Southern Songliao Basin Research Articles

Coupling relationship between reservoir diagenesis and hydrocarbon accumulation in Lower Cretaceous Yingcheng Formation of Dongling, Changling fault depression, Songliao Basin, Northeast China

ABSTRACT The Lower Cretaceous Yingcheng Formation in the southern Songliao Basin is the typical tight oil sandstone in China. In order to better predict the petrophysical properties of the tight sandstone reservoirs in the Lower Cretaceous Yingcheng Formation, Songliao Basin, Northeast China, the diagenesis and porosity evolution was investigated using a suite of petrographic and geochemical techniques including thin section analysis, scanning electron microscopy, mercury intrusion and fluid inclusion analysis, on a set of selected tight sandstone samples. Combined with the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge, the matching relationship between reservoir porosity evolution and hydrocarbon accumulation history is analyzed. The result showed that the tight sandstone reservoirs characterized of being controlled by deposition, predominated by compaction, improved by dissolution and enhanced by cementation. The hydrocarbon accumulation period was investigated using a suite of hydrocarbon generation and expulsion history, microfluorescence determination and temperature measurement technology. According to the homogenization temperature of the inclusions and the history of burial evolution, Yingcheng Formation has mainly two phases hydrocarbon accumulation. The first phase of oil and gas is charged before the reservoir is tightened, the oil and gas generated by Shahezi source rocks enter the sand body of Yingcheng Formation, influenced by the carrying capability of sand conducting layer, oil and gas is mainly conducted by the better properties and higher connectivity sand body and enriched in the east, which belongs to the type of densification after hydrocarbon accumulation. The second phase of oil and gas charge after densification, which belongs to the type of densification before the hydrocarbon accumulation.

Discovery of the Cretaceous-Paleogene compressional structure and basin properties of the southern Songliao Basin

Discovery of the Cretaceous-Paleogene compressional structure and basin properties of the southern Songliao Basin

Constraints on S-wave velocity structures of the lithosphere in mainland China from broadband ambient noise tomography

In this study, a 3-D S-wave velocity model of the lithosphere in mainland China down to 150 km depth was determined from ambient noise data. First, we collected 2-year continuous waveforms recorded by 1031 broadband stations of the China Regional Seismic Network and NECESSArray. Then, by applying the procedures of noise cross-correlation and time-frequency domain phase-weighted stacking, we obtained the interstation empirical Green's functions of Rayleigh waves. We measured the group and phase velocity dispersions at periods of 5–125 s from the EGFs and inverted both group and phase velocities for the S-wave velocity model. Our model revealed that the lithospheric thickness in the northern Songliao basin is ~70–110 km. A weak high-velocity anomaly that is discontinuous in the vertical direction was detected beneath the southern Songliao basin, which may indicate that the lithosphere is delaminated in this area. The model also revealed that deep (>150 km) high-velocity lithospheric roots exist beneath the Ordos and Sichuan basins. The lithospheric thicknesses in eastern parts of the Northeast China Block, the North China Craton, and the South China Block are only 60–100 km, which may be the result of a series of deep processes caused by the subduction of the Pacific plate to the Eurasian plate that produced strong lithospheric thinning in eastern China. The lithospheric thickness within the Tanlu fault belt is generally thinner than that in the adjacent areas of the fault belt, and the S-wave velocity of the mantle lithosphere beneath the fault belt is also lower than that beneath the adjacent areas, indicating that the Tanlu fault belt is a deep fault belt cutting through the lithosphere.

Pore throat size distribution and oiliness of tight sands-A case study of the Southern Songliao Basin, China

In this study, several methods such as mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) for characterizing the pore throat size distribution of tight sandstone reservoirs were selected. The differences of mercury injection parameters such as maximum interconnected pore throat radius, median radius, mercury injection curve, and pore throat distribution curve of oil-bearing and oil-free samples were compared. By designing the algorithm program and using the mercury injection data, the coefficient of converting the T2 relaxation time to the pore-throat radius was corrected, and the pore throat distribution of different oil saturation samples were analyzed. Environmental scanning electron microscope (ESEM) was also used to observe the microscopic pores and the oil in them. Using the above data, we systematically analyzed the control of pore throat distribution on the oiliness. It is found that the oiliness depends on the maximum interconnected pore throat radius, and the oil saturation depends on the oil migration front pressure and pore volume. The oiliness of tight sand has good relationship with pore throat size distribution, and the better reservoir petrophysical properties result in better oiliness. Statistics on the pore throat size distribution of oil-bearing tight sands were carried out and main intervals of pore throat size distribution of oil-bearing reservoirs were given in this paper. This understanding provides a basis for searching sweet spots in the future exploration of the tight oil.

