Subei Basin Research Articles

Paleoclimatic and provenance implications of magnetic parameters from the Miocene sediments in the Subei Basin

Thick sediments from foreland basins usually provide valuable information for understanding the relationships between mountain building, rock denudation, and sediment deposition. In this paper, we report environmental magnetic measurements performed on the Miocene sediments in the Subei Basin, northeastern Tibetan Plateau. Our results show two different patterns. First, the bulk susceptibility and SIRM, ARM, and HIRM mainly reflect the absolute-concentration of magnetic minerals; all have increased remarkably since 13.7 Ma, related to provenance change rather than climate change. Second, the ratios of IRM100mT/SIRM, IRM100mT/IRM30mT, and IRM100mT/IRM60mT, together with the redness and S ratio, reflect the relative-concentration of hematite, being climate-dependent. Their vertical changes correlate in general with the long-term Miocene climatic records of marine oxygen isotope variations, marked by the existence of higher ratios between 17 and 14 Ma. This may imply that global climate change, rather than uplift of the Tibetan Plateau, played a dominant role in the long-term climatic evolution of the Subei area from the early to middle Miocene.

Anomalous distribution of steranes in deep lacustrine facies low maturity-maturity source rocks and oil of Funing formation in Subei Basin

Test analyses such as pyrolysis, soluble organic matter extraction, group component separation and GC-MS were conducted to 22 source rock samples and 28 crude oil samples from deep lacustrine facies of Funing Formation in Subei Basin. Source rocks stayed at the low maturity-mature stage (Ro:0.58%–0.71%), while crude oil stayed at the mature stage (Rc:0.71%–0.88%). The Pr/Ph values of samples in the research area ranged between 0.16 and 0.62. These samples could be classified as Sterane/Hopane>1 and Sterane/Hopane<1. For all the samples, the C29 Steranes content was high; had inverse “L” distribution in ααα20RC27-ααα20RC28-ααα20RC29 regular sterane; αα20R-Sterane played a dominant role; abundances of ββ- and 20S-Sterane were low. As for samples of Sterane/Hopane>1, G/C30H ranged between 0.63 and 2.56, and the Sterane isomerization was very low. As for samples with Sterane/Hopane<1, G/C30H ranged between 0.04 and 0.46 and the Sterane isomerization was higher than the former. Abnormal distribution of Sterane isomerization was rarely influenced by thermal dynamic effects and sources, but was mainly influenced by the sedimentary environment. A lot of references reported Sterane isomerization with abnormally high abundance under the high-salinity environment. However, the finding obtained by the research that the higher water salinity corresponded to the lower degree of Sterane isomerization was discovered for the first time. The C29 Sterane abundance was high and the C29/C27 regular sterane ratio was constant and would not vary with changes of environmental parameters and biological source parameters. Sterane content was not correlated with tricyclic terpene of algae sources, but was positively correlated with ETR of the aquatic organism source, while it had very good positive correlation with Gammacerane. In addition, samples with the high Sterane content had high abundance in Carotene, C24+alkyl-cyclohexane and C21+isoprenoid alkanes. Through profound analysis and reference survey, it is found that the abnormally high abundance of C29 Sterane of samples in the research area may be correlated with halophilic protozoon in salinized deepwater lakes.

