary rocks. The micro/nanopore structures of these tight rocks were investigated with the application of optical microscopy, scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), gas adsorption (GA) and nuclear magnetic resonance (NMR). The results indicated that the pore systems were connected by nanoscale throats dominated the storage spaces of the lacustrine tight oil reservoirs, while there were obvious differences among these three tight rocks, including pore types, pore size and movable fluid distribution. (i) The terrigenous sandstones, which were represented by the Triassic Chang 7 tight sandstones in the Ordos Basin and Cretaceous Quantou tight sandstones in the Songliao Basin, were mainly arkoses, and their storage space was mainly composed of dissolution pores and intraclay mineral pores. Feldspar, rock fragments and carbonate cements were the majority of the dissolved components, and the diameter of dissolution pores ranged from 1 micron to 50 microns. Abundant intrakaolinite and illite/smectite mixed layers pores were developed, and the pore size was 10 nm to 500 nm. The MICP and GA data suggested that storage spaces were connected by throats with diameters of 10 nm˜300 nm. (ii) The endogenous carbonate rocks, which were represented by the Jurassic Da'anzhai limestones in the Sichuan Basin, were the tightest rocks with porosities of less than 5% and permeabilities of less than 0.01×10-3 μm². The calcite dissolution pores and fractures with diameters of 10 nm˜500 nm were the most important storage spaces. The majority of pore systems were connected by throats with diameters of 6 nm˜100 nm based on the MICP and GA data. (iii) The tight mixed sedimentary rocks, which were represented by the Permian Lucaogou Formation in the Junggar Basin, were complex in lithologic composition, and dolostones and dolomite sandstones were the most important exploration targets. The interdolomite pores were the dominant storage spaces, in which abundant illite/smectite mixed layers were filled, and the pore size ranged from 50 nm to 50 microns. The MICP and GA data showed that the storage space was dominated by throats with diameters of 10 nm˜200 nm, and their volumetric contributions could reach over 70%. These results could provide a reference for future tight oil research and exploration in China.

Utilizing borehole electrical image and conventional logs to characterize petrology of mixed volcanic and sedimentary rocks in Jiamuhe Formation at JL2 Wellfield, Zhongguai Uplift, Junggar Basin, NW China

Lower Permian Jiamuhe Formation of JL2 Wellfield, Zhongguai Uplift, Junggar Basin, was constituted by mixed volcanic and sedimentary rocks. The rock types were various, and it is difficult to characterize the petrology just relying on limited cores in oil-bearing intervals. Based on core photos, petrological characteristics were synthetically interpreted using borehole electrical image and conventional logs in this paper. Six types of common volcanic rocks (rhyolite, dacite, andesite, basalt, tuff, and breccia) and three types of clastic sedimentary rocks (conglomerate, sand, and silt-clay) were represented by borehole electrical image log and conventional natural gamma ray (GR), resistivity (RT), neutron (CNL), and bulk density (DEN) logs. Two special volcanic textures, pillow structure and natural fracture, were also described in electrical images. Serial cross-plots in pairs of four conventional logs were formed to distinguish typical rocks, and the distinction between volcanic breccias and sedimentary conglomerates was analyzed in electrical images. Borehole electrical image and conventional logs can be efficiently used to supplement coring and petrological information, which provides important insights on the paleogeology of mixed volcanic and sedimentary rocks in Jiamuhe Formation at JL2 Wellfield.

Response of the weathering carbon sink in terrestrial rocks to climate variables and ecological restoration in China

The weathering carbon sink (CS) of rocks has a sensitive response to different influencing factors, and it is important to accurately distinguish this response in the global carbon cycle. However, no quantitative analysis of the response mechanism has been performed. In this study, the CS of the 12 types of terrestrial rocks in China from 2000 to 2014 is estimated using the GEM-CO2 model. The relative contribution rates of climate change and ecological restoration to the CS are quantitatively evaluated using the Lindeman-Merenda-Gold model. Results showed that: (1) The CS of terrestrial rocks in China was 17.69 Tg C yr−1, and the CS flux (CSF) was 2.53 t C km−2 yr−1; mixed sedimentary rocks had the highest CS (6.89 Tg C yr−1), and carbonate rocks had the highest CSF (5.8 t C km−2 yr−1). (2) The average annual CSF slightly decreased at a rate of 5.4 kg C km−2 yr−1; the areas of the CSF that decreased in the south were the areas where water budget decreased significantly, and it was the areas with a reduced water budget and ecological deterioration in the north. (3) The relative contribution rates of water budget and precipitation reached 57% and 35%, respectively; the response of the CSF to temperature was evident in areas with low or decreasing temperatures, and the influence of fractional vegetation cover (FVC) on the CSF in low value area was evident. (4) Mixed sedimentary rocks and carbonate rocks displayed a more evident reduction trend in the CSF than other rocks. This research verified the applicability of the GEM-CO2 model in China and presented a scientific basis for quantitative assessment of the impact of climate change and ecological restoration on the CSF.