Impact of laminae on pore structures of lacustrine shales in the southern Songliao Basin, NE China

Evaluating the influences of laminae on shale pore structures is vital to understand the enrichment mechanism of lacustrine shale gas. This study investigates lacustrine shales from the lower Cretaceous Shahezi Formation in the Changling Sag, southern Songliao Basin, NE China, using mineralogical and geochemical analyses, CO2 & N2 adsorption, and high-pressure mercury intrusion porosimetry (MIP), to compare pore structure characteristics between laminated and massive shales, and elucidate their main controlling factors. Our results show that, laminae in lacustrine shales from the Shahezi Formation include those clay-rich, organic-rich, and silty ones, and minor carbonate-rich ones. Micropores in both laminated and massive shales show tri-modal distributions with main peaks at 0.47–0.54 nm, 0.57–0.63 nm, and 0.80–0.85 nm, respectively; however, the laminated shales exhibit great variations in micropore volumes among different samples. Mesopores within the laminated shales are smaller than those in the massive shales, and their pore volumes (PV) are about 1/3 less than those of the massive shales. This is due to that the laminated shales are rich in slit-like pores whereas the massive shales have more plate-like and ink-bottle pores. Thermal maturity (Ro) and total organic carbon (TOC) contents are the essential factors controlling the development of micropores and mesopores in the laminated shales, however they show weak effects on macropore development in this formation; this may be related to that shales from the Shahezi Formation generally have low TOC contents and type III kerogen. Clay minerals and quartz both have weak relevance to pore development in this formation.

Study on the relationship between lithology and reservoir characteristics of Yingcheng Formation in the southern Songliao Basin

In order to deeply understand the relationship between the microstructural characteristics of volcanic reservoirs and the lithology of volcanic rocks, it provides important theoretical significance for the effective development of volcanic reservoir oil and gas fields. The microscopic characteristics of the volcanic reservoirs of the Yingcheng Formation in the Songliao Basin were analyzed by means of rock section determination, pore structure parameters analysis and casting thin section analysis. The results show that the target layer has low porosity and permeability, and the volcanic pore structure of different lithologies is different. Volcanic lava generally develops original gas pores, phenocryst dissolved pores, matrix dissolved pores, and tectoclase, while volcanic clastic rocks generally have developmental dissolution pores as the main feature. The micro-heterogeneity of volcanic reservoirs is generally strong, and the development of tectonic fractures is an important factor in changing the reservoir performance of volcanic rocks.

Mechanisms for the accumulation of deep gas in the southern Songliao Basin, China

The Lower Cretaceous Yingcheng Formation in the southern Songliao Basin contains inorganic CO2, hydrocarbon, and mixed-gas reservoirs and is overlain by Denglouku Formation sandstone hosting secondary hydrocarbon gas reservoirs. We measured the diffusion coefficients of CH4 and CO2 in volcanic rock samples collected from five wells in the Yingcheng Formation. Our results show that the diffusion coefficient of CH4 is greater than that of CO2. Following the accumulation of hydrocarbon gas in the Yingcheng Formation, the region was affected by movement along deep-seated faults, resulting in the upward migration of mantle-derived inorganic CO2 into hydrocarbon gas reservoirs in volcanic rocks of the Yingcheng Formation, displacing pre-existing hydrocarbon gas. The hydrocarbon gas was transported upward into Denglouku Formation sandstone (which has favorable physical properties for gas storage) and was then enriched to form secondary gas reservoirs.

