Raoyang Sag Research Articles

A Method for Predicting the Action Sites of Regional Mudstone Cap Rock Affecting the Diversion of Hydrocarbons Transported along Oil Source Faults

Regional mudstone cap rock has an important influence on the oil and gas distribution of the oil source faults below it. Therefore, studying the influence of these mudstone cap rocks on the hydrocarbon distribution pattern is fundamental to understanding the oil and gas distribution of the lower generation and upper reservoir reservoirs in the Bohai Bay Basin. This study classified two types of hydrocarbon diversion from oil source faults: blockage diversion and seepage diversion. To locate them, we established a method to predict the areas with blockage diversion and seepage diversion separately by superimposing the sealing and leakage parts of the regional mudstone cap rock with the regions of the connected distribution of sand bodies and the favorable hydrocarbon transport sites of the oil source faults, respectively. We used this approach to predict the locations where hydrocarbons are diverted by the oil source faults under the regional mudstone cap rocks in the first and second sections of the Dongying Formation (E3d1-2) in the Liuchu area of the Raoyang Sag, Bohai Bay Basin. The results show that the regional mudstone cap rock’s blockage diversion occurs mainly in the south-central area of Liuchu, with a localized distribution in the northern part. The seepage diversion site is primarily located in the northeastern area and is also found locally in the west. Both diversions are beneficial for the accumulation of hydrocarbons from the source rocks of the first member of the Shahejie Formation (E3s1) to the upper second member of the Dongying Formation (E3d2U). The latter can also accumulate hydrocarbons in the Guantao Formation (N1g). The results align with the hydrocarbon distribution, demonstrating the feasibility of our method to predict various oil source fault diversion sites under the regional mudstone cap rock. This prediction method offers valuable guidance for exploring the lower generation and upper reservoir hydrocarbon accumulations in hydrocarbon-bearing basins.

Development and influencing factors of middle‐deep tight sandstone reservoir ‘sweet spots’ in fault basin—Taking the Eocene Shahejie Formation in Suning area of Raoyang Sag in Bohai Bay basin as an example

This study takes the tight sandstone of the lower sub‐member of the first member of the Eocene Shahejie Formation (Es1x) in the Suning area, Raoyang Sag, Bohai Bay Basin. Furthermore, it determines the rock type of the middle‐deep tight sandstone through lithologic profiling, core observation and log data analysis. The genetic control factors of tight sandstone in the study area are analysed using conventional thin section, pore‐casted thin section and scanning electron microscope, and the characteristics and genetic control factors of tight sandstone reservoirs are summarized. The research results show that the tight sandstones in this area are primarily feldspathic sandstone and feldspathic lithic sandstone, with high quartz contents, around 61.50%. The average porosity of the reservoir in the study area is 4.69%, and the average permeability is 2.92 mD, which are characteristics of low‐porosity and low‐permeability reservoirs. Through research, it has been found that the distribution of sedimentary facies and diagenesis in the study area control the development and distribution of ‘sweet spots’. Both compaction and cementation reduce the physical properties of the reservoir, while dissolution can increase porosity, thereby increasing the physical properties of the reservoir; dissolution resulted in an increase of approximately 3.57%–3.69% in the porosity of the tight reservoir in the study area, with an average porosity increase of 3.63%, and is the main controlling factor for the development of ‘sweet spots’ in the reservoir. The results provide an important basis for guiding the exploration and development of tight sandstone oil and gas in the lower sub‐member of the first member of the Eocene Shahejie Formation (Es1x)in the Suning area.

Seismic sedimentology response characteristics of nearshore terminal fan in the suning area of Raoyang Sag, Bohai Bay Basin, NE China

A nearshore terminal fan is a special water system formed in arid environments. The characterisation of its thin-channel sand bodies has long been a challenge restricting oil and gas exploration. This study takes the Suning area of the Raoyang Sag as an example and uses the principles of seismic sedimentology to conduct seismic sedimentary research on the nearshore terminal fan of the first member of the Palaeogene Shahejie Formation (Es1) based on three-dimensional seismic, logging, and core analysis. Seven fourth-order sequences (SQV7) were identified within Es1, deposited by a fluvial river system terminating at the contracting bank of a lake. Prograding terminal fan sedimentary facies on a gentle slope zone were observed in the root mean square seismic attributes after spectral decomposition. We have successfully resolved the sandstone within the studied terminal fan system using a 90° phase conversion of the seismic data and red-green-blue (RGB) fusion of the various seismic attributes. The upper subsegment of the Shahejie Formation developed extensive nearshore terminal fan sedimentation, and the seismic sedimentological response characteristics were mainly channel-like and strip-shaped geomorphic systems deposited on gentle slope zones, indicating distributary channels and distal basin sedimentation. This study enriches our understanding of nearshore fans and provides ideas for predicting favourable sand bodies in this type of sedimentary facies.