Assessment of Geothermal Resources in Petroliferous Basins in China

In order to speed up the development and utilization of hydrothermal energy, it is essential to assess the potential of geothermal resources in petroliferous basins. In this paper, the distribution of reservoirs (aquifers) and the characteristics of geothermal fields have been studied systematically based on geological, geophysical, well drilling, temperature, and sample test data obtained from the major petroliferous basins of China. It has been found that some of the porous sandstone formations in these petroliferous basins are major geothermal reservoirs and are extensively thick and widely distributed. In general, the geothermal gradient in China is higher in the eastern basins and lower in the western basins. On average, the geothermal gradient is above 30 °C/km in the Bohai Bay, Songliao, and Subei basins. The geothermal resource abundance is also higher in eastern China and in the Beibuwan basin in southern China, and the geothermal source forming condition is better, followed by the Ordos, Qaidam, and Sichuan basins in Central China. Other potential basins include the Tarim and Junggar basins in western China, where the geothermal gradient ranges between 21 and 22 °C/km, on average. In this paper, three methods, stochastic simulation, unit volumetric, and analogy, were used for the assessment of geothermal resources. Using the stochastic simulation and unit volumetric methods, the geothermal resources, annual recovered geothermal resources, geothermal water resources, and thermal energy of water in 11 basins or blocks of up to 4000 m deep were calculated. Grading evaluation criteria were established by considering the heterogeneity of geothermal reservoirs. The results showed that the petroliferous basins are very rich in geothermal resources. The annual recoverable resources reach 1626.8 × 106 tons of standard coal, in which grade I, grade II, and grade III resources are 641.9 × 106, 298.6 × 106, and 686.3 × 106 tons of standard coal, respectively. The results demonstrate that the development and utilization of geothermal energy in oilfields has a huge potential for industrial production and family use, and a great significance for the development of green oilfields. With the high demand of heat, the eastern oilfields with high geothermal resource abundance should be the first to be considered for the production and utilization of geothermal energy, followed by the central and western oilfields.

Sedimentary Evolution and Transgressions of the Western Subei Basin in Eastern China since the Late Pliocene

AbstractSedimentary sequences in the Subei Basin are complex and have been affected by interactions between the ocean and rivers since the Late Pliocene, including the Yellow River, Huaihe River, and the Yangtze River. This sedimentary evolution, in particular the timing of Pleistocene transgressions, has long been a matter of controversy owing to the lack of precise chronological evidence. The aim of this study is to explore the evolution of the sedimentary environment throughout the past 3.00 Ma in this region on the basis of a comprehensive analysis of particle size and foraminifera and ostracods collected in the TZK9 core from the Subei Basin combined with geochronological studies of magnetostratigraphy, AMS14C and optically stimulated luminescence (OSL). The results show that fluvial facies in the sedimentary environment from 3.00 to 1.01 Ma. There were fluvial facies and reflects six sea‐level high stands from 1.01 to 0.25 Ma. The study area was affected by four large‐scale transgressions since 0.25 Ma. The four marine sedimentary layers known as DU7 (buried at 48–52 m), DU5 (buried at 35–41 m), DU3 (buried at 16–23 m), and DU1 (buried at 2–4 m) are recorded in the MIS7 (210–250 ka), MIS5, MIS3, and Holocene, respectively. The magnitude of the DU5 transgression was identical to that of the DU3 transgression, both were larger than the DU7 transgression, and the DU1 transgression was the weakest. The variation of transgression strength reflects the influence of global changes in sea level, tectonic subsidence, shell ridges, and sand dams. In the TZK9 core, we found evidence of seven sea‐level high stands from the Early–Middle Pleistocene, and the first one caused by regional rapid subsidence and could be traced back to 0.83‐0.84 Ma. The sea‐level high stands and the age of the first one recorded above was different from other cores in eastern China, this was caused by the lack of absolute age control and the differences in paleotopography during this period. This study reconstructs sedimentary evolution, determines the transgression and its age, establishes the chronology since the Late Pliocene, and provides a scientific framework for further paleoenvironmental and tectonic studies. The results of this study highlight the important role that local tectonics and global sea level play in the sedimentary evolution and transgressions that have occurred in the western Subei Basin.