Lithology identification and reservoir characteristics of the mixed siliciclastic-carbonate rocks of the lower third member of the Shahejie formation in the south of the Laizhouwan Sag, Bohai Bay Basin, China

In the Shahejie Formation in the D oilfield area of the Laizhouwan Sag, Bohai Bay Basin, a mixed sedimentary structure of terrigenous clastic and carbonate rocks has developed. Scholars at home and abroad have performed much research on clastic and carbonate lithology identification. However, studies on lithology identification and reservoir characteristics of mixed siliciclastic-carbonate rocks in the study area have not been conducted thus far. Due to the complex logging response characteristics of mixed sedimentary rocks, it is difficult for conventional lithology identification methods to clearly reflect the lithological characteristics of mixed siliciclastic-carbonate rocks. In this study, the sensitivity of acoustic time difference, natural gamma ray, density, deep lateral resistivity and compensated neutron logging curves to each lithology was analysed using the conventional cross-plot method in the D oilfield. On this basis, a decision tree method is used to further screen and identify the lithology layer by layer, and the process and results of identifying a mixed lithology with logging curves are clearly reported. In addition, imaging logging and core data are used to verify the identified lithology, and a lithology identification template and technical method consisting of conventional logging cross plots, decision trees, imaging logging and core calibration are established to improve the accuracy of lithology identification of mixed rocks. Moreover, the influence of different lithologies on the reservoir characteristics in the study area is analysed.

Seismic recognition and quantitative characterization of siliciclastic–carbonate mixed sedimentary rocks in the Bohaiwan Basin, Northeast of China

Mixed sediments often occur in continental lake basins due to the coupling effects of the climate, hydrodynamic conditions, tectonic processes, and material abundances of provenances, and the mixed sedimentary rock formed in this process has dual characteristics. The response of mixed sedimentary rock to seismic waves is different from that of conventional sandstones or carbonate rocks and easily causes errors in identification and characterization by conventional research methods. In this paper, taking the mixed sedimentary rock in the southern part of the Laizhouwan Sag in the Bohaiwan Basin as an example, the petrological characteristics of the mixed sedimentary rock are studied and its sedimentary model is established. On this basis, a comparative study of the seismic waveform characteristics is carried out. Compared with sandstone, the waveform continuity of mixed sedimentary rock is poor, the amplitudes are weak and the frequencies are low, but opposite results are found compared with the conglomerate formed by gravity flow at the bottom. Contrast analysis of the acoustic impedance shows that the value of the acoustic impedance of mixed sedimentary rocks is higher than that of sandstone and is generally greater than 6.2 × 106 kg/m3 m/s. Therefore, the threshold value between sandstone and mixed rock is set to 6 × 106 kg/m3 m/s in inversions to delineate the boundary between mixed sedimentary rock and sandstone. Finally, using seismic attribute analysis and inversion technology, the scales of the horizontal and vertical distributions of the mixed sedimentary rock platform are predicted. The range of the platform is 7100 m in length, 3100 m in width, and 15.6 km2 in area. Longitudinally, the mixed sedimentary rock layer spans the third layer of the third member of Shahejie Formation (ES33).

Fractal characteristics of the pore structures of fine-grained, mixed sedimentary rocks from the Jimsar Sag, Junggar Basin: Implications for lacustrine tight oil accumulations

The tight reservoir property and oil accumulation are greatly affected by the complexity and heterogeneity of pore structure. Fine-grained, mixed sedimentary formation samples from the Permian Lucaogou Formation in the Jimsar Sag were systematically studied focusing on their fractal characteristics and effects on the storage space, minerals and diagenesis, and its implication for tight oil accumulation. Fractal dimensions were calculated by the MICP model equation. The lacustrine fine-grained, mixed sedimentary samples are characterized by single and multi-fractal structures. Compared with the multi-fractal reservoir, the pore-throat structure of single fractal reservoir is more uniform and the pore throat size is relatively smaller. Minerals and diagenesis have apparent influences on fractal characteristics of the lacustrine MSR tight oil reservoir. The enrichment of terrestrial minerals can increase the heterogeneity of storage space and might be the reason for the multi-fractal pore structures. The enrichment of tuff in mixed sedimentary rock (MSR) can greatly improve the property and reduce the fractal dimension of tight reservoirs through dissolution. Large-pore filling calcite has dual effects on destroying reservoir space and reducing heterogeneity, which makes the weak correlation between calcite and fractal dimension. Fractal characteristics implicate that pore structures play a very significant role in controlling the accumulation of tight oil. MSRs with D or De more than 2.586 and porosity (average pore-throat radius) less than 6.3% (0.05 μm) are mostly incapable of effectively accumulating oil.