Micro formation mechanism of oil–water inversion in reservoirs based on water film theory—An example from the Dawangzhuang Oilfield in Bohai Bay Basin

In oil fields, the inverted oil–water phenomenon of “water above oil”, which cannot be explained by the traditional principle of gravity differentiation. In hydrophilic reservoirs, water films easily form on rock particle surface. Water films reduce the effective pore throat radius and the space between pores and throats, generate disjoining pressure, and significantly reduces the seepage capacity of pore fluids, which leading to the phenomenon of oil water inversion. Based on laboratory experiments, a model of the relationship between the disjoining pressure -water film thickness was established, and the dynamic mechanism of oil–water inversion was studied using water-film theory. The main factor controlling oil–water inversion is the interaction between the dynamic (buoyancy) and resistance (capillary force and disjoining pressure) forces of oil–water migration and the configuration relationship with the average pore throat radius. The effects of the water film thickness and pore throat radius on the disjoining pressure and capillary resistance were quantitatively analysed, and the critical pore radius model for oil–water inversion was determined. Combining the capillary pressure curve and pore throat distribution, a practical and feasible theoretical technology and method for judging whether oil–water inversion occurs in a reservoir was established. 22 sets of experiments were conducted to verify the reliability and sensitivity of the disjoining pressure - water film thickness model. Taking the Dawangzhuang Oilfield, Raoyang Sag, Bohai Bay Basin, China as an example, through laboratory experiments, specific strata and reservoir data were obtained in the study area, and the disjoining pressure, capillary resistance, and buoyancy were calculated using the disjoining pressure - water film thickness model. By comparing the relationship between the buoyancy and the sum of the equilibrium disjoining pressure and capillary resistance, combined with the distribution of the pore throat radius, it is determined whether there is oil water inversion phenomenon. The practical application based on the theory and method of water film thickness has strong promotion potential for medium–low porosity and permeability reservoirs with the phenomenon of oil water inversion distribution of “water above oil ” whcih can effectively guide oil fields to further select favorable exploration and development targets.

Lower limit of effective reservoir physical properties and controlling factors of medium-deep clastic reservoirs: a case study of the Dawangzhuang area in Raoyang sag, Bohai Bay Basin

With the continuous improvement of oil and gas exploration in the middle and shallow layers, the deep Paleogene in Raoyang sag has become an important exploration field of “increasing reserves and production” in North China. The recognition and control factors of deep effective reservoirs are unclear, which restricts the recognition effect of oil and gas exploration. The key to identify effective reservoirs is to determine the lower limit standard of reservoir physical properties. The single method for calculating the lower limit of physical properties has limitations. The lower limit of the effective reservoir physical properties of the Shahejie Formation in the Dawangzhuang area was obtained by comprehensively using physical property, well logging and oil test data. The data were analysed by oil testing method, metre oil production index test method and frequency curve intersection method. On this basis, combined with the cast thin-section observations, scanning electron microscopy and other test results, the comprehensive control of effective reservoir development of the Shahejie Formation in the Dawangzhuang area of the Raoyang sag was studied from various factors, such as formation pressure, sand body thickness and diagenesis. The results show that the lower limit of porosity was 9.73, 9.44 and 8.85% at depths of Es1, Es2 and Es3, respectively. The lower limit of permeability was 1.21 × 10–3, 1.18 × 10–3 and 0.59 × 10–3 μm2, respectively. Effective reservoirs are easier to form in areas with formation pressure coefficient greater than 1.2. Formation overpressure inhibits compaction and promotes dissolution. The proportion of effective reservoirs of sand bodies with thicknesses greater than 2 m can reach more than 75%. The influence of diagenesis on the reservoir is mainly manifested in compaction and cementation making the reservoir compact. The porosity reduction rate caused by compaction can reach 20–75%, while dissolution makes the reservoir form secondary pores. The average pore throat radius of secondary pores can reach 4 ~ 6.3 µm. This study makes use of the applicability of different methods, which is more instructive for predicting the effective reservoir of the Shahejie Formation in the study area. In addition, the research results provide a reference for the development mechanism of medium-deep clastic reservoirs.