Heavy-mineral-based provenance and environment analysis of a Pliocene series marking a prominent transgression in the south Yellow Sea

Detrital minerals in sediments can provide insight into sedimentary provenance and depositional environments. Here we report a record of heavy minerals of a 300.1 m long core recovered from the western south Yellow Sea, which was dated back to the late Pliocene (~3.5 Ma). Results of detrital mineral analysis show that sediments are dominated mainly by silicate minerals sourced from the Chinese mainland through river inputs. Heavy minerals suggest that marine transgression occurred around 76–84 m depth of the core depth, about at 792–840 ka BP. The upper part of the core (0–76 m) is dominantly marine and the lower part (below 84 m) is terrestrial. From the Miocene to Early Pleistocene, the Yellow Sea area generally subsided; the deposition center began to move from west to east, therefore the content of fine sand increased relatively in the core sediment. Meanwhile, under the influence of depression evolution in the Subei Basin, the paths of rivers were shifted, as a result the contents of augite and garnet changed, sediments in the study area were dominated by proximal sediments sourced from the Subei Basin, possibly of mixed sediments of the Yellow River and Yangtze River. The lower part of the core was characterized by a dramatic increase in augite with hornblende and epidote as dominant minerals. In the Early Pleistocene, under the background of subsidence in the Yellow Sea area, while transgression occurred, rivers shifted southward. Comparison with the mineral composition of adjacent core NHH01 and surface sediments from the region suggests the Yellow River dominated the study area, and the old Yangtze River input being occasionally dominant. The heavy mineral assemblage was dominated by hornblende and epidote, as well as mica, garnet and authigenic pyrite in some sediment layers.

Fragments of asthenosphere incorporated in the lithospheric mantle underneath the Subei Basin, eastern China: Constraints from geothermobarometric results and water contents of peridotite xenoliths in Cenozoic basalts

Anhydrous, medium/coarse-grained spinel bearing mantle xenoliths from the Subei Basin, Eastern China have mineral arrangements that reflect low energy geometry. Because of clinopyroxene modal contents, they are grouped into cpx-rich lherzolites (cpx ≥ 14percentage), lherzolites (8 < cpx < 14%), cpx-poor lherzolites (6 < cpx ≤ 8%) and harzburgites (cpx up to 4%), without relevant textural differences from the most fertile to the most depleted lithotypes. The cpx-rich lherzolites have mineral chemistry close in composition to Primitive Mantle (PM), whereas cpx-poor lherzolite (and lherzolite) and harzburgite groups cannot be considered as a result of direct melting of the PM source. In addition, the high LREE, Th and U contents, coupled with a Sr enrichment of clinopyroxenes in these lithotypes, indicate the circulation of a silicate melt (with crustal components) in a variably depleted mantle sector well before the entrapment of the xenoliths by the host basalt.Despite the large differences in refractory lithophile element contents (i.e. Ca, Al and REE), the equilibrium temperatures never exceed 1021 °C with a constant difference (<200 °C) from harzburgites to cpx-rich lherzolites. Measured mineral water contents indicate that the whole rock contains, on average, 19 ± 7 ppm of H2O without any systematic variation among rock types nor correlation with Al2O3, light-REE and Yb (or Y) contents of cpx. The cpx H2O contents of cpx-rich lherzolites (41–96 ppm) are, on average, one order of magnitude lower than those theoretically expected (214–530 ppm) for a residuum after a maximum of 3% of PM fractional (≈ bulk) melting in the spinel stability field.The proposed dehydration model suggests that the cold highly refractory harzburgites and cpx-poor lherzolites (and lherzolites?) may represent old cratonic lithospheric mantle modified at depth by the interaction with silicate melts, which may also have involved crustal components. In turn, cpx-rich lherzolites constitute fragments of upwelling fertile asthenosphere, which caused the removal/erosion of the lowermost part of the lithospheric mantle. This asthenosphere portion may have been incorporated in the lithospheric region since the Jurassic and it may have progressively cooled down after one (or more) partial melting episodes. The water depletion can be accounted for a continuous loss by diffusion during the subsolidus chemical-physical readjustment, well after (>5My, based on modelled H2O solid-solid diffusion rate) the occurrence of the last melting episode.