Geochemical characterization and quantitative evaluation of shale oil reservoir by two-dimensional nuclear magnetic resonance and quantitative grain fluorescence on extract: A case study from the Qingshankou Formation in Southern Songliao Basin, northeast China

Abstract Understanding the content and geochemical characteristics of shale oil are two significant aspects of shale oil exploration and evaluation. This paper details the analysis of 32 shale samples extracted by sealed coring from the first member of the Upper Cretaceous Qingshankou Formation (K2qn1) in the Southern Songliao Basin, northeast China. Geochemical techniques including Rock-Eval pyrolysis, contents of total organic carbon (TOC) and clay mineral identification by X-ray diffraction (XRD) were performed on the samples. A total of 13 samples were further analyzed using two-dimensional nuclear magnetic resonance (2D NMR) and the quantitative grain fluorescence on extract (QGF-E) technique to determine the content and properties of different types of organic matter (OM) in the shale reservoir. The OM content and spectral intensity obtained from the NMR and QGF results were positively correlated with TOC content and pyrolysis S1 of the samples, respectively. In addition, the characteristic fluorescence parameters (R1, R2) of the shale extract and crude oil obtained by total scanning fluorescence (TSF) indicated that the crude oil in the studied shale reservoir was a thick-medium oil in the medium mature stage. The comprehensive experiment with the combined use of high-frequency 2D NMR and QGF technologies realized the quantitative characterization of shale oil composition and maturity, and demonstrated advantages of nondestructive, rapid and quantitative detection of OM, especially oil content in shale. New shale oil evaluation parameters obtained from these experiments, therefore, are more significant than traditional measurements such as Soxhlet extraction and pyrolysis.

Spatial-temporal coupling between high-quality source rocks and reservoirs for tight sandstone oil and gas accumulations in the Songliao Basin, China

The spatial-temporal relationship between high-quality source rocks and reservoirs is a key factor when evaluating the formation, occurrence, and prospectivity of tight oil and gas reservoirs. In this study, we analyze the fundamental oil and gas accumulation processes occurring in the Songliao Basin, contrasting tight oil sand reservoirs in the south with tight gas sand reservoirs in the north. This is done using geochemical data, constant-rate and conventional mercury injection experiments, and fluid inclusion analyses. Our results demonstrate that as far as fluid mobility is concerned, the expulsion center coincides with the overpressure zone, and its boundary limits the occurrence of tight oil and gas accumulations. In addition, the lower permeability limit of high-quality reservoirs, controlled by pore-throat structures, is 0.1 × 10−3 μm2 in the fourth member of the Lower Cretaceous Quantou Formation (K1q4) in the southern Songliao Basin, and 0.05 × 10−3 μm2 in the Lower Cretaceous Shahezi Formation (K1sh) in the northern Songliao Basin. Furthermore, the results indicate that the formation of tight oil and gas reservoirs requires the densification of reservoirs prior to the main phase of hydrocarbon expulsion from the source rocks. Reservoir “sweet spots” develop at the intersection of high-quality source rocks (with high pore pressure) and reservoirs (with high permeability).

A comprehensive approach for fracability evaluation in naturally fractured sandstone reservoirs based on analytical hierarchy process method

AbstractThe tight sandstone gas reservoir in southern Songliao Basin is naturally fractured and is characterized by its low porosity and permeability. Large‐scale hydraulic fracturing is the most effective way to develop this tight gas reservoir. Quantitative evaluation of fracability is essential for optimizing a fracturing reservoir. In this study, nine fracability‐related factors, particularly mechanical brittleness, unconfined compressive strength (UCS), mineral brittleness, cohesion, internal friction angle (IFA), natural fracture, fracture toughness, horizontal stress difference, and fracture barrier were obtained based on a series of petrophysical and geomechanical experiments. Taking above factors into consideration, a modified comprehensive evaluation model is proposed based on analytic hierarchy process (AHP) method. The UCS and IFA were removed from the AHP model based on the results of factor correlation analysis. The transfer matrix in the weighting procedure was applied to improve the consistency of judgment matrix, and the fuzzy matrix was employed to promote the objectiveness of final decision. The fracability evaluation of four reservoir intervals in Jinshan gas field was analyzed. Field fracturing tests were conducted to verify the feasibility of the proposed evaluation model. Results showed that the tubing pressure curve is more fluctuated in the reservoir interval with more developed natural fractures, and gas production is higher in the reservoir interval with greater fracability coefficient. The field test data coincide with the results of the proposed evaluation model.

