Lithofacies Types Research Articles

Lithofacies characteristics and their controlling effects on reservoirs in buried hills of metamorphic rocks: A case study of late Paleozoic units in the Aryskum depression, South Turgay Basin, Kazakhstan

Metamorphic rock reservoirs are special reservoirs that are quite different from traditional sedimentary rock reservoirs and contain rich hydrocarbon reserves. However, their lithofacies characteristics and reservoir properties are relatively underexplored, and the key factor in reservoirs is lithofacies. Based on core samples, thin sections, whole-rock analysis and well-log and seismic data, we evaluated the lithologies, petrological characteristics and reservoir properties of metamorphic rocks and then explored the main factors controlling reservoir formation and metamorphic rock quality in the Aryskum depression, South Turgay Basin, Kazakhstan. In this area, four types of lithofacies are identified: granitic gneiss (high DEN and CN, low GR), leptite (high GR and CN), gabbro (low GR and CN) and granite (high GR and low CN); among these rocks, granitic gneiss is widely distributed. The reservoir spaces of metamorphic rocks are divided into fractures and pores produced via metamorphism and structural movements. Fractures can be subdivided into structural fractures (SFs), dissolution fractures (DFs), cleavage microfissures (CMfs) and microfissures (Mfs), and pores include dissolved intergranular pores (DInterPs), dissolved intragranular pores (DIntrPs) and vugs (Vgs). Numerous SFs and DFs provide the main reservoir spaces for hydrocarbons. The reservoir quality of metamorphic rocks is primarily controlled by faults, fractures, lithofacies, minerals, dissolution and paleogeomorphology. Lithofacies and minerals are the internal factors that control dissolution and fragmentation, while paleogeomorphology and faults are the external factors that control fracture development. Leptite and granitic gneiss are high-quality reservoir lithofacies, and high paleogeomorphology areas and fault zones are the optimal locations for fractures and pores. Reservoir spaces undergo a series of repeated changes from formation to development, filling and reformation, and a comprehensive reservoir evolution model is proposed to illustrate the distribution of high-quality reservoirs. This study has scientific significance for investigating the reservoir-forming mechanisms of metamorphic rocks and practical applications for exploring and developing oil and gas fields in areas with metamorphic rocks.

Sedimentary characteristics and model of gravity flows in the eocene Liushagang Formation in Weixi'nan depression, South China Sea

Gravity flows have been well investigated in deep-marine settings but less so in deep-lacustrine environments. To further understand the sedimentary characteristics and processes of gravity flows in a lacustrine rift basin, sublacustrine fans in the Weixi'nan Depression in the South China Sea were identified and studied by integrating core, well logging and 3D seismic data. Several aspects of gravity flow deposits were discussed in this study: (1) classification: seven lithofacies types were further grouped into four lithofacies assemblages/associations associated with emplacement by slumps, gravel-rich debris flows, high-density turbidity currents (which may transform into muddy debris flows) and low-density turbidity currents. (2) Proximal vs. distal position: the quantitative analysis of the lithofacies associations indicated that the gravity flow deposits are dominated by gravel-rich debris flow deposits and high-density turbidites in the proximal parts of the sublacustrine fans, whereas low-density turbidites dominate in the basin center. (3) Major triggers (humid climate and/or slope failures): the presence of active structures and a humid climate favored the formation of gravity flows, which originated from delta collapses and/or floods. (4) Lateral and vertical lithofacies distributions: the coarse-grained deposits represent a spectrum of different categories of gravity flows. In proximity to the slope break zone, slumps transformed into gravel-rich debris flows through liquefaction and deformation. As the gravity flows advanced toward the basin, the gravel-rich debris flows gradually became diluted with the surrounding water and transformed into high-density turbidity currents. When the flows reached the deep basin plain, the high-density turbidity currents transformed into low-density turbidity currents because the coarse grains could no longer remain in suspension. (5) An alternative mechanism: when the flows eroded the muddy substrate/basement, an alternative mechanism involving reverse-flow transformation occurred. In some cases, high-density turbidity currents transformed into muddy debris flows because the eroded light material became segregated toward the rear of the turbidity current and inhibited turbulence. This sedimentary model of gravity flows can be used for oil exploration in the study area and similar lacustrine rift basins.