The Research on Complex Lithology Identification Based on Well Logs: A Case Study of Lower 1st Member of the Shahejie Formation in Raoyang Sag

Lithology identification is the basis for sweet spot evaluation, prediction, and precise exploratory deployment and has important guiding significance for areas with low exploration degrees. The lithology of the shale strata, which are composed of fine-grained sediments, is complex and varies regularly in the vertical direction. Identifying complex lithology is a typical nonlinear classification problem, and intelligent algorithms can effectively solve this problem, but different algorithms have advantages and disadvantages. Compared were the three typical algorithms of Fisher discriminant analysis, BP neural network, and classification and regression decision tree (C&RT) on the identification of seven lithologies of shale strata in the lower 1st member of the Shahejie Formation (Es1L) of Raoyang sag. Fisher discriminant analysis method is linear discriminant, the recognition effect is poor, the accuracy is 52.4%; the accuracy of the BP neural network to identify lithology is 82.3%, but it belongs to the black box and can not be visualized; C&RT can accurately identify the complex lithology of Es1L, the accuracy of this method is 85.7%, and it can effectively identify the interlayer and thin interlayer in shale strata.

Lithofacies Characteristics and Controlling Factors of Fine-Grained Sedimentary Rocks in the Lower First Member of the Shahejie Formation in the Northern Lixian Slope of Raoyang Sag, China

In order to further explore the potential of unconventional oil and gas in the northern Lixian Slope of Raoyang Sag, the lithofacies types and their controlling factors of the lower first member of the Shahejie Formation (Es1L) in the Xiliu area were analyzed through the comprehensive application of core log studies, micro-sections and their magnified analyses, mineral suits recognitions with XRD, and major and organic geochemistry. The results show that the lacustrine sedimentary rocks of Es1L in the Xiliu area comprise five lithofacies types: medium organic silty mixed fine-grained rock, medium organic massive clay rock, high organic shale calcareous mixed fine-grained rock, high organic matter shale calcareous clay rock, and high organic matter shale limestone. The vertical variation of the characteristic element reflects that the deposition in Es1L has different stages; that is, the bottom of Es1L is formed with low source influence, high water salinity, and strong reduction conditions, while the upper fine-grained rocks are formed in a sedimentary environment with high source influence, low water salinity, and weak reduction conditions. The change of the sedimentary environment controls the development of lithofacies and the occurrence of organic matter. These results advocate for the basic research of fine-grained sedimentary rocks in the faulted lacustrine basin aid for further unconventional oil and gas exploration.

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

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

Method and Application of Reservoir Unit Division in Continental Faulted Lacustrine Basin: An Example from the L70 Fault Block in Raoyang Sag.

The continental faulted lacustrine basin is an important type of continental oil and gas basin with typical compound oil and gas accumulation characteristics. The traditional petroleum geological theory usually makes a general analysis of multiple reservoirs. It uses the structural model to understand the reservoir and has a vague understanding of the reservoir. The actual exploration is inconsistent with the traditional theory, which restricts exploration and development. In this case, taking the lower submember of Sha 1 of Shahejie Formation of L70 fault block in Raoyang sag as an example, the concept of reservoir unit is put forward, and the reservoir is redivided using the "four diagrams and four steps" method, the oil-water distribution is clarified, the type of reservoir unit is defined, and multiple potential reservoir units are proposed. It enriches and perfects the traditional petroleum geological theory, provides a new research method and idea for petroleum exploration and development, and ushers in new opportunities for the developed old oil fields.