Origin of the carbonaceous debris and its implication for mineralization within the Qianjiadian uranium deposit, southern Songliao Basin

Appearance and coal petrology of the carbonaceous debris (CD) from the Qianjiadian uranium deposit, southern Songliao Basin are illustrated in order to reveal the relationship between the CD and uranium mineralization, response of the CD and uranium mineralization to diabase dikes, and origin of the CD. Three types of CD have been identified in the Yaojia Formation: CD serving as trapped sediments, CD serving as laminae of cross bedding, and CD occurring in lumpy sandstones. With an average content of 91.2% in the CD, organic macerals are primarily composed of homoeollinite, desmocollinite and inertinite. Inorganic macerals of the CD mainly include pyrite with an average content of 8.8%. Average contents of the desmocollinite, inertinite and pyrite from the CD in the ore bearing section are higher than that in the non-ore bearing section, respectively. The study demonstrates that the CD rich in inertinite and desmocollinite facilitates uranium mineralization. Uranium is frequently adsorbed in the desmocollinite and inertinite, and coffinite is embedded in the inertinite, inferring that roles of the CD in uranium mineralization are both adsorption and reduction. Vitrinite reflectance (VR) of the CD ranges from 0.24% to 0.513% with an average value of 0.335%. The VR of the CD in the ore bearing section is generally larger than that in the non-ore bearing section. Uranium mineralization and enlargement of the VR are both attributed to the diabase dikes. The CD originated from the Late Cretaceous wildfire-related woody plants, and it has been transported into fluvial channels and preserved in sandstones.

The Impacts of Matrix Compositions on Nanopore Structure and Fractal Characteristics of Lacustrine Shales from the Changling Fault Depression, Songliao Basin, China

The Lower Cretaceous Shahezi shales are the targets for lacustrine shale gas exploration in Changling Fault Depression (CFD), Southern Songliao Basin. In this study, the Shahezi shales were investigated to further understand the impacts of rock compositions, including organic matters and minerals on pore structure and fractal characteristics. An integrated experiment procedure, including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission-scanning electron microscope (FE-SEM), low pressure nitrogen physisorption (LPNP), and mercury intrusion capillary pressure (MICP), was conducted. Seven lithofacies can be identified according to on a mineralogy-based classification scheme for shales. Inorganic mineral hosted pores are the most abundant pore type, while relatively few organic matter (OM) pores are observed in FE-SEM images of the Shahezi shales. Multimodal pore size distribution characteristics were shown in pore width ranges of 0.5–0.9 nm, 3–6 nm, and 10–40 nm. The primary controlling factors for pore structure in Shahezi shales are clay minerals rather than OM. Organic-medium mixed shale (OMMS) has the highest total pore volumes (0.0353 mL/g), followed by organic-rich mixed shale (ORMS) (0.02369 mL/g), while the organic-poor shale (OPS) has the lowest pore volumes of 0.0122 mL/g. Fractal dimensions D1 and D2 (at relative pressures of 0–0.5 and 0.5–1 of LPNP isotherms) were obtained using the Frenkel–Halsey–Hill (FHH) method, with D1 ranging from 2.0336 to 2.5957, and D2 between 2.5779 and 2.8821. Fractal dimensions are associated with specific lithofacies, because each lithofacies has a distinctive composition. Organic-medium argillaceous shale (OMAS), rich in clay, have comparatively high fractal dimension D1. In addition, organic-medium argillaceous shale (ORAS), rich in TOC, have comparatively high fractal dimension D2. OPS shale contains more siliceous and less TOC, with the lowest D1 and D2. Factor analysis indicates that clay contents is the most significant factor controlling the fractal dimensions of the lacustrine Shahezi shale.

Evaluation of pore structures in volcanic reservoirs: a case study of the Lower Cretaceous Yingcheng Formation in the Southern Songliao Basin, NE China