Interpreting the Subsurface Lithofacies at High Lithological Resolution by Integrating Information From Well‐Log Data and Rock‐Core Digital Images

AbstractSpectral facies interpretation and classification methods have been proposed to improve the sophistication of interpretation of the subsurface heterogeneity. In the spectral facies interpretations, the intensity values of the RGB spectrum and the local entropy from rock‐core digital images are used, and the results are compared to conventional electrofacies and expert petrophysical interpretations. During the classification, a practically applicable model that identifies the more detailed types of lithofacies is constructed by using a multilayer neural network model, with the interpreted spectral facies and well‐log data from the corresponding depths used as response and explanatory variables, respectively. Core digital images and five types of well‐log data from the Satyr 5 well in Western Australia are applied for the actual implementation. Through comparative interpretations, three spectral facies are identified as separable lithofacies (i.e., shale, shaly‐sandstone, and sandstone lithofacies), which is supported by detailed HyLogger mineralogy along the tested cores. On the other hand, two electrofacies (i.e., shale‐dominant and sand‐dominant facies) are identified by a conventional method. In the classification based on the spectral facies, the trained multilayer neural network model showed high prediction accuracy for all the lithofacies. Based on these observations, it is confirmed that more precise lithofacies interpretation and classification can be conducted with the developed methods. The developed methods have the potential to improve subsurface characterization when high lithological resolution is essential.

Pore Structure and Fractal Characteristics of Different Shale Lithofacies in the Dalong Formation in the Western Area of the Lower Yangtze Platform

The purpose of this article was to quantitatively investigate the pore structure and fractal characteristics of different lithofacies in the upper Permian Dalong Formation marine shale. Shale samples in this study were collected from well GD1 in the Lower Yangtze region for mineral composition, X-ray diffraction (XRD), and nitrogen adsorption–desorption analysis, as well as broad-ion beam scanning electron microscopy (BIB-SEM) observation. Experimental results showed that the TOC (total organic carbon) content and vitrinite reflectance (Ro) of the investigated shale samples were in the ranges 1.18–6.45% and 1.15–1.29%, respectively, showing that the Dalong Formation shale was in the mature stage. XRD results showed that the Dalong Formation shale was dominated by quartz ranging from 38.4% to 54.3%, followed by clay minerals in the range 31.7–37.5%, along with carbonate minerals (calcite and dolomite), with an average value of 9.6%. Based on the mineral compositions of the studied samples, the Dalong Formation shale can be divided into two types of lithofacies, namely siliceous shale facies and clay–siliceous mixed shale facies. In siliceous shale facies, which were mainly composed of organic pores, the surface area (SA) and pore volume (PV) were in the range of 5.20–10.91 m2/g and 0.035–0.046 cm3/g, respectively. Meanwhile, the pore size distribution (PSD) and fractal dimensions were in the range 14.2–26.1 nm and 2.511–2.609, respectively. I/S (illite-smectite mixed clay) was positively correlated with SA, PV, and fractal dimensions, while illite had a negative relationship with SA, PV, and fractal dimensions. I/S had a strong catalytic effect on organic matter for hydrocarbon generation, which was beneficial to the development of organic micropores, so I/S was conducive to pore structure complexity and the increase in SA and PV, while illite easily filled organic pores, which was not beneficial to the improvement of pore space. In clay–siliceous mixed shale facies, which mainly develop inorganic pores such as intergranular pores, SA and PV were in the range of 6.71–11.38 m2/g and 0.030–0.041 cm3/g, respectively. Meanwhile, PSD and fractal dimensions were in the range of 14.3–18.9 nm and 2.563–2.619, respectively. Quartz and I/S showed weak positive correlations with SA, PV, and fractal dimensions. The various compact modes between quartz particles and the disorder of I/S were conducive to the complexity of pore structure and the improvement of SA and PV. The research findings can provide a reference for the optimization and evaluation of shale gas favorable area of the Lower Yangtze Platform.

Shale pore structure characteristics of the high and low productivity wells, Jiaoshiba shale gas field, Sichuan Basin, China: Dominated by lithofacies or preservation condition?

Abstract Both the characteristics of lithofacies and tectonic movements are important factors in controlling the development, destruction, and preservation of pores in shale reservoirs. However, the main factors that control the structure of shale pores are complex and highly disputed, restricting the understanding of the mechanisms that lead to the accumulation of shale gas. In this study, mineral composition and geochemical analyses, high resolution field emission scanning electron microscopy (FE-SEM), and low-pressure gas adsorption (N2 and CO2) were conducted to evaluate the pore systems of different types shale in the Jiaoshiba area, Sichuan Basin. A total of three shale groups, including silica-rich high production shales, clay-rich low production shales, and silica-rich low production shales were identified on the basis of lithofacies and gas-bearing characteristics. The highest values of TOC, pore volume, and specific surface area are found in the silica-rich high production shales, with an average of 4.182%, 33.04 × 10−3 cm3/g, and 38.71 m2/g, respectively. The total pore volume and specific surface area in the clay-rich low production shales are 19.85% and 28.62% lower than that of silica-rich high production shales. This suggests that the types of lithofacies have a more prominent influence on the specific surface area of the shales, while the pore volume plays a smaller role. However, the pressure in silica-rich low production shales is often released due to strong tectonic deformation. The OM pores in the silica-rich low production shales therefore tend to be both compressed and closed, with low surface porosity and of round shape. The total pore volume and specific surface area of silica-rich low production shales are 34.63% and 22.0% lower than that of the silica-rich high production shales. This indicates that tectonic movement has a significant influence on the shale pore volume, while specific surface area has a smaller influence.