Numerical Data Modelling and Classification in Marine Geology by the SPSS Statistics

The paper focuses on the geostatistical analysis of the data set on the Philippine archipelago. The research question is understanding variability in several geospatial parameters (geology, geomorphology, tectonics and bathymetry) in different segments of the study area. The initial data set was generated in QGIS by digitizing 25 cross-sectioning profiles. The data set contained information on the geospatial parameters in the samples by profiles. Modelling and statistical analysis were performed in SPSS IBM Statistics software. The analysis of the topography shows strong variability of the elevations in the samples with the extreme depths in the central part of the study area (profile 13 with -9,400 m) and highest elevations in its south-western part (profile 17 with 1950 m). The analysis of the geological classes and lithology shows maximal samples of the basic volcanic rocks (40,40%) followed by mixed sedimentary consolidated rocks (31,90 %). Pairwise analysis of the sediment thickness and slope aspect demonstrates correlation between these two variables with the maximal sediment layer in the profiles 1-4 crossing the Philippines. The hierarchical dendrogram clustering of the bathymetry by three approaches shown maximal correlation of 5 clusters containing profile groups: 12-18 (centre), 22-25 (south-west), 1-2 (north), 7-8 (north-east), 19-21 (south-west). Other profiles show lesser similarities in the bathymetric patterns. The forecasting models were computed for the geospatial variables showing gradual increase in the gradient angles southwards and increased values for the sediment thickness in the north. Technically, the results proved effectiveness of the SPSS application of the geological data modelling. The paper focuses on the geostatistical analysis of the data set on the Philippine archipelago. The research question is understanding variability in several geospatial parameters (geology, geomorphology, tectonics and bathymetry) in different segments of the study area. The initial data set was generated in QGIS by digitizing 25 cross-sectioning profiles. The data set contained information on the geospatial parameters in the samples by profiles. Modelling and statistical analysis were performed in SPSS IBM Statistics software. The analysis of the topography shows strong variability of the elevations in the samples with the extreme depths in the central part of the study area (profile 13 with -9,400 m) and highest elevations in its south-western part (profile 17 with 1950 m). The analysis of the geological classes and lithology shows maximal samples of the basic volcanic rocks (40,40%) followed by mixed sedimentary consolidated rocks (31,90 %). Pairwise analysis of the sediment thickness and slope aspect demonstrates correlation between these two variables with the maximal sediment layer in the profiles 1-4 crossing the Philippines. The hierarchical dendrogram clustering of the bathymetry by three approaches shown maximal correlation of 5 clusters containing profile groups: 12-18 (centre), 22-25 (south-west), 1-2 (north), 7-8 (north-east), 19-21 (south-west). Other profiles show lesser similarities in the bathymetric patterns. The forecasting models were computed for the geospatial variables showing gradual increase in the gradient angles southwards and increased values for the sediment thickness in the north. Technically, the results proved effectiveness of the SPSS application of the geological data modelling.

Characteristics and formation mechanism of multi-source mixed sedimentary rocks in a saline lake, a case study of the Permian Lucaogou Formation in the Jimusaer Sag, northwest China

Abstract The Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin in China is an uncommon set of mixed sedimentary rocks. The mixed sediment content of the Lucaogou Formation is unusual due to the large amounts of tuffaceous material, which makes the system more complex than the commonly mixed siliciclastic-carbonate rocks. In this study, the volcanic material acts as a third end-member component in a multi-source mixed sedimentary system. Therefore, a new type of mixed siliciclastic-carbonate-tuffaceous rocks has been introduced. Methods including core observations, thin section analysis, grain size measurements, and X-ray diffraction are used to investigate the characteristics of mineral components, sedimentary structures and lithological assemblages in upper and lower sections of the formation. These analyses are then used to determine the lateral distribution area of mixed sedimentation. Sedimentary mixing processes originally described for siliciclastic-carbonate systems are used and expanded here for the multi-source mixed rocks in a saline lacustrine basin. Controlling factors on mixed sedimentation included climate change, lake level fluctuation, tectonic activity and sediment supply rate mentioned in previous studies, as well as variations of energy levels in different parts of the lake. Mixed-depositional models for the lower and upper sections of the Lucaogou Formation in saline lacustrine basin have been established, respectively.

Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz, northern Iran: implications for palaeoweathering, provenance, and tectonic setting

Combined petrographic and geochemical data of the siliciclastic sedimentary rocks from the Shemshak Group in the northeastern Alborz Mountains, north of Iran are described, together with their implications for palaeoweathering, their provenance, and tectonic setting. Based on field observations and modal composition, the sandstones are classified as litharenites. The chemical index of alteration (CIA) indicated that the source terrains underwent a moderate intensity of chemical weathering. The index of chemical variation (ICV) values indicated that the Shemshak Group rocks were immature and related to a source area with an active tectonic regime. Major, trace and rare earth element (REE) data suggested the domination of mixed sedimentary (recycled) and igneous rocks in the source area of the Shemshak Group. Petrographic and geochemical characteristics of Shemshak Group rocks suggest an active continental margin (ACM), which corresponds to the collision of the Iran plate with the Turan plate.