Different pore types and variable pore structures can determine reservoir potentials of volcanic rocks. Therefore, it is of great importance to understand the pore structures of volcanic reservoirs. With the Lower Cretaceous Yingcheng volcanic rocks, this study attempts to investigate and evaluate the micro pore structures of a volcanic reservoir through: (1) measuring porosity and permeability of core samples; (2) identifying pore types within thin sections; (3) characterizing pore structures with pressure-controlled mercury injection. The Lower Cretaceous Yingcheng volcanic rocks exhibits low porosities (on average 7.39%) and very low permeability (on average 0.283 × 10−6 m2). Three major pore types were identified from the Lower Cretaceous Yingcheng volcanic reservoir, including primary pores, secondary pores and fractures. Additionally, the pore structures of the Lower Cretaceous Yingcheng volcanic reservoir can be classified into three types. Type I pore structure primarily occurs in the volcanic breccia, crystal tuff and tuffites. The corresponding pore system is dominated by primary intergranular pores, intragranular dissolved pores and devitrified micropores with porosity and permeability higher than 5.4% and 0.05 × 10−6 µm2, respectively. Corresponding mercury intrusion curves are characterized by low displacement pressure, high maximum mercury saturation and mercury withdrawal efficiency. Type II pore structure is commonly observed in the tuff and tuffites, while primary intergranular pores and secondary micropores dominate pore system with porosity of 2.5–5.4% and permeability of 0.012–0.05 × 10−6 m2. The mercury intrusion curves are similar to the type I. Type III pore structure is also pervasively developed in the crystal tuff and tuffites. Intragranular dissolved micropores are most important ones with porosity and permeability less than 2.5% and 0.01 × 10−6 m2, respectively. Different from type I and II pore structure, the mercury intrusion curves of the type III have high-displacement pressure, low maximum mercury saturation and mercury withdrawal efficiency, indicating poor reservoir quality.

Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir of Fuyu Oil Layer in the Southern Songliao Basin, Northeast China

AbstractThe diagenesis mechanism and the physical properties of a terminal fan reservoir are determined by nuclear magnetic resonance (NMR), X-ray diffraction and scanning electron microscopy. The main provenance directions are NE and SE, and the two oppositely directed fans converge to form a small catchment basin. The mudstone color is red or purplish red, which accounts for 60% of the total rock. The sandstones are lithic-feldspar sandstones and feldspar-lithic sandstones, with a smaller quartz component relative to the adjacent sandstone formations. The reservoir mainly consists of intergranular pores (51%), intragranular pores (22%), corrosion pores (20%), micro-fractures (5%) and clay matrix pores (2%). The porosity of the reservoir is only 13%, and the throats are fine with high displacement pressure. The diagenetic processes included compaction, cementation, replacement, and dissolution, and the most influential factor on the reservoir porosity was compaction. The detrital rock cement mainly consists of clay minerals (48%), quartz (23%), carbonate (19%), feldspar (7%) and dawsonite (3%). Among them, the mixed I/S layer has the most content and the most important cementation. In addition, a small amount of dawsonite is found in the pores of the sandstone, which is a unique mineral that is related to the background of inorganic CO2. The main diagenesis factors that affected this sandstone’s porosity were compaction, early quartz overgrowth and calcite cementation, which reduced the porosity from 40% to approximately 8%. Although dissolution and fracture increased the porosity (from 8% to 26%), clay- and carbonate-mineral cementation during the late diagenesis period had a dramatic effect, forming a typical low-porosity and low-permeability reservoir.

Analysis of continental shale gas accumulation conditions in a rifted basin: A case study of Lower Cretaceous shale in the southern Songliao Basin, northeastern China

Abstract The analysis of shale gas accumulation conditions is the basis of shale gas exploration. Previous research has mainly focused on the Upper Cretaceous shale in the Songliao Basin of northeastern China, and few studies have analyzed the Lower Cretaceous shale in this area. Based on an integrated analysis of the sedimentary facies, shale samples and seismic interpretation from the Lower Cretaceous Shahezi, Yingcheng and Denglouku Formations of the Fulongquan Depression in the Songliao Basin, the distribution of the shale and the generation, accumulation and preservation conditions of the shale gas are evaluated and discussed. Core observations and sedimentary facies analysis show that the sedimentary facies types in the Shahezi, Yingcheng and Denglouku Formations are mainly fan delta, braided river delta, sublacustrine fan and lacustrine facies. The semideep lake, shallow lake and delta front sedimentary subfacies are favorable for the deposition of shale. The study of the shale distribution indicates that the shale was mainly deposited near the maximum flooding surface of the third-order sequences, and the shales in the Shahezi and Yingcheng Formations are darker and thicker than those in the Denglouku Formation. In addition, the shale in this study area is laterally distributed in the depression belt and the steep slope belt near the boundary faults of the basin. According to the geochemistry tests, rock properties and mineral constituents, the shales in the Shahezi and Yingcheng Formations have favorable gas generation potential. Compared with the properties of the major gas-producing shales in China and the United States, the shales in the Shahezi and the Yingcheng Formations are thick and have a favorable organic matter type, high average organic matter abundance, high organic matter thermal maturity that reached a high maturation stage, high porosity and high gas contents; these properties meet the standards of industrial exploitation, and these shales can therefore be considered favorable targets for unconventional energy exploration.