Study on the lithofacies of the second member of Tengger formation in Jiergalangtu of Erlian Basin

Through analyzing coring well observation and in combination with drilling data log sheet and the analysis of the test data, combined with the regional sedimentary tectonic evolution background of the second member of Tengger formation in Jiergalangtu of Erlian basin, the results show that lithic arkose and feldspathic lithic sandstone are developed in the study area. There are abundant sedimentary structures, mainly including parallel bedding, laminated bedding and massive bedding, etc. According to the analysis of sedimentary characteristics, nine lithofacies types and four vertical assemblages are summarized, which respectively reflect different microfacies environments: subaqueous distributary channel subaqueous distributary channel interestuarine bar sheet sand.

Architecture of a sandstone uranium reservoir and the spatial distribution of its internal carbonaceous debris: A case study of the Zhiluo Formation, eastern Ordos Basin, northern China

The sandstone of the Middle Jurassic Zhiluo Formation, in the eastern Ordos Basin, provides the opportunity for conducting a critical case-study of the architecture and internal carbonaceous debris (CD) distribution of a sandstone uranium reservoir located in a field outcrop in Shiwan. The 16–24 m thick outcrop section is a sandstone-dominated subaqueous distributary channel succession, in which six lithofacies types (Se, St, Sm, Sp, Sh, and Fl) occur in a predictable order of repeated fining-upward cycles. These facies emphasize that the dominant structures had been deposited under lower flow regime conditions. Various groups of associated facies have been identified as six distinct architectural elements (intrachannel units, longitudinal bar, transverse bar, diagonal bar, dune complex, and abandoned channel deposits) within a framework of five orders of bounding surfaces. These elements, with specific lithofacies associations and spatial relationships, indicate that the subaqueous distributary channel had involved in four stages of formation: channel-occurring, channel-flooding, channel-waning, and channel-abandonment. Moreover, as one of the most significant reductants in the sandstone succession, the CD occurs in the form of channel lags and is concentrated in the middle and lower parts of the lag deposit zones. The spatial distribution of CD within the studied succession is controlled by the sandstone architecture and the CD-rich zones are preserved exclusively upon the basal surfaces of intrachannel units, longitudinal and transverse bars. This study can be helpful for exploring the distribution of metallogenic flow field as well as the restriction of the reductants on interlayer oxidation.

Sedimentary evolution and chemostratigraphy of the post-Sturtian cap carbonate-like Dolomie Tigrée Formation (Katanga Supergroup) in the Democratic Republic of the Congo

This study provides new constraints on the paleoenvironmental, eustatic and carbon isotopic models of the cap carbonate-like rocks of the Dolomie Tigrée Formation in the Katanga region, Democratic Republic of the Congo. The study integrates detailed lithofacies analysis combined with sequence stratigraphy to reveal the high-resolution architecture framework of deposition. Eight lithofacies types are defined: shale, siltstone and sandstone (LF1), dolosiltite (LF2), dolarenite (LF3), massive dolomudstone (LF4), laminated dolomudstone (LF5), intraclastic dolopackstone (LF6), laminar dolomudstone (LF7), and fenestral dolopackstone (LF8). These lithofacies types show that the Dolomie Tigrée carbonate succession accumulated on a distally steepened slope to proximal carbonate ramp setting. The sequence stratigraphy analysis revealed 17 elementary parasequence sets (5th order) that are grouped into 3 distinct successive marine transgressive cycles (PSQ1 to PSQ3) of 4th order magnitude. Each cycle records, from the base to the top, a dominant transgressive system tract, vertically-upward evolving to short time highstand and lowstand system tracts bounded by a sequence boundary. PSQ1 records an early marine transgression marked by the negative shift in δ13C values near −5‰ at the base, then returned to positive values in the upper part of PSQ1 and PSQ2. This carbon isotopic excursion is here interpreted as the hydrological and tectonic reorganizations of outer-ramp-derived allopadic materials from deeper to shallow carbonate-oversaturated reservoirs, coupled with a glacial-isostatic rebound, which enhanced the development of biogenic carbonates in a proximal carbonate ramp setting. Our results show that the changes of sea level triggered the carbonate deposition occurring during the marine transgression. This trangression was related to extensional tectonics of the Nguba rift basin, which were possibly enhanced by the isostatic deglacial rebound in the aftermath of the Sturtian glaciation event.

