2017 Vol. 28, No. 6
Fine-grained sedimentary rocks are defined as rocks which mainly compose of fine grains ( < 62.5 μm). The detailed studies on these rocks have revealed the need of a more unified, comprehensive and inclusive classification. The study focuses on fine-grained rocks has turned from the differences of inorganic mineral components to the significance of organic matter and microorganisms. The proposed classification is based on mineral composition, and it is noted that organic matters have been taken as a very important parameter in this classification scheme. Thus, four parameters, the TOC content, silica (quartz plus feldspars), clay minerals and carbonate minerals, are considered to divide the fine-grained sedimentary rocks into eight categories, and the further classification within every category is refined depending on subordinate mineral composition. The nomenclature consists of a root name preceded by a primary adjective. The root names reflect mineral constituent of the rock, including low organic (TOC < 2%), middle organic (2% < TOC < 4%), high organic (TOC > 4%) claystone, siliceous mudstone, limestone, and mixed mudstone. Primary adjectives convey structure and organic content information, including massive or limanited. The lithofacies are closely related to the reservoir storage space, porosity, permeability, hydrocarbon potential and shale oil/gas sweet spot, and are the key factor for the shale oil and gas exploration. The classification helps to systematically and practicably describe variability within fine-grained sedimentary rocks, what's more, it helps to guide the hydrocarbon exploration.
Based on the observation of the well cores, thin section and FESEM, combined with X-ray diffraction, physical property testing and geochemical indicators, the reservoir characteristics and the controlling factors of the shale oil enrichment of the Es4s-Es3x shale in the Jiyang depression were detailed analyzed. Studies show that carbonate and clay minerals are dominated in the shale. According to the triangle chart, the TOC content (2% and 4%), carbonate and clay minerals, nine lithofacies have been identified. The reservoir space types are rich in the shale, in which, the laminated fractures, recrystallization intracrystalline pores and organic pores are high-quality reservoir spaces. The shale oil enrichment is mainly determined by the hydrocarbon-producing potential and reservoir capacity. The hydrocarbon-producing capacity is controlled by the organic geochemistry indicators, especially the TOC content for the study area, and the thickness of the organic-rich shale. The reservoir capacity is mainly affected by the lithofacies, the TOC content and the structural activities. In addition, the shale oil production is influenced by the fracability of the shale, which is mainly controlled by the lithofacies, structural activities, formation pressure, etc. The shale oil reservoir evaluation should focus on the TOC content, the thickness of the organic-rich shale, lithofacies and structural factor.
Fine-grained rocks (FGR) are the important source rocks and reservoirs of shale hydrocarbon which is the prospect hotspot at present. Widely distributed fine-grained sediments (FGS) of the upper fourth member of Shahejie Formation in Dongying depression are taken as an example to study the space-time evolution and controlling factor of FGS in this paper. Based on the analysis of well cores, thin sections, inorganic and organic geochemistry indicators, FGR are divided into 7 types of lithofacies. Through the study of 'point-line-plane', this study shows that FGS has the characteristics of rhythum, diversity and succession. The first stage is characterized by clayey FGS (massive claystone). The second stage is characterized by carbonate FGS (low-TOC laminated limestone) and dolomitic FGS (dolomitic-silty shale) formed by transgression. The third stage is characterized by organic-rich carbonate FGS (middle/high-TOC laminated limestone) distributed in cycle. The fourth stage is characterized by FGS mixed carbonate and siliciclastic sediments (calcareous-silty shale). A variety of space-time evolution of FGS are controlled by multiple factors including tectonism, climate and lake conditions.