Optimal matching between CO2 sources in Jiangsu province and sinks in Subei‐Southern South Yellow Sea basin, China

AbstractAn understanding of CO2 sources, sinks, and their optimal matching relationships is helpful when making an initial assessment or making decisions regarding CO2 sequestration. One of the most populous and developed provinces in China, Jiangsu, is facing tremendous pressure to reduce CO2 emissions. This study assessed CO2 emissions from large, stationary CO2 sources and the CO2 geological storage capacity for the Jiangsu province, and studied their optimal geographical matching relationships. The results of the study show that major, large, stationary sources have a total of 730.75 Mt/year CO2 emissions, and the majority of them are located in southern Jiangsu. The Subei‐Southern South Yellow Sea basin, with a total storage capacity of 5.21 × 104 Mt, can be subdivided into 28 storage blocks based on the faults. Source‐sink geographical matching shows that the locations of the sources and the matched sinks are approximately translational. When the entire Subei‐Southern South Yellow basin was chosen as an option for CO2 sinks, the sinks that matched to the northern Jiangsu CO2 sources were mainly located in the onshore northern Subei basin. The northern sources tended to match the north sinks of the northern Subei basin. The CO2 from the southern Jiangsu should be stored in the southern Subei basin and the offshore Southern South Yellow basin. When only the offshore storage blocks are optional CO2 sinks, the CO2 sources in the northern and central Jiangsu mainly match the sinks in the northwestern Southern South Yellow basin. The CO2 sources from southern Jiangsu mainly match the sinks in the southern and eastern Southern South Yellow basin. © 2018 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Assessment of Mesozoic and Upper Paleozoic source rocks in the South Yellow Sea Basin based on the continuous borehole CSDP-2

Abstract Being lack of core data and measured data, although the marine oil & gas exploration in the South Yellow Sea Basin has been going on for 57 years, no commercial hydrocarbon reservoirs have been found so far. The quantitative source rocks evaluation based on geochemical data and fluid inclusions containing hydrocarbon benefits from the completion of borehole CSDP-2 in 2016, a continuous coring well of the Chinese Continental Shelf Drilling Program (CSDP), which indicates that the marine origin oil-gas exploration in the South Yellow Sea Basin enters into a new stage. As revealed by the well CSDP-2, three sets of effective hydrocarbon source rocks in the Mesozoic and Upper Paleozoic have been identified, which are distributed in the Lower Triassic Qinglong Formation (T1q), Upper Permian Longtan-Dalong Formation (P3l-d) and Middle Permian Gufeng-Qixia Formation (P2g-q) respectively. Among them, organic-rich source rocks mainly concentrate in the lower part of the P3l-d and the middle & upper part of the P2g-q, and the P3d mudstone also works as good source rock. The P3l-d mudstone with high organic abundance is the chief source rock in the South Yellow Sea Basin, whose maximum hydrocarbon-generating intensity may reaches up to 795.09 × 104 t/km2 and hydrocarbon-expulsing intensity remains at the level of 165.03 × 104 t/km2. A noteworthy feature of the P2g-q source rocks is the strong vertical heterogeneity, with the hydrocarbon-expulsing intensity of the upper part being 34.14 × 104 t/km2 while that of the lower part being 18.07 × 104 t/km2. In contrast, the organic abundance of argillaceous limestone in the T1q is mainly poor to medium and its hydrocarbon-expulsing intensity is the weakest one in the three sets of hydrocarbon source rocks, only about 12.7 × 104 t/km2. Taking into consideration the joint influence of depositional environment, organic matter parent source and biological productivity, the overall quality of source rocks in the South Yellow Sea Basin is at the similar level to the Lower Yangtze Subei Basin, but lower than that of the Upper Yangtze Sichuan Basin. The most significant oil & gas information is that multiple direct oil indications and high value abnormalities of gas logging have been discovered in the well CSDP-2. Moreover, the Mesozoic and Upper Paleozoic marine strata in the South Yellow Sea Basin have experienced three hydrocarbon charging stages, indicating a broad prospect of marine oil & gas exploration.