Controls on and geological models of hydrocarbon accumulation in gentle slopes caused by exceptionally weak tectonic activities: A case study of the Lixian Slope, Raoyang Sag, Eastern China

The Lixian Slope is located in the western part of the Raoyang Sag, Bohai Bay Basin, Northern China. This slope was developed due to exceptionally weak tectonic activities; therefore, it is aptly called a weak tectonic slope. The slope gradients, faults, sandbodies, source rocks, and oil-source correlation were evaluated. During the development of the Lixian Slope, its gradients were usually low (generally less than 1.00°). Most of the faults have low fault throws (usually less than 30 m). Moreover, the faults of this slope have poor vertical transporting capacity. The sandstone-to-strata ratios for different Paleogene strata of the Lixian Slope are generally less than 40%, suggesting that the connectivity of sandbodies in the Paleogene strata is poor. In addition, research on the sources of crude oil indicates that the oil in most slope areas originated from Es1x source rocks. The study of the controls on hydrocarbon accumulation in the Lixian Slope indicates the following: 1) the planar distribution of oil and gas reservoirs in the Lixian Slope is strictly controlled by both the Es1x and Ek–Es4 source rocks, the Es1x in particular; 2) the sandbodies, faults, and low gradients control the planar enrichment of hydrocarbons; and 3) the faults and source rocks control the vertical distribution of oil and gas reservoirs. According to the characteristics of and controls on hydrocarbon accumulation, a hydrocarbon accumulation model suitable for a weak tectonic slope is proposed. Finally, a few recommendations for future explorations are suggested. The Es2 and Es1 members, especially those near the NNE–NEE trending faults, are critical target intervals for future exploration operations.

Quantitative characterization of hydrocarbon charging driving force and resistance during reservoir-forming stages: a case from the Maxi subsag, Raoyang sag

Oil and gas migration and accumulation are different manifestations of driving force and resistance during the evolution process of oil-gas pool formation; thus, determining the relationship between the driving force and resistance during oil-gas pool formation is of great significance in exploring for oil and gas in subsags. Taking the Maxi subsag in the Raoyang sag as the study object and simulating surplus paleopressure evolution and quantitatively characterizing the lower limits of displacement pressure of the effective reservoirs, the evolution process of driving force and resistance of hydrocarbon charging during various stages was reproduced, and the relationship between the driving force and resistance of hydrocarbon charging in the subsag was determined. The paleopressure evolution of the subsag underwent three stages: initial accumulation—release—reaccumulation. The lower limit of the effective reservoir displacement pressure gradually increased as the physical properties of reservoirs worsened. The hydrocarbon driving in the subsag can be divided into three types: overpressure driving, buoyancy driving and hybrid driving. The subsag region had two periods of oil-gas pool formation with the different overpressure driving range and hybrid driving range.

Coupling Relationship between Lithofacies and Brittleness of the Shale Oil Reservoir: A Case Study of the Shahejie Formation in the Raoyang Sag

Analyzing the characteristics of rock brittleness in low-permeability mudstone and shale (MS) formations is imperative for efficient hydraulic fracturing stimulation. Rock brittleness depends on the mineral composition, organic matter abundance, and bedding structure. Based on the MS from Shahejie Formation mineral composition (clay mineral, felsic mineral, and calcareous mineral contents), total organic content, and bedding structure (laminated, laminar, and massive), six types of lithofacies were identified: clay-rich MS, felsic-rich MS, calcareous-rich MS, clay MS, felsic MS, and calcareous MS. The quartz, feldspar, calcite, and dolomite of the Shahejie Formation are brittle minerals. Consequently, lithofacies with high felsic and calcareous mineral contents are more brittle. In addition, laminated and laminar MS are also conducive to hydraulic fracturing. Therefore, laminated, organic-rich, and calcareous-rich MS are the dominant lithofacies for hydraulic fracturing in the Shahejie Formation. The lithofacies and brittleness index were predicted by the response characteristics between mineral compositions and logging curves. The 3521–3552 m section of well B11x is dominated by calcareous-rich MS with developed laminae, representing a favorable section for hydraulic fracturing. Fragile minerals and oil are widely developed in the lower part of the lower 1st member of the Shahejie Formation (Es1L) in the southwestern part of Zhaohuangzhuang-Suning, where hydraulic fracturing can be used to increase shale oil production.