Depositional environment and pore structure of mixed lithofacies shale of the Longmaxi Formation in the DM Block, the Southern Sichuan Basin, China

Distinguishing the differences of pore characters between different mixed lithofacies shales is helpful for improving shale gas development efficiencies. In this study, the targeted Longmaxi shale ( L1) of the Southern Sichuan Basin was selected as the research object. Two kinds of mixed lithofacies shale were identified by analyzing total organic carbon and X-ray diffraction results. The forming depositional environment of mixed lithofacies shale was researched by elements analyses. Pores in different mixed lithofacies shale were observed using field emission scanning electronic microscope. Low-field nuclear magnetic resonance and low-temperature nitrogen adsorption were conducted to analyze pore characters of different mixed lithofacies shale. The results showed that L1 were mainly composed of organic rich clay–siliceous mixed shales (OR-M-1) and organic extreme rich calcareous–siliceous mixed shales (OER-M-3). OR-M-1 and OER-M-3 were formed in high paleo-producing dysoxic–oxic seawater and anoxic waterbody, respectively. Micro-pore and meso-pore volumes of OER-M-3 were greater than those of OR-M-1 while macro-pore volume of OER-M-3 was lower than that of OR-M-1. Meso-pore surficial and structural complexities of OER-M-3 were greater than those of OR-M-1. OER-M-3 were greater in oil-wetting micro-pore structural complexities while lower in water-wetting micro-pore structural complexities, compared with OR-M-1. The inherent relationships between lithofacies type and sedimentary environment, total organic carbon, as well as pore characters, respectively, were quite close.

Facies and Architectural Element Analysis of Braided Fluvial Succession: The Tertiary Sawah tambang Sandstone, Sawahlunto, Indonesia

The Sawahtambang Sandstone Formation of Sawahlunto, Indonesia has hitherto never been studied in detail to ascertain its depositional processes and its palaeoenvironment. The Sawahtambang Sandstone has more than 394 m thickness which consists of conglomeratic sandstone, sandstone, and clay-silt intercalation within which six lithofacies types (St, Sp, Sh, Ss, Fl, and Fm) appear creating iterated fining upward sequence. Herewith, this paper aims to replenish the avantgarde sedimentological interpretation of the Sawahtambang Sandstone in the intermontane basin of Indonesia based on outcrops along the Trans-Sumatra Highway. The study is attained through facies identification based on stratigraphic measuring section or outcrop profile, studio analysis consists of log analysis and superposition reconstruction, and architectural element delineation according to facies association. Structures in clay-silt facies consist of lamination and fissile whereas in the sand grained include planar and trough cross-bedding, planar lamination and granule lag, which depicts a lower-flow-regime sand channel. The architectural elements that made from facies association are identified into five distinct elements (multi-storey channel, single storey channels, sand bedforms, laminated sand sheet, and floodplain fines), which present a fluvial environment. The relation of the architectural elements demonstrates that the Sawahtambang Sandstone uphold a record of the braided river system that flowed from southwest to northeast in the southwestern part of Ombilin Basin. The dominance of coarse-grained (channel and sandy bedform) element over clay-silt grained (laminated sand sheet and floodplain fines) element and the extensive appearance of thick amalgamated channel elements reinforce the interpretation of a low-sinuous braided fluvial system in which the stacking pattern shows the channel bodies accumulated both lateral via lateral accretion of point bar and via vertical amalgamated streams.