Ichnology and sedimentology of the trace fossil-bearing fluvial red beds from the lowermost member of the Paleocene Funing Formation in the Jinhu Depression, Subei Basin, East China

Abstract The lowermost member of the Paleocene Funing Formation, which is composed of bioturbated fluvial red beds, is extensively distributed throughout the Jinhu Depression, Subei Basin, East China. Despite its economic value as one of the most important oil-bearing reservoirs in the Subei Basin, the sedimentology, ichnology and paleoenvironment of this formation in the Jinhu Depression remain poorly understood. In this work, we investigated the ichnofossils and sedimentary facies, analyzed the ichnofacies and potential tracemakers and reconstructed the paleoenvironment of the Jinhu Depression based on cores from the lowermost member of the Paleocene Funing Formation. This formation contains a moderately diverse ichnofauna (9 ichnotaxa of the Scoyenia ichnofacies) that includes meniscate burrows (Taenidium barretti, Taenidium satanassi and Beaconites coronus), simple horizontal cylindrical burrows (Palaeophycus tubularis, Palaeophycus heberti and Planolites isp.), simple vertical or subvertical burrows (Skolithos linearis and Skolithos isp.) and plant root traces. The other complex branched and chambered burrows are left in open nomenclature. These ichnofossils are widespread throughout the three facies associations identified in this study: (1) ephemeral fluvial channel, (2) crevasse splay and (3) distal floodplain facies associations. Multiple lines of sedimentological and ichnological evidence, namely, the predominance of massive and upper flow regime structures, the occurrence of invertebrate burrows (e.g., Skolithos and Taenidium) in the lower and middle parts of the channel deposits and the dramatic changes in the color and mottling in channel and overbank sediments, indicate that the river discharge was short lived and that the water table was highly variable. The variations in the discharge and water level are attributed to the influence of seasonal rainfall. The cyclic nature of intense rainfall events and dry periods suggests that the Subei Basin was likely located in a semiarid region with a seasonal rainfall pattern during the early Paleocene. This study also reveals that the ephemeral fluvial channel is the facies association with the highest potential to yield oil-bearing reservoirs in the Paleocene red beds of the Jinhu Depression. Additionally, the influence of bioturbation on the reservoir quality in this region should be considered in future research.

Methodology for predicting reservoir breakdown pressure and fracture opening pressure in low-permeability reservoirs based on an in situ stress simulation

Using numerical simulations of the paleostress field during the time of fracture formation, we predict the occurrence of fractures in the reservoirs of the second member of the Funing Formation in the Tongcheng fault zone, Subei basin, East China. We use sonic velocity measurements from rock cores and microseismic monitoring to determine the in situ stress, which is combined with fracture data to calculate the magnitude of the in situ stress at the well site. By determining the mechanical parameters of the rock, we use the finite element method (FEM) to develop a geomechanical model and conduct numerical simulations of the in situ stress field. Based on the occurrence of fractures and the numerical simulation results of the in situ stress field, we predict the opening pressure and opening sequence of natural fractures in the reservoirs. The results indicate that the maximum horizontal principal stress in the second member of the Funing Formation in the Tongcheng fault zone is oriented ENE. During the injection process, the ENE-trending fractures open first, and the ESE-trending fractures open later. The opening pressure of the fractures increases as the angle between the fracture strike and the maximum horizontal principal stress increases, and the burial depth of the fractures also has a relatively positive correlation with the opening pressure. At the structurally high site (1850 m–2350 m), the opening pressure of the fractures is in the range of 25 MPa–50 MPa, and at the structurally low site (3300 m–4000 m), the opening pressure of the fractures is in the range of 45 MPa–75 MPa. By calculating the actual fracture pressure of the reservoirs, we propose using different injection pressures in different blocks to ensure the high and stable production of oil from wells.