BIOMARKER GEOCHEMISTRY OF PALEOGENE CRUDE OILS AND SOURCE ROCKS FROM THE RAOYANG SAG, BOHAI BAY BASIN, NE CHINA: AN OIL – SOURCE ROCK CORRELATION STUDY

This study presents a systematic geochemical analysis of Paleogene crude oils and source rocks from the Raoyang Sag in the Jizhong sub‐basin of the Bohai Bay Basin (NE China). The geochemical characteristics of fifty‐three oil samples from wells in four sub‐sags were analysed using gas chromatography (GC) and gas chromatography – mass spectrometry (GC‐MS). Twenty core samples of mudstones from Members 1 and 3 of the Eocene‐Oligocene Shahejie Formation were investigated for total organic carbon (TOC) content and by Rock‐Eval pyrolysis and GC‐MS to study their geochemistry and hydrocarbon generation potential. The oils were tentatively correlated to the source rocks.The results show that three groups of crude oils can be identified. Group I oils are characterized by high values of the gammacerane index and low values of the ratios of Pr/Ph, Ts/Tm, 20S/(20S+20R) C29 steranes, ββ/(ββ+αα) C29 steranes, C27 diasteranes/ C27 regular steranes and C27/C29 steranes. These oils have the lowest maturity and are interpreted to have originated from a source rock containing mixed organic matter deposited in an anoxic saline lacustrine environment. The biomarker parameter values of Group III oils are the opposite to those in Group I, and are interpreted to indicate a highly mature, terrigenous organic matter input into source rocks which were deposited in suboxic to anoxic freshwater lacustrine conditions. The parameter values of Group II oils are between those of the oils in Groups I and III, and are interpreted to indicate that the oils were generated from mixed organic matter in source rocks deposited in an anoxic brackish–saline or saline lacustrine environment.The results of the source rock analyses show that samples from Member 1 of the Shahejie Formation were deposited in an anoxic, brackish – saline or saline lacustrine environment with mixed organic matter input and are of low maturity. Source rocks in Member 3 of the Shahejie Formation were deposited in a suboxic to anoxic, brackish – saline or freshwater lacustrine environment with a terrigenous organic matter input and are of higher maturity. Correlation between rock samples and crude oils indicates that Group I oils were probably derived from Member 1 source rocks, while Group III oils were more likely generated by Member 3 source rocks. The Group II oils with transitional characteristics are likely to have a mixed source from both sets of source rocks.

A New Evaluation Method of Shale Oil Sweet Spots in Chinese Lacustrine Basin and Its Application

Compared with the marine shale from North America, Chinese lacustrine basins have more complex geological and reservoir conditions, which makes the selection of sweet spot intervals in the shale oil reservoir particularly difficult. It is thus crucial to accurately predict the shale oil sweet spots for effective exploration and development of shale oil in a lacustrine basin. In this paper, we propose an innovative evaluation method of shale oil sweet spots, which considers five primary parameters (i.e., total oil content, movable oil ratio, reservoir pressure coefficient, permeability, and rock brittleness index) to construct a comprehensive weighting factor, which is used to quantitatively identify a favorable shale oil reservoir. This method firstly constructs an evaluation function for each of the parameters, and then calculates a comprehensive weighting factor to determine the shale oil sweet spot. Statistical results show that the oil production of formation testing intervals have a good positive correlation with the average value of the comprehensive weighting factor of the intervals, which verifies the feasibility of the method. Based on this method, one of the key exploratory wells, Qiang21 in the Raoyang Sag of Bohai Bay basin, was selected as a case study and was determined to be a sweet spot interval of the shale oil reservoir in the upper third member of the Shahejie Formation. This study provides a new way to obtain a favorable exploration interval of shale oil reservoirs and serves shale oil development.

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

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

Diagenesis and Origination of Carbonate Cements in Deeply Buried Sandstones of the Eocene Es3 Member, Raoyang Sag, Bohai Bay Basin, China