Characterizing the Micropores in Lacustrine Shales of the Late Cretaceous Qingshankou Formation of Southern Songliao Basin, NE China

Characterizing the Micropores in Lacustrine Shales of the Late Cretaceous Qingshankou Formation of Southern Songliao Basin, NE China

Study of sedimentary characteristic in Denglouku formation, southern Songliao basin, Northeast China

Based on geological and geophysical data (e.g., core observation, well-logging curves, logging, analysis assay, and seismic data), the sedimentary facies, characteristics and distributions of the Lower Cretaceous Denglouku formation in the southeast uplift region of southern Songliao Basin are analyzed. The study shows that four types of sandbodies developed in Denglouku formation: alluvial fan, fan delta, braided river and braided river delta. In the early stage of the transition period named D1 period, the deposition is significantly controlled by the fault depression, resulting in rapid unloading of the sediments. Five large-scale divided fault-depressions and two small fault depressions have been developed. The ancient uplift at the periphery of the fault depression refers to the main development area of alluvial fan depositional system, Fan delta deposits are distributed around the fault depression, and the fan delta front extends into the lake. During D2 (the end of the transition period), several geological processes exist: (1) the alluvial fan, fan delta, braided river, braided river delta and lake sediments developed; (2) the unified sediment unloading area is formed. Taking the community of the floodplain and shore-shallow lake contact zone as a boundary, the western part of the boundary mainly developed alluvial fan and braided river sedimentary system from the provenance highland in the western and northern, the east part mainly developed lacustrine facies massively. With the debris sediments of provenance highland injected into the lake basin successively, it constituted retrograde fan deltas in different scales. Among them, the fan delta front and braided delta front subfacies are the main enrichment reservoirs for oil and gas, which is the favorable facies of oil and gas exploration in the Denglouku formation in the future.

Sedimentary characteristics and pattern of distributary channels in shallow water deltaic red bed succession: A case from the Late Cretaceous Yaojia formation, southern Songliao Basin, NE China

Shallow water delta developed during the Late Cretaceous greenhouse climate depositing in the red bed succession of the Yaojia Formation in the Southern Songliao Basin, NE China. The shallow water delta deposits provide an opportunity to understand the influence of high discharge variability in a semi-arid climate on the fluvial patterns, internal sedimentary details, and reservoir quality of distributary channels. The sedimentary process of the shallow water delta in the Yaojia Formation was controlled by a dual-stage process, including a high-flow stage and a low-flow stage within a semi-arid paleoclimate. During the high-flow stage, crevasse channels formed through avulsion of major distributary channels on the delta plain. Subaqueous terminal distributary channels and mouth bars formed at the delta front during progradation of the delta. During the low-flow stage after flood events, crevasse channels were abandoned and frequently exposed becoming reddish in color. This process can be reason for the classification for two main types of distributary channels: major distributary channels and crevasse channels in the delta plain. Sand bodies in distributary channels are distinguished by sedimentary structures indicate critical and supercritical flow conditions. Based on well logs and 3D seismic data, the sand distribution of was analyzed from the sandstone isopach map of the 2nd sand bed in the first member of the Yaojia Formation. The framework sand belts develop coincides with distribution of the major distributary channels. Based on detailed observational and reservoir test data from cores, the distribution of major distributary channels revealed a belt of high-quality reservoirs in the shallow water delta. This study provides a new perspective on the sedimentary pattern of distributary channels in shallow water deltas with high discharge variability, which has important implications for reservoir exploration in lacustrine basins.