Quantitative Prediction of Lower Order Faults Based on the Finite Element Method: A Case Study of the M35 Fault Block in the Western Hanliu Fault Zone in the Gaoyou Sag, East China

AbstractLower order faults (LOFs) are characterized by small‐scale, strong concealment and difficult to identify in seismic data, which affects the accuracy of LOF interpretation. Due to the quality of the seismic data in the M35 fault block of the Gaoyou sag, Subei basin, the direction of the LOFs cannot be accurately identified, leading to the unclear relationship between the injection and production wells in the local oil fields and affecting the development of the remaining oil. In this paper, the formation mechanism of LOFs in the M35 fault block is determined from analysis of the fault activity and tectonic evolution, and a finite element geomechanical model is established to simulate the paleostress field during the formation of the LOFs. Based on the relationship between the areal density of LOFs and the stress parameters, the regions of LOF development are predicted by the minimum principal stress, the maximum and minimum principal stress difference (Δσ), normal stress (σn), and the strain energy density; the shear stresses are used to predict the dip direction of the LOFs. Through the coordinate system transformation of the principal stress directions, the strikes of the LOFs are quantitatively predicted. Finally, the reliability of the prediction results is verified by the latest structural map of the M35 fault block. The method proposed in this paper can be widely applied to the quantitative prediction of multiple parameters of LOFs in complex fault blocks.

Bioturbation influence on reservoir rock quality: A case study of Well Bian-5 from the second member Paleocene Funing Formation in the Jinhu sag, Subei basin, China

The bioturbated horizons identified from Member Two of the Paleocene Funing Formation (E1f2) in the Jinhu sag, have not been previously assessed to ascertain their influence on reservoir quality. Owing to that, conventional thin section, pulse decay permeametry, pressure decay porosimetry, Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS) methods were used to analyze the significance of bioturbation in controlling porosity and permeability on selected cored intervals from one well. Intensely bioturbated core slab samples A and B selected from Bian-5 Well are dominantly controlled by Ophiomorpha and Macaronichnus, and Macaronichnus, respectively. Results showed that Ophiomorpha and Macaronichnus in sample A exhibited sediment cleaning activity with better sorted grains in burrow fillings. Mud and clay were removed from the burrow filling and directed towards burrow lining and/or host sediment by burrowing organism resulting in well sorted burrow fill matter; the net effect, enhanced isotropy and increased porosity in burrows fills. Macaronichnus ichnofabrics in sample B also exhibited sediment cleaning activity causing reduction of local porosity in host sediment and an outcome of permeability improvement in burrows. Reservoir quality of bioturbated sandstone of Bian-5 well in the Jinhu sag is, therefore, dependent on activity of burrowing organism, burrow structure, bioturbation extent as well as the existence or nonexistence of burrow fill or lining.

Influence of sedimentary facies on reservoir quality and distribution of diagenetic features in the Funing Formation, Wanglongzhuang Oilfield, Subei Basin, Eastern China

The Paleogene clastic sediments of the Funing Formation, which is characterized by slump-fan, delta-front, and beach-bar deposits, are important targets for oil and gas exploration in the Wanglongzhuang Oilfield, Subei Basin, eastern China. Three categories of reservoir quality are distinguished on the basis of petrographic and petrophysical experiments such as nuclear magnetic resonance (NMR) and mercury injection (MI). A paragenetic sequence for diagenetic evolution of each sedimentary facies has been reconstructed to determine the mechanisms for the variations in reservoir quality of the formation and the influence of the sedimentary facies on the distribution of diagenetic features in each of its members. The maturity of the organic material, textural relationships, and the distribution pattern of diagenetic minerals suggest that the study area underwent mainly mesodiagenesis. The mesodiagenesis A and mesodiagenesis B stages induced different diagenetic features in the target reservoir; the beach-bar sand of the E1f2 unit and the delta-front sand of the E1f4 unit underwent mostly only cementation; the delta-front deposits of the E1f3 unit, however, suffered more from mechanical and chemical compaction than from cementation. The loss of porosity in the fluxoturbidites of E1f4 due to compaction was just as large as the loss by cementation. The gravity-flow channel, sand-bar, and distributary-channel deposits have a higher porosity and permeability than the sand-beach and estuarine-bar deposits.