Diagenesis is one of the most predominant factors controlling reservoir quality in the deeply buried siliciclastic sandstones of the third member in the Eocene Shahejie Formation (Es3), in the Raoyang Sag, the Bohai Bay Basin. In this study, thin section, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectrum, carbon and oxygen isotopes, and fluid inclusion analyses are used to restructure paragenetic sequences and detect origins of carbonate cements recorded in this deeply buried member. Based on petrographic analyses, the Es3 sandstones are identified as lithic arkoses and feldspathic litharenites at present, but derived from original arkoses and lithic arkoses, respectively. Geohistorically, the Es3 sandstones have undergone two diagenetic episodes of eogenesis and mesogenesis. Events observed during eogenesis include chemical compaction, leaching of feldspar, development of chlorite coating and kaolinite, precipitation of the first generation of quartz overgrowth (QogI), dissolution of feldspar, and precipitation of calcite and nonferroan dolomite cement. Mesogenetic alterations include chemical compaction, precipitation of kaolinite aggregate and the second generation of quartz overgrowth (QogII), precipitation of ankerite, development of I/S and illite, and formation of pyrite. Carbon and oxygen isotopic data show that calcite cements are characterized by 13C ( δ 13 C PDB ranging from -0.7‰ to 1.0‰ with an average of 0.1‰) and 18O ( δ 18 O SMOW varying from 12.3‰ to 19.0‰ with an average of 16.2‰); these stable isotopic data combined with Z value (from 114.69 to 122.18) indicate skeletal debris ( δ 13 C PDB ranging from -1.2‰ to -1.1‰ with an average of -1.15‰; δ 18 O SMOW varying from 23.0‰ to 23.2‰ with an average of 23.1‰) and ooids in adjacent carbonate beds involved in meteoric water and seawater from outside jointly served as the carbon sources. For nonferroan dolomite, the δ 13 C PDB value of -4.1‰ is a little bit negative than the calcite, and the δ 18 O SMOW of 14.3‰ is coincident with the calcite, which suggest the nonferroan dolomites come from the diagenetic fluids with a similar oxygen isotopic composition to that of the calcite but modified by the external acidic δ 13C-depleted water. However, the ankerites are actually rich in 12C ( δ 13 C PDB ranging from -10.0‰ to -1.2‰, mean = − 4.3 ‰ ) and 16O ( δ 18 O SMOW varying from 10.1‰ to 19.4‰, mean = 14.9 ‰ ), when combined with the distribution of cutting down along the direction pointing to sand-body center from the margin and microthermometric temperature (Th’s) data mainly varying between 115.2°C and 135.5°C with an average of 96.0°C, indicating the main origination from the Es3 source rocks with effective feldspar buffer action for the acidic fluids in the margins of the Es3 sandstones. In addition, the necessary elements for ankerite such as Fe2+, Ca2+, and Mg2+ ions also come from organic matter and clay minerals during thermal maturation of the Es3 source rocks. The study provides insights into diagenetic processes and origination of carbonate cements in the Es3 sandstones; it will facilitate the cognition of predictive models of deeply buried sandstone reservoirs to some extent, which can reduce the risks involved in oil and gas exploration and development.

Supporting technology for fast drilling and its application in the central deep buried hill zone in the Raoyang sag

Supporting technology for fast drilling and its application in the central deep buried hill zone in the Raoyang sag

Cyclostratigraphy and astronomical tuning during the Oligocene in the Jizhong Depression, Bohai Bay Basin, northeastern China

The Dongying Formation (Ed) and the first member of the Shahejie Formation (Es1) in the central Raoyang Sag cover about 1705 m-long sedimentary series of terrestrial lacustrine infill. We employed the gamma-ray (GR) log data as a paleoclimatic and paleoenvironmental proxy to conduct the high-resolution cyclostratigraphy research in well W33 and W62. Prominent sedimentary cycles in decameters and meters were revealed by multi-taper (MTM) power spectra, evolutionary fast Flourier transformation (FFT) and correlation coefficient analyses in the Es1 and Ed intervals. The 405 kyr long eccentricity, 125 kyr and 100 kyr short eccentricity, 40 kyr obliquity, and 23 kyr precession can be recognized with a wavelength ratio of approximately 20:5:2:1, respectively. By tuning the 405 kyr filter output in well W33 to the orbital target curve of La2010d astronomical solution, we established an astronomical time scale (ATS) that provided absolute age controls during the Oligocene and shed light on the event correlation among isolated depressions in the Bohai Bay Basin. Sediment accumulation rates (SARs) were derived from the extracted 405 kyr long eccentricity and were tested with an average of 17 cm/kyr. Age boundaries of Ng/Ed1, Ed1/Ed2, Ed2/Ed3, Ed3/Es1 and Es1/Es2 are 23.03 Ma, 24.34 Ma, 25.53 Ma, 29.05 Ma and 33.10 Ma, respectively. Based on the ATS and SARs, we discussed the depositional evolution within age boundaries, cause of high SARs and origin of formation overpressure. After the largest lake transgression, shallow-water deltas and fluvial plains transitionally dominate the central Raoyang Sag with shallowing water depth. Thin interlayers of mudstones and sandstones originate from shallow-water deltaic systems with low sand-to-mud ratios. Under high SARs enhanced by mountain uplift and low sand-to-mud ratios, formation overpressure results mainly from high porosities in under-compacted mudstone intervals.