Facies Analysis of Eocene Sediment of Umuahia Area, Southeastern Nigeria

This research work is the detailed facies analysis of the depositional environments and paleogeographic setting of the Eocene sedimentary sequence (Ameki Formations) exposed in the Umuahia area and paleoclimate during that periods. The study area was mainly concentrated around Amaudara inUmuahia South and Ekeoha in Umuahia North. And the co-ordinate are as follows,location-1 0.5°30.80N, 0.7°26.93E, location-2 0.5°30.39N, 0.7°26.62E, location-3 0.5° 32.83N, 0.7°27.24 E and location-4 0.5°32.19 N, 0.7°26.13 E.
 The aim of the study is to analyze the detailed sedimentary facies and describe the depositional environment in other to predict the depositional environment of the Eocene sediment (Ameki Formation) of the study area, which is underlain by rock unit of Ameki and predominately contains Laterite, mudstone, siltstone, claystone, sandstone and shale and Burrows were identified.
 The rock sequence consist of reddish lateritic material, highly weathered mudstone capped with ripped bedded kaolinite clay unit, light grey claystone, cross-bedded sandstone with claystone, whitish sandstone, siltystone, fine-medium grained sandstone with pockets of mudclast capped with ferruginized ground and dark grey shale.
 On the basis of gross lithology, sand-silt-clay percentage, color, texture and assemblage of sedimentary structure, eight distinct lithofacies type were recognized, grey shale facie (Gs), clay stone facie (Cs), cross-bedded sandstone facie (Cbs), mudstone facie (Mf), lateritic facie (Lf), mudstone facie (Bms), ferruginized sandstone facie (Fsf), sandstone facie (Bsf) are recognized within the lithosuccesion.
 From the analysis, the facies are grouped into two facie association on the basis of grain size. The Fine-grained facies association (FFA) which consist of Gs, Cbs, Cs, Mf and Fst and the Medium to Fine-grained facies association (MFA) which also consist of Bms, Bsf and Lf. It also shows medium grained sand, moderately sorted to well sorted sandstone, Skewness ranged from symmetrical to positive skewed and kurtosis showed leptokurtic.
 Deduction from facies analysis and grain size analysis shows that Ameki Formation consist of foraminifera and Mollusca which indicate that Ameki Formation was deposited in the estuarine(Marine) environment.

Origin of deep-water fine-grained sediments as revealed from the Lower Cretaceous rifting basin sequence in the Lingshan Island, Yellow Sea, Eastern China

Understanding the sediment dynamics responsible for the deposition of fine-grained sediments is essential for studying fine-grained sediment origin and its distribution, which is of a great significance for unconventional oil and gas exploration and development. Fine-grained sediments from a Lower Cretaceous rift-basin sequence in the Lingshan Island, the middle of the Dengta section in the Lingshandao Formation were studied. On the basis of detailed outcrop investigation, thin section observation, and laboratory measurements, seven types of lithofacies and three lithofacies associations were identified in the fine-grained sediment sequence. The sediment dynamics were deduced from a comprehensive examination and interpretcation of the sedimentary structures and lithofacies associations. The fine-grained sediments in the research area belong to a silt dominated subclass, which is composed mainly of thin-bedded siltstones with normal grading and mudstones with horizontal lamination. Suspension settling is responsible for the development of horizontal lamination in some mudstone. The Bouma-like lithofacies associations are developed by downslope fine-grained turbidity currents. The hybrid event bed-like lithofacies associations are formed by hybrid flow, while turbidity currents transformed to muddy debris flows downslope. The interbedded thin sandstone and mudstone banded lithofacies associations may be resulted from a downslope transitional flow between a turbidity current and a muddy debris flow. The vertical facies stacking patterns of the fine-grained sediment sequence indicatec that the fine-grained gravity flow may have been evolved from a transitional flow to a debris flow then to a low-density turbidity flow and finally to a hybrid flow. Lithofacies associations deposited by hybrid flow and transitional flow are potential sweet spots for unconventional oil and gas exploration in such fine-grained sediment formation.

Sedimentological and diagenetic control on the reservoir quality of deep-lacustrine sedimentary gravity flow sand reservoirs of the Upper Triassic Yanchang Formation in Southern Ordos Basin, China