Hierarchy and subsurface correlation of muddy baffles in lacustrine delta fronts: a case study in the X Oilfield, Subei Basin, China

Muddy baffles are one of the major geological factors controlling the underground fluid flow as well as the remaining oil distribution. This study used detailed drilling data from the E1f1 reservoir in the X Oilfield, Subei Basin, China, to investigate the hierarchical muddy baffles developed on lacustrine delta fronts and their precise cross-well correlation. According to the theories on allogenic and autogenic cycles as well as the genesis and scales of muddy baffles, five orders of muddy baffles were classified, which provided various degrees of difficulty when attempting interwell correlation. Under the guidance of a reliable stratigraphic model, the precise cross-well correlation of muddy baffles could be achieved and the key point of establishment of this stratigraphic model was to calculate foreset angles of hierarchical muddy baffles during deposition. During calculation, the relationships between sediment flow direction, higher-order strata dipping direction, well types and well trajectory direction should all be taken into consideration and some other angles (referred to here as θ1, θ2 and θ3) should also be measured. Such new methods of cross-well correlation for hierarchical muddy baffles could greatly reduce the uncertainties and multiple solutions, which in turn would be significant for efficient development and oil recovery enhancement in the reservoirs.

Quantitative multiparameter prediction of fault-related fractures: a case study of the second member of the Funing Formation in the Jinhu Sag, Subei Basin

In this paper, the analysis of faults with different scales and orientations reveals that the distribution of fractures always develops toward a higher degree of similarity with faults, and a method for calculating the multiscale areal fracture density is proposed using fault-fracture self-similarity theory. Based on the fracture parameters observed in cores and thin sections, the initial apertures of multiscale fractures are determined using the constraint method with a skewed distribution. Through calculations and statistical analyses of in situ stresses in combination with physical experiments on rocks, a numerical geomechanical model of the in situ stress field is established. The fracture opening ability under the in situ stress field is subsequently analyzed. Combining the fracture aperture data and areal fracture density at different scales, a calculation model is proposed for the prediction of multiscale and multiperiod fracture parameters, including the fracture porosity, the magnitude and direction of maximum permeability and the flow conductivity. Finally, based on the relationships among fracture aperture, density, and the relative values of fracture porosity and permeability, a fracture development pattern is determined.

Flow units classification for geostatisitical three-dimensional modeling of a non-marine sandstone reservoir: A case study from the Paleocene Funing Formation of the Gaoji Oilfield, east China

AbstractFlow units classification can be used in reservoir characterization. In addition, characterizing the reservoir interval into flow units is an effective way to simulate the reservoir. Paraflow units (PFUs), the second level of flow units, are used to estimate the spatial distribution of continental clastic reservoirs at the detailed reservoir description stage. In this study, we investigate a nonroutine methodology to predict the external and internal distribution of PFUs. The methodology outlined enables the classification of PFUs using sandstone core samples and log data. The relationships obtained between porosity, permeability and pore throat aperture radii (r35) values were established for core and log data obtained from 26 wells from the Funing Formation, Gaoji Oilfield, Subei Basin, China. The present study refines predicted PFUs at logged (0.125-m) intervals, whose scale is much smaller than routine methods. Meanwhile, three-dimensional models are built using sequential indicator simulation to characterize PFUs in wells. Four distinct PFUs are classified and located based on the statistical methodology of cluster analysis, and each PFU has different seepage ability. The results of this study demonstrate the obtained models are able to quantify reservoir heterogeneity. Due to different petrophysical characteristics and seepage ability, PFUs have a significant impact on the distribution of the remaining oil. Considering these allows a more accurate understanding of reservoir quality, especially within non-marine sandstone reservoirs.