Seismic geomorphology of shoal-water deltaic and mixed carbonate-siliciclastic beach-bar systems in hanging wall of rift basins: Paleogene of the Raoyang Sag, Bohai Bay Basin, China

Recent studies of ancient rift basins focus on the tectonosedimentary models, which emphasize the control of tectonics on the distribution and evolution of depositional systems in rift basins, whereas these studies seldom address stratigraphic dominated depositional models. Compared to footwall depositional systems, hanging wall depositional systems are especially underexplored due to their fine-grained, thin-layered, and widely distributed features. We integrated seismic data, cores, and well logs to define the dispersal of hanging wall depositional systems that are related to the fluctuation of lacustrine level in the late synrift and postrift stages. We identified one third-order sequence (SQ1) on the seismic reflection data and subdivided five fourth-order sequences (SSQ1–SSQ5) based on the features of parasequence sets. The strata in SQ1 record a complete regressive and progressive lacustrine process. We interpreted shoal-water deltaic systems and mixed carbonate-siliciclastic beach-bar systems in the SQ1. Within SSQ1 and SSQ2, sediments fed from the northwest, west, and southwest, and developed shoal-water deltaic systems with an average width range of the distributary channel belts within 680–1220 m. In SSQ3, beach-bar systems gradually changed from clastic beach bars to mixed carbonate-siliciclastic beach bars in a basinward direction. Clastic beach-bar systems and mixed carbonate-siliciclastic beach-bar systems were extending north–northeast with an average length of approximately 3.5–8 km and a width of 1–4 km. For those in SSQ4 and SSQ5, shoal-water deltaic systems prograded into the lacustrine, and distributary channel belts occur with an average width range within 520–880 m. We developed a spatial and temporal quantitative evaluation of deltaic and beach-bar reservoirs within a third-order sequence on the hanging wall dip slope, and it has potential worldwide implications for other hanging wall dip slopes in rift basins.

Lacustrine conditions control on the distribution of organic-rich source rocks: An instance analysis of the lower 1st member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin

The lower 1st Member (Es1L) of the Oligocene Shahejie Formation in the Raoyang Sag, the particularly noteworthy source rock interval in the Jizhong Subbasin, were analyzed with sedimentological and geochemical techniques to describe the distribution of organic-rich source rocks and reveal the lacustrine environment and lake types during source rocks deposition. Results of the Raoyang Sag demonstrate that the organic-rich source rocks are mainly distributed in the north-central region with a maximum thickness of up to 120 m in the Maxi Subsag and gradually decreases to the southern tectonic regions such as Liuxi Subsag. These horizontal and vertical heterogeneities are attributed to depositional environments and lake types during the sedimentary period of the Es1L. Lakes in the Raoyang Sag are generally characterized by saline water during the Es1L deposition. The samples representing the south part of the Raoyang Sag display high pristane/phytane (Pr/Ph), C19/C23 TT, and C24 Tet/C26 TT ratios as well as low gammacerane index, 4-methyl sterane index, and sterane/hopane ratio. These findings suggest that hydrological open lake type has unstable water-column stratification, less reducing bottom water, and significant terrigenous supply. However, the samples representing the north-central part of the Raoyang Sag display high gammacerane index, 4-methyl sterane index, and sterane/hopane ratio as well as low Pr/Ph, C19/C23TT, and C24 Tet/C26 TT ratios. A strong correlation exists between δ13C and δ18O, indicating hydrological closed lake-type with excellent water stratification, anoxic bottom water, and enhanced algae productivity. This lake-type is of great significance for continuing research on organic matter enrichment in the Bohai Bay Basin.