Abstract The Upper Triassic deep-lacustrine sedimentary gravity flow sandstones (Yanchang Formation, Southern Ordos Basin) form an important oil reservoir, however, it is considered as a low permeability to tight reservoir. This study combines petrographic, mineralogical and geochemical data in order to better constrain the controls on reservoir quality. Two major lithofacies types (lithofacies 1 - sandy debrite; lithofacies 2 - turbidite) were identified and their origin relating to deep-lacustrine gravity flows have been addressed. The gravity flow sandstones are significantly diagenetically altered, including mechanical compaction, authigenic clay formation, carbonate and quartz cementation and dissolution. In addition, volcanic tuffaceous dust alteration is one of the important processes during burial, which provided silica for quartz cementation and that easily transformed to authigenic smectite. It also formed poorly crystallised kaolinite and random mixed-layer I/S clays. The relatively negative δ13C (with an average of −2.3‰) and δ18O values (with an average of −17.0‰) of carbonate cements in the gravity flow sandstones reflect decarboxylation of organic matter as one of the important carbonate sources. The factors controlling reservoir quality relate to depositional and diagenetic processes. Depositional factors (e.g. grain size, grain sorting) had a stronger influence on lithofacies 2 sandstones compared to lithofacies 1, which made lithofacies 2 sandstones more easily compact and lose most of its primary porosity. Samples with less intergranular volume (IGV) indicate that mechanical compaction plays an important role in reducing reservoir quality. However, cementation (clay minerals and carbonate cementation) affected sandstones of lithofacies 1 to a stronger degree, heavily destroying reservoir quality while in this lithofacies dissolution (e.g. feldspar, volcanic fragments and minor quartz) significantly improved reservoir characteristics in contrast to lithofacies 2 sandstones. Quartz cements could stabilize the framework and preserve the open porosity, however, they are present with relatively low contents and have limited effect in influencing the reservoir quality. In summary, lithofacies 1 sandstones possess a slightly better reservoir potential in the study area. All these factors have great implications for tight oil exploration and production of the Yanchang Formation in the Southern Ordos Basin which can serve as an example for other tight sandstone gravity flow reservoirs elsewhere in the world.

Lithofacies and reservoir characteristics of Permian volcanic rocks in the Sichuan Basin

In order to provide basis for further exploration and deployment of volcanic gas reservoirs in the Sichuan Basin, we classified Permian volcanic rocks in this basin using drilling, mud logging, assay, logging and seismic data comprehensively. Then, the lithological characteristics, lithofacies characteristics, lithofacies distribution and reservoir characteristics of different types of volcanic rocks were studied. Besides, the main factors controlling the development of volcanic reservoirs were determined. And the following research results were obtained. First, volcanic rocks in the Sichuan Basin can be comprehensively classified into 4 categories (i.e., volcanic lava, volcaniclastic lava, volcaniclastic rocks and sedimentary volcaniclastic rocks), and basalt, breccia lava, tuffaceous breccia lava, tuff, sedimentary tuff are mainly developed. Second, 3 types of volcanic lithofacies are extensively developed in the Sichuan Basin, including overflow facies, effusive facies and volcanic sedimentary facies. Overflow facies is mainly dominated by basalt, effusive facies is mainly composed of volcaniclastic lava, and volcanic sedimentary facies is mainly composed of tuff and sedimentary tuff. Third, volcanic reservoirs are lithologically composed of breccia lava and basalt. Breccia lava reservoirs are pore-type reservoirs of moderate porosity and low permeability with devitrified micropores and dissolution pores as main reservoir space. In basalt, fractures and dissolution pores are mainly developed, and they are porous-fractured reservoirs with lower matrix porosity. Fourth, the factors controlling volcanic reservoirs mainly include lithology, lithofacies, later fluid alteration and tectonism. Volcaniclastic lava is devitrified in the process of rapid condensation, and a great number of dispersion-shape micropores are formed to provide favorable conditions for further dissolution and pore enhancement in the later stage. Post-diagenesis effect (e.g. dissolution) and fracture development make the reservoir properties better. In conclusion, the areal distribution of volcanic reservoirs in the Sichuan Basin is mainly controlled by the lithofacies. Porous-fractured basalt reservoirs are mainly developed in the Ya'an–Leshan–Pingshan area in the southwestern Sichuan Basin, and pore-type volcaniclastic lava reservoirs are mainly developed in the Chengdu–Jianyang–Santai area.

Application of Fisher discriminant method in the facies division of lake facies-Taking the third member of Shahejie Formation in Dongying Sag as an example

ABSTRACTA large number of lacustrine mudstones were formed in the third member of Shahejie Formation in the Paleogene in the DongyingSag. It is the main source rock and lithologic reservoir development layer in Dongying area. Based on the viewpoints of the predecessors, core observation, and logging data analysis, the lithologic characteristics of the third member of Shahejie Formation (including mineral content, sedimentary structure and organic matter content) were summarized. With the logging method, the logging curves such as GR, AC and R25 were selected to establish the recognition mode. Fisher discriminant analysis was performed using SPSS software to establish a quantitative partitioning scheme. The experimental results show that the lacustrine mudstones in the third member of Shahejie Formation in Dongying Sag can be subdivided into seven types of lithofacies, each of which has different identification characteristics; the discriminant equation can be established through discriminant analysis to distinguish different types of facies. It provides a research idea for the identification and prediction of lithofacies for the well sections lacking coring data and logging data.