Authigenic minerals related to wettability and their impacts on oil accumulation in tight sandstone reservoirs: An example from the Lower Cretaceous Quantou Formation in the southern Songliao Basin, China

Oil accumulation, being difficult and complicated, is an important issue in petroleum exploration researches. Authigenic minerals, such as carbonate cements and chlorite, can in certain reservoirs alter the wettability of some pore spaces from water-wet to oil-wet. Generally, these oil-wet pore spaces are favorable for oil accumulation. The alteration of reservoir wettability induced by authigenic minerals and the impacts on oil accumulation in tight sandstone reservoirs were investigated using a suite of mineralogical and geochemical characterization techniques, including thin section observation, SEM-EDS, XRD, QEMSEM, CL, quantitative grain fluorescence (QGF), fluorescence spectral analysis, contact angle measurement and sealed coring oil saturation testing on the fourth member of the Lower Cretaceous Quantou Formation (K1q4) in the southern Songliao Basin, China. The study shows that the tight sandstone reservoirs are compositionally immature with detrital grains distributed homogeneously. Quartz, carbonates and clay minerals that show heterogeneous distribution characteristics are the major authigenic minerals in some parts of the intergranular pores. The detrital mineral assemblage suggests that the reservoir rocks at deposition and before diagenesis had characterized by strong water-wet properties. With the development of authigenic minerals, carbonate cements and authigenic chlorite tend to alter the wettability of some parts of the existing pore spaces from water-wet to oil-wet. In the K1q4 sandstone reservoirs, oil prefers to accumulate in the cemented residual pore spaces around carbonate cements and chlorite. Reservoirs containing about 4–5% carbonate cements are suggested to be more preferable to oil accumulation. These reservoirs are mainly located between sandstone-mudstone interfaces and central parts of the sand bodies. Chlorites have mainly two effects: on one hand, chlorite alters the wettability of existing pore spaces and provides preferential accumulation sites for oil in the tight sandstone reservoirs, on the other hand it can reduce the adhesion of oil through forming “clay-oil flocs” and promote further migration. Consequently, the reservoirs with moderate amount of carbonate cements or chlorite always show relative high oil saturation in the K1q4 tight sandstones.

Late Cretaceous–Cenozoic thermochronology in the southern Songliao Basin, NE China: New insights from apatite and zircon fission track analysis

The Late Cretaceous–Cenozoic thermochronology in the southern Songliao Basin was determined using apatite and zircon fission track analysis data. It is crucial to deciphering the evolution of the Songliao Basin and changes in the movement direction of the Pacific Plate. We present eight new apatite and eight corresponding zircon fission track data from borehole samples across the Baixingtu uplift. The apatite fission track (AFT) thermochronological modeling reveals that the basin underwent two distinct compression- and extension-related uplift stages of rapid cooling during the Late Cretaceous–Cenozoic: stage I during the Late Cretaceous–Early Paleogene (∼70–52 Ma), stage II in the Late Paleogene–Early Neogene (∼38–10 Ma). The AFT data vary from 76.0 ± 5.0 to 25.3 ± 1.9 Ma and the zircon binomial peak fitting ages ranged from 79 to 61 Ma, all younger than the stratigraphic ages (∼92–88 Ma) of their host rocks. One sample had similar fission track ages (apatite 76.0 ± 5.0 Ma and zircon 79.0 ± 3.0 Ma) within reasonable error, representing a magmatic event. The data were divided into four groups. (i) 79–76 Ma: magmatic cooling event, (ii) 69–61 Ma: uplift of the whole region and exhumation and erosion, (iii) 37–34 Ma and (iv) ∼25 Ma: various uplift events related to tectonic inversion, causing partial uplift, exhumation and erosion in the basin. This created favorable conditions for formation and storage of mineral resources, especially sandstone uranium ore deposits. Our results combined with previous research suggest that the variation in uplift and exhumation began in the basin’s SE part and propagated to the NW ∼5–8 Ma later. The tectonic inversion in the southern Songliao Basin at least began ∼37–34 Ma. This corresponds with the time of the Paleo-Pacific Plate shifting to NWW motion and the India–Asia collision. These regional-scale uplift, exhumation and inversion events were probably responses to the subduction and turning of the Paleo-Pacific Plate from the southeast and the collision of the Indian subcontinent from the southwest. It is important to study the tectonic inversion in the whole east of China since the Late Cretaceous and the change of the direction of the Pacific plate movement.