Exploration achievement of lithological oil reservoirs in the south of Haian depression, the Subei Basin and its enlightenment

Exploration achievement of lithological oil reservoirs in the south of Haian depression, the Subei Basin and its enlightenment

Evaluation of geological risk and hydrocarbon favorability using logistic regression model with case study

Abstract A robust and effective evaluation on geological risk and favorability of hydrocarbon occurrence contributes to a meaningful exploration decision. This paper systematically reviewed and discussed the evaluation units, evaluation variables and evaluation models relevant to geological risk and favorability evaluation, and presented a data-driven procedure using logistic regression method. In this method, quantitative logistic regression relationship between the hydrocarbon occurrence and key geologic factors are obtained by integrating available geoscience information and current exploration results. It offers significant convenience to derive the probability of hydrocarbon occurrence in a straightforward and simple form with a more objective and reproductive result. The proposed method and workflow are further illustrated using an example from the first member of Dainan Formation in Yong'an area of Gaoyou Sag of Subei Basin in China. This case study not only quantified the geological risk and favorability of specific exploration target, but also revealed their spatial variation and highlighted the favorable area for further exploration by the resultant maps. The evaluation results and actual observation displayed a good correspondence and the overall evaluation accuracy was 94.0%, indicating that such an approach could capture the essential spatial characteristics of hydrocarbon accumulations and provide a useful tool in resources appraisal.

The Synthesis and Decomposition of Signals and Its Application in Subei Basin Exploration

Subei basin is the most promising onshore oil and gas bearing basin in South China. With the deepening of exploration, subtle hydrocarbon reservoirs have gradually become the major target of exploration. Seismic record often shows low signal to noise ratio (SNR), resulting that conventional seismic records have three shortcomings in the identification of subtle reservoirs: difficult to identify small faults; difficult to show the distribution law of sand body; and difficult to find traps. In order to solve this problem, we conducted the research on signal synthesis and decomposition. The research results showed that seismic record of different frequency bands can be restored from original seismic record and both of them contain real stratigraphic information. Based on this, when a certain band or several bands in the original seismic record is affected by noise and result in the reduction of SNR of seismic record, seismic information seriously affected by noise can be abandoned, leaving only less affected seismic information to obtain seismic record with higher SNR. In the collection of actual seismic record, the low and high band seismic information is seriously affected by noise, while medium-band seismic information is less affected. Therefore, based on this, the medium-band seismic information can be restored from the original seismic record to be new record, which is called predominant frequency band seismic record. In this paper, based on the research result, the predominant frequency band seismic record was applied to the two areas of Subei basin and the result showed the research result can be used as a good instruction on well placement and the improvement of drilling success rate.

Natural enhancement and mobility of oil reservoirs by supercritical CO2 and implication for vertical multi-trap CO2 geological storage

The accumulation and production of both deep mantle-derived CO2 and light oil were discovered in the Huangqiao reservoir, which is located in the Subei Basin, East China. The Huangqiao reservoir shows that both CO2 and oil are entrapped in and produced from vertical multi-traps. The effects of deep CO2 on the accumulation and production of oil under natural conditions and the implications for CO2 geological storage are investigated in detail. The fluid inclusions in quartz or calcite veins from the Silurian Fentou Formation (S2-3f), Permian Longtan Formation (P2l) and Triassic Qinglong Formation (T1q) have homogenization temperatures (Ths) that display peak ranges of 180°C–190 °C, 170°C–180 °C and 150°C–160 °C, respectively. The Ths are higher than the formation temperatures. The calcite veins have light carbon and oxygen isotope compositions and have high 87Sr/86Sr ratios and positive Eu anomalies. These characteristics reveal the activities of deep CO2-rich hydrothermal fluids in the basin strata. The feldspar in the S2-3f and P2l sandstone reservoirs underwent significant dissolution because of the presence of CO2-rich fluids, forming of a large amount of secondary pores in the sandstone reservoirs and enhancing the reservoirs’ physical properties. The measured porosity reaches 12.3%. CO2 in a supercritical state extracted or dissolved light petroleum components out of sedimentary rocks and then carried them towards reservoirs. Thus, CO2 and oil co-accumulated in the enhanced reservoirs, e.g. S2-3f, P2l and T1q. During drilling development, this supercritical CO2 naturally increased the mobility of oil, resulting in the co-production of highly pure CO2 and light oil. Based on the unique features of the Huangqiao CO2-oil reservoir, we propose a new CO2 geological storage model, namely, a vertical multi-trap geological storage model, which can either entrap large volumes of CO2 or lower the risk of long-term storage.