Lithofacies classification and origin of the Eocene lacustrine fine-grained sedimentary rocks in the Jiyang Depression, Bohai Bay Basin, Eastern China

There are multiple sets of lacustrine source rocks in the Paleogene of the Jiyang Depression, Bohai Bay Basin, Eastern China, which are main targets in the terrestrial shale oil exploration. Compared with marine shales, lacustrine fine-grained rocks are more complex in their mineralogical compositions, sedimentary structures and vertical stacking patterns. Therefore, it is of great theoretical and practical significance to carry out petrological characterization and lithofacies classification in lacustrine fine-grained rocks. Based on observations in cores and thin sections as well as elemental and organic geochemical analyses, this study systematically analyzes petrological characteristics of Eocene lacustrine fine-grained sedimentary rocks in the Jiyang Depression from perspectives of mineralogical compositions, sedimentary structures and organic matter abundance. Three end members (carbonate minerals, clay minerals and terrigenous clastic minerals), together with organic matter abundance and sedimentary structure, are employed to establish a lithofacies classification scheme that is suitable for fine-grained rocks in the Jiyang Depression, Eastern China. A total of 7 lithofacies types are recognized, such as the organic matter-rich laminated calcareous mudstone lithofacies and the moderately organic matter-rich layered calcareous mudstone lithofacies. Through analysis of lithofacies from the bottom to the top of the drilling well, sixth-order sequence units are found to have good correspondence to lithofacies assemblages. Based on consideration of magnetic susceptibility, mineral content, and geochemical proxies, these sixth-order sequence units are divided into three types of lithofacies assemblages, namely the Type A lithofacies assemblage corresponding to the dominantly rising base level, Type B lithofacies assemblage corresponding to the dominantly falling base level, and Type C lithofacies assemblage with the isostatic base level. Among them, the Type C lithofacies assemblage is the most favorable for shale oil exploration and development. Results of this study are expected to not only deepen understanding of lithofacies and its genesis and origin of lacustrine fine-grained rocks, but also provide a theoretical basis for shale oil exploration and development.

Linkage between calcirudite lithofacies and storm-generated depositional processes in the Upper Cambrian Series, Shandong Province, eastern China

Ideal carbonate tempestites consist of five units from base to top: (i) calcirudite with a scoured base and basal gutter casts; (ii) normally graded calcarenite; (iii) parallel-laminated calcarenite; (iv) hummocky cross bedded to wavy-laminated calcarenite-calcisiltite; and, (v) horizontal-laminated mudstone. However, the calciruditic part remains relatively poorly documented in terms of its lithofacies characteristics and relationship to storm-generated depositional processes. Based on outcrop observation, we address this knowledge gap by analyzing the petrologic and sedimentary properties of over 100 intervals of calcirudite from carbonate tempestites in Upper Cambrian strata in Shandong Province, eastern China. Fair-weather deposits formed in a carbonate ramp recording three episodes of relative sea-level rise and fall, corresponding to the Middle, lower-Upper and upper-Upper Cambrian Series, respectively. Calcirudite tempestite intervals, which are embedded in fair-weather deposits, are classified into five types of lithofacies according to their petrologic and sedimentary structures: horizontal- to imbricate-aligned calcirudite (LF1); perpendicularly-aligned calcirudite (LF2); well-rounded sand-supported massive calcirudite (LF3); poorly-rounded matrix-supported massive calcirudite (LF4); and, poorly-rounded tightly-stacked calcirudite (LF5). These lithofacies are interpreted, respectively, as gravel shoal deposits linked to storm waves and storm-enhanced tides, gutter cast-infilling deposits linked to storm vortices, high-density turbidites, debrites, and collapse deposits mainly linked to storm back-flow. Storm depositional successions are dominantly constructed by single beds of LF1, and to a lesser extent by scour-based successions comprising LF1, LF3, LF4, or LF5 passing upward into graded gravelly calcarenite to calcarenite, capped with parallel- to hummocky cross-stratified calcisiltite to calcarenite. The considerable variation in the tempestite successions suggests that storm-generated depositional processes differed between the inner ramp, mid-ramp, and out-ramp sub-environments. Significantly, this study recognizes carbonate gravelly shoal deposits, which are seldom documented in the literature. Moreover, the abundance of gutter casts strongly supports the importance of storm vortices as a depositional process in carbonate tempestite generation. The linkages between carbonate tempestite lithofacies and storm-generated depositional processes demonstrated herein can be used to improve interpretations of paleoenvironments and paleogeography in other carbonate successions around the world and through geological time.

