Sedimentary Period Research Articles

Accumulation conditions and key exploration & development technologies of heavy oil in Huanxiling oilfield in Liaohe depression, Bohai Bay Basin

The Huanxiling oilfield is located in the southern part of the western slope of the western sag in Liaohe depression. The west side of this oilfield is connected with two sets of high-quality source rocks of Member 3 and Member 4 of Shahejie Formation in Qingshui sub-sag. The oilfield has fan delta, turbidite fan and other types of reservoirs, it also has cap rock of thick mudstone in Member 3 and Member 4 of Shahejie Formation. Under background of the warped basement, the warped fault-block draped compound trap zone are developed, which includes nine types of trap. From perspective of hydrocarbon accumulation, the slope of this area has always been the target area for hydrocarbon migration and accumulation. Inclusion analysis shows that there are multiple stages of hydrocarbon charging in this area, and the main reservoir forming period is the sedimentary period of Member 3 of Shahejie Formation and the sedimentary period of Dongying Formation. High-quality source-reservoir-cap conditions ensure large-scale hydrocarbon accumulation in this area. Based on the theory of compound hydrocarbon accumulation, many types of oil and gas reservoirs, including light oil reservoir and heavy oil reservoir, have been found in this area, with total reserves of 500 million tons. In view of the oilfield characterized by large reservoir burial span, multiple oil-bearing strata, strong heterogeneity and various types of oils, multi-batch seismic data processing & interpretation technology and thin reservoir inversion technology based on geological model are established in the preliminary exploration period, steam-flooding physical simulation technology of heavy oil, oil-reservoir fine description technology of thermal recovery heavy oil, steam huff and puff technology of ordinary heavy oil and steam-flooding technology of mid-deep buried heavy oil are developed in the development period, and technologies such as separate-layer injection, selective injection, sand control and lifting of heavy oil are matched and improved. These technology series provides technical guarantee for efficient exploration and development of Huanxiling oilfield.

Effects of lacustrine organic-rich fine-grained sedimentary rocks on the accumulation of tight oil

In order to establish the distribution pattern of fine-grained sedimentary rocks and clarify the coupling relationship between lacustrine organic-rich fine-grained sedimentary rocks and tight oil reservoir, the Chang7 Member of Triassic Yanchang Formation in Longdong area was taken as an example to be studied by core description and log interpretation, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Lacustrine organic-rich fine-grained sedimentary rocks were divided into shale and mudstone according to their lithology and texture. The results show that the shale is mainly developed in Chang73 Sub-member, while the mudstone is developed in each sub-member of Chang7 Member. The organic-rich fine-grained sedimentary rocks in Bazhu-Maling-Qingcheng-Huachi area are the thickest in Longdong area. During the Chang73 sedimentary period, the shale was widely distributed in the center of the lake. In the Chang72 sedimentary period, the mudstone and turbidite were interstratified and superimposed. During the Chang71 sedimentary period, the mudstone was only distributed as thin layers between the thick turbidite. The Chang7 tight oil is widely distributed in the turbidite sand bodies, and it is mainly sourced from the Chang73 shale. The distribution of tight oil is controlled by the development of organic-rich fine-grained sedimentary rocks.

Provenance analysis and tectonic setting of the Upper Jurassic Series in Mohe Basin, northeast China: Implication for the closure of Mongol–Okhotsk Ocean

The Mohe Basin, to the south of Mongol–Okhotsk suture belt (MOSB) closely, is a good window into investigations of the evolution of the Mongol–Okhotsk Ocean during the Late Mesozoic. Sandstone samples of the Upper Jurassic Series in the Mohe Basin, Mohe, and Kaikukang formations are studied in detail in terms of petrography, geochronology, and geochemistry. All samples have low compositional and textural maturity, which are located in the areas of dissected arc and basement uplift in Dickinson's discrimination diagrams. LA‐ICP‐MS U–Pb zircon dating yields the youngest concordant ages of 173 ± 4 Ma and 157 ± 3 Ma for the Mohe and Kaikukang formations, respectively, which constrain the maximum sedimentary periods of them. The peak ages of all concordant ages correspond identically to the regional magmatic activities on both the Siberian Craton and Erguna Block. All samples demonstrate geochemical characteristics not so different from the upper continental crust, whose provenance is felsic in lithology and is situated in active continental margins. Thus, a Mohe foreland basin and its binary provenance are strongly suggested, with the Erguna Block – a southern source and the Siberian Craton – a northern one. Based on all the evidence above, a simple tectonic model of the evolution of the Mongol–Okhotsk Ocean is raised, that is, from the Triassic to the Middle Jurassic; the Mongol–Okhotsk oceanic plate continued to subduct under the Siberian Craton and the Erguna Block, triggering intense magmatism in these areas. In the Late Jurassic, the rapid closure of the Mongol–Okhotsk Ocean caused the basement uplift and intense denudation which provided abundant sources of dissected arc and basement materials for the Mohe Basin.

Clay Minerals and Element Geochemistry of Clastic Reservoirs in the Xiaganchaigou Formation of the Lenghuqi Area, Northern Qaidam Basin, China

We performed mineralogical and geochemical analyses of core samples from the Lenghuqi area in the northern marginal tectonic belt of the Qaidam Basin. The clay mineralogy of the Xiaganchaigou Formation sandstone is dominated by I + I/S + C types and characterized by high illite, a higher mixed-layer illite/smectite and chlorite, lesser smectite, and an absence of kaolinite. The clay minerals reflect that the Oligocene sedimentary basin formed in an arid-semi-arid climate with weak leaching and chemical weathering, and that diagenesis occurred in a K+- and Mg2+-rich alkaline environment. Measured major oxide concentrations show clear correlations. The lower Xiaganchaigou Formation is representative of a dry and cold freshwater sedimentary environment, whereas the upper Xiaganchaigou Formation is warmer and more humid. Trace element and rare earth element variations indicate that the paleoclimate conditions of the lower Xiaganchaigou Formation sedimentary period were relatively cold and dry, while the upper Xiaganchaigou Formation formed under warmer and more humid climate conditions. These findings reflect a global climate of a cold and dry period from the late Eocene to early Oligocene, and a short warming period in the late Oligocene.

Trace Elemental Geochemistry and Depositional Environment of Shale Oil Reservoir Rocks within the Permian Lucaogou Formation, Jimusaer Sag

By analyzing the geochemical indicators of trace elements and rare earth elements in shale oil cores, this paper clarifies the depositional environment of the Lucaogou Formation, and it provides some guidance for the exploration and deployment of shale oil in the Jimsar Sag. The average value of Th/U in the Lucaogou Formation is 1.89, which of V/(V+Ni) is 0.74, which of V/Cr is 3.05, which of V/Sc is 8.31, and w(Ni) are less than 40×10−6, indicating the terrestrial reduced environment. The Sr/Ba ratio changes frequently, ranging from 0.55 to 25.62, averaging 4.42, reflecting a higher paleo-salinity.87Sr/86Sr ratio ranges from 0.705286 to 0.705784, averaging 0.705486, and it is substantially lower than the average value of the strontium isotope of the crustaceous silicon-aluminum rock. Besides, 143Nd/144Nd ratio ranges from 0.512419 to 0.512722, and sediments have the characteristics of the origin of the mantle, reflecting the intracontinental rift formation environment. The analysis of rare earth elements indicates that the sediments has the characteristics of LREE relative enrichment, Ce weak positive anomaly and Eu negative anomaly, and furtherly excluded the possibility of marine sedimentary environment. Comprehensive analysis shows the sedimentary period of the Lucaogou Formation is a reduced and salty intracontinental rift lacustrine environment.

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in the Upper Yangtze Area: Taking Jiaoye-1 Well in the Jiaoshiba Block as an Example

Organic matter is the material basis of shale hydrocarbon generation. The current organic matter content in shale is controlled by the original sedimentary organic matter abundance. Therefore, the study of the enrichment mechanism of sedimentary organic matter in shale has become an important issue to be solved. The Upper Yangtze area is the important exploration and exploitation area of marine shale gas in China. The shale of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Yangtze area is the research object. Choosing redox indicator and biological productivity indicator, the study explores the enrichment mechanism of sedimentary organic matter from two aspects, sealing of water and volcanic activity. The results show that excess siliceous mineral in the shale of the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area is bioorigin. Excess siliceous mineral can be used as one of the indicators of biological productivity. On the one hand, layer phenomenon occurred since the strong water sealing during the sedimentary period of Wufeng and the lower section of the Longmaxi Formation, which results in the high content of oxygen in surface water. On the other hand, the active volcanic activity brought volcanic ash which was beneficial to biological reproduction. Both of these factors led to higher biological productivity during this period. At the same time, the strong sealing of water made the lower layer of the water more reductive, and the active volcanic activity caused climate change, enhancing the reduction of the lower layer of the water, which made the rich organic matter deposited from the surface water well preserved. In the sedimentary period of the upper section of the Longmaxi Formation 1st member in the Upper Yangtze area, on the one hand, due to the weakened sealing of water, the oxygen content of the upper water decreased. On the other hand, the volcanic activity weakened until it stopped, and the source of volcanic ash rich in nutrient elements decreased. These two aspects led to lower biological productivity during this period. At the same time, the weaker water sealing could lead to a decrease in the reduction of the lower layer of the water, and the gradual cessation of volcanic activity no longer affected the climate, causing the destruction of sedimentary organic matter by oxidation.

Geochemistry and sedimentology of the Upper Ordovician–lower Silurian black shale in the northern margin of the Upper Yangtze Platform, South China: implications for depositional controls on organic-matter accumulation

Organic-rich black shales from the Upper Yangtze Platform from the Upper Ordovician and lower Silurian are considered excellent source rocks and unconventional reservoirs of hydrocarbons in South China. This research combined geochemical analyses and detailed sedimentological observations to investigate the depositional controls on organic-matter abundance in the prolific black mudstone of the Wufeng Formation and Long-1 Member of the lower Silurian Longmaxi. Six primary lithofacies deposited at variable marine water depths and under different water column chemistries were identified from a ∼50 m-long profile based on microscopic observations of sediment texture and structure, and analyses of total organic carbon (TOC) content, major and trace-element abundances, and pyrite framboid size. Our results showed that TOC content is strongly correlated with terrigenous SiO2 content and the non-detrital components of V, U and Mo, suggesting that the accumulation of organic matter in the studied shale was controlled by terrigenous clast fluxes and anoxic water conditions. However, the weak covariance relationship between TOC content and productivity proxies, including P/Ti and Ba/Al, demonstrates that the accumulation of organic matter was not controlled by primary productivity. The three high-productivity lithofacies of the Wufeng Formation and Long-1 Member have low Co*Mn values, indicating active marine upwelling during sedimentary periods. The northern margin of the Upper Yangtze Platform was an open basin system influenced by the Kwangsian Orogeny and global sea-level changes, but oxygen-depleted bottom waters in the basin favoured the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich black shales.

High Precision Sequence Division and Geological Significance of Nantun Formation in Wunan Subsag

Based on the data of core and logging, high precision sequence stratigraphic framework of Nantun Formation of Wunan subsag, Wuerxun Depression in Hailar Basin and sedimentary systems distribution characteristics which is under its control are studied. It shows that there are five types of sequence boundaries in Nantun Formation, and it can further be broken into 3 three-level and 9 four-level sequence. According to the variation of remaining layer thickness of each sequence, the change rule of deposition centers in Nantun Formation appears transferring to west gradually, and from down to up the formation distribution characteristics changes from ‘medium thickness to east and west thin’ to ‘west thick- east thin’. During the sedimentary period, four sedimentary systems mainly develops: braided river delta, fan delta, sublacustrine fan and lake. The plane distribution of sedimentary system has the characteristics of ‘east-west zoning’. The fan delta sedimentary system mainly develops in the western Wuxi slope zone, and the braided river delta mainly develops in the eastern Wudong gentle slope zone. The plane distribution characteristics of each sedimentary system in different periods have obvious inheritance.

Activity of hydrothermal fluid at the bottom of a lake and its influence on the development of high-quality source rocks: Triassic Yanchang Formation, southern Ordos Basin, China

Evidence for hydrothermal activity, such as siliceous rocks, iron dolomite layer deposits, and hydrothermal mineral combinations of ilmenite + pyrite + anhydrite, has been found in the high-quality source rocks from the Chang 7 layer in the Triassic Yanchang Formation from the southern margin of the Ordos Basin, China. In this paper, elemental geochemical analysis is carried out on a large number of samples collected from an outcrop geological section and drill core from the Chang 7 layer. We use elemental ratios to study the hydrothermal activity; the Al/(Al + Fe + Mn) ratio reflects the paleoproductivity, and the P/Al and P/Ti ratios and total S contents reflect the redox environment. The relationships between the hydrothermal activity and paleoproductivity, redox conditions and organic matter content are discussed. The correlation coefficients between the hydrothermal index Al/(Al + Fe + Mn) and the (Fe + Mn)/Ti ratio and total organic carbon are as high as –0.88 and 0.87, respectively, demonstrating a close relationship between hydrothermal activity and source-rock deposition. The correlation coefficients between the Al/(Al + Fe + Mn) ratio and the P/Al and P/Ti ratios, and total S contents are –0.81, –0.80, and –0.89, respectively, supporting this relationship. Hydrothermal activity can promote productivity at the lake surface and the development of a bottom-water anoxic environment. The paleoproductivity level of the Chang 7 sedimentary period is relatively high and represents an oxygen-poor anaerobic environment. The hydrothermal activity in the southern margin of the basin is strong, whereas the activity in the north is gradually weakened. Studies have shown that the hydrothermal activity at the bottom of the lake plays an important role in promoting the large-scale development of the Chang 7 high-quality source rocks. The organic matter abundance in samples with hydrothermal alteration are greater than in samples elsewhere.

Development of Upwelling during the Sedimentary Period of the Organic-Rich Shales in the Wufeng and Longmaxi Formations of the Upper Yangtze Region and Its Impact on Organic Matter Enrichment

This study uses logging data, mineral component content, total organic carbon (TOC) content, and microscopic characteristics of the organic-rich shales in the Wufeng and Longmaxi Formations, as well as data reported by other researchers, to demonstrate that upwelling has played an important role in the organic matter enrichment. The results show that (1) the organic-rich shales of Well N211 in the Upper Yangtze region are located in the Wufeng Formation and the lower Longmaxi Formation, with a burial depth between 2308–2357 m. (2) The organic-rich shales are enriched in biogenic silica. (3) Based on paleogeographic location and characteristics of organisms, this study determines that upwelling occurred during the deposition of the organic-rich shales in the Wufeng and Longmaxi Formations, promoting the enrichment of organic matter in the shales. (4) The upwelling intensity gradually increased from the sedimentary period of the organic-rich shales in the mid-lower Wufeng Formation to the sedimentary period of the Guanyinqiao Member, and then decreased gradually from the sedimentary period of the Guanyinqiao Member to the sedimentary period of the organic-rich shales in the Longmaxi Formation, and leads to the different enrichment of organic matter in the vertical direction. The different developments of upwelling led to the coexistence of both high and low TOC contents in the Guanyinqiao Member along the vertical direction.

Sedimentary model reconstruction and exploration significance of Permian He 8 Member in Ordos Basin, NW China

Abstract Based on the Late Paleozoic geological background and the latest exploration achievements of the Ordos Basin and North China platform, it is concluded that during the sedimentary period of Permian He 8 Member, the area in concern had multiple material sources, multiple river systems, flat terrain, shallow sedimentary water, widely distributed fluvial facies sand body and no continuous lake area, so alluvial river sedimentary system developed in the whole region. Based on stratigraphic correlation and division, and a large number of drilling and outcrop data, a comprehensive analysis of lithofacies and sedimentary facies types and distribution was carried out to reconstruct the ancient geographic pattern of the He 8 Member sedimentary period. The results of paleogeography restoration show that the area of Ordos Basin was the “runoff area” in the sedimentary slope in the western part of the North China platform during the sedimentary period of He 8 Member, the whole region was mainly alluvial plain sedimentation featuring alternate fluvial facies, flood plain facies and flood-plain lake facies. According to the results of flume deposition simulation experiment, a new sedimentary model of “alluvial river & flood-plain lake” is established, which reveals the genesis of large area gravel sand body in He 8 Member of this area and provides geological basis for the exploration of tight gas in the south of the basin.

Sequence stratigraphy and lithofacies paleogeography of the Middle–Upper Cambrian Xixiangchi Group in the Sichuan Basin and its adjacent area, SW China

Abstract Based on isotopic, lithologic and electrical data and logging cycle analysis technique, stratigraphic sequences in the Middle–Upper Cambrian Xixiangchi Group in the Sichuan Basin and its adjacent area are divided, and its sedimentary characteristics and evolution are analyzed. The Xixiangchi Group can be divided into 5 third-order sequences (Sq1–Sq5), of which sequences I to III (Sq1–Sq3) are relatively complete, sequences IV and V are denuded in the Late Cambrian because of the Dian-Chuan paleo-uplifts. Third-order sequences of the Xixiangchi Group in this area have the characteristics of thin in the west and thick in the east, showing that the Caledonian paleo-uplift is a synsedimentary paleo-uplift and the paleogeomorphology in the platform is a gentle slope. Sequence I develops high stand systems tract and transgressive systems tract. The other third-order sequences are dominated by highstand systems tracts, and the transgressive systems tracts last shortly in time and are limited in area. The basic features of evaporative–restricted platform of gentle slope type developed continuously in the sedimentary period of the Xixiangchi Group, its sedimentary environment of “high in west and low in east” and the change of micro paleogeomorphology in the platform control the continuous development of sedimentary facies. Open platform is developed only in sequence I and sequence II, and the inner beach of the platform and the edge beach of the platform are mostly developed in sequence II and sequence III. It indicates that there are two platform margin zones in the study area, a relatively stable, large-scale platform marginal zone in NE Guizhou–Western Hunan and Hubei, and a moving and small-scale platform marginal zone in North Chongqing–Western Hubei.

New insights into multiple provenances evolution of the Jurassic from heavy minerals characteristics in southern Junggar Basin, NW China

Abstract The method of random forest was used to classify the heavy mineral assemblages of 2 418 Jurassic samples in the southern Junggar Basin, and determine the distribution of the heavy mineral assemblages from the same provenance systems. Based on the analysis of heavy minerals assemblages, ZTR index, sedimentary characteristics, U-Pb zircon ages, whole-rock geochemical and paleocurrent direction analysis, the study reveals that five important provenances were providing sediments to the southern Junggar Basin in the Jurassic period: The North Tianshan (NTS), Central Tianshan (CTS), Bogda Mountains, Zhayier Mountains and Kalamaili Mountains. During the Early Jurassic, NTS-CTS, Kalamaili Mountains and Zhayier Mountains are primary provenances, Bogda Mountains started to uplift and supply clastic materials in the Middle Jurassic. There are three sedimentary area in the Jurassic of southern Junggar Basin: the western part, the central part and the eastern part. In the western part, the clastic materials of the Early Jurassic was mainly from NTS blocks and Zhayier Mountains, and the sediments were dominantly derived from the Zhayier Mountains during the Middle–Late Jurassic. In the central part, the main provenance of the Early Jurassic switched from NTS to CTS. In the Xishanyao Formation, the main source went back to NTS again. The NTS was the primary provenance during the sedimentary periods of Toutunhe Formation and Qigu Formation. In the eastern part, the contribution of CTS and Kalamaili Mountains were considered as major provenances in the Early Jurassic−Xishanyao Formation, small proportion of sediments were from NTS. The Bogda mountains uplifted and started to provide sediments to the Junggar Basin in the sedimentary period of Xishanyao Formation, and became the major source during the Toutunhe Formation period, with small amount of sediments from CTS. The provenance from CTS was hindered during the sedimentary period of Qigu Formation owing to the uplifting of the Bogda mountains, and the sediments were mainly from the Bogda mountains and NTS.

Paleo-sedimentary environmental restoration and its significance of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin, NW China

Abstract Paleo-sedimentary environment of Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, including the paleoclimate, paleo-salinity and paleo-redox conditions were restored through geochemical elements analysis of 289 samples collected from the outcrop sections around and wells drilled in the basin and using a series of identification indexes of paleo-climate, paleo-salinity and paleo-redox conditions, such as CaO/MgO·Al2O3, Sr/Cu, Rb/Sr, Rb/K2O, Th/U, V/(V+Ni), the content of element B tested from the mudstone. Comprehensive analysis shows that in sedimentary period of the Chang 7, the paleo-climate was warm temperate to subtropical climate with temperature higher than 15 °C, the water body was continental brackish water to freshwater, and the sediments were deposited under strong reduction conditions. Suitable temperature, extensively deep lake basin and strongly reductive paleo-sedimentary environment led to the blooming, enrichment and preservation of organic matter in the submember Chang 73. As a result, a set of high-quality source rock was formed, laying material foundation for large-scale accumulation of shale oil.

Sedimentary facies and sequence stratigraphy in the Lower Triassic Jialingjiang Formation, Sichuan Basin, China

The Lower Triassic Jialingjiang Formation is an important oil and gas exploration target in the Sichuan Basin. The lithofacies mainly consists of clastic rocks, carbonates, and evaporates, which represents a transition from continental to marine depositional environment. The Jialingjiang Formation of the Sichuan Basin is a typical epeiric carbonate platforms. The study area contains four types of facies, including mixed tidal flat facies, open platform facies, restricted platform facies, and evaporitic platform facies. Based on the analysis of cores and logging data, two types of type II sequence boundaries were identified, which are, respectively, local exposure unconformity boundary and lithology and lithofacies transformation boundary. The Jialingjiang Formation is divided into three third-order sequences and six system tracts. Thus, we establish a connected multi-well sequence stratigraphic framework based on typical single-well sequence stratigraphic division. The sedimentary evolution of the Jialingjiang Formation was divided into three major sedimentary periods and six stages, with rising and falling of the sea level, mixed tidal flat, open platform, and restricted platform prograded first, and then restricted platform and evaporitic platform retrograded. It also shows that the open platform shoal and restricted platform tidal flat have oolitic limestone and algal dolomite with good-quality physical properties and are the favorable reservoir facies belt for hydrocarbon exploration and development.

Provenance Characteristics of Nantun Formation in Baer Depression and its Dominate of the Depositional System

Fine depicting of antiquity source environment is of great significance of depositional systems restoration, finding the dominant sandstone development zone and forecasting exploration target areas. This paper uses data such as palaeo-geomorphology, seismic attributes, heavy minerals and sandstone present, to analyse and depict the source system of Nantun Formation sedimentary period from different angles and levels. It shows that the provenance of Nantun period in Baer Depression is mainly from the slope zone around the depression and low bulges in the basin, converging from the basin to the center. The direction of the main source of N1 and N2 sedimentary period have obviously inheritance. There are 9 major source systems in Nantun formation of Baer depression which is developing sedimentary environment such as fan dalta and sublacustrine facies.

Shale gas reservoir characterization: A typical case in the Southeast Chongqing of Sichuan Basin, China.

The Lower Silurian Longmaxi Shale in Southeast Chongqing of Sichuan Basin in China is considered to be a potential shale gas reservoir by many scholars in recent years. The special shale gas well, namely, Pengye-1 well, was selected as a case study to evaluate the characteristics of the shale gas reservoir. A series of experiments were performed to analyze the geochemical, mineralogical, and petrophysical features and gas content using samples of the Longmaxi Shale from Pengye-1 well. The results show that the organic and inorganic porosities of these samples are range of 0.08–2.73% and 0.06–2.65%, with the average of 1.10% and 1.76%, respectively. The inorganic pores primarily contribute to the porosity until the TOC content is more than 3%. Organic matter plays an important role in adsorbed gas content. The adsorbed gas is dominant in the Longmaxi Shale of Pengye-1 well, which ranges from 0.46 to 2.24 cm3/g, with an average of 1.38 cm3/g. The free gas content ranges from 0.45 to 0.84 cm3/g with an average of 0.68 cm3/g, and is 24.4–49.7 percent of total gas with an average of 37.5%. The bottom part of the Longmaxi Shale is the most favorable for shale gas exploring, which is higher of brittleness mineral content, porosity and gas content. Compare with the other five shales in America, the Lower Silurian Longmaxi Shale is derived from older sedimentary periods with significantly higher thermal maturity and has experienced several periods of intense tectonic, which are unfavorable for the shale gas enrichment.

Sequence subdivision and development characteristics of low-accommodation non-marine basins: a case study of the Yanchang Formation in the Ordos Basin

ABSTRACTThe third-order sequences of the Yanchang Formation in the Ordos Basin were determined by a comprehensive analysis of surface outcrops, core, logging and seismic data. Natural gamma curves were studied by the band-pass method. The hidden stratigraphic sequence and six lithological cycles were extracted. By examining sequence boundaries and lithological cycles, we identified six third-order (including SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6) and three second-order sequences in the Yanchang Formation. The sequence stratigraphy of the Ordos Basin was found to have the following special development characteristics: (1) the third-order sequence growths were totally stable, and the thickness of the Ordos Basin was relatively stable; (2) thick sand bodies did not form in lowstand systems because of underdeveloped slope breaks; (3) the third-order sequences had diverse features at different sedimentary periods; (4) the distribution of the depositional systems had various characteristics in different tectonic belts, and the entire sequence had the filling characteristics of multiprovenances and multicycles. The low-accommodation Ordos Basin in the Upper Triassic was limited by slow subsidence, gentle basement paleorelief, stable tectonic settings and sufficient sediment supply. The characteristics of the basin included stable developed sequences, limited preserved lowstand systems tracts, multiple depocentres and subsiding centres, development of many delta systems in the north and south of the basin controlled by the gradient of the basement paleorelief, and laterally stretching sediments. Results further indicated that the controlling factors of the third-order stratigraphic sequences of Yanchang Formation in the low-accommodation lacustrine Ordos Basin included sediment supply (provenance), climate and lake-level fluctuation. Overall, our case study indicated that the development characteristics and controlling factors of sequence stratigraphy in a low-accommodation lake basin provided a comprehensive method of establishing the sequence stratigraphic framework of a low-accommodation non-marine basin. Our results also predicted that the SQ3, SQ4 and SQ5 of the Qiaochuan and Zhidan areas favoured oil and gas prospecting of the Yanchang Formation.

Change in Sediment Provenance Near the Current Estuary of Yellow River Since the Holocene Transgression

Sedimentary sequence and sediment provenance are important factors when it comes to the studies on marine sedimentation. This paper studies grain size distribution, lithological characteristics, major and rare earth elemental compositions, micropaleontological features and 14C ages in order to examine sedimentary sequence and sediment provenance of the core BH6 drilled at the mouth of the Yellow River in Bohai Sea. According to the grain size and the micropaleontological compositions, 4 sedimentary units have been identified. Unit 1 (0–8.08 mbsf) is of the delta sedimentary facies, Unit 2 (8.08–12.08 mbsf) is of the neritic shelf facies, Unit 3 (12.08–23.85 mbsf) is of near-estuary beach-tidal facies, and Unit 4 (23.85 mbsf–) is of the continental lake facies. The deposits from Unit 1 to Unit 3 have been found to be marine strata formed after the Holocene transgression at about 10 ka BP, while Unit 4 is continental lacustrine deposit formed before 10 ka BP. The provenances of core BH6 sediments show properties of the continental crust and vary in different sedimentary periods. For Unit 4 sediments, the source regions are dispersed while the main provenance is not clear, although the parent rock characteristics of a few samples are similar to the Luanhe River sediments. For Unit 3, sediments at 21.1–23.85 mbsf have been mainly transported from the Liaohe River, while sediments above 21.1 mbsf are mainly from the Yellow River and partially from the Liaohe River. For Unit 2, the sediments have been mainly transported from the Yellow River, with a small amount from other rivers. For Unit 1, the provenance is mainly the Yellow River catchment. These results help in better understanding the evolution of the Yellow River Delta.

Orbital cyclicity in sedimentary sequence and climatic indications of C-O isotopes from Lower Cretaceous in Qingxi Sag, Jiuquan Basin, NW China

Sedimentary deposits of the Lower Cretaceous Xiagou Formation form the most significant potential hydrocarbon reservoirs in the Qingxi Sag, Jiuquan Basin (NW China). Zircon U-Pb ages of the dated basalts at the top of the Xiagou Formation give an isochron age of 115.6 Ma, and the sedimentation interval of the Xiagou Formation was speculated to range from about 125/124 Ma to 115 Ma based on paleontological research and stratigraphic correlation analysis. Here we use GR logging data as a palaeoenvironmental and palaeoclimatic proxy to conduct a detailed cyclostratigraphic study of five selected wells. Power spectra, evolutionary fast Fourier transformation and wavelet analysis all reveal significant sedimentary cycles in the Xiagou Formation. The ratios of cycle wavelengths in these stratigraphic units are 33.82 m : 7.91 m : 3.06 m : 1.79 m, which is similar to the ratio of orbital targets of 20 : 5 : 2 : 1. The ratio of 20 : 5 : 2 : 1 is interpreted as Milankovitch cycles of 405 kyr long eccentricity, 100 kyr short eccentricity, 37 kyr obliquity, and 22 kyr precession cycles respectively. A high-resolution astronomical time scale is constructed by tuning the stratigraphy into target curves of orbital cycles respectively. Based on the astronomical time scale, the absolute ages of 55 samples were estimated, which are used for subsequent stable carbon and oxygen isotope stratigraphy analysis. The analysis results of the five studied wells in the Qingxi Sag indicate: (1) a negative trend of δ13C values upwards in the Xiagou Formation, and (2) negative δ18O values with a positive trend upwards. Both relatively heavy values and pronounced covariances of δ13C values and δ18O values indicate an arid-evaporation-controlled climate during the sedimentary period of the Lower Cretaceous Xiagou Formation, Qingxi Sag, Jiuquan Basin. Moreover, positive covariances of SQK1g2+3 indicate extremely high temperature, and negative covariances of SQK1g1 indicate a relatively low temperature.