Following shale gas, shale oil has become another highlight in unconventional hydrocarbon exploration and development. A large amount of shale oil has been produced from a host of marine shale in North America in recent years. In China, lacustrine shale, as the main source rock of conventional oil and gas, should also have abundant oil retained in place. In this study, geochemical and geologic characteristics of lacustrine shale from Es3L sub-member in Bonan sag were characterized by using total organic carbon (TOC), Rock-Eval pyrolysis, X-ray diffraction, and ∆log R method. The results show that the Es3L sub-member shale have TOC contents ranging from 0.5 wt.% to 9.3 wt.%, with an average of 2.9 wt.%. The organic matter is predominantly Type Ⅰ kerogen, with minor amounts of Type Ⅱ1 kerogen. The temperature of maximum yield of pyrolysate (Tmax) values ranges from 424 to 447 ℃, with an average of 440 ℃, and vitrinite reflectance (Ro%) ranges from 0.7% to 0.9%, indicating most of shales are thermally mature. The dominant minerals of Es3L shale in Bonan sag are carbonates (including calcite and dolomite), averaging 51.82 wt.%, and the second minerals are clay (mostly are montmorillonite-illite-mixed layer and illite) and quartz, averaging about 18 wt.%. Finally, its shale oil resources were evaluated by using the volumetric method, and the evaluation result shows that the shale oil resource is up to 5.94 billion tons, and mostly Class Ⅰ resource. Therefore, the exploration of the lacustrine shale oil of Es3L in Bonan sag should be strengthened.
Shale parasequence analysis is an important part of sequence stratigraphy sudies. This paper proposed a systematic research method for analyzing shale parasequences including their delineation, division, characteristics and origins. The division method is established on the basis of lithofacies. Multi-method analysis and mutual verification were implemented by using auxiliary indicators (such as mineral compositions, geochemical indicators and wavelet values). A typical shale parasequence comprises a lower interval of deepening water-depth and an upper interval of shallowing water-depth (e.g., a shale parasequence including a high-total organic carbon (TOC) shale-low-TOC limy shale). Abrupt increases in pyrite content, TOC value, relative hydrocarbon generation potential ((S1+S2)/TOC), and wavelet values are indicative of parasequence boundaries. The proposed research method was applied to study the upper fourth member of the Shahejie Formation in the Dongying depression, Bohai Bay Basin. Results show that there were seven types of parasequences developed. A singular and a dual structured parasequences were identified. Three factors controlling the development of the shale parasequences were identified including relative lake level change, terrestrial input and transgression. The development of high-TOC ( > 2%) shale parasequences was mainly controlled by biological and chemical sedimentation. The low-TOC ( < 2%) shale parasequences were mainly deposited by chemical sedimentation. The diversities of shale parasequences were caused by four major controlling factors including climate, relative lake level change, terrestrial input and emergency (e.g., transgression).
The Paleozoic Wufeng-Longmaxi shale is one of the main horizons for shale gas exploration in Sichuan Basin. Outcrop, core and thin section observations, X-ray diffraction analysis, trace element geochemistry and other methods have been used to understand the sedimentary characteristics and identify hydrocarbon source rocks in suitable sedimentary paleoenvironments in the Wufeng-Longmaxi shale in northern Guizhou Province. The thickness of the Wufeng-Longmaxi Formation ranges from 20 to 200 m and it was mainly deposited on a deep-water shelf. The TOC content is high, up to 5.75%. The main non-organic minerals are detrital quartz and clay minerals, with a little plagioclase feldspar, potassium feldspar, calcite, dolomite and pyrite. There is also biogenic microcrystalline quartz. Six lithofacies have been identified: siliceous shale, clay shale, calcareous shale, silty shale, carbonaceous shale, and muddy siltstone. Using biological Ba, V/(V+Ni), TOC, V/Cr, B, Sr/Ba and other indicators, we estimate primary productivity, redox conditions and paleosalinity and show that the early stage of Wufeng-Longmaxi deposition occurred under strong anoxic conditions, high paleosalinity and yielded a high TOC content and an excellent potential shale gas source. The anoxic environment was destroyed at the late stages of Wufeng-Longmaxi deposition, the TOC content decreased, so that it is likely to be a high quality source rock. Organic pores acted as the key reservoir space in the shales, and the pores are mainly mesopose, with most pore diameters less than 20 nm. The siliceous shale has high TOC content and brittle mineral (quartz) content making it an important exploration target for shale oil and gas exploration.
The Lower Permian Lucaogou Formation in the Jimsar sag, Junggar Basin is a typical tight-oil reservoir in China. For effective exploration and production, the formation of a high-quality reservoir must be thoroughly studied. In this work, the tight-oil reservoir was examined using a variety of methods, including core and thin-section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The tight-oil reservoirs were primarily deposited in saline lake environments, which are dominated by variable admixture of dolomite, quartz, feldspar, tuff, calcite and pyrite. Nine main lithofacies were identified: (1) siliceous mudstone, (2) dolomitic siliceous mudstone, (3) dolomitic mudstone, (4) intraclast packstone/grainstone, (5) ooid grainstone, (6) bioclast grainstone, (7) dolomitic siltstone, (8) mixed siliclastic and intraclast grainstone and (9) brecciated dolomitic mudstone. The pore types are classified into four categories: primary intergranular, moldic, intercrystalline and fracture pores. The properties of tight-oil reservoirs are quite poor, with low porosity (ave. 7.85%) and permeability (ave. 0.110 mD) and a small pore-throat radius (ave. 0.086 ǌm). The tight-oil reservoirs are dominated by the aggradation of a repetitive meter-scale sedimentary facies succession that records distinct lacustrine expansions and contractions. These tight carbonates have also undergone significant diagenetic alterations, such as dolomitization, dissolution, neomorphism and fracture created intercrystalline and moldic pores, vug and fractures; chemical and mechanical compaction and carbonate cementation have decreased the reservoir quality. Variations in reservoir quality in the Jimusar sag are due to a combination of lithofacies type, high-frequency cyclic depositional architecture, dissolution intensity, dolomitization and tectonic related deformation. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Jimusar fine-grained rocks.
Fine-grained carbonate rocks, which extensively occur in the Eocene strata in the Shulu sag, Bohai Bay Basin, North China, represent an unconventional, fine-grained carbonate reservoir. However, previous studies have ignored the complexity of the lithofacies components and their formation mechanisms. Fine-grained carbonate rocks are typical reservoirs in which hydrocarbons form and gather. A better understanding of the nature of these rocks is extremely important for evaluating the quality of unconventional, fine-grained carbonate reservoirs. Various lithofacies components were discriminated in this study with a combination of petrographic observations and carbon isotope analyses. These fine-grained carbonate rocks comprise terrigenous, biogenic and diagenetic materials. Terrigenous input and biologically induced precipitation are the main sources of the materials in the lake. Five lithofacies were identified based on the observations of sedimentary features (core and thin section) and mineralogical data: (1) varve-like laminated calcilutite, (2) graded laminated calcilutite, (3) interlaminated calcisiltite-calcilutite, (4) massive calcilutite, and (5) massive calcisiltite-calcarenite. Their origins were recorded by various lithofacies components, which are controlled by the interactions of physical, chemical and biological processes. This study indicated that the lithology of the bedrocks was the key factor controlling carbonate accumulation. The tectonics and climate can influence the weathering and types of lithofacies. Primary productivity controlled the precipitation of the endogenic calcite. These factors jointly determined the abundant fine-grained carbonate rocks that have accumulated in the Shulu sag.
This paper was conducted on the shale reservoir in the Dongying depression in the Shahejie Formation of Paleogene. Based on detailed core description, rock thin sections, argon ion polishing, scanning electron microscopy (SEM) analysis, CT scanning and carbon-oxygen isotopic test, the formation mechanism of the inter-laminar fractures (or seams) and their development conditions were discussed by comprehensive analysis of the diagenetic evolution features and the reservoir space evolution characteristics. The results show that the laminated composite pattern in Dongying depression was dominated by vertical distribution of laminated calcite and clay minerals. Contrasted to the traditional understanding, development degree of inter-laminar fractures was much lower. Pure fluid between layers cannot exist without framework minerals, and the pre-existing well connected pore or horizontal overpressure micro seam was favorable paths of fluid migration. From the points of inter-laminar fractures origin mechanism and its evolution, the so called seams, were much more complex than traditional understanding that hydrocarbon generation raised the pore pressure and led to the parallel microfractures. But the inter-laminar fractures had little relationship with that whether the lamination was developed or bedding boundary was clear or not. In fact, the shale reservoir inter-laminar fractures were not rigorous fracture. They were controlled by pre-existing inter-granular pore (or seam) or dissolved pore, which distributed along carbonate laminae boundary and were connected by later super pressure, dissolution and the seepage force. And the development conditions were burial depth (bigger than 3 000 m) and the Ro value ( > 0.6%). Discussion on its origin and evolution has important petroleum geological significance on optimizing reservoir segment and favorable target areas selection.
The conception of special geologic bodies has been applied in this study to define and illustrate economic oil and gas reservoirs in sandbodies, which is helpful for hydrocarbon forecast and exploration. Using the comprehensive analysis of drilling, logging, seismic, seismic attribute extraction, and logging constrained inversion in 3D data volume, the special geologic bodies of the Shahejie Formation (Ps1s) in northern Qikou sag, which have good hydrocarbon potential, are identified and described under the constraint of sequence stratigraphic framework. The dimensions, geometry, and spatial distribution features of the special geologic bodies, as well as their inner architectures and sequence-structure patterns, are also ascertained. The geologic bodies evolved from relative centralization in the middle part of the lacustrine basin in the early stage of Ps1s Formation, to several dispersive isolated parts in the later stage. This shows a small cycle interval (~2.5 Ma) at the end of the whole transgressive depositional process of Ps1 (~7.5 Ma) during the expansion of the lake and relative lake level rise, with the control of sediments supply and fault-related subsidence, which effect the evolution of the sedimentary system. According to the relevance and regularity of the geologic bodies' development, different types of potential profitable reservoir traps, including the lithologic lenticular traps, lithologic updip pinchout traps, and structural-lithologic composite traps, can be preserved in the study area. In addition, the internal architecture models provide scientific basis for further hydrocarbon exploration in the frontier basin without enough data.
This paper investigated the porosity controlling factors for tight sandstone reservoir in the Daniudi gas field, Ordos Basin based on an integrated petrographic, petrophysical and geostatistical analyses, and proposed a comprehensive prediction model for reservoir porosity. Compaction was found to be a key factor for causing reservoir densification. The degree of sandstone compaction appears to be affected by grain sizes and sorting. Under normal compaction conditions (e.g., cement content less than 6%, and with no dissolution), the variation in reservoir porosity with burial depth can be well correlated with grain compositions, grain sizes, and sorting. Based on qualitative examination of the controlling factors for reservoir porosities, geostatistics were used to quantify the effects of various geological parameters on reservoir porosities. A statistical model for comprehensive prediction of porosity was then established, on the assumption that the present reservoir porosity directly relates to both normal compaction and diagenesis. This model is easy to use, and has been validated with measured porosity data. The porosity controlling factors and the comprehensive porosity prediction can be used to quantify effects of the main controlling factors and their interaction on reservoir property evolution, and may provide a reference model for log interpretation.
Based on observations made on cores and cuttings from several wells in the lowermost part of the third member of the Shahejie Formation, several rock types, specifically clast-supported rudstone, matrix-supported rudstone, mixed-source rudstone, calcisiltite/calcarenite, massive calcilutite and laminated calcilutite, have been identified in the Shulu sag. According to the sedimentary structures and distribution characteristics of these rocks, the carbonate breccias fall into two categories, based on their origins: one formed by fan-delta channel sedimentation, whereas the other formed by earthquake-induced slump fan deposition. Clast-supported rudstone and matrix-supported rudstone are the main lithologies deposited by braided rivers in the fan delta plain and front, of which the pore space is mainly dissolution pores within gravels and tectonic fissures. Clast-supported rudstone, matrix-supported rudstone and mixed-source rudstone are the main lithologies of the earthquake-induced slump fans. These carbonate breccias developed along with soft-sediment deformation structures, which are interpreted as seismites and are widely distributed in the sag, in which intercrystalline pores, intergranular pores and fissures created from diagenetic shrinkage are developed. The two kinds of rudstones have different reservoir characteristics and oil/gas testing results. The rudstones generated in the fan delta have higher porosity and permeability, as well as better oil/gas testing results. Thus, they are key targets for petroleum exploration.
The Silurian stratigraphic sequence has recently become one of the most important exploration targets in the Tarim Basin, with a considerable amount of profitable hydrocarbon pools discovered in the central Tarim Basin. Previous exploration activities indicate that the Silurian stratigraphic sequence in the eastern Tarim Basin has great hydrocarbon exploration potential. The Silurian reservoirs comprise a set of tight marine sandstones, whose diagenetic sequence and genetic mechanism are still poorly understood. The complex relationship of hydrocarbon generation, the timing of the peak expulsion of the source rocks and the evolution of the reservoirs remains unclear. An integrated description and analysis have been carried out on core samples from eleven wells selected from the eastern Tarim Basin. A range of petrographic and geochemical analyses were conducted. By using an integrated approach with thin-section petrography, scanning electron microscopy (SEM), cathodoluminescence (CL), carbon and oxygen isotope geochemistry, formation water analysis, X-ray diffractometry (XRD), electron probe microanalysis and fluid inclusion microthermometry, the genesis and occurrence of individual diagenetic events were documented to reconstruct the diagenetic sequence and diagenetic model for the Silurian sandstone. Additionally, the tight nature of the Silurian reservoirs can mainly be attributed to the compaction processes and cementation. In particular, the destructiveness of the compactional processes to the original porosity is far greater than that from the cementation. Furthermore, fluid inclusion analyses also indicate that the Silurian sandstone has experienced three phases of hydrocarbon charge. The first two phases occurred during the eodiagenesis stage (from the Late Silurian to the Early Devonian and from the Late Carboniferous to the end of the Late Permian), when the Silurian sandstone was not tight and had a porosity of greater than 20%. The third phase occurred during the stage B of mesodiagenesis (since the Late Cretaceous), when the Silurian sandstone was fully tight.
Based on the sedimentary and tectonic background of the Termit Basin, this paper focuses on the Upper Cretaceous Yogou Formation and uses organic geochemistry, logging, oil testing and seismic data to analyze the primary control factors of the hydrocarbon accumulation and establish corresponding model in order to predict favorable exploration target zones of hydrocarbon reservoirs. This study demonstrates that the Upper Cretaceous Yogou Formation is a self-generation and self-accumulation type reservoir. The Yogou Formation hydrocarbon reservoirs in the Koulele area are controlled by four factors: (1) the source rock is controlled by a wide range of YS1-YS2 marine shale, (2) the sandstone reservoir is controlled by the YS3 underwater distributary channel and storm dunes, (3) migration of hydrocarbons is controlled by faults and the regional monocline structure, and (4) the accumulation of hydrocarbons is controlled by lateral seal. The structures in the western Koulele area are primarily reverse fault-blocks with large throws, and the structures in the east are dominantly fault-blocks with small throws (co-rotating and reverse) and a fault-nose. In the western Koulele area, where the facies are dominated by storm dunes on a larger scale, it is easier to form lithologic reservoirs of sandstone lens. In the eastern Koulele area, high-quality channel sandstone reservoirs, fault-blocks with small throws, and the monocline structure benefit for the formation of updip pinch out lithologic traps, fault lithologic reservoirs and fault-nose structural reservoirs. Future exploration targets should be focused in the western storm dunes zone and eastern distributary channel sand zone with small fault-blocks.
With the breakthrough of exploration in Well TP16-1, the lower Kepingtage Formation becomes a key target for petroleum exploration of deep clastic reservoir in Tahe area. In this paper we focused on the research of the reservoir characteristics and its controlling factors in two sub-member formations (S1k11 and S1k13). Based on X-ray diffraction, conventional physical properties data (porosity and permeability) and reservoir storage space data (casting thin section and scanning electron microscope), we determined that the S1k1 Formation belongs to extra-low porosity and permeability reservoir, although the upper S1k13 Formation shows relative better physical characteristic than the lower S1k11 Formation. The development of storage space in the study area is controlled by sedimentary microfacies, diagenesis process. Reservoirs in S1k1 Formation are mainly located in channel (S1k11 sandstones) and sand flat (S1k13 sandstones). The sand flat sediments with a more coarse grain size compared with the channel. In diagenesis, compaction is the major controlling factor for reducing the porosity, followed by cementation. Dissolution of diagenesis is the major controlling factor in enhancing the reservoir porosities. Compared with channel (S1k11) sandstones, sand flat sandstones (S1k13) have better reservoir quality for its weaker compaction, cementation and stronger dissolution. On the basis of sedimentary characteristics (grain size and subfacies), physical property (porosity and permeability) and reservoir storage space, we divide the S1k1 reservoir into three categories (Ⅰ, Ⅱ and Ⅲ). Type Ⅰ reservoir is high quality reservoir. It is mainly distributed in the south area of S1k11 and S1k13 reservoir. Type Ⅱ is moderate reservoir. It is located in the middle of S1k11 reservoir and in the north of S1k13 reservoir. Type Ⅲ is the poor reservoir. It is only located in the north of S1k11 reservoir.
Grain size analysis is a common method in the study of sedimentology. For the consolidated sedimentary rocks, the traditional methods are rock slice observation and image analysis. In recent years, laser particle size analyzer is used widely in particle size analysis of sedimentary rock. Unlike the pretreatment of loose samples, the rock samples must be crushed, added acid to wipe out cement, and washed. However, in the step of washing, most of the fines component (less than 63 μm) in the suspended state should be inevitably lost. It will significantly affect the accuracy of particle size analysis, especially for siltstone. This paper presents a siltstone sample pretreatment method which core step is washing acid by centrifuge. Compared with traditional decantation method, the results show that the median particle size reduced 33.2 μm on average. Compared with the precipitation method which is commonly used for handling loose samples, the change of solid-liquid separation time is from 12 h to 10 min, while the average reduction of median particle size is about 15 μm. The grain size value corresponded to the cumulative volume of 10%/90% reduced 2.5 μm/20.3 μm on average. The percentage of the clay component less than 2 μm increased 2.88% on average. The fine particle (2-4 μm) and silt component (4-63 μm) increased 1.71% and 5.56% on average. Based on this method, two kinds of similar lacustrine siltstone were analyzed. They are tempestite and beach bar which are difficult to identify in the Lijin sub-depression, Dongying depression, Shengli oilfield, China. The final grain-size probability plot of tempestite is the type of "one saltation component and three suspension components". The content of suspension components can reach to 80%-90%. The beach bar is the type of "one saltation component and two suspension components". The content of suspension components can reach to 40%-45%. They both have the characteristics of high slope which means well sorting. But they can be distinguished based on the suspension sedimentary characteristics which were preserved by maximum degree in this kind of sample pretreatment method.
The beach-bar reservoir has become an important exploration target in China, but its depositional mechanism and controlling factors have not yet been fully modeled. They have become an inhibitory factor for the exploration and development of beach-bar reservoirs. The depositional mechanism of beach-bars and their controlling factors have been studied by means of a flume experiment including seven runs under controlled boundary conditions which were the water level (Run 1, Run 2 and Run 3), wave parameters (Run 1, Run 4 and Run 5) and initial slope (Run 1, Run 6 and Run 7). The experiment revealed that the development of beach-bar was controlled by water level, wave parameters and initial slope. The deposited locations of distal bar and nearshore bar were controlled by the water level. Two beach-bars were migrated downward when the water level falls (Run 1, Run 2 and Run 3). The width and thickness of distal bar and nearshore bar were controlled by wave parameters, especially the wave height. They increased with the scale of wave. But, the maximum thickness is limited by the water level (Run 1, Run 4 and Run 5). The distance between distal bar and nearshore bar was controlled by the initial slope. It became shorter with the steeper slope. Distal bar and nearshore bar changed into one bar when the initial gradient was greater than 1/20 (Run 1, Run 6 and Run 7). The results suggest formative mechanism and controlling factors related to beach-bars.
As one of the most important source rocks and reservoirs of unconventional natural gas, the sedimentary environment and mode of peat swamp (the predecessor of coal seam) is important to the coal seam's spatial distribution, material composition, hydrocarbon generation potential, reservoir physical properties, etc. To reveal the depositional characteristics and history of environmental change in a terrestrial basin during a period of peat accumulation, the Middle Jurassic aged #7 coal from Gaoquan in the Qaidam Basin (NW China) was investigated using sedimentology, maceral composition, geochemistry and sequence stratigraphy. Based on identification of the sedimentary shoreline break belt, wave energy depletion point and position of wave base, the peat swamp system can be subdivided into (1) lakeside plain, (2) low energy lakeshore, (3) high energy lakeshore, and (4) shallow lake subfacies. A new method for determining coal facies is proposed based on the combination of environmental parameters including oxidation-reduction levels, energy conditions and the influence of terrigneous sediments. The evolution of the coal seam shows that peat was deposited mainly in the low energy lakeshore and lakeside plain subfacies. Five types of sequence stratigrpahic surface and two types of parasequence were identified. Forced lake regressions and normal lake regressions are attributed as the causes of sequence boundaries. The sequence stratigraphic framework comprises six sequences and corresponding system tracts, and the curve of base-level for each demonstrates a characteristic initial period of slow rising followed by fast rising and then returning to slow rising. A model indicating the relationship among base-level changes, coal facies evolution, and the environmental features in the swamp is proposed that shows the environmental features of the swamp were controlled by both base-level changes and coal facies. Accompanying depositional environment changes from a lakeside plain to lakeshore and shallow lake caused by increasing rate of base-level rise, water paleosalinity, acidity and the percentage of woody plants decrease, and the bog type alters from the low marsh to raised bog.
Beach-bars are well developed in the fourth member of the Shahejie Formation in the Shubei area, western depression of the Liaohe oilfield. The fourth member is composed of two three-order sequences, including a lower Sequence 1 (SQ1) and an upper Sequence 2 (SQ2). SQ2, which is the focus of this study, comprises a lowstand systems tract (LST) and a transgressive system tract (TST). Three types of beach-bars have been recognized: shallow-littoral lacustrine sandy beach-bars, semi-deep lacustrine storm-related sandy beach-bars and shallow-littoral lacustrine carbonate beach-bars. Paleo-environments during the deposition of SQ2, including the paleo-geomorphology, paleo-water depth and paleo-source have been reconstructed to determine the formation mechanisms of the different types of beach-bars. The shallow-littoral lacustrine sandy beach-bars formed mainly by waves and currents and were mostly deposited in the LST, where water depths ranged from 3 to 9 m and the terrestrial clast supply is sufficient. The storm-related sandy beach-bars developed in the TST in the southern part of the study area near previously massively deposited sand bodies where there was sufficient water depth to preserve them. The carbonate beach-bars formed primarily in the TST in the Shubei low buried hill belt where there was a lack of terrestrial clast supply and complex, uneven geomorphology that easily gave rise to a lagoonal environment.