Early Carboniferous extension regime in South Tianshan: Constraints from alluvial fan of Bashisuogong Formation in Sepabayi area in Xinjiang, China

The Bashisuogong Formation, exposed along the southern margin of the Chinese South Tianshan and north‐western edge of the Tarim Block, is divided into the lower member conglomerates and sandstones and upper member limestones. In the present contribution, four lithofacies types are recognized and further grouped into two distinct lithofacies sub‐associations based on sedimentary features of the alluvial fan of the Bashisuogong Formation in Sepabayi area. The debris flow products, with lenticular clast‐supported conglomerate, dominate the proximal fan. Couplets composed of conglomerate and sandstone represent the middle fan. The sedimentary features and palaeocurrent data indicate that clasts are sourced from the north‐east adjacent area. We propose that the Bashisuogong Formation, with an upward‐fining facies sequence pattern in this study, reveals an extensional regime during the Early Carboniferous. This fits within the context of synchronous volcanism and Bashisuogong Formation in north‐west Tarim. This tectonic setting resulted in the onset of the Carboniferous to Permian sedimentary cycle. The sedimentation centres were confined to Tuogemaiti‐Wushi band, where the alluvial fan was deposited. The Carboniferous transgressive succession controlled by the former graben shoulders thins progressively south‐eastward in north‐west Tarim.

Depositional cycles of the Lower Miocene Rudeis Formation (southwestern offshore margin of the Gulf of Suez, Egypt): Implications for reservoir evaluation

The Lower Miocene Rudeis Formation in the Zeit Bay Field (Gulf of Suez, Egypt) has been studied by analysis of cores from six boreholes. The objective was to detail the sedimentary cycles in the formation and their implications for the reservoir characteristics, as well as to obtain a more detailed insight into the precise age by establishing a foraminifer zonation. The formation, which was found to be some 60–191 m thick, is composed almost exclusively of various types of carbonates. These contain foraminifers that prove a Burdigalian age, and that also provide insight into the depositional environments, which turned out to form three belts. These belts represent an intertidal lagoon, barrier shoals, and a reef complex, respectively, indicating a down-dip depositional profile of a shallow marine rimmed carbonate shelf. Analysis of the cyclicity pattern of the Rudeis Formation, supported by facies analysis and CycloLog curves, makes it possible to distinguish three superimposed 3rd-order depositional cycles, which are subdivided into several higher-order cycles that are strongly controlled by eustatic sea-level fluctuations. A set of lithofacies isopach maps has been constructed for these cycles with the objective to facilitate interpretation of the spatial distribution of the reservoir quality for hydrocarbons. The maps show that the reservoir quality of the Rudeis Formation is heterogeneous because of the highly variable characteristics of the various lithofacies types.

The fluid mud flow deposits represent mud caps of Holocene hybrid event beds from the widest and gentlest shelf

The internal fabric of mud caps, which are on top of sediment gravity flow deposits, remains unexplored so far. We identified four hybrid event beds on one of the widest and gentlest shelf (East China Sea Shelf). Three types of lithofacies, including massive sand, chaotic mud and homogeneous mud, were distinguished. Homogeneous mud represents mud caps of these hybrid event beds. Scanning electron microscopy performed in both homogeneous mud and chaotic mud has revealed the presence of mud aggregates (mud clasts) conforming sand-size clasts. Chemical analyses of the aggregates show the presence of abundant gypsum/anhydrite and clay mineral components as well as relicts of quartz, muscovite and feldspar, suggesting an origin related to erosional events and an intraclast origin. The vertical shortening, deformation together with the pore spaces of mud clasts indicate a shallow burial depth of erosion, where mud sediments were still unconsolidated and water-saturated. No background sedimentation is present in the mud caps, because we did not observe any bioturbated interval in core images and X-radiographs. The lack of laminated muds and synsedimentary trace fossils in the homogeneous muds suggest an origin related to fluid mud flows. As proved on previous experiments, the mud caps could be related to head erosion and shearing of the muddy debris flow/mud flow's upper surface which generated a slower moving fluid mud flow. Our study further confirms that no laminated sand/silt (H4 division) would be present in hybrid event beds with mud-dominated H3 division. Head erosion of muddy debris flow/mud flow would not allow laminated H4 division to form, because the fluid mud flow derived from the parent mud-dominated debris flow was mainly composed of grains smaller than 30 μm which can effectively damp turbulence effectively. Grain size analysis performed on samples from fluid mud flow deposit fall into pelagites/hemipelagites area (T area), suggesting that fluid mud flow deposits are also a candidate for T area, and the area does not necessarily reflect a deposition under un-